ICD GPS 200Cw/IRNs 1_2_3_4 200C
User Manual: 200C
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UNCLASSIFIED REVISIONS DESCRIPTION DATE ICD-GPS-200, Revision C, Initial Release 10 October 1993 IRN-200C-001 13 October 1995 IRN-200C-002 25 September 1997 IRN-200C-003 11 October 1999 IRN-200C-004 12 April 2000 DISTRIBUTION STATEMENT A APPROVED FOR PUBLIC RELEASE; DISTRIBUTION IS UNLIMITED. APPROVALS AUTHORIZED SIGNATURES REPRESENTING DATE GPS NAVSTAR JPO Signature on file 15 December 1994 SMC/CZ (AFMC) ROCKWELL INTERNATIONAL Signature on file 16 November 1993 SPACE SYSTEMS DIVISION ROCKWELL INTERNATIONAL Signature on file 15 November 1993 COLLINS AVIONICS & COMMUNICATIONS DIVISION INTERNATIONAL BUSINESS MACHINES (IBM) Signature on file 02 December 1993 FEDERAL SYSTEMS COMPANY MARTIN-MARIETTA Signature on file * 05 August 1994 ASTRO SPACE DIVISION * An asterisk affixed to the approval signature indicates that the approval is subject to exceptions taken in the "Letter of Exception" contained in Appendix I of this document. INTERFACE CONTROL DOCUMENT UNLESS OTHERWISE SPECIFIED: DIMENSIONS ARE IN INCHES. TOLERANCES ON: DECIMALS ANGLES XX = ±0.03 ±0° 30' XXX = ±0.01 THIS DOCUMENT SPECIFIES TECHNICAL REQUIREMENTS AND NOTHING HEREIN CONTAINED SHALL BE DEEMED TO ALTER THE TERMS OF ANY CONTRACT OR PURCHASE ORDER BETWEEN ALL PARTIES AFFECTED. DR BY CHK BY APPROVALS 10 Oct 1993 ARINC RESEARCH CORPORATION 2250 E. Imperial Highway, Suite 450 El Segundo, CA 90245-3509 ICD TITLE Navstar GPS Space Segment / Navigation User Interfaces SIZE CODE IDENT NO. DRAWING NO. A OVYX1 ICD-GPS-200 SCALE: UNCLASSIFIED N/A REV: C SHEET i UNCLASSIFIED (This page intentionally left blank.) ICD-GPS-200C 10 OCT 1993 ii UNCLASSIFIED UNCLASSIFIED REVISION RECORD LTR DESCRIPTION DATE APPROVED Initial Release 25 Jan 1983 A Incorporates IRN-200NC-001, IRN-200NC-002, and IRN200NC-003 25 Sep 1984 B Incorporates IRN-200A-001A 30 Nov 1987 C Incorporates IRN-200B-001 thru IRN-200B-007 10 Oct 1993 C Re-formatted in Microsoft Word 6.0 in GEMS compatible format 10 Oct 1993 12 Jan 1996 C Changed distribution status to Public Release 25 Sep 1997 20 Oct 1997 NC IRN-200C-001 ICD-GPS-200C 13 OCT 1995 iii UNCLASSIFIED UNCLASSIFIED (This page intentionally left blank.) ICD-GPS-200C 10 OCT 1993 iv UNCLASSIFIED UNCLASSIFIED Page Revision Record Pages Revision Pages Revision i IRN-200C-004 83-84 Original ii-iv Original 85 IRN-200C-004 v IRN-200C-004 86-88 Original vi-vii Original 89 IRN-200C-003 viii-ix IRN-200C-004 90 IRN-200C-004 x-xi IRN-200C-003 91-93 IRN-200C-003 xii Original 94 Original xiii IRN-200C-004 95 IRN-200C-002 xiv Original 96 Original 1-2 Original 97 IRN-200C-003 3-4 IRN-200C-002 98 IRN-200C-004 5 IRN-200C-004 99-100 Original 6 Original 101 IRN-200C-004 7 IRN-200C-004 102-102b IRN-200C-003 8-14 Original 103 IRN-200C-002 15 IRN-200C-002 104-106 Original 16-25 Original 107 IRN-200C-004 26 IRN-200C-003 108-111 Original 27-33 Original 112-113 IRN-200C-004 34 IRN-200C-004 114-116 Original 35-40 Original 117-118b IRN-200C-002 41 IRN-200C-003 119 IRN-200C-002 42 IRN-200C-002 120 Original 43 IRN-200C-004 121 IRN-200C-001 44 Original 122-123 IRN-200C-004 45 IRN-200C-004 124 Original 46-50 Original 125 IRN-200C-003 51-64 IRN-200C-002 126-128 Original 64a-64b IRN-200C-003 129 IRN-200C-001 65-66 Original 130-134 IRN-200C-004 67-73 IRN-200C-002 135-138 Original 74-75 IRN-200C-003 76-76b IRN-200C-002 77 IRN-200C-004 78 Original 79 IRN-200C-002 80-82 IRN-200C-004 IRN-200C-004 ICD-GPS-200C 12 April 2000 v UNCLASSIFIED UNCLASSIFIED (This page intentionally left blank.) ICD-GPS-200C 10 OCT 1993 vi UNCLASSIFIED UNCLASSIFIED TABLE OF CONTENTS 1 SCOPE.............................................................................................................................................................. 1 1.1 Scope ................................................................................................................................................... 1 1.2 Key Dates................................................................................................................................................. 1 1.3 ICD Approval and Changes ...................................................................................................................... 1 2 APPLICABLE DOCUMENTS ........................................................................................................................ 3 2.1 Government Documents ........................................................................................................................... 3 2.2 Non-Government Documents.................................................................................................................... 4 3 REQUIREMENTS ........................................................................................................................................... 5 3.1 Interface Definition................................................................................................................................... 5 3.2 Interface Identification.............................................................................................................................. 5 3.2.1 Ranging Codes................................................................................................................................ 5 3.2.1.1 P-Code .................................................................................................................................. 7 3.2.1.2 Y-Code ................................................................................................................................. 7 3.2.1.3 C/A-Code.............................................................................................................................. 7 3.2.1.4 Non-Standard Codes.............................................................................................................. 7 3.2.2 NAV Data..................................................................................................................................... 10 3.2.3 L-Band Signal Structure................................................................................................................ 10 3.3 Interface Criteria..................................................................................................................................... 10 3.3.1 Composite Signal.......................................................................................................................... 10 3.3.1.1 Frequency Plan.................................................................................................................... 11 3.3.1.2 Correlation Loss .................................................................................................................. 11 3.3.1.3 Carrier Phase Noise ............................................................................................................. 11 3.3.1.4 Spurious Transmissions ....................................................................................................... 12 3.3.1.5 Phase Quadrature ................................................................................................................ 12 3.3.1.6 User-Received Signal Levels ............................................................................................... 12 3.3.1.7 Equipment Group Delay ...................................................................................................... 14 3.3.1.7.1 Group Delay Uncertainty ................................................................................................ 14 3.3.1.7.2 Group Delay Differential ................................................................................................ 14 3.3.1.8 Signal Coherence................................................................................................................. 14 ICD-GPS-200C 10 OCT 1993 vii UNCLASSIFIED UNCLASSIFIED 3.3.1.9 Signal Polarization .............................................................................................................. 15 3.3.2 PRN Code Characteristics ............................................................................................................. 15 3.3.2.1 Code Structure .................................................................................................................... 17 3.3.2.2 P-Code Generation .............................................................................................................. 17 3.3.2.3 C/A Code Generation .......................................................................................................... 28 3.3.3 Navigation Data ............................................................................................................................ 33 3.3.4 GPS Time and SV Z-Count ........................................................................................................... 33 4 NOT APPLICABLE....................................................................................................................................... 37 5 NOT APPLICABLE....................................................................................................................................... 39 6 NOTES ........................................................................................................................................................... 41 6.1 Acronyms............................................................................................................................................... 41 6.2 Definitions.............................................................................................................................................. 43 6.2.1 User Range Accuracy.................................................................................................................... 43 6.2.2 SV Block Definitions .................................................................................................................... 43 6.2.2.1 Developmental SVs............................................................................................................. 43 6.2.2.2 Operational SVs .................................................................................................................. 43 6.2.2.2.1 Block II SVs.............................................................................................................. 44 6.2.2.2.2 Block IIA SVs ........................................................................................................... 44 6.2.2.2.3 Block IIR SVs ........................................................................................................... 44 6.2.3 Operational Interval Definitions .................................................................................................... 44 6.2.3.1 Normal Operations .............................................................................................................. 44 6.2.3.2 Short-term Extended Operations .......................................................................................... 44 6.2.3.3 Long-term Extended Operations .......................................................................................... 45 6.2.4 GPS Week Number....................................................................................................................... 45 6.3 Supporting Material................................................................................................................................ 45 6.3.1 Received Signals........................................................................................................................... 45 6.3.2 Extended Navigation Mode (Block II/IIA)..................................................................................... 47 6.3.3 Block IIA Mode (Block IIR).......................................................................................................... 48 6.3.4 Autonomous Navigation Mode (Block IIR) ................................................................................... 48 10 APPENDIX I: LETTERS OF EXCEPTION .............................................................................................. 49 10.1 Scope ................................................................................................................................................. 49 10.2 Applicable Documents.......................................................................................................................... 49 IRN-200C-004 ICD-GPS-200C 12 April 2000 viii UNCLASSIFIED UNCLASSIFIED 10.3 Letters of Exception.............................................................................................................................. 49 20 APPENDIX II: GPS NAVIGATION DATA STRUCTURE FOR DATA ID NO. 2 ................................. 65 20.1Scope ..................................................................................................................................................... 65 20.2Applicable Documents............................................................................................................................ 65 20.2.1 Government Documents.............................................................................................................. 65 20.2.2 Non-Government Documents ...................................................................................................... 65 20.3Requirements ......................................................................................................................................... 66 20.3.1 Data Characteristics .................................................................................................................... 66 20.3.2 Message Structure....................................................................................................................... 66 20.3.3 Message Content......................................................................................................................... 79 20.3.3.1 Telemetry Word (TLM)..................................................................................................... 79 20.3.3.2 Handover Word (HOW) .................................................................................................... 79 20.3.3.3 Subframe 1........................................................................................................................ 82 20.3.3.3.1 Subframe 1 Content ................................................................................................. 82 20.3.3.3.1.1 Transmission Week Number .............................................................................. 82 20.3.3.3.1.2 Code(s) on L2 Channel....................................................................................... 83 20.3.3.3.1.3 SV Accuracy...................................................................................................... 83 20.3.3.3.1.4 SV Health .......................................................................................................... 84 20.3.3.3.1.5 Issue of Data, Clock (IODC) .............................................................................. 85 20.3.3.3.1.6 Data Flag for L2 P-Code .................................................................................... 85 20.3.3.3.1.7 (Reserved).......................................................................................................... 85 20.3.3.3.1.8 Estimated Group Delay Differential.................................................................... 85 20.3.3.3.1.9 SV Clock Correction .......................................................................................... 86 20.3.3.3.2 Subframe 1 Parameter Characteristics ...................................................................... 86 20.3.3.3.3 User Algorithms for Subframe 1 Data ...................................................................... 86 20.3.3.3.3.1 User Algorithm for SV Clock Correction............................................................ 88 20.3.3.3.3.2 L1 - L2 Correction ............................................................................................. 90 20.3.3.3.3.3 Ionospheric Correction ....................................................................................... 91 20.3.3.3.3.4 Example Application of Correction Parameters................................................... 91 20.3.3.4 Subframes 2 and 3 ............................................................................................................. 91 20.3.3.4.1 Content of Subframes 2 and 3 .................................................................................. 93 IRN-200C-004 ICD-GPS-200C 12 April 2000 ix UNCLASSIFIED UNCLASSIFIED 20.3.3.4.2 Subframe 2 and 3 Parameter Characteristics ............................................................. 95 20.3.3.4.3 User Algorithm for Ephemeris Determination .......................................................... 97 20.3.3.4.3.1 Curve Fit Intervals ............................................................................................. 97 20.3.3.4.3.2 Parameter Sensitivity.........................................................................................101 20.3.3.4.3.3 Coordinate Systems...........................................................................................101 20.3.3.4.3.3.1 ECEF Coordinate System ............................................................................101 20.3.3.4.3.3.2 Earth-Centered, Inertial (ECI) Coordinate System ........................................102 20.3.3.4.3.4 Geometric Range ..............................................................................................102 20.3.3.4.4 NMCT Validity Time ...........................................................................................102a 20.3.3.5 Subframes 4 and 5 ..........................................................................................................102b 20.3.3.5.1 Content of Subframes 4 and 5 ...............................................................................102b 20.3.3.5.1.1 Data ID and SV ID............................................................................................104 20.3.3.5.1.2 Almanac Data ...................................................................................................106 20.3.3.5.1.3 SV Health .........................................................................................................109 20.3.3.5.1.4 (Reserved).........................................................................................................112 20.3.3.5.1.5 (Reserved).........................................................................................................112 20.3.3.5.1.6 Anti-Spoof (A-S) Flags and SV Configurations .................................................112 20.3.3.5.1.7 Almanac Reference Week .................................................................................113 20.3.3.5.1.8 Universal Coordinated Time (UTC) Parameters.................................................113 20.3.3.5.1.9 Ionospheric Data ...............................................................................................114 20.3.3.5.1.10 Special Messages ............................................................................................117 20.3.3.5.1.11 (Deleted).........................................................................................................118 20.3.3.5.1.12 NMCT ............................................................................................................118 20.3.3.5.2 Algorithm Related to Subframe 4 and 5 Data.........................................................118a 20.3.3.5.2.1 Almanac .........................................................................................................118a 20.3.3.5.2.2 Almanac Reference Time ..................................................................................121 20.3.3.5.2.3 Almanac Time Parameters.................................................................................121 20.3.3.5.2.4 Universal Coordinated Time (UTC)...................................................................122 20.3.3.5.2.5 Ionospheric Model ............................................................................................125 20.3.3.5.2.6 NMCT Data ......................................................................................................125 IRN-200C-003 ICD-GPS-200C 11 OCT 1999 x UNCLASSIFIED UNCLASSIFIED 20.3.4 Timing Relationships .................................................................................................................129 20.3.4.1 Paging and Cutovers.........................................................................................................129 20.3.4.2 SV Time vs. GPS Time.....................................................................................................129 20.3.4.3 Speed of Light..................................................................................................................129 20.3.4.4 Data Sets ..........................................................................................................................130 20.3.4.5 Reference Times...............................................................................................................132 20.3.5 Data Frame Parity ......................................................................................................................135 20.3.5.1 SV/CS Parity Algorithm ...................................................................................................135 20.3.5.2 User Parity Algorithm ......................................................................................................135 IRN-200C-003 ICD-GPS-200C 11 OCT 1999 xi UNCLASSIFIED UNCLASSIFIED LIST OF FIGURES Figure 3-1 Space Vehicle/NAV User Interfaces............................................................................................ 6 Figure 3-2 Generation of Codes and Modulating Signals ............................................................................ 16 Figure 3-3 X1A Shift Register Generator Configuration ............................................................................. 18 Figure 3-4 X1B Shift Register Generator Configuration ............................................................................. 19 Figure 3-5 X2A Shift Register Generator Configuration ............................................................................. 20 Figure 3-6 X2B Shift Register Generator Configuration ............................................................................. 21 Figure 3-7 P-Code Generation.................................................................................................................... 24 Figure 3-8 P-Code Signal Component Timing ............................................................................................ 25 Figure 3-9 G1 Shift Register Generator Configuration................................................................................ 29 Figure 3-10 G2 Shift Register Generator Configuration................................................................................ 30 Figure 3-11 C/A-Code Generation ............................................................................................................... 31 Figure 3-12 C/A-Code Timing Relationships................................................................................................ 32 Figure 3-13 Time Line Relationship of HOW Message................................................................................. 35 Figure 6-1 User Received Minimum Signal Levels..................................................................................... 46 Figure 10-1 Letters of Exception.................................................................................................................. 51 Figure 20-1 Data Format.............................................................................................................................. 67 Figure 20-2 TLM and HOW Formats ........................................................................................................... 80 Figure 20-3 Sample Application of Correction Parameters............................................................................ 92 Figure 20-4 Ionospheric Model ...................................................................................................................126 Figure 20-5 Example Flow Chart for User Implementation of Parity Algorithm ...........................................137 ICD-GPS-200C 10 OCT 1993 xii UNCLASSIFIED UNCLASSIFIED LIST OF TABLES Table 3-I Code Phase Assignments ............................................................................................................ 8 Table 3-II Composite L1 Transmitted Signal Phase.................................................................................... 13 Table 3-III Received Minimum RF Signal Strength..................................................................................... 13 Table 3-IV P-Code Reset Timing................................................................................................................ 26 Table 3-V Final Code Vector States........................................................................................................... 27 Table 20-I Subframe 1 Parameters ............................................................................................................. 87 Table 20-II Ephemeris Data Definitions ...................................................................................................... 94 Table 20-III Ephemeris Parameters............................................................................................................... 96 Table 20-IV Elements of Coordinate Systems ............................................................................................... 98 Table 20-V Data IDs and SV IDs in Subframes 4 and 5 ..............................................................................105 Table 20-VI Almanac Parameters ................................................................................................................108 Table 20-VII NAV Data Health Indications...................................................................................................110 Table 20-VIII Codes for Health of SV Signal Components .............................................................................111 Table 20-IX UTC Parameters ......................................................................................................................115 Table 20-X Ionospheric Parameters ............................................................................................................116 Table 20-XI (Deleted)..................................................................................................................................119 Table 20-XII IODC Values and Data Set Lengths..........................................................................................131 Table 20-XIII Reference Times ......................................................................................................................134 Table 20-XIV Parity Encoding Equations .......................................................................................................136 IRN-200C-004 ICD-GPS-200C 12 April 2000 xiii UNCLASSIFIED UNCLASSIFIED (This page intentionally left blank.) ICD-GPS-200C 10 OCT 1993 xiv UNCLASSIFIED UNCLASSIFIED 1. SCOPE 1.1 Scope. This Interface Control Document (ICD) defines the requirements related to the interface between the Space Segment (SS) of the Global Positioning System (GPS) and the Navigation User Segment (US) of the GPS. 1.2 Key Dates. The major milestones for which integration data shall be provided are: a. (TBD) 1.3 ICD Approval and Changes. ARINC Research Corporation has been designated the Interface Control Contractor (ICC), and is responsible for the basic preparation, approval, distribution, and retention of the ICD in accordance with YEN 75-13A. The following signatories must approve this ICD to make it effective: a. Space Segment Contractors BLOCK I/II/IIA Rockwell International, Space Systems Division Block IIR Martin Marietta, Astro Space Division b. Control Segment Contractor International Business Machines, Federal Systems Company c. User Segment Contractors Rockwell International, Collins Avionics & Communications Division d. Navstar GPS Joint Program Office SMC/CZ (AFMC) ICD-GPS-200C 10 OCT 1993 1 UNCLASSIFIED UNCLASSIFIED Initial signature approval of this ICD can be contingent upon a letter of exception delineating those items by paragraph numbers that are not a part of the approval. Such letters of exception can be prepared by any of the signatories and must be furnished to the ICC for inclusion in Appendix I of the approved and officially released version of the ICD. Changes to the approved version of this ICD can be initiated by any of the signatories and must be approved by all above signatories. The ICC is responsible for the preparation of the change paper, change coordination, and the change approval by all signatories in accordance with YEN 75-13A. Designated signatories can approve proposed changes to this ICD without any increase in the scope of a specific contract by so specifying in a letter of exception. Such letters of exception must be furnished to the ICC for inclusion in the released version of the approved change and in Appendix I of the subsequent revised issues of the ICD. Whenever all the issues addressed by a letter of exception are resolved, the respective signatory shall so advise the ICC in writing. When some (but not all) of the exceptions taken by a signatory are resolved, the signatory shall provide the ICC with an updated letter of exception. Based on such notifications -- without processing a proposed interface revision notice (PIRN) for approval -- the ICC will omit the obsolete letter of exception from the next revision of the ICD and will substitute the new one (if required). Review cycles for all Proposed Interface Revisions Notices (PIRNs) is 45 days after receipt by individual addressees unless a written request for a waiver is submitted to the ICC. Reviewing parties with delinquent responses will be charged with an automatic letter of exception. ICD-GPS-200C 10 OCT 1993 2 UNCLASSIFIED UNCLASSIFIED 2. APPLICABLE DOCUMENTS 2.1 Government Documents. The following documents of the issue specified contribute to the definition of the interfaces between the GPS Space Segment and the GPS Navigation User Segment, and form a part of this ICD to the extent specified herein. Specifications Federal None Military None Other Government Activity None Standards Federal None Military None Other Publications ICD-GPS-203*, Navstar GPS Selective Availability and Anti-Spoofing current issue Requirements (SECRET) (U) ICD-GPS-224*, Navstar GPS Selective Availability and Anti-Spoofing current issue Receiver Design Requirements (SECRET) (U) ICD-GPS-225*, Navstar GPS Selective Availability and Anti-Spoofing current issue Host Application Equipment Design Requirements with the Precise Positioning Service Security Module (SECRET) (U) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 3 UNCLASSIFIED UNCLASSIFIED YEN 75-13A Interface Control Working Group 16 Nov 1979 with Change Notice 1 * ICD-GPS-203 is the applicable document for the signatories of this document (ICD-GPS-200); for all other organizations the applicable document is ICD-GPS-224 and/or ICD-GPS-225. In case of conflict between this document (ICD-GPS-200) and either ICD-GPS-203, ICD-GPS-224, or ICD-GPS-225, this document shall govern. 2.2 Non-Government Documents. The following documents of the issue specified contribute to the definition of the interfaces between the GPS Space Segment and the GPS Navigation User Segment and form a part of this ICD to the extent specified herein. Specifications None Other Publications None IRN-200C-002 ICD-GPS-200C 25 SEP 1997 4 UNCLASSIFIED UNCLASSIFIED 3. REQUIREMENTS 3.1 Interface Definition. As shown in Figure 3-1, the interface between the GPS Space Segment (SS) and the GPS navigation User Segment (US) consists of two radio frequency (RF) links: L1 and L2. Utilizing these links, the space vehicles (SVs) of the SS shall provide continuous earth coverage for signals which provide to the US the ranging codes and the system data needed to accomplish the GPS navigation (NAV) mission. These signals shall be available to a suitably equipped user with RF visibility to an SV. The related selective availability (SA) and anti-spoofing (A-S) requirements are defined in ICD-GPS-203 and/or in ICD-GPS-224 and/or in ICD-GPS-225 (see note in paragraph 2.1). 3.2 Interface Identification. The carriers of the L-band links are modulated by up to two bit trains, each of which normally is a composite generated by the Modulo-2 addition of a pseudo-random noise (PRN) ranging code and the downlink system data (referred to as NAV data). 3.2.1 Ranging Codes. Three PRN ranging codes are transmitted: the precision (P) code which is the principal NAV ranging code; the Y-code, used in place of the P-code whenever the A-S mode of operation is activated; and the coarse/acquisition (C/A) code which is used primarily for acquisition of the P (or Y) code (denoted as P(Y)). Appropriate code-division-multiplexing techniques allow differentiating between the SVs even though they all transmit at the same L-band frequencies. The SVs will transmit intentionally "incorrect" versions of the C/A and the P(Y) codes where needed to protect the users from receiving and utilizing anomalous NAV signals as a result of a malfunction in the SV's reference frequency generation system. These two "incorrect" codes are termed nonstandard C/A (NSC) and non-standard Y (NSY) codes. IRN-200C-004 ICD-GPS-200C 12 April 2000 5 UNCLASSIFIED UNCLASSIFIED GPS SPACE SEGMENT (SS) ON-BOARD COMPUTER PROGRAM (OBCP) SPACE VEHICLE (SV) L1 L2 GPS USER SEGMENT (US) Figure 3-1. Space Vehicle/NAV User Interfaces ICD-GPS-200C 10 OCT 1993 6 UNCLASSIFIED UNCLASSIFIED 3.2.1.1 P-Code. The PRN P-code for SV ID number i is a ranging code, Pi(t), of 7 days in length at a chipping rate of 10.23 Mbps. The 7 day sequence is the Modulo-2 sum of two sub-sequences referred to as X1 and X2i; their lengths are 15,345,000 chips and 15,345,037 chips, respectively. The X2i sequence is an X2 sequence selectively delayed by 1 to 37 chips thereby allowing the basic code generation technique to produce a set of 37 mutually exclusive P-code sequences of 7 days in length. Of these, 32 are designated for use by SVs, while the remaining 5 are reserved for other purposes (e.g. ground transmitters, etc.). Assignment of these code phase segments by SVID number (or other use) is given in Table 3-I. 3.2.1.2 Y-code. The PRN Y-code, used in place of the P-code when the A-S mode of operation is activated, is defined in ICD-GPS-203 and/or in ICD-GPS-224 and/or in ICD-GPS-225 (see note in paragraph 2.1). 3.2.1.3 C/A Code. The PRN C/A Code for SV ID number i is a Gold code, Gi(t), of 1 millisecond in length at a chipping rate of 1023 Kbps. The Gi(t) sequence is a linear pattern generated by the Modulo-2 addition of two subsequences, G1 and G2i, each of which is a 1023 chip long linear pattern. The epochs of the Gold code are synchronized with the X1 epochs of the P-code. As shown in Table 3-I, the G2i sequence is a G2 sequence selectively delayed by 5 to 950 chips, thereby generating a set of 36 mutually exclusive C/A-codes. Assignment of these by SV-ID (or other use) is also given in Table 3-I. 3.2.1.4 Non-standard Codes. The NSC and NSY codes, used to protect the user from a malfunction in the SV's reference frequency system (reference paragraph 3.2.1), are not for utilization by the user and, therefore, are not defined in this document. IRN-200C-004 ICD-GPS-200C 12 April 2000 7 UNCLASSIFIED UNCLASSIFIED Table 3-I. Code Phase Assignments (sheet 1 of 2) SV ID No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 * ** *** GPS PRN Signal No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Code Phase Selection Code Delay Chips C/A(G2i) (X2i) C/A P ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 5 6 7 8 17 18 139 140 141 251 252 254 255 256 257 258 469 470 471 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 2 3 4 5 1 2 1 2 3 2 3 5 6 7 8 9 1 2 3 6 7 8 9 9 10 8 9 10 3 4 6 7 8 9 10 4 5 6 First 10 Chips Octal* C/A First 12 Chips Octal P 1440 1620 1710 1744 1133 1455 1131 1454 1626 1504 1642 1750 1764 1772 1775 1776 1156 1467 1633 4444 4000 4222 4333 4377 4355 4344 4340 4342 4343 4343 In the octal notation for the first 10 chips of the C/A code as shown in this column, the first digit (1) represents a "1" for the first chip and the last three digits are the conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 1 are: 1100100000). C/A codes 34 and 37 are common. PRN sequences 33 through 37 are reserved for other uses (e.g. ground transmitters). ⊕ = "exclusive or" NOTE: The code phase assignments constitute inseparable pairs, each consisting of a specific C/A and a specific P code phase, as shown above. ICD-GPS-200C 10 OCT 1993 8 UNCLASSIFIED UNCLASSIFIED Table 3-I. Code Phase Assignments (sheet 2 of 2) SV ID No. 20 21 22 23 24 25 26 27 28 29 30 31 32 *** *** *** *** *** * ** *** GPS PRN Signal No. 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34** 35 36 37** Code Phase Selection Code Delay Chips C/A(G2i) (X2i) C/A P ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ ⊕ 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 472 473 474 509 512 513 514 515 516 859 860 861 862 863 950 947 948 950 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 4 5 6 1 4 5 6 7 8 1 2 3 4 5 4 1 2 4 7 8 9 3 6 7 8 9 10 6 7 8 9 10 10 7 8 10 First 10 Chips Octal* C/A First 12 Chips Octal P 1715 1746 1763 1063 1706 1743 1761 1770 1774 1127 1453 1625 1712 1745 1713 1134 1456 1713 4343 4343 In the octal notation for the first 10 chips of the C/A code as shown in this column, the first digit (1) represents a "1" for the first chip and the last three digits are the conventional octal representation of the remaining 9 chips. (For example, the first 10 chips of the C/A code for PRN Signal Assembly No. 1 are: 1100100000). C/A codes 34 and 37 are common. PRN sequences 33 through 37 are reserved for other uses (e.g. ground transmitters). ⊕ = "exclusive or" NOTE: The code phase assignments constitute inseparable pairs, each consisting of a specific C/A and a specific P code phase, as shown above. ICD-GPS-200C 10 OCT 1993 9 UNCLASSIFIED UNCLASSIFIED 3.2.2 NAV Data. The system data, D(t), includes SV ephemerides, system time, SV clock behavior data, status messages and C/A to P (or Y) code handover information, etc. The 50 bps data is Modulo-2 added to the P(Y)and C/A- codes; the resultant bit-trains are used to modulate the L1 and L2 carriers. For a given SV, the data train D(t), if present, is common to the P(Y) and C/A codes on both the L1 and L2 channels. The content and characteristics of data ID number 2 are given in Appendix II of this document. Data ID number 1 is no longer in use. 3.2.3 L-Band Signal Structure. The L1 link consists of two carrier components which are in phase quadrature with each other. Each carrier component is bi-phase shift key (BPSK) modulated by a separate bit train. One bit train is the Modulo-2 sum of the P(Y)-code and NAV data, while the other is the Modulo-2 sum of the C/A-code and the NAV data. The L2 link is BPSK modulated by only one of those two bit trains; the bit train to be used for L2 modulation is selected by ground command. A third modulation mode is also selectable on the L2 channel by ground command: it utilizes the P(Y)-code without the NAV data as the modulating signal. For a particular SV, all transmitted signal elements (carriers, codes and data) are coherently derived from the same on-board frequency source. 3.3 Interface Criteria. The criteria specified in the following define the requisite characteristics of the SS/US interface. 3.3.1 Composite Signal. The following criteria define the characteristics of the composite L-band signals. ICD-GPS-200C 10 OCT 1993 10 UNCLASSIFIED UNCLASSIFIED 3.3.1.1 Frequency Plan. The L-band signals shall be contained within two 20.46-MHz bands centered about L1 and L2. The carrier frequencies for the L1 and L2 signals shall be coherently derived from a common frequency source within the SV. The nominal frequency of this source -- as it appears to an observer on the ground -- is 10.23 MHz. The SV carrier frequency and clock rates -- as they would appear to an observer located in the SV -are offset to compensate for relativistic effects. The clock rates are offset by ∆ f/f = -4.4647E-10, equivalent to a change in the P-code chipping rate of 10.23 MHz offset by a ∆ f = -4.5674E-3 Hz. This is equal to 10.22999999543 MHz. The nominal carrier frequencies (f0) shall be 1575.42 MHz, and 1227.6 MHz for L1 and L2, respectively. 3.3.1.2 Correlation Loss. Correlation loss is defined as the difference between the SV power received in a 20.46 MHz bandwidth and the signal power recovered in an ideal correlation receiver of the same bandwidth. On the L1 and L2 channels, the worst case correlation loss occurs when the carrier is modulated by the sum of the P(Y) code and the NAV data stream. For this case, the correlation loss apportionment shall be as follows: 1. SV modulation imperfections 0.6 dB 2. Ideal UE receiver waveform distortion 0.4 dB (due to 20.46 MHz filter) 3.3.1.3 Carrier Phase Noise. The phase noise spectral density of the unmodulated carrier shall be such that a phase locked loop of 10 Hz one-sided noise bandwidth shall be able to track the carrier to an accuracy of 0.1 radians rms. ICD-GPS-200C 10 OCT 1993 11 UNCLASSIFIED UNCLASSIFIED 3.3.1.4 Spurious Transmissions. In-band spurious transmissions shall be at least 40 dB below the unmodulated L1 and L2 carriers over the allocated 20.46 MHz channel bandwidth. 3.3.1.5 Phase Quadrature. The two L1 carrier components modulated by the two separate bit trains (C/A-code plus data and P(Y)-code plus data) shall be in phase quadrature (within ±100 milliradians) with the C/A signal carrier lagging the P signal by 90 degrees. Referring to the phase of the P carrier when Pi(t) equals zero as the "zero phase angle", the P(Y)- and C/A-code generator output shall control the respective signal phases in the following manner: when Pi(t) equals one, a 180-degree phase reversal of the P-carrier occurs; when Gi(t) equals one, the C/A carrier advances 90 degrees; when the Gi(t) equals zero, the C/A carrier shall be retarded 90 degrees (such that when Gi(t) changes state, a 180-degree phase reversal of the C/A carrier occurs). The resultant nominal composite transmitted signal phases as a function of the binary state of the modulating signals are as shown in Table 3-II. 3.3.1.6 User-Received Signal Levels. The SV shall provide L1 and L2 navigation in accordance with the minimum levels specified in Table 3-III into a 3 dBi linearly polarized user receiving antenna (located near ground) at worst normal orientation, when the SV is above a 5-degree elevation angle. Additional related data is provided as supporting material in paragraph 6.3.1. ICD-GPS-200C 10 OCT 1993 12 UNCLASSIFIED UNCLASSIFIED Table 3-II. Composite L1 Transmitted Signal Phase Nominal Composite L1 Signal Phase* 0° -70.5° +109.5° 180° * Code State P C/A 0 1 0 1 0 0 1 1 Relative to 0, 0 code state with positive angles leading and negative angles lagging. Table 3-III. Received Minimum RF Signal Strength Signal Channel P(Y) C/A L1 -163.0 dBW -160.0 dBW L2 -166.0 dBW or -166.0 dBW ICD-GPS-200C 10 OCT 1993 13 UNCLASSIFIED UNCLASSIFIED 3.3.1.7 Equipment Group Delay. Equipment group delay is defined as the delay between the L-band radiated output of a specific SV (measured at the antenna phase center) and the output of that SV's on-board frequency source; the delay consists of a bias term and an uncertainty. The bias term is of no concern to the US since it is included in the clock correction parameters relayed in the NAV data, and is therefore accounted for by the user computations of system time (reference paragraph 20.3.3.3.3.1). The uncertainty (variation) of this delay as well as the difference between the L1 vs the L2 delays are defined in the following. 3.3.1.7.1 Group Delay Uncertainty. The effective uncertainty of the group delay shall not exceed 3.0 nanoseconds (two sigma). 3.3.1.7.2 Group Delay Differential. The group delay differential between the radiated L1 and L2 P(Y) signals is specified as consisting of random plus bias components. The mean differential is defined as the bias component and will be either positive or negative. For a given navigation payload redundancy configuration, the absolute value of the mean differential delay shall not exceed 15.0 nanoseconds. The random variations about the mean shall not exceed 3.0 nanoseconds (two sigma). 3.3.1.8 Signal Coherence. All transmitted signals for a particular SV shall be coherently derived from the same on-board frequency standard; all digital signals shall be clocked in coincidence with the PRN transitions for the Psignal and occur at the P-signal transition speed. On the L1 channel the data transitions of the two modulating signals (i.e., that containing the P(Y)-code and that containing the C/A-code) shall be such that the average time difference between the transitions does not exceed 10 nanoseconds (two sigma). ICD-GPS-200C 10 OCT 1993 14 UNCLASSIFIED UNCLASSIFIED 3.3.1.9 Signal Polarization The transmitted signal shall be right-hand circularly polarized (RHCP). For the angular range of ±14.3 degrees from boresight, L1 ellipticity shall be no worse than 1.2 dB for Block II/IIA and shall be no worse than 1.8 dB for Block IIR Svs. L2 ellipticity shall be no worse than 3.2 dB for Block II/IIA SVs and shall be no worse than 2.2 dB for Block IIR over the angular range of ±14.3 degrees from boresight. 3.3.2 PRN Code Characteristics. The characteristics of the P- and the C/A-codes are defined below in terms of their structure and the basic method used for generating them. The characteristics of the Y-code are defined in ICD-GPS-203 and/or ICD-GPS-224 and/or ICD-GPS-225 (see note in paragraph 2.1). Figure 3-2 depicts a simplified block diagram of the scheme for generating the 10.23 Mbps Pi(t) and the 1.023 Mbps Gi(t) patterns (referred to as P- and C/A-codes respectively), and for Modulo-2 summing these patterns with the NAV bit train, D(t), which is clocked at 50 bps. The resultant composite bit trains are then used to modulate the L-band carriers. IRN-200C-002 ICD-GPS-200C 25 SEP 1997 15 UNCLASSIFIED UNCLASSIFIED EPOCH DETECT X1 EPOCH EPOCH RESET 20 50 Hz Z-COUNT 1 KHz ZCOUNTER X1 CODE GENERATOR 1.023 MHz 10 GOLD CODE GENERATOR EPOCH DETECT X1(t) Gi(t) RESET COMMAND GENERATOR Gi(t) RECLOCKING DEVICE D(t) FORMATTED DATA X2i (t) Pi(t) REMOTE COMMAND EPOCH RESET CODE SELECT DEVICE X2 CODE GENERATOR 10.23 MHz FREQUENCY SOURCE D(t) DATA ENCODER Pi(t) D(t) Pi(t) Figure 3-2. Generation of Codes and Modulating Signals ICD-GPS-200C 10 OCT 1993 16 UNCLASSIFIED UNCLASSIFIED 3.3.2.1 Code Structure. The Pi(t) pattern (P-code) is generated by the Modulo-2 summation of two PRN codes, X1(t) and X2(t - iT), where T is the period of one P-code chip and equals (1.023 x 107)-1 seconds, while i is an integer from 1 through 37. This allows the generations of 37 unique P(t) code phases (identified in Table 3-I) using the same basic code generator. The linear Gi(t) pattern (C/A-code) is the Modulo-2 sum of two 1023-bit linear patterns, G1 and G2i. The latter sequence is selectively delayed by an integer number of chips to produce 36 unique G(t) patterns (defined in Table 3-I). 3.3.2.2 P-Code Generation. Each Pi(t) pattern is the Modulo-2 sum of two extended patterns clocked at 10.23 Mbps (X1 and X2i). X1 itself is generated by the Modulo-2 sum of the output of two 12-stage registers (X1A and X1B) short cycled to 4092 and 4093 chips respectively. When the X1A short cycles are counted to 3750, the X1 epoch is generated. The X1 epoch occurs every 1.5 seconds after 15,345,000 chips of the X1 pattern have been generated. The polynomials for X1A and X1B, as referenced to the shift register input, are: X1A: 1 + X6 + X8 + X11 + X12, and X1B: 1 + X1 + X2 + X5 + X8 + X9 + X10 + X11 + X12. Samples of the relationship between shift register taps and the exponents of the corresponding polynomial, referenced to the shift register input, are as shown in Figures 3-3, 3-4, 3-5 and 3-6. ICD-GPS-200C 10 OCT 1993 17 UNCLASSIFIED UNCLASSIFIED POLYNOMIAL X1A: 1 + X6 + X8 + X11 + X12 STAGE NUMBERS 0 1 2 3 4 5 6 7 8 9 10 11 12 0 0 0 1 0 0 1 0 0 1 0 0 1 2 3 4 5 6 7 INITIAL CONDITIONS 8 9 10 11 TAP NUMBERS SHIFT DIRECTION Figure 3-3. X1A Shift Register Generator Configuration ICD-GPS-200C 10 OCT 1993 18 UNCLASSIFIED OUTPUT 12 UNCLASSIFIED POLYNOMIAL X1B: 1 + X1 + X 2 + X 5 + X8 + X 9 + X 10 + X 11 + X 12 STAGE NUMBERS 0 1 2 3 4 5 6 7 8 9 10 11 12 0 0 1 0 1 0 1 0 1 0 1 0 1 2 3 4 5 6 7 INITIAL CONDITIONS 8 9 10 11 TAP NUMBERS SHIFT DIRECTION Figure 3-4. X1B Shift Register Generator Configuration ICD-GPS-200C 10 OCT 1993 19 UNCLASSIFIED OUTPUT 12 UNCLASSIFIED POLYNOMIAL X2A: 1 + X1 + X3 + X4 + X5 + X7 + X8 + X9 + X10 + X11 + X12 STAGE NUMBERS 0 1 2 3 4 5 6 7 8 9 10 11 12 1 0 1 0 0 1 0 0 1 0 0 1 1 2 3 4 5 6 7 INITIAL CONDITIONS 8 9 10 11 TAP NUMBERS SHIFT DIRECTION Figure 3-5. X2A Shift Register Generator Configuration ICD-GPS-200C 10 OCT 1993 20 UNCLASSIFIED OUTPUT 12 UNCLASSIFIED POLYNOMIAL X2B: 1 + X2 + X3 + X4 + X8 + X9 + X12 STAGE NUMBERS 0 1 2 3 4 5 6 7 8 9 10 11 12 0 0 1 0 1 0 1 0 1 0 1 0 1 2 3 4 5 6 7 INITIAL CONDITIONS 8 9 10 11 TAP NUMBERS SHIFT DIRECTION Figure 3-6. X2B Shift Register Generator Configuration ICD-GPS-200C 10 OCT 1993 21 UNCLASSIFIED OUTPUT 12 UNCLASSIFIED The state of each generator can be expressed as a code vector word which specifies the binary sequence constant of each register as follows: (a) the vector consists of the binary state of each stage of the register, (b) the stage 12 value appears at the left followed by the values of the remaining states in order of descending stage numbers, and (c) the shift direction is from lower to higher stage number with stage 12 providing the current output. This code vector convention represents the present output and 11 future outputs in sequence. Using this convention, at each X1 epoch, the X1A shift register is initialized to code vector 001001001000 and the X1B shift register is initialized to code vector 010101010100. The first chip of the X1A sequence and the first chip of the X1B sequence occur simultaneously in the first chip interval of any X1 period. The natural 4095 chip cycles of these generating sequences are shortened to cause precession of the X1B sequence with respect to the X1A sequence during subsequent cycles of the X1A sequence in the X1 period. Re- initialization of the X1A shift register produces a 4092 chip sequence by omitting the last 3 chips (001) of the natural 4095 chip X1A sequence. Re-initialization of the X1B shift register produces a 4093 chip sequence by omitting the last 2 chips (01) of the natural 4095 chip X1B sequence. This results in the phase of the X1B sequence lagging by one chip for each X1A cycle in the X1 period. The X1 period is defined as the 3750 X1A cycles (15,345,000 chips) which is not an integer number of X1B cycles. To accommodate this situation, the X1B shift register is held in the final state (chip 4093) of its 3749th cycle. It remains in this state until the X1A shift register completes its 3750th cycle (343 additional chips). The completion of the 3750th X1A cycle establishes the next X1 epoch which re-initializes both the X1A and X1B shift registers starting a new X1 cycle. The X2i sequences are generated by first producing an X2 sequence and then delaying it by a selected integer number of chips, i, ranging from 1 to 37. Each of the X2i sequences is then Modulo-2 added to the X1 sequence thereby producing up to 37 unique P(t) sequences. ICD-GPS-200C 10 OCT 1993 22 UNCLASSIFIED UNCLASSIFIED The X2A and X2B shift registers, used to generate X2, operate in a similar manner to the X1A and X1B shift registers. They are short-cycled, X2A to 4092 and X2B to 4093, so that they have the same relative precession rate as the X1 shift registers. X2A epochs are counted to include 3750 cycles and X2B is held in the last state at 3749 cycle until X2A completes its 3750th cycle. The polynomials for X2A and X2B, as referenced to the shift register input, are: X2A: 1 + X1 + X3 + X4 + X5 + X7 + X8 + X9 + X10 + X11 + X12, and X2B: 1 + X2 + X3 + X4 + X8 + X9 + X12. (The initialization vector for X2A is 100100100101 and for X2B is 010101010100). The X2A and X2B epochs are made to precess with respect to the X1A and X1B epochs by causing the X2 period to be 37 chips longer than the X1 period. When the X2A is in the last state of its 3750th cycle and X2B is in the last state of its 3749th cycle, their transitions to their respective initial states are delayed by 37 chip time durations. At the beginning of the GPS week, X1A, X1B, X2A and X2B shift registers are initialized to produce the first chip of the week. The precession of the shift registers with respect to X1A continues until the last X1A period of the GPS week interval. During this particular X1A period, X1B, X2A and X2B are held when reaching the last state of their respective cycles until that X1A cycle is completed (see Table 3-IV). At this point, all four shift registers are initialized and provide the first chip of the new week. Figure 3-7 shows a functional P-code mechanization. Signal component timing is shown in Figure 3-8, while the end-of-week reset timing and the final code vector states are given in Tables 3-IV and 3-V, respectively. ICD-GPS-200C 10 OCT 1993 23 UNCLASSIFIED UNCLASSIFIED 10.23 MHz X1 EPOCH SET X1A EPOCH C R X1A REGISTER 1 6 12 I A 4092 DECODE 6, 8, 11, 12 3750 RESUME CLOCK CONTROL HALT Z-COUNTER 403,200 C I 1 SET X1B EPOCH R X1B REGISTER 3749 B 12 7 DAY RESET 4093 DECODE 1, 2, 5, 8, 9, 10, 11, 12 REGISTER INPUTS END/WEEK CLOCK CONTROL HALT C - CLOCK I - INPUT R - RESET TO INITIAL CONDITIONS ON NEXT CLOCK 3750 C I 1 SET X2A EPOCH R X2A REGISTER C 12 4092 DECODE 1, 3, 4, 5, 7, 8, 9, 10, 11, 12 START/WEEK RESUME X2 EPOCH 37 ENABLE A X1 END/WEEK HALT Pi B CLOCK CONTROL C I X2B REGISTER 1 2 12 R X2i SET X2B EPOCH 3749 1 i 37 C 4093 DECODE 2, 3, 4, 8, 9, 12 X2 SHIFT REGISTER Figure 3-7. P-Code Generation ICD-GPS-200C 10 OCT 1993 24 UNCLASSIFIED UNCLASSIFIED 0 1 2 3 0 1 2 3 0 X1 EPOCHS 37 Chips 74 Chips X2 EPOCHS P Epoch TIME 0 1.5 sec 3.0 sec 4.5 sec 7 days 14 days Figure 3-8. P-Code Signal Component Timing ICD-GPS-200C 10 OCT 1993 25 UNCLASSIFIED UNCLASSIFIED Table 3-IV. P-Code Reset Timing (Last 400 µsec of 7-day period) Code Chip X1A-Code X1B-Code X2A-Code X2B-Code 1 345 1070 967 • • • • • • • • • • • • 3023 3367 4092 3989 • • • • • • • • • • • • 3127 3471 4092 4093 • • • • • • • • • • • • 3749 4093 4092 4093 • • • • • • • • • • • • 4093 4092 4093 4092* * Last Chip of Week. IRN-200C-003 ICD-GPS-200C 11 OCT 1999 26 UNCLASSIFIED UNCLASSIFIED Table 3-V. Final Code Vector States Vector State for 1st Chip Code X1A Chip Number Vector State 4091 100010010010 4092 000100100100 4092 100101010101 4093 001010101010 4091 111001001001 4092 110010010010 4092 000101010101 4093 001010101010 following Epoch 001001001000 X1B 010101010100 X2A 100100100101 X2B 010101010100 NOTE: First Chip in each sequence is output bit whose leading edge occurs simultaneously with the epoch. ICD-GPS-200C 10 OCT 1993 27 UNCLASSIFIED UNCLASSIFIED 3.3.2.3 C/A-Code Generation. Each Gi(t) sequence is a 1023-bit Gold-code which is itself the Modulo-2 sum of two 1023-bit linear patterns, G1 and G2i. The G2i sequence is formed by effectively delaying the G2 sequence by an integer number of chips ranging from 5 to 950. The G1 and G2 sequences are generated by 10-stage shift registers having the following polynomials as referred to in the shift register input (see Figures 3-9 and 3-10). G1 = X10 + X3 + 1, and G2 = X10 + X9 + X8 + X6 + X3 + X2 + 1. The initialization vector for the G1 and G2 sequences is 1111111111. The G1 and G2 shift registers are initialized at the P-coder X1 epoch. The G1 and G2 registers are clocked at 1.023 MHz derived from the 10.23 MHz P-coder clock. The initialization by the X1 epoch phases the 1.023 MHz clock to insure that the first chip of the C/A code begins at the same time as the first chip of the P-code. The effective delay of the G2 sequence to form the G2i sequence is accomplished by combining the output of two stages of the G2 shift register by Modulo-2 addition (see Figure 3-11). Thirty-six of the possible combinations are selected, one to correspond to each of the 36 different P-codes. Table 3-I contains a tabulation of the G2 shift register taps selected and their corresponding P-code X2i and PRN signal numbers together with the first several chips of each resultant PRN code. Timing relationships related to the C/A code are shown in Figure 3-12. ICD-GPS-200C 10 OCT 1993 28 UNCLASSIFIED UNCLASSIFIED POLYNOMIAL G1: 1 + X3 + X10 STAGE NUMBERS INPUT 0 1 2 3 4 5 6 7 8 9 10 1 1 1 1 1 1 1 1 1 1 1 2 3 4 5 6 7 8 INITIAL CONDITIONS 9 OUTPUT 10 TAP NUMBERS SHIFT DIRECTION Figure 3-9. G1 Shift Register Generator Configuration ICD-GPS-200C 10 OCT 1993 29 UNCLASSIFIED UNCLASSIFIED POLYNOMIAL G2: 1 + X2 + X3 +X6 + X8 + X9 + X10 STAGE NUMBERS INPUT 0 1 2 3 4 5 6 7 8 9 10 1 1 1 1 1 1 1 1 1 1 1 2 3 4 5 6 7 8 INITIAL CONDITIONS 9 OUTPUT 10 TAP NUMBERS SHIFT DIRECTION Figure 3-10. G2 Shift Register Generator Configuration ICD-GPS-200C 10 OCT 1993 30 UNCLASSIFIED UNCLASSIFIED 3 10 X1 EPOCH I S C 10.23 MHz G1 G1 REGISTER SYNCH 10 2 I C S SYNCH 20 G EPOCH 1 Kbps 3 6 8 9 10 G2 REGISTER 1023 DECODE Gi G2i PHASE SELECT LOGIC 50 bps TO DATA ENCODER REGISTER INPUTS C I S - CLOCK INPUT SET ALL ONES Figure 3-11. C/A-Code Generation ICD-GPS-200C 10 OCT 1993 31 UNCLASSIFIED UNCLASSIFIED X1 Epoch @ 2/3 bps 1023 1023 1023 1023 1023 1023 BIT Gold Code @ 1023 Kbps 0 1 etc. 1 msec 2 18 19 0 Gold Code Epochs @ 1000/sec Data @ 50 cps 20 msec Figure 3-12. C/A-Code Timing Relationships ICD-GPS-200C 10 OCT 1993 32 UNCLASSIFIED UNCLASSIFIED 3.3.3 Navigation Data. The content and format of the NAV data for data ID number 2 are given in Appendix II of this document (reference paragraph 20.3.3.5.1.1). Data ID number 1 is no longer in use. 3.3.4 GPS Time and SV Z-Count. GPS time is established by the Control Segment and is referenced to a UTC (as maintained by the U.S. Naval Observatory) zero time-point defined as midnight on the night of January 5, 1980/morning of January 6, 1980. The largest unit used in stating GPS time is one week defined as 604,800 seconds. GPS time may differ from UTC because GPS time shall be a continuous time scale, while UTC is corrected periodically with an integer number of leap seconds. There also is an inherent but bounded drift rate between the UTC and GPS time scales. The OCS shall control the GPS time scale to be within one microsecond of UTC (Modulo one second). The NAV data contains the requisite data for relating GPS time to UTC. The accuracy of this data during the transmission interval shall be such that it shall relate GPS time (maintained by the MCS of the CS) to UTC (USNO) within 90 nanoseconds (one sigma). This data is generated by the CS; therefore, the accuracy of this relationship may degrade if for some reason the CS is unable to upload data to a SV. At this point, it is assumed that alternate sources of UTC are no longer available, and the relative accuracy of the GPS/UTC relationship will be sufficient for users. Range error components (e.g. SV clock and position) contribute to the GPS time transfer error, and under normal operating circumstances (two frequency time transfers from SV(s) whose navigation message indicates a URA of eight meters or less), this corresponds to a 97 nanosecond (one sigma) apparent uncertainty at the SV. Propagation delay errors and receiver equipment biases unique to the user add to this time transfer uncertainty. ICD-GPS-200C 10 OCT 1993 33 UNCLASSIFIED UNCLASSIFIED In each SV the X1 epochs of the P-code offer a convenient unit for precisely counting and communicating time. Time stated in this manner is referred to as Z-count, which is given as a 29-bit binary number consisting of two parts as follows: a. The binary number represented by the 19 least significant bits of the Z-count is referred to as the time of week (TOW) count and is defined as being equal to the number of X1 epochs that have occurred since the transition from the previous week. The count is short-cycled such that the range of the TOW-count is from 0 to 403,199 X1 epochs (equaling one week) and is reset to zero at the end of each week. The TOWcount's zero state is defined as that X1 epoch which is coincident with the start of the present week. This epoch occurs at (approximately) midnight Saturday night-Sunday morning, where midnight is defined as 0000 hours on the Universal Coordinated Time (UTC) scale which is nominally referenced to the Greenwich Meridian. Over the years the occurrence of the "zero state epoch" may differ by a few seconds from 0000 hours on the UTC scale since UTC is periodically corrected with leap seconds while the TOWcount is continuous without such correction. To aid rapid ground lock-on to the P-code signal, a truncated version of the TOW-count, consisting of its 17 most significant bits, is contained in the hand-over word (HOW) of the L-Band downlink data stream; the relationship between the actual TOW-count and its truncated HOW version is illustrated by Figure 3-13. b. The ten most significant bits of the Z-count are a Modulo 1024 binary representation of the sequential number assigned to the current GPS week (see paragraph 6.2.4). The range of this count is from 0 to 1023 with its zero state being defined as the GPS week number zero and every integer multiple of 1024 weeks, thereafter (i.e. 0, 1024, 2048, etc.). IRN-200C-004 ICD-GPS-200C 12 April 2000 34 UNCLASSIFIED UNCLASSIFIED P(Y)-CODE EPOCH (END/START OF WEEK) X1 EPOCHS 1.5 sec 0 403,192 403,196 1 2 3 4 5 6 7 8 403,199 DECIMAL EQUIVALENTS OF ACTUAL TOW COUNTS SUBFRAME EPOCHS 6 sec 100,799 0 1 2 3 DECIMAL EQUIVALENTS OF HOW-MESSAGE TOW COUNTS NOTES: 1. TO AID IN RAPID GROUND LOCK-ON THE HAND-OVER WORD (HOW ) OF EACH SUBFRAME CONTAINS A TRUNCATED TIME-OF-WEEK (TOW) COUNT 2. THE HOW IS THE SECOND WORD IN EACH SUBFRAME (REFERENCE PARAGRAPH 20.3.3.2). 3. THE HOW-MESSAGE TOW COUNT CONSISTS OF THE 17 MSBs OF THE ACTUAL TOW COUNT AT THE START OF THE NEXT SUBFRAME. 4. TO CONVERT FROM THE HOW-MESSAGE TOW COUNT TO THE ACTUAL TOW COUNT AT THE START OF THE NEXT SUBFRAME, MULTIPLY BY FOUR. 5. THE FIRST SUBFRAME STARTS SYNCHRONOUSLY WITH THE END/START OF WEEK EPOCH. Figure 3-13. Time Line Relationship of HOW Message ICD-GPS-200C 10 OCT 1993 35 UNCLASSIFIED UNCLASSIFIED (This page intentionally left blank.) ICD-GPS-200C 10 OCT 1993 36 UNCLASSIFIED UNCLASSIFIED 4. NOT APPLICABLE ICD-GPS-200C 10 OCT 1993 37 UNCLASSIFIED UNCLASSIFIED (This page intentionally left blank.) ICD-GPS-200C 10 OCT 1993 38 UNCLASSIFIED UNCLASSIFIED 5. NOT APPLICABLE ICD-GPS-200C 10 OCT 1993 39 UNCLASSIFIED UNCLASSIFIED (This page intentionally left blank.) ICD-GPS-200C 10 OCT 1993 40 UNCLASSIFIED UNCLASSIFIED 6. NOTES 6.1 Acronyms AI - Availability Indicator AODO - Age of Data Offset A-S - Anti-Spoofing Autonav - Autonomous Navigation BPSK - Bi-Phase Shift Key CS - Control Segment DN - Day Number EAROM - Electrically Alterable Read-Only Memory ECEF - Earth-Centered, Earth-Fixed ECI - Earth-Centered, Inertial ERD - Estimated Range Deviation GPS - Global Positioning System HOW - Hand-Over Word ICC - Interface Control Contractor ICD - Interface Control Document ID - Identification IODC - Issue of Data, Clock IODE - Issue of Data, Ephemeris LSB - Least Significant Bit LSF - Leap Seconds Future MCS - Master Control Station MSB - Most Significant Bit NAV - Navigation NDUS - Nudet Detection User Segment NMCT - Navigation Message Correction Table NSC - Non-Standard C/A-Code NSY - Non-Standard Y-code OBCP - On-Board Computer Program OCS - Operational Control Segment PRN - Pseudo-Random Noise RF - Radio Frequency IRN-200C-003 ICD-GPS-200C 11 OCT 1999 41 UNCLASSIFIED UNCLASSIFIED RMS - Root Mean Square SA - Selective Availability SEP - Spherical Error Probable SS - Space Segment SV - Space Vehicle SVN - Space Vehicle Number TBD - To Be Determined TBS - To Be Supplied TLM - Telemetry TOW - Time Of Week UE - User Equipment URA - User Range Accuracy URE - User Range Error US - User Segment USNO - U.S. Naval Observatory UTC - Universal Coordinated Time WGS 84 - World Geodetic System 1984 WN - Week Number IRN-200C-002 ICD-GPS-200C 25 SEP 1997 42 UNCLASSIFIED UNCLASSIFIED 6.2 Definitions 6.2.1 User Range Accuracy. User range accuracy (URA) is a statistical indicator of the ranging accuracies obtainable with a specific SV. URA is a one-sigma estimate of the user range errors in the navigation data for the transmitting satellite. It includes all errors for which the Space and Control Segments are responsible. It does not include any errors introduced in the user set or the transmission media. While the URA may vary over a given subframe fit interval, the URA index (N) reported in the NAV message corresponds to the maximum value of URA anticipated over the fit interval. 6.2.2 SV Block Definitions. The following block definitions are given to facilitate discussion regarding the capability of the various blocks of GPS satellites to support the SV-to-US interface. 6.2.2.1 Developmental SVs. The original concept validation satellites developed by Rockwell International and designated as satellite vehicle numbers (SVNs) 1-11 are termed "Block I" SVs. These SVs were designed to provide 3-4 days of positioning service without contact from the CS. These SVs transmitted a configuration code of 000 (reference paragraph 20.3.3.5.1.6). There are no longer any active Block I SVs in the GPS constellation. The last Block I SV was decommissioned in 1995. 6.2.2.2 Operational SVs. The operational satellites are designated Block II, Block IIA and Block IIR SVs. Characteristics of these SVs are provided below. Modes of operation for these SVs and accuracy of positioning services provided are described in paragraphs 6.3.2 through 6.3.4. These SVs all transmit a configuration code of 001 (reference 20.3.3.5.1.6). The navigation signal provides no direct indication of the type of the transmitting SV. IRN-200C-004 ICD-GPS-200C 12 April 2000 43 UNCLASSIFIED UNCLASSIFIED 6.2.2.2.1 Block II SVs. The first block of full scale operational SVs developed by Rockwell International are designated as SVNs 13-21 and are termed "Block II" SVs. These SVs were designed to provide 14 days of positioning service without contact from the CS. 6.2.2.2.2 Block IIA SVs. The second block of full scale operational SVs developed by Rockwell International are designated as SVNs 22-40 and are termed "Block IIA" SVs. These SVs were designed to provide 180 days of positioning service without contact from the CS. 6.2.2.2.3 Block IIR SVs. The block of operational replenishment SVs developed by Martin Marietta are designated as SVNs 41-66 and are termed "Block IIR" SVs. These SVs will provide at least 14 days of positioning service without contact from the CS when the SVs are operating in the Block IIA mode and will provide a minimum of 180 days of positioning service without contact from the CS when operating in autonomous navigation (Autonav) mode. 6.2.3 Operational Interval Definitions. The following three operational intervals have been defined. These labels will be used to refer to differences in the interface definition as time progresses from SV acceptance of the last navigation data upload. 6.2.3.1 Normal Operations. The SV is undergoing normal operations whenever the fit interval flag (reference paragraph 20.3.3.4.3.1) is zero. 6.2.3.2 Short-term Extended Operations. The SV is undergoing short-term extended operations whenever the fit interval flag is one and the IODE (reference paragraph 20.3.4.4) is less than 240. ICD-GPS-200C 10 OCT 1993 44 UNCLASSIFIED UNCLASSIFIED 6.2.3.3 Long-term Extended Operations. The SV is undergoing long-term extended operations whenever the fit interval flag is one and the IODE is in the range 240-255. Note: the DoD Navigation User Segment and Time Transfer User have no requirement to operate, and may not operate properly, whenever any SV is operating in long-term extended operations. 6.2.4 GPS Week Number. The GPS week numbering system is established with week number zero (0) being defined as that week which started with the X1 epoch occurring at midnight UTC(USNO) on the night of January 5, 1980/ morning of January 6, 1980. The GPS week number continuously increments by one (1) at each end/start of week epoch without ever resetting to zero. Users must recognize that the week number information contained in the Nav Message may not necessarily reflect the current full GPS week number (see paragraphs 20.3.3.3.1.1, 20.3.3.5.1.7, and 20.3.3.5.2.4). 6.3 Supporting Material 6.3.1 Received Signals. The guaranteed minimum user-received signal levels are defined in paragraph 3.3.1.6. As additional supporting material, Figure 6-1 illustrates the minimum power of the near-ground user-received L1 and L2 signals as a function of SV elevation angle using the following assumptions: (a) the signal is measured at the output of a 3 dBi linearly polarized receiving antenna; (b) the SV is above a 5 degree elevation angle; (c) the received signal levels are observed within the in-band allocation defined in paragraph 3.3.1.1; (d) the atmospheric path loss is 2.0 dB; and (e) the SV attitude error is 0.5 degrees (towards reducing signal level). The actual SV attitude error will not exceed ±0.5 degrees after the SV has stabilized to its final orbital state. IRN-200C-004 ICD-GPS-200C 12 April 2000 45 UNCLASSIFIED UNCLASSIFIED RECEIVED POWER AT 3dB i LINEARLY POLARIZED ANTENNA (dBw) -157 C/A - L1 -160 P - L1 -163 P - L2 or -166 C/A - L2 0o 5o 20o 40o 60o 80o 90o 100o USER ELEVATION ANGLE (DEG) Figure 6-1. User Received Minimum Signal Levels ICD-GPS-200C 10 OCT 1993 46 UNCLASSIFIED UNCLASSIFIED Higher received signals levels can be caused by such factors as SV attitude errors, mechanical antenna alignment errors, transmitter power output variations due to temperature variations, voltage variations and power amplifier variations, and due to a variability in link atmospheric path loss. The maximum received signal levels as a result of these factors is not expected to exceed -155.5 dBw and -153.0 dBw, respectively, for the P(Y) and C/A components of the L1 channel, nor -158.0 dBw for either signal on the L2 channel. This estimate assumes that the receiving antenna characteristics are as described above, the atmospheric loss is 0.6 dB and the SV attitude error is 0.5° (towards increased signal level). 6.3.2 Extended Navigation Mode (Block II/IIA). The Block II and IIA SVs are capable of being uploaded by the CS with 182 days of navigation data to support a 180 day positioning service. Due to memory retention limitations, the Block II SVs may not transmit correct data for the entire 180 days but are guaranteed to transmit correct data for at least 14 days to support short-term extended operations. Under normal conditions the CS will provide daily uploads to each SV, which will allow the SV to maintain normal operations as defined in paragraph 6.2.3.1 and described within this ICD. During normal operations, the SVs will have a user range error that is at or below a level required to support a positioning accuracy of 16 meters spherical error probable (SEP). In addition, the almanac data, UTC parameters and ionospheric data will be maintained current to meet the accuracy specified in this ICD. If the CS is unable to upload the SVs (the CS is unavailable or the SV is unable to accept and process the upload), each SV will individually transition to short-term extended operations and eventually to long-term extended operations (based on time from each SV's last upload) as defined in paragraphs 6.2.3.2 and 6.2.3.3, and as further described throughout this ICD. As time from upload continues through these three operational intervals, the user range error of the SV will increase, causing a positioning service accuracy degradation. The rate of accuracy degradation is slow over the short-term extended operations interval, such that at the end of this interval (approximately 14 days after upload) the US will be able to achieve a positioning accuracy of 425 meters SEP. The rate of accuracy degradation increases in the long-term extended interval, such that by the 180th day after the last upload, the positioning errors will have grown to 10 kilometers SEP. During these intervals the URA will continue to provide the proper estimate of the user range errors. ICD-GPS-200C 10 OCT 1993 47 UNCLASSIFIED UNCLASSIFIED During short-term and long-term extended operations (approximately day 2 through day 182 after an upload), the almanac data, UTC parameters and ionospheric data will not be maintained current and will degrade in accuracy from the time of last upload. 6.3.3 Block IIA Mode (Block IIR). The Block IIR SVs, when operating in the Block IIA mode, will perform similarly to the Block IIA SVs and will provide at least 14 days of positioning service (through short-term extended operations) without contact from the CS. 6.3.4 Autonomous Navigation Mode. The Block IIR SV, in conjunction with a sufficient number of other Block IIR SVs, operates in an Autonav mode when commanded by the CS. Each Block IIR SV in the constellation determines its own ephemeris and clock correction parameters via SV-to-SV ranging, communication of data, and on-board data processing which updates data uploaded by the CS. In the Autonav mode the Block IIR SV will maintain normal operations as defined in paragraph 6.2.3.1 and as further described within this ICD, and will have a user range error that is at or below a level required to support 16 meter SEP accuracy. If the CS is unable to upload the SVs, the Block IIR SVs will maintain normal operations for period of at least 180 days after the last upload. In the Autonav mode, the almanac data, UTC parameters and ionospheric data are still calculated and maintained current by the CS and uploaded to the SV as required. If the CS is unable to upload the SVs, the almanac data, UTC parameters and ionospheric data will not be maintained current and will degrade in accuracy from the time of the last upload. ICD-GPS-200C 10 OCT 1993 48 UNCLASSIFIED UNCLASSIFIED 10. APPENDIX I. LETTERS OF EXCEPTION 10.1 Scope. As indicated in paragraph 1.3, initial signature approval of this document, as well as approval of subsequent changes to the document, can be contingent upon a "letter of exception". This appendix depicts such "letters of exception" when utilized by any signatory of this document in the initial approval cycle and/or in the change approval process. The ICC will omit such letters of exception from subsequent revisions of this document based on written authorization by the respective signatory (without processing a proposed interface revision notice (PIRN) for approval). When some (but not all) of the exceptions taken by a signatory are resolved, the signatory shall provide the ICC with an updated letter of exception for inclusion in the next ICD revision (without processing a PIRN for approval). 10.2 Applicable Documents. The documents listed in Section 2.0 shall be applicable to this appendix. 10.3 Letters of Exception. If signature approval of this document -- as affixed to the cover page -- is marked by an asterisk, it indicates that the approval is contingent upon the exceptions taken by that signatory in a letter of exception. Any letter of exception which is in force for the revision of the ICD is depicted in Figure 10-1. Signatories for whom no letter of exception is shown have approved this version of the document without exception. ICD-GPS-200C 10 OCT 1993 49 UNCLASSIFIED UNCLASSIFIED (This page intentionally left blank.) ICD-GPS-200C 10 OCT 1993 50 UNCLASSIFIED UNCLASSIFIED Space Systems Division Rockwell International Corporation 2600 Westminster Boulevard P.O. Box 3644 Seal Beach, California 90740-7644 Rockwell International In reply refer to 93MA3728 Date: September 22, 1993 To: The ARINC Companies 11770 E. Warner Ave., Suite 210 Fountain Valley, CA 92708 Subject: Rockwell International Letter of Exception to ICD GPS-200 PIRN-200A-006NC, dated November 21,1986. Attention: Thomas R. Denigan Reference: ARINC Companies FAX dated September 20, 1993, T. R. Denigan to D. L. Butler, same subject. It is Rockwell’s position that the statement requested in the subject letter of exception be incorporated, as written, in the next revision to ICD-200. The ‘B’ revision of ICD-200 incorporated only the last portion of the requested change “....the initialization vector for X2A is 100100100101 and for X2B is 010101010100.” It is felt that the first portion of the sentence, “Using the same convention identified for X1A and X1B,...” will aid the reader of the ICD in understanding the derivation of the X2A and X2B terms. ROCKWELL INTERNATIONAL Space Systems Division Signature on file Signature on file W. L Young, Manager Engineer Contracts & Proposals F. E. Cooper, Chief cc: GPS Program D. L. Butler W. F. Fratzke Figure 10-1. Letter of Exception (sheet 1 of 15) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 51 UNCLASSIFIED UNCLASSIFIED Collins Avionics & Communications Division Rockwell International Corporation 350 Collins Road NE Cedar Rapids, IA 52498 (319) 395-1000 Rockwell International September 23, 1993 ARINC Research Corporation 11770 Warner Avenue, Suite 210 Fountain Valley, CA 92708 Attention: Mr. Tom Denigan Subject: Review of ICD-GPS-200B Outstanding Letters of Exception in Preparation of ICD-GPS-200C Dear Mr. Denigan: A review of Rockwell International’s Collins Avionics & Communications Division, outstanding Letters of Exception as listed in IRN-200B-007 to ICD-GPS-200B shows 2 Letters of Exception that have been satisfied or are no longer pertinent: sheet 46, (letter dated March 31, 1987) sheet 53 & 54, (letter dated September 10, 1986) The following letters as listed in IRN-200B-007 to ICD-GPS-200B are still pertinent and are to be included in any revised ICD-GPS-200 releases: sheet 54a, 54c (letter dated March 27, 1991) sheet 56g (letter dated September 23, 1992) Sincerely, Signature on file C.S, Olson Program Manager Figure 10-1. Letter of Exception (sheet 2 of 15) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 52 UNCLASSIFIED UNCLASSIFIED Collins Avionics & Communications Division Rockwell International Corporation 350 Collins Road NE Cedar Rapids, IA 52498 (319) 395-1000 Rockwell International 2458b/0048b March 27, 1991 ARINC Research Corporation 4410 East Fountain Blvd., Suite 100 Colorado Springs, CO 80916 Attention: Ms. Cheryl Abendschan Subject: Rockwell CACD Letter of Exception against PIRN-200B-001B (as corrected by ARINC memo on typos and oversights, 19 MAR 91) The subject PIRN-200B-001B documents the extended navigation capabilities inherent in the Space Segment/User Segment interface as a result of the incorporation of mission package software release OR5.10 into the Control Segment. CACD’s approval of this PIRN, which indicates concurrence that the PIRN accurately reflects the as-built Control and Space Segments in the area of extended navigation, is given with exception. CACD takes exception because: 1. The extended navigation capabilities documented by the PIRN are not a requirement of the GPS Phase III User Equipment (UE) contract F04701-85-C-0038, the GPS UE Phase IV contract F04701-90-C-0092, or the GPS MAGR Contract F04701-91-C-0003. Therefore, the GPS User Equipment, which has been and will be developed under these contracts, do no operate in compliance with extended navigation. 2. While we may technically comment on the impact to the UE as a result of OR5.10 implementation, we cannot (as PIRN approval might otherwise indicate) verify that OR5.10 implements the extended navigation requirements identified in the PIRN. In summary, “Long Term Extended” operations from 15 to 180 days following an upload from the Control Segment are not supported by the Phase III GPS UE. Attachment 1 contains the detailed comments on the PIRN items to which CACD takes exception. Sincerely, Signature on file J. L. Arnold GPS Programs Manager Enclosure cc: Lt. Jim Dagley Capt. Greg Laushine Figure 10-1. Letter of Exception (sheet 3 of 15) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 53 UNCLASSIFIED UNCLASSIFIED ATTACHMENT 1 Rockwell CACD Exceptions to PIRN-200B-001B PIRN ITEM Rockwell CACD Comment 17, 18 These PIRN items document the change in the definition of the Week Number in subframe 1, word 3 due to the implementation of long term extended ephemeris curve fits which cross GPS week boundaries. The new definition states that the week number is the ten most significant bits of the Z-count and will represent the GPS week of the start of the data set transmission interval. Previously, the week number always represented the current GPS week of transmission. CACD takes exception to this redefinition because: 1. The PIRN introduces an inconsistency with the definition of Z-count given in paragraph 3.3.4, page 33 which states that “the ten most significant bits of the Z-count are a binary representation of the sequential number assigned to the present GPS week (Module 1024).” 2. Since the GPS week being transmitted by the Space Segment could vary from SV to SV (depending upon time of upload) and since the transmitted GPS week could be different from the current GPS week by one week starting on day 29 after an upload, the GPS UE could navigate using the wrong GPS week. Use of the wrong GPS week could cause navigation interruptions and could result in the incorrect time-tagging of the satellite data. The user could therefore be provided with navigation data which is marked valid when, in fact, it is not valid. This is an unacceptable situation. 22,29,31, 57a, 57b,58, 60 These PIRN items document the change in the definition of the ephemeris fit interval flag for a value equal to 1 and its corresponding relationship to the IODC/IODE during extended operations. The GPS UE supports only a fit interval of 6 hours when the fit interval flag equals 1. The newly defined ephemeris fit intervals of 8, 14, 26, 50, 74, 98, 122, and 146 hours for Long Term Extended Operations are not supported and the UE will compute 6 hour curve fits whenever these are in effect. Figure 10-1. Letters of Exception (sheet 4 of 15) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 54 UNCLASSIFIED UNCLASSIFIED ATTACHMENT 1 (cont.) Rockwell CACD Exceptions to PIRN-200B-001B PIRN ITEM Rockwell CACD Comment 39, 40, 50, 54 These PIRN items describe the almanac data sets and how the almanac URE grows throughout extended operations. The GPS UE requirements for almanac-based direct P-code TTFF (time to first fix) are only applicable to Normal Operations in which the almanac parameters have been updated within the last six days. Extended operations may jeopardize missions of those users who require efficient almanac-based direct P-code TTFFs. 43 This PIRN item documents the fact that the health summary in subframe 5, page 25 is only updated at the time of almanac upload. During extended operations the health summary may become outdated due to the length of time since the last upload. If the health summary becomes outdated and does not accurately reflect the status of the GPS constellation, the TTFF for the GPS UE may be delayed. This is due to the time wasted on the possible acquisition of unhealthy SVs which were marked “healthy” by the health summary. Also, attempts to acquire healthy SVs which are marked unhealthy will not be made. As a result, extended operations may jeopardize missions of those users who require efficient TTFF. 45,46 These PIRN items document the changes for the UTC parameter data sets during extended operations. The GPS UE uses the UTC parameters to provide the user with precise time. Exception is taken because: 1. CACD is not confident that the accuracy of the UTC parameters can be maintained throughout extended operations. This accuracy is specified as 90 ns (one sigma) on ICD-GPS-200B page 32. As a result, extended operations may jeopardize missions of those users who require precise UTC. 2. Since the GPS UE does not account for the degraded accuracy of the UTC parameters as a function of time during extended operations, the user may be provided with an incorrect estimate of his time accuracy which again may jeopardize his mission. CACD believes the UTC parameters’ accuracy, as a function of time, should be specified in some GPS system specification. Figure 10-1. Letters of Exception (sheet 5 of 15) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 55 UNCLASSIFIED UNCLASSIFIED Collins Avionics & Communications Division Rockwell International Corporation 350 Collins Road NE Cedar Rapids, IA 52498 (319) 395-1000 Rockwell International September 23, 1992 ARINC Research Corporation 11770 Warner Avenue, Suite 210 Fountain Valley, CA 92708 Attention: Mr. Peter Fyfe Subject: PIRN-200B-009A Rockwell CACD Letter of Exception Dear Mr. Fyfe: The subject PIRN-200B-009A which documents the changes to the Space Segment/User Segment interface for the Block IIR SVs is hereby approved by Rockwell CACD with the following exception: The PIRN states that UTC parameters (PIRN items 4, 20), ionospheric model parameters (PIRN items 21, 27a), and almanac data (PIRN items 22a, 22b) will degrade when the Block IIR SVs do not receive an upload from the Control Segment. Since the IIR SVs indicate “normal operations” (curve fit interval flag of 4 hours) at all times, dome user segment requirements cannot be met during “normal operations” in the absence of Control Segment uploads. These affected requirements are precise UTC time transfer and almanac-based direct P-code time to first fix. The affected User Equipment (UE) is that designed and developed by Rockwell CACD under the GPS UE Phase III Contract F04701-85-C-0038, GPS UE Phase IV Contract F04701-90C-0092, and GPS MAGR Contract F04701-91-C-0003. Sincerely, Signature on file C. S. Olson Program Manager CSS/jk Figure 10-1. Letters of Exception (sheet 6 of 15) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 56 UNCLASSIFIED UNCLASSIFIED Collins Avionics & Communications Division Rockwell International Corporation 350 Collins Road NE Cedar Rapids, IA 52498 (319) 395-1000 Rockwell International June 12, 1995 ARINC Research Corporation 2250 East Imperial Highway, Suite 450 El Segundo, CA 90245-3509 Attention: Mr. Thomas Denigan Subject: PIRN-200C-001 Rockwell CACD Letter of Exception Enclosure: Approval sheet for PIRN-300C-001 References: ARINC Research Corporation letter dated January 16, 1995; Subject: PIRN-200C-001 to ICD-GPS-200C The subject PIRN-200C-001 is approved by Rockwell CACD with the following exception: The effect of Item 2, “Change Section 20.3.3.5.2.2, page 121” of this PIRN is that computed almanac age in Rockwell CACD government User Equipment (UE) will be approximately 14 hours older than actual almanac age. Since almanac age computation is for display/output purposes only there is no impact to receiver operation or navigation solution accuracy. CACD computes an almanac time-of-transmission that is nominally the multiple of 212 seconds truncated from 3.5 days prior to the almanac reference time, toa. Item 2 of subject PIRN-200C-001 documents the change of toa from being nominally the multiple of 212 seconds truncated from 3.5 days (84 hours) after the first valid transmission time for an almanac set to being nominally the multiple of 212 seconds truncated from 70 hours after the first valid transmission time for an almanac set. Therefore, Rockwell CACD government UE will compute and output an incorrect almanac age by approximately 14 hours. Figure 10-1. Letters of Exception (sheet 7 of 15) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 57 UNCLASSIFIED UNCLASSIFIED The second sentence of paragraph 20.3.3.5.2.2, “The almanac is updated often enough to ensure that GPS time, t, shall differ from toa by less than 3.5 days during the transmission period”, must not change. This is to ensure the proper resolution of the GPS week number associated with the almanac. Technical questions concerning this matter should be referenced to Lawrence Burns at (319)395-2616. Sincerely, Signature on file Craig Olson GPS Program Manager Figure 10-1. Letters of Exception (sheet 8 of 15) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 58 UNCLASSIFIED UNCLASSIFIED International Business Machines Corporation 800 N. Frederick Avenue Gaithersburg, MD 20879 RD001001935 October 1, 1993 Mr. Tom Denigan ARINC Research Corporation 11770 Warner Avenue, suite 210 Fountain Valley, CA 92780 Subject: IBM “Letter of Exception” against ICD-GPS-200B as Modified by IRN-200B-001B Reference: 1. ICD-GPS-200B, dated November 30, 1987 2. IRN-200B-001B, dated April 15,, 1991 3. IRN-200B-002, dated July 26, 1991 4. IRN-200B-003, dated December 2, 1991 5. IRN-200B-004, dated December 5, 1991 6. IRN-200B-005, dated December 16, 1991 7. IRN-200B-006, dated December 9, 1992 8. IRN-200B-007, dated July 19, 1993 9. IBM letter 020689-2, dated February 6, 1989 10. Contract F04701-90-C-0009 Dear Mr. Denigan: With the release of the referenced IRN-200B-001B through -007, this letter represents the current IBM letter of exception against ICD-GPS-200B, replacing Reference 9. The IBM contract does not support the following: Block I end of data transmission (Paragraph 20.3.2) The option of repeated almanacs for 12 or fewer SVs (Paragraphs 20.3.3.5.1.2, 20.3.3.5.1.3) The use of pages 2, 3, 4, 5, 7, 8, 9, and 10 of subframe 4 for purpose other than almanac data for SVs 25 through 32 (Paragraphs 20.3.3.5.1, 20.3.3.5.1.1, 20.3.3.5.1.3, Table 20-V) Figure 10-1. Letters of Exception (sheet 9 of 15) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 59 UNCLASSIFIED UNCLASSIFIED -2Mr. Tom Denigan RD001001935 October 1, 1993 Questions and coordination related to the technical content of ICD-GPS200 should be addressed to Ming Kang Chien at 301/240-6449. Very truly yours, Signature on file Magdalena V. Clyne Contract Administrator cc: Capt. B. Schrimsher, SMC/CZGD Lt. R. Layton, SMC/CZET 1 1 Figure 10-1. Letters of Exception (sheet 10 of 15) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 60 UNCLASSIFIED UNCLASSIFIED MARTIN MARIETTA ASTRO SPACE POST OFFICE BOX 8555 PHILADELPHIA, PENNSYLVANIA 19101 17 August 1994 GPS IIR-CM-1046 ARINC Research Corporation 2250 E. Imperial Highway, Suite 450 El Segundo, CA 90245-3509 Attention: Ms. Pat Alexander Subject: Approval of ICD-GPS-200, Revision C Reference: Contract F04701-89-C-0073 ICD-GPS-200, Revision C dated 10 October 1993 Dear Ms. Alexander: Martin Marietta Astro Space approves with exception ICD-GPS-200, Revision C as evidenced by the attached signed approval sheet. The areas of exception are both general and specific in nature. General Areas of Exceptions Martin Marietta takes exception to specific changes in requirements originally conveyed in IRN Nos. IRN-200B-001, IRN-200B-004, IRN-200B-005, and IRN-200B-006 of ICD-GPS200B. The principal reason for these exceptions is that Martin Marietta’s contract does not include requirements for Extended Navigation, User Range Accuracy bin structure, some aspects of the Time of Almanac requirements and the Spherical Error Probability of the navigation signals received by the navigation users. Detailed reasons for these exceptions are given below. 1. Extended Navigation Martin Marietta takes exception to the application of Extended Navigation (EN) requirements, or the attribution of EN performance or EN performance verification to the Block IIR SV. Such application or attribution may inadvertently be construed from the overall context. The Block IIA operational mode called Long Term Extended Operations/”Extended Navigation” is undefined and not required in the Block IIR contract. There is no Block IIR requirement to support 180 days in the Block IIA mode without regular CS contacts and uploads. The Block IIR contract defines the Block IIA mode in the context of ICD-GPS-200B dated 30 November 1987 which describes a 14 day autonomy capability. Figure 10-1. Letters of Exception (sheet 11 of 15) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 61 UNCLASSIFIED UNCLASSIFIED 17 August 1994 GPS IIR-CM-1046 Page 2 The Block IIR design supports 180 day autonomy in the context of the Autonomous Navigation mode and 14 day autonomy in the Block IIA mode. It may be possible to construct upload databases that will allow operation beyond 14 days with the current design, but this is not a current requirement. The Block IIR Space Vehicle does not support such an upload design, performance description, and performance verification. Section 20.3.2, sentence 2 states “Block IIR SVs are designed to have sufficient memory to store 182 days of upload NAV data in the Block IIA mode ...” Martin Marietta takes exception to a “182 day” NAV data storage requirement in the Block IIA mode. The Block IIR design and validation plan is required to provide performance and memory margin computed on 14 day storage in the IIA mode as defined in Section 20.3.4.4 and 20.3.4.5 of the ICD-GPS-200B dated 30 November 1987. Sections 20.3.4.4 and 20.3.4.5 with new Tables 20-XII and 20-XIII, define different “days spanned”, “fit intervals”, and “transmission intervals” compared to the 30 November 1987 ICD-GPS-200B. These changes are generated by some of the Extended Navigation upload characteristics for the Block IIA SV which supports “182 days” of data. Martin Marietta takes exception to evaluation and validation of the Block IIR design performance under these modified sections and tables. 2. URA Bin Structure Martin Marietta takes exception to items relating to URA index to ranges of URA in meters appears to require the Block IIR SV in the Autonomous Navigation (AN) mode to transition from on index to the next at exactly the values of URA indicated. This specification of URA bins is not a defined requirement for Block IIR. The Block IIR AN design is based on the equations and the ‘no better than’ descriptions present in ICD-GPS-200B, in accordance with the Block IIR implementation. When in the AN mode, the Martin Marietta design estimates URA on board the SV and converts the result to the index in the NAV user message by rearranging the equations and solving for the index. As a result, our design approximates, but does not exactly match, the description when Block IIR is in the AN mode. When Block IIR is in the IIA mode, the Martin Marietta design does match the URA bin description exactly because we broadcast the index uploaded from the CS and the CS estimates URA. 3. Spherical Error Probable (SEP) Martin Marietta takes exception to the URE statement in 6.3.4 since it implies Martin Marietta responsibility to relate Block IIR URE to 16 meters SEP. Figure 10-1. Letters of Exception (sheet 12 of 15) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 62 UNCLASSIFIED UNCLASSIFIED 17 August 1994 GPS IIR-CM-1046 Page 3 IRN-200B-006 added section 6.3.4 on “Autonomous Navigation Mode.” The third sentence states that the Block IIR SV in the Autonav mode “... will have a user range error that is at or below a level required to support 16 meter SEP accuracy.” The Block IIR SV constellation, when authorized to operate in the Autonav mode, is required to provide 6 meters (1 sigma) URE. The 6 meter requirement is defined in SV Segment Specification (SS-SS-500). Martin Marietta has been advised that this independently derived 6 meter URE requirement does support the 16 meter SEP system requirement for a nominal geometric dilution of precision. The 16 meter SEP accuracy in the user equipment output is dependent on geometry of the SVs chosen by the URE for the solution. The 16 meter SEP accuracy is not a requirement for Block IIR Space Vehicle. 4. Time of Almanac (Toa) Martin Marietta takes exception to paragraph 20.3.3.5.2.2, since it implies that the SV is required to ensure that time of almanac (Toa) values be the same for a given data set (when the SV health is changed by the CS) or that Toa differ for successive data sets (which contain changes in SV health). This is a CS responsibility. Martin Marietta is concerned about the ambiguous CS/SV requirement to ensure that the described Toa values are presented to Users in Appendix II, paragraph 20.3.3.5.2.2. ICD-GPS-200, Revision C deletes ‘The CS shall ensure’ in paragraph 20.3.3.5.2.2. This is a change from ICD-GPS-200B dated 11/30/87 which is applied to our contract. If so deleted, ICD-GPS-200B will be mute as to who ensures that “All Toa values in SF4&5 shall be the same for a given almanac data set and shall differ for successive data sets which contain changes in almanac parameters or SV health.’ The Block IIR design is not required to, and does not, affect or check Toa based on the SV health settings described by this section. The Block IIR design depends on CS uploads for SV health and the relationship of SV health data to almanac reference time. In an independent process, CS uploads for Toa values and almanacs are inputs to the on-board generation of Toa values when the Block IIR design propagates almanacs to remain within 3.5 days of GPS time. If the CS provides a valid upload, the Block IIR on-board processing will maintain that all Toa values in SF4&5 will be the same for a given almanac data set and will differ for successive data sets which contain changes in almanac parameters. Figure 10-1. Letters of Exception (sheet 13 of 15) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 63 UNCLASSIFIED UNCLASSIFIED 17 August 1994 GPS IIR-CM-1046 Page 4 Specific Exceptions In addition, Martin Marietta takes exceptions to the specific requirements listed below: Paragraph # 20.3.3.3.1.1 Table 20-V 6.2.3 - 6.2.3.3 6.3.2, 6.3.3 6.3.4 6.3.4 20.3.2 20.3.2 20.3.3.4.1 20.3.3.5.1.2 20.3.3.5.1.2 20.3.3.5.2.2 20.3.4.1 20.3.4.4 Table 20-XII 20.3.4.4 Table 20-XII Description Change of week number: Extended Navigation (EN). SV ID Nos. in Note 4 (EN). Definitions. CS responsibility. N/A to Block-IIR. Extended Navigation Mode description. “... and will have ... 16 meter SEP accuracy.” Orbit parameters. Narrative on almanac. N/A to Block-IIR. Martin Marietta uses ICD-GPS-401 method. “... 182 days of uploaded NAV data in the Block IIA ...” “(d) if a control ... subframes will indicate ID = 1 ...” IRN excludes Block-IIR. CS requirement deleted. (Related to extended nav.). Propagation of Toa requirements. “For Block II ... transmission interval.” Propagation of Toa requirements. Reqmt for subframe changes at frame boundary. Table 20-XII: Ext. Nav related. IODC Requirements. Ext. Nav related. Two hour data sets are not tested. Transmission intervals. Ext. Nav related. Note that if Martin Marietta has taken earlier exception to a change in any requirements in a previous revision of this document, Martin Marietta continues to take exception to that change. The retraction of an exception will be accomplished by a letter explicitly stating that the exception is no longer valid. If there are any questions of a technical nature concerning the contents of this letter, please contact Dave Levin at (610) 354-3022. All other questions or comments should be addressed to the undersigned at (610) 354-1710. Very truly yours, MARTIN MARIETTA ASTRO SPACE Signature on file D. Supow Manager, GPS Contracts GPS PMO cc: Maj. Paul Schubert (CZEP) /dd Figure 10-1. Letters of Exception (sheet 14 of 15) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 64 UNCLASSIFIED UNCLASSIFIED Lockheed Martin Federal Systems, Inc. 700 N. Frederick Avenue Gaithersburg, MD 20879-3328 Telephone 301-240-7500 In reply refer to: GOSC96000912 September 5, 1996 ARINC Research Corporation 2250 E. Imperial Highway, Suite 450 El Segundo, CA 90245-3509 Attention: Mr. Soon K. Yi Subject: PIRN-200C-002 Reference: 1. Contract F04606-95-D-0239 2. ARINC letter RO/SMS/ES/SE/96-025, dated July 9, 1996 Dear Mr. Yi: PIRN-200C-002 has been reviewed by Lockheed Martin Federal Systems, the Control Segment Contractor. This PIRN is approved subject to the contractual exception below: The changes contained in this PIRN are not within the baseline of our GPS OCS Support Contract. Enclosed is the signed approval sheet requested in the referenced letter. If you have any questions, please contact Ming Kang Chien at (301)240-6449. Very truly yours, Signature on file C. T. Thomas Contract Administrator Enclosure cc: Maj. P. Schubert Capt. F. Wylie Capt. J. Gravitt Capt. J. Varljen D. Munk D. Greer SMC/CZGD SMC/CZEA SMC/CZE SMC/CZEP SMC/CZGP SM-ALC/PKLX Figure 10-1. Letters of Exception (sheet 15 of 16). IRN-200C-003 ICD-GPS-200C 11 OCT 1999 64a UNCLASSIFIED UNCLASSIFIED Government Systems 350 Collins Road NE Cedar Rapids, Iowa 52498 Tel. 319.295.1000 Rockwell Collins August 27, 1999 ARINC Incorporated 2250 East Imperial Highway, Suite 450 El Segundo, CA 90245-3509 Attention: Mr. Soon K. Yi Reference: ARINC memo ATE/SMS/OPS/GJP/99-038, dated 28 July 99 Dear Mr. Yi: Subject: Rockwell Collins, Inc. Letter of Exception against PIRN-200C-003 Revision A, as defined by the referenced ARINC memo The subject PIRN documents the use of an Earth Centered Inertial (ECI) frame to define the location of the satellites and account for satellite motion during signal transit time. There are also numerous typo corrections. Also the Navigation Message Correction Term (NMCT), also commonly referred to as WAGE, is partially covered. Rockwell Collins approves this PIRN with the following exceptions: 1) Implementation of older style corrections for satellite motion during signal transit time (paragraph 20.3.3.4.3.3 and 20.3.3.4.3.4) shall be permitted in User Equipment. 2) Use of NMCT data (paragraph 20.3.3.5.2.6) is not mandatory and will be determined by the manufacturers of User Equipment, based on required accuracy. Sincerely, Signature on file A. Caslavka, Director Navigation Systems js Figure 10-1. Letters of Exception (sheet 16 of 16). IRN-200C-003 ICD-GPS-200C 11 OCT 1999 64b UNCLASSIFIED UNCLASSIFIED 20. APPENDIX II. GPS NAVIGATION DATA STRUCTURE FOR DATA ID NO. 2 20.1 Scope. This appendix describes the specific GPS navigation (NAV) data structure denoted by data ID number 2. This data ID number, when transmitted as part of the NAV data, shall be represented by the two-bit binary notation as 01. Data ID number 1 is no longer in use. 20.2 Applicable Documents. 20.2.1 Government Documents. In addition to the documents listed in paragraph 2.1, the following documents of the issue specified contribute to the definition of the NAV data related interfaces and form a part of this Appendix to the extent specified herein. Specifications None Standards None Other Publications None 20.2.2 Non-Government Documents. In addition to the documents listed in paragraph 2.2, the following documents of the issue specified contribute to the definition of the NAV data related interfaces and form a part of this Appendix to the extent specified herein. Specifications None Other Publications none ICD-GPS-200C 10 OCT 1993 65 UNCLASSIFIED UNCLASSIFIED 20.3 Requirements 20.3.1 Data Characteristics. The data stream shall be transmitted by the SV on the L1 and L2 channels at a rate of 50 bps. The data stream, when present, shall be common to both of those L-band frequencies, irrespective of the PRN ranging code(s) used. 20.3.2 Message Structure. As shown in Figure 20-1, the message structure shall utilize a basic format of a 1500 bit long frame made up of five subframes, each subframe being 300 bits long. Subframes 4 and 5 shall be subcommutated 25 times each, so that a complete data message shall require the transmission of 25 full frames. The 25 versions of subframes 4 and 5 shall be referred to herein as pages 1 through 25 of each subframe. Each subframe shall consist of ten words, each 30 bits long; the MSB of all words shall be transmitted first. Each subframe and/or page of a subframe shall contain a telemetry (TLM) word and a handover word (HOW), both generated by the SV, and shall start with the TLM/HOW pair. The TLM word shall be transmitted first, immediately followed by the HOW. The latter shall be followed by eight data words. Each word in each frame shall contain parity (reference Section 20.3.5). ICD-GPS-200C 10 OCT 1993 66 UNCLASSIFIED UNCLASSIFIED DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 1 MSB FIRST 3 SECONDS WORD 2 WORD 3 WORD 4 WORD 5 71 SUBFRAME NO. PAGE NO. 1 N/A 1 31 TLM 22 BITS 61 HOW C P 73 77 83 91 121 WN t P 22 BITS P 10 BITS 23 BITS*** P L2 P DATA FLAG - 1 BIT C/A OR P ON L2 - 2 BITS URA INDEX - 4 BITS SV HEALTH - 6 BITS 2 MSBs IODC - 10 BITS TOTAL DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 6 1 N/A 181 24 BITS*** P WORD 8 197 16 BITS*** TGD MSB FIRST 3 SECONDS WORD 7 151 P 24 BITS*** WORD 9 211 219 P 8 BITS WORD 10 241 toc 16 BITS P af2 8 BITS 8 LSBs 271 af1 16 BITS P af0 22 BITS IODC - 10 BITS TOTAL *** RESERVED P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 1 of 11) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 67 UNCLASSIFIED t P UNCLASSIFIED DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 1 SUBFRAME NO. PAGE NO. 2 N/A WORD 2 1 WORD 3 31 TLM 61 HOW C P t 22 BITS MSB FIRST 3 SECONDS P 22 BITS WORD 4 69 91 107 121 ∆n Crs IODE 8 BITS WORD 5 P 16 BITS 8 BITS 16 BITS P P 24 BITS MSBs LSBs M0 - 32 BITS TOTAL DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 6 151 CUC 2 N/A 16 BITS 8 BITS MSBs e - 32 BITS TOTAL WORD 7 167 WORD 8 181 P 211 24 BITS MSB FIRST 3 SECONDS P CUS 16 BITS LSBs WORD 9 227 8 BITS 241 P 271 24 BITS P 287 toe t 16 BITS MSBs A - 32 BITS TOTAL WORD 10 LSBs FIT INTERVAL FLAG - 1 BIT AODO - 5 BITS P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 2 of 11) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 68 UNCLASSIFIED P UNCLASSIFIED DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 1 SUBFRAME NO. PAGE NO. 3 N/A WORD 2 1 WORD 3 31 61 TLM HOW C P t 22 BITS MSB FIRST 3 SECONDS 77 Cic P 22 BITS WORD 4 91 8 BITS 16 BITS WORD 5 P 121 24 BITS MSBs N/A WORD 7 P WORD 8 WORD 9 211 Crc 16 BITS WORD 10 241 271 • 8 BITS P 24 BITS Ω P P 24 BITS MSBs LSBs MSB FIRST 3 SECONDS 181 24 BITS P i 0 - 32 BITS TOTAL 150 BITS 3 8 BITS LSBs DIRECTION OF DATA FLOW FROM SV 151 Cis P 16 BITS Ω0 - 32 BITS TOTAL WORD 6 137 LSBs IODE 8 BITS 279 IDOT 14 BITS t ω - 32 BITS TOTAL i0 - 32 BITS TOTAL P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 3 of 11) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 69 UNCLASSIFIED P UNCLASSIFIED DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 1 SUBFRAME NO. PAGE NO. 5 1 THRU 24 MSB FIRST 3 SECONDS WORD 2 WORD 3 WORD 4 WORD 5 61 1 31 TLM 63 69 e HOW C P t 22 BITS 91 P 16 BITS 22 BITS toa 8 P BITS 99 121 δi • Ω P 16 BITS 8 P BITS 16 BITS DATA ID - 2 BITS SV ID - 6 BITS SV HEALTH DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 6 WORD 7 151 5 1 THRU 24 WORD 8 181 A 24 BITS P WORD 9 211 Ω0 MSB FIRST 3 SECONDS 241 ω P 24 BITS WORD 10 P 24 BITS 271 M0 279 290 t P P 24 BITS 8 MSBs 3 LSBs af0 - 11 BITS TOTAL af1 - 11 BITS TOTAL P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED NOTE: PAGES 2, 3, 4, 5, 7, 8, 9 & 10 OF SUBFRAME 4 HAVE THE SAME FORMAT AS PAGES 1 THROUGH 24 OF SUBFRAME 5 Figure 20-1. Data Format (sheet 4 of 11) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 70 UNCLASSIFIED UNCLASSIFIED DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 1 SUBFRAME NO. PAGE NO. WORD 2 WORD 3 WORD 4 WORD 5 61 1 31 TLM 5 MSB FIRST 3 SECONDS 63 t 22 BITS 91 121 SV HEALTH SV HEALTH toa WNa 6 BITS/SV 6 BITS/SV 8 8 P P P SV SV SV SV SV SV SV SV BITS BITS 1 2 3 4 5 6 7 8 HOW C P 25 69 P 22 BITS DATA ID - 2 BITS SV (PAGE) ID - 6 BITS DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 6 MSB FIRST 3 SECONDS WORD 7 WORD 8 WORD 9 WORD 10 271 151 5 25 181 211 241 274 SV HEALTH SV HEALTH SV HEALTH SV HEALTH 6 BITS/SV 6 BITS/SV 6 BITS/SV 6 BITS/SV P P P P SV SV SV SV SV SV SV SV SV SV SV SV SV SV SV SV 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 19 BITS** t 3 BITS *** ** RESERVED FOR SYSTEM USE *** RESERVED P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 5 of 11) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 71 UNCLASSIFIED P UNCLASSIFIED DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 1 SUBFRAME NO. PAGE NO. 4 1, 6, 11, 16 & 21 MSB FIRST 3 SECONDS WORD 2 WORD 3 WORD 4 WORD 5 61 1 31 TLM 63 69 91 121 HOW C P t 22 BITS 16 BITS*** P 22 BITS 24 BITS*** P 24 BITS*** P P DATA ID - 2 BITS SV (PAGE) ID - 6 BITS DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 6 WORD 7 151 4 1, 6, 11, 16 & 21 WORD 8 181 24 BITS*** P WORD 9 211 24 BITS*** MSB FIRST 3 SECONDS P 241 24 BITS*** P 8*** BITS WORD 10 249 16 BITS*** 271 P 22 BITS** t ** RESERVED FOR SYSTEM USE *** RESERVED P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 6 of 11) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 72 UNCLASSIFIED P UNCLASSIFIED DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 1 SUBFRAME NO. PAGE NO. 4 12, 19, 20, 22, 23 & 24 MSB FIRST 3 SECONDS WORD 2 WORD 3 WORD 4 WORD 5 61 1 31 TLM 63 69 91 121 HOW C P t 22 BITS 16 BITS*** P 22 BITS 24 BITS*** P 24 BITS*** P P DATA ID - 2 BITS SV (PAGE) ID - 6 BITS DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 6 WORD 7 151 4 12, 19, 20, 22, 23 & 24 WORD 8 181 24 BITS*** P WORD 9 211 24 BITS*** MSB FIRST 3 SECONDS P 241 24 BITS*** P 8*** BITS WORD 10 249 16 BITS** 271 P 22 BITS** t ** RESERVED FOR SYSTEM USE *** RESERVED P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 7 of 11) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 73 UNCLASSIFIED P UNCLASSIFIED DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 1 SUBFRAME NO. PAGE NO. WORD 2 WORD 3 WORD 4 WORD 5 61 1 31 TLM 4 MSB FIRST 3 SECONDS 63 69 t 22 BITS 91 99 107 121 129 137 β0 β3 α1 α3 β2 α0 α2 β1 8 8 8 8 8 8 P 8 P 8 P BITS BITS BITS BITS BITS BITS BITS BITS HOW C P 18 77 P 22 BITS DATA ID - 2 BITS SV (PAGE) ID - 6 BITS DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 6 WORD 7 151 18 WORD 8 181 A1 4 211 219 227 WORD 9 241 249 257 WORD 10 271 24 BITS WNt tot DN ∆tLS ∆tLSF 8 P 8 P 8 8 8 8 8 BITS BITS BITS BITS BITS BITS BITS MSBs LSBs P 24 BITS MSB FIRST 3 SECONDS P 279 14 BITS** t WNLSF A0 - 32 BITS TOTAL ** RESERVED FOR SYSTEM USE P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 8 of 11) IRN-200C-003 ICD-GPS-200C 11 OCT 1999 74 UNCLASSIFIED P UNCLASSIFIED DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 1 SUBFRAME NO. PAGE NO. WORD 2 WORD 3 WORD 4 WORD 5 61 1 31 63 69 TLM 4 MSB FIRST 3 SECONDS HOW C P 25 t 22 BITS 91 A-SPOOF & SV CONFIG P SV SV SV SV 1 2 3 4 22 BITS P 121 A- SPOOF & SV CONFIG SV SV SV SV SV SV 5 6 7 8 9 10 P A- SPOOF & SV CONFIG SV SV SV SV SV SV 11 12 13 14 15 16 P DATA ID - 2 BITS SV (PAGE) ID - 6 BITS DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 6 MSB FIRST 3 SECONDS WORD 7 WORD 8 WORD 9 WORD 10 227 151 A- SPOOF & SV CONFIG 4 25 SV SV SV SV SV SV 17 18 19 20 21 22 181 P 211 A- SPOOF & SV CONFIG SV SV SV SV SV SV 23 24 25 26 27 28 229 A-SPOOF & SV CONFIG P SV SV SV SV 29 30 31 32 SV 25 241 271 SV HEALTH 6 BITS/SV P P SV SV SV SV 26 27 28 29 2 BITS ** SV HEALTH 6 BITS/SV SV SV SV 30 31 32 t P SV HEALTH - 6 BITS ** RESERVED FOR SYSTEM USE P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED 4 BITS ** Figure 20-1. Data Format (sheet 9 of 11) IRN-200C-003 ICD-GPS-200C 11 OCT 1999 75 UNCLASSIFIED UNCLASSIFIED DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 1 SUB FRAME NO. PAGE NO. WORD 2 31 TLM 4 WORD 3 61 1 71 HOW t 22 BITS WORD 4 WORD 5 69 63 C P 13 MSB FIRST 3 SECONDS P 22 BITS 91 E R D 1 E R D 2 E R D 3 6 B I T S 6 B I T S 2 M S B S P 121 E R D 3 E R D 4 E R D 5 E R D 6 E R D 7 4 L S B S 6 B I T S 6 B I T S 6 B I T S 2 M S B S P E R D 7 E R D 8 E R D 9 4 L S B S 6 B I T S 6 B I T S E R D 1 0 E R D 1 1 6 B I T S 2 M S B S P DATA ID - 2 BITS SV (PAGE) ID - 6 BITS AVAILABILITY INDICATOR - 2 BITS DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 6 WORD 7 151 4 13 WORD 8 181 E R D 1 1 E R D 1 2 E R D 1 3 E R D 1 4 E R D 1 5 4 L S B S 6 B I T S 6 B I T S 6 B I T S 2 M S B S P MSB FIRST 3 SECONDS WORD 9 211 E R D 1 5 E R D 1 6 E R D 1 7 E R D 1 8 E R D 1 9 4 L S B S 6 B I T S 6 B I T S 6 B I T S 2 M S B S P WORD 10 241 E R D 1 9 E R D 2 0 E R D 2 1 E R D 2 2 E R D 2 3 4 L S B S 6 B I T S 6 B I T S 6 B I T S 2 M S B S P 271 E R D 2 3 E R D 2 4 E R D 2 5 E R D 2 6 E R D 2 7 4 L S B S 6 B I T S 6 B I T S 6 B I T S 2 M S B S P E R D 2 7 E R D 2 8 E R D 2 9 E R D 3 0 4 L S B S 6 B I T S 6 B I T S 6 B I T S t P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 10 of 11) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 76 UNCLASSIFIED P UNCLASSIFIED DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 1 SUBFRAME NO. PAGE NO. 4 14, 15 & 17** MSB FIRST 3 SECONDS WORD 2 WORD 3 WORD 4 WORD 5 61 1 31 TLM 63 69 91 121 HOW C P t 22 BITS 16 BITS** P 22 BITS 24 BITS** P 24 BITS** P P DATA ID - 2 BITS SV (PAGE) ID - 6 BITS DIRECTION OF DATA FLOW FROM SV 150 BITS WORD 6 WORD 7 151 14, 15 & 17** 4 WORD 8 181 24 BITS** P WORD 9 211 24 BITS** MSB FIRST 3 SECONDS P WORD 10 241 24 BITS** P 271 24 BITS** P 22 BITS** t ** THE INDICATED PORTIONS OF WORDS 3 THROUGH 10 OF PAGES 14 AND 15 ARE RESERVED FOR SYSTEM USE, WHILE THOSE OF PAGE 17 ARE RESERVED FOR SPECIAL MESSAGES PER PARAGRAPH 20.3.3.5.1.10 P = 6 PARITY BITS t = 2 NONINFORMATION BEARING BITS USED FOR PARITY COMPUTATION (SEE PARAGRAPH 20.3.5) C = TLM BITS 23 AND 24 WHICH ARE RESERVED Figure 20-1. Data Format (sheet 11 of 11) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 76a UNCLASSIFIED P UNCLASSIFIED (This page intentionally left blank.) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 76b UNCLASSIFIED UNCLASSIFIED Block II and IIA SVs are designed with sufficient memory capacity for storing 182 days of uploaded NAV data. However, the memory retention of these SVs will determine the duration of data transmission. Block IIR SVs are designed to have sufficient memory to store 182 days of uploaded NAV data in the Block IIA mode and to store 210 days of CS data needed to generate NAV data on-board in the Autonav mode. Alternating ones and zeros will be transmitted in words 3 through 10 in place of the normal NAV data whenever the SV cannot locate the requisite valid control or data element in its on-board computer memory. The following specifics apply to this default action: (a) the parity of the affected words will be invalid, (b) the two trailing bits of word 10 will be zeros (to allow the parity of subsequent subframes to be valid -- reference paragraph 20.3.5), (c) if the problem is the lack of a data element, only the directly related subframe(s) will be treated in this manner, (d) if a control element cannot be located, this default action will be applied to all subframes and all subframes will indicate ID = 1 (i.e., an IDcode of 001) in the HOW (reference paragraph 20.3.3.2) (Block IIR SVs indicate the proper subframe ID for all subframes.) and (e) certain failures of data which may occur in the SV memory or during an upload will cause the SV to transmit in non-standard codes (NSC and NSY) which would preclude normal use by the US. Normal NAV data transmission will be resumed by the SV whenever a valid set of elements becomes available. IRN-200C-004 ICD-GPS-200C 12 April 2000 77 UNCLASSIFIED UNCLASSIFIED Block II SVs are uploaded with 182 days of NAV data. However, the EAROM retentivity for these SVs is designed and guaranteed for only 14 days. Therefore, Block II SV memory is most likely to fail sometime during long-term extended operations after repeated write operations. In the case of memory failure, the SV will transmit alternating ones and zeros in word 3-10 as specified in the above paragraph. Block IIA SVs are also uploaded with 182 days of data. However, the EAROM retentivity for these SVs is designed and guaranteed for 180 days. The memory retentivity for the Block IIR SVs is designed and guaranteed for 210 days. Although the data content of the SVs will be temporarily reduced during the upload process, the transmission of valid NAV data will be continuous. The data capacity of specific operational SVs may be reduced to accommodate partial memory failures. ICD-GPS-200C 10 OCT 1993 78 UNCLASSIFIED UNCLASSIFIED 20.3.3 Message Content. The format and contents of the TLM word and the HOW, as well as those of words three through ten of each subframe/page, are described in the following subparagraphs (message content related to SA is defined in ICD-GPS-203 and/or ICD-GPS-224 and/or in ICD-GPS-225 -- see note in paragraph 2.1). The timing of the subframes and pages is covered in Section 20.3.4. 20.3.3.1 Telemetry Word. Each TLM word is 30 bits long, occurs every six seconds in the data frame, and is the first word in each subframe/page. The format shall be as shown in Figure 20-2. Bit 1 is transmitted first. Each TLM word shall begin with a preamble, followed by the TLM message, two reserved bits, and six parity bits. The TLM message contains information needed by the authorized user as defined in ICD-GPS-203 and/or ICD-GPS224 and/or in ICD-GPS-225 (see note in paragraph 2.1), and by the CS, as described in the related SS/CS interface documentation. 20.3.3.2 Handover Word (HOW). The HOW shall be 30 bits long and shall be the second word in each subframe/page, immediately following the TLM word. A HOW occurs every 6 seconds in the data frame. The format and content of the HOW shall be as shown in Figure 20-2. The MSB is transmitted first. The HOW begins with the 17 MSBs of the time-of-week (TOW) count. (The full TOW count consists of the 19 LSBs of the 29-bit Zcount). These 17 bits correspond to the TOW-count at the X1 epoch which occurs at the start (leading edge) of the next following subframe (reference paragraph 3.3.4). IRN-200C-002 ICD-GPS-200C 25 SEP 1997 79 UNCLASSIFIED UNCLASSIFIED TLM Word 1 = Reserved Bits MSB Preamble 1 TLM Message 1 0 0 0 1 0 1 1 1 2 3 4 5 6 7 8 Parity LSB 9 10 11 12 13 1 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 HOW Solved for bits to preserve parity check with zeros in bits 29 and 30 Anti-Spoof Flag “Alert” Flag MSB Parity LSB Subframe ID TOW-Count Message (Truncated) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 0 0 20 21 22 23 24 25 26 27 28 29 30 Figure 20-2. TLM and HOW Formats IRN-200C-004 ICD-GPS-200C 12 April 2000 80 UNCLASSIFIED UNCLASSIFIED Bit 18 is an "alert" flag. When this flag is raised (bit 18 = "1"), it shall indicate to the unauthorized user that the SV URA may be worse than indicated in subframe 1 and that he shall use that SV at his own risk. The authorized user shall refer to ICD-GPS-203 and/or ICD-GPS-224 and/or ICD-GPS-225 (see note in paragraph 2.1). Bit 19 is an anti-spoof (A-S) flag. A "1" in bit-position 19 indicates that the A-S mode is ON in that SV. Bits 20, 21, and 22 of the HOW provide the ID of the subframe in which that particular HOW is the second word; the ID code shall be as follows: Subframe 1 2 3 4 5 ID Code 001 010 011 100 101 IRN-200C-004 ICD-GPS-200C 12 April 2000 81 UNCLASSIFIED UNCLASSIFIED 20.3.3.3 Subframe 1. The content of words three through ten of subframe 1 are defined below, followed by related algorithms and material pertinent to use of the data. 20.3.3.3.1 Subframe 1 Content. The third through tenth words of subframe 1 shall each contain six parity bits as their LSBs; in addition, two non-information bearing bits shall be provided as bits 23 and 24 of word ten for parity computation purposes. The remaining 190 bits of words three through ten shall contain the clock parameters and other data described in the following. The clock parameters describe the SV time scale during the period of validity. The parameters in a data set shall be valid during the interval of time in which they are transmitted and shall remain valid for an additional period of time after transmission of the next data set has started. The timing information for subframes, pages, and data sets is covered in Section 20.3.4. 20.3.3.3.1.1 Transmission Week Number. The ten MSBs of word three shall contain the ten MSBs of the 29-bit Zcount as qualified herein. These ten bits shall be a Modulo 1024 binary representation of the current GPS week number at the start of the data set transmission interval (see paragraph 3.3.4(b)). The GPS week number increments at each end/start of week epoch. For Block II SVs in long-term extended operations, beginning approximately 28 days after upload, the transmission week number may not correspond to the actual GPS week number due to curve fit intervals that cross week boundaries. IRN-200C-004 ICD-GPS-200C 12 April 2000 82 UNCLASSIFIED UNCLASSIFIED 20.3.3.3.1.2 Code(s) on L2 Channel. Bits 11 and 12 of word three shall indicate which code(s) is (are) commanded ON for the L2 channel, as follows: 00 = Reserved, 01 = P code ON, 10 = C/A code ON. 20.3.3.3.1.3 SV Accuracy. Bits 13 through 16 of word three shall give the URA index of the SV (reference paragraph 6.2.1) for the unauthorized user. The URA index (N) is an integer in the range of 0 through 15 and has the following relationship to the URA of the SV: URA INDEX 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 0.00 2.40 3.40 4.85 6.85 9.65 13.65 24.00 48.00 96.00 192.00 384.00 768.00 1536.00 3072.00 6144.00 URA (meters) < URA ≤ 2.40 < URA ≤ 3.40 < URA ≤ 4.85 < URA ≤ 6.85 < URA ≤ 9.65 < URA ≤ 13.65 < URA ≤ 24.00 < URA ≤ 48.00 < URA ≤ 96.00 < URA ≤ 192.00 < URA ≤ 384.00 < URA ≤ 768.00 < URA ≤ 1536.00 < URA ≤ 3072.00 < URA ≤ 6144.00 < URA (or no accuracy prediction is available - unauthorized users are advised to use the SV at their own risk.) ICD-GPS-200C 10 OCT 1993 83 UNCLASSIFIED UNCLASSIFIED For each URA index (N), users may compute a nominal URA value (X) as given by: • If the value of N is 6 or less, X = 2(1 + N/2), • If the value of N is 6 or more, but less than 15, X = 2(N - 2), • N = 15 shall indicate the absence of an accuracy prediction and shall advise the unauthorized user to use that SV at his own risk. For N = 1, 3, and 5, X should be rounded to 2.8, 5.7, and 11.3 meters, respectively. 20.3.3.3.1.4 SV Health. The six-bit health indication given by bits 17 through 22 of word three refers to the transmitting SV. The MSB shall indicate a summary of the health of the NAV data, where 0 = all NAV data are OK, 1 = some or all NAV data are bad. The five LSBs shall indicate the health of the signal components in accordance with the codes given in paragraph 20.3.3.5.1.3. The health indication shall be given relative to the "as designed" capabilities of each SV (as designated by the configuration code - see paragraph 20.3.3.5.1.6). Accordingly, any SV which does not have a certain capability will be indicated as "healthy" if the lack of this capability is inherent in its design or if it has been configured into a mode which is normal from a user standpoint and does not require that capability. Additional SV health data are given in subframes 4 and 5. The data given in subframe 1 may differ from that shown in subframes 4 and/or 5 of other SVs since the latter may be updated at a different time. ICD-GPS-200C 10 OCT 1993 84 UNCLASSIFIED UNCLASSIFIED 20.3.3.3.1.5 Issue of Data, Clock (IODC). Bits 23 and 24 of word three in subframe 1 shall be the two MSBs of the ten-bit IODC term; bits one through eight of word eight in subframe 1 shall contain the eight LSBs of the IODC. The IODC indicates the issue number of the data set and thereby provides the user with a convenient means of detecting any change in the correction parameters. Constraints on the IODC as well as the relationship between the IODC and the IODE (issue of data, ephemeris) terms are defined in paragraph 20.3.4.4. Short-term and Long-term Extended Operations. Whenever the fit interval flag indicates a fit interval greater than 4 hours, the IODC can be used to determine the actual fit interval of the data set (reference section 20.3.4.4). 20.3.3.3.1.6 Data Flag for L2 P-Code. When bit 1 of word four is a "1", it shall indicate that the NAV data stream was commanded OFF on the P-code of the L2 channel. 20.3.3.3.1.7 (Reserved) 20.3.3.3.1.8 Estimated Group Delay Differential. Bits 17 through 24 of word seven contain the L1-L2 correction term, TGD, for the benefit of "L1 only" or "L2 only" users; the related user algorithm is given in paragraph 20.3.3.3.3. IRN-200C-004 ICD-GPS-200C 12 April 2000 85 UNCLASSIFIED UNCLASSIFIED 20.3.3.3.1.9 SV Clock Correction. Bits nine through 24 of word eight, bits one through 24 of word nine, and bits one through 22 of word ten contain the parameters needed by the users for apparent SV clock correction (toc, af2, af1, af0). The related algorithm is given in paragraph 20.3.3.3.3. 20.3.3.3.2 Subframe 1 Parameter Characteristics. For those parameters whose characteristics are not fully defined in Section 20.3.3.3.1, the number of bits, the scale factor of the LSB (which shall be the last bit received), the range, and the units shall be as specified in Table 20-I. 20.3.3.3.3 User Algorithms for Subframe 1 Data. The algorithms defined below (a) allow all users to correct the code phase time received from the SV with respect to both SV code phase offset and relativistic effects, (b) permit the "single frequency" (L1 or L2) user to compensate for the effects of SV group delay differential (the user who utilizes both frequencies does not require this correction, since the clock parameters account for the induced effects), and (c) allow the "two frequency" (L1 and L2) user to correct for the group propagation delay due to ionospheric effects (the single frequency user may correct for ionospheric effects as described in paragraph 20.3.3.5.2.5). ICD-GPS-200C 10 OCT 1993 86 UNCLASSIFIED UNCLASSIFIED Table 20-I. Subframe 1 Parameters No. of Bits** Scale Factor (LSB) Code on L2 2 1 discretes Week No. 10 1 week L2 P data flag 1 1 discrete SV accuracy 4 SV health 6 1 discretes TGD 8* 2-31 seconds IODC 10 toc 16 24 af2 8* 2-55 sec/sec2 af1 16* 2-43 sec/sec af0 22* 2-31 seconds Parameter * ** *** Effective Range*** Units (see text) (see text) 604,784 seconds Parameters so indicated shall be two's complement, with the sign bit (+ or -) occupying the MSB; See Figure 20-1 for complete bit allocation in subframe; Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor. ICD-GPS-200C 10 OCT 1993 87 UNCLASSIFIED UNCLASSIFIED 20.3.3.3.3.1 User Algorithm for SV Clock Correction. The polynomial defined in the following allows the user to determine the effective SV PRN code phase offset referenced to the phase center of the antennas (∆tsv) with respect to GPS system time (t) at the time of data transmission. The coefficients transmitted in subframe 1 describe the offset apparent to the two-frequency user for the interval of time in which the parameters are transmitted. This estimated correction accounts for the deterministic SV clock error characteristics of bias, drift and aging, as well as for the SV implementation characteristics of group delay bias and mean differential group delay. Since these coefficients do not include corrections for relativistic effects, the user's equipment must determine the requisite relativistic correction. Accordingly, the offset given below includes a term to perform this function. The user shall correct the time received from the SV with the equation (in seconds) t = tsv - ∆tsv t = GPS system time (seconds), tsv = effective SV PRN code phase time at message transmission time (seconds), ∆tsv = SV PRN code phase time offset (seconds). (1) where The SV PRN code phase offset is given by ∆tsv = af0 + af1(t - toc) + af2(t - toc)2 + ∆tr (2) where af0, af1 and af2 are the polynomial coefficients given in subframe 1, toc is the clock data reference time in seconds (reference paragraph 20.3.4.5), and ∆tr is the relativistic correction term (seconds) which is given by ∆tr = Fe(A)1/2 sin Ek. The orbit parameters (e, A, Ek) used here are described in discussions of data contained in subframes 2 and 3, while F is a constant whose value is F = − 2µ c2 1 2 = - 4.442807633 (10)-10 sec/(meter)1/2, ICD-GPS-200C 10 OCT 1993 88 UNCLASSIFIED UNCLASSIFIED where µ = 3.986005 x 1014 meters 3 second 2 = value of Earth's universal gravitational parameters meters second = speed of light. c = 2.99792458 x 108 Note that equations (1) and (2), as written, are coupled. While the coefficients af0, af1 and af2 are generated by using GPS time as indicated in equation (2), sensitivity of tsv to t is negligible. This negligible sensitivity will allow the user to approximate t by tSV in equation (2). The value of t must account for beginning or end of week crossovers. That is, if the quantity t - toc is greater than 302,400 seconds, subtract 604,800 seconds from t. If the quantity t - toc is less than -302,400 seconds, add 604,800 seconds to t. The control segment will utilize the following alternative but equivalent expression for the relativistic effect when estimating the NAV parameters: → ∆tr = − → 2 R •V c2 where → R is the instantaneous position vector of the SV, → V is the instantaneous velocity vector of the SV, and c is the speed of light. (Reference paragraph 20.3.4.3). → → It is immaterial whether the vectors R and V are expressed in earth-fixed, rotating coordinates or in earth-centered, inertial coordinates. IRN-200C-003 ICD-GPS-200C 11 OCT 1999 89 UNCLASSIFIED UNCLASSIFIED 20.3.3.3.3.2 L1 - L2 Correction. The L1 and L2 correction term, TGD, is initially calculated by the CS to account for the effect of SV group delay differential between L1 and L2 based on measurements made by the SV contractor during SV manufacture. The value of TGD for each SV may be subsequently updated to reflect the actual on-orbit group delay differential. This correction term is only for the benefit of "single-frequency" (L1 or L2) users; it is necessitated by the fact that the SV clock offset estimates reflected in the af0 clock correction coefficient (see paragraph 20.3.3.3.3.1) are based on the effective PRN code phase as apparent with two frequency ionospheric corrections. Thus, the user who utilizes the L1 frequency only shall modify the code phase offset in accordance with paragraph 20.3.3.3.3.1 with the equation (∆tSV)L1 = ∆tSV - TGD where TGD is provided to the user as subframe 1 data. For the user who utilizes L2 only, the code phase modification is given by (∆tSV)L2 = ∆tSV - γTGD where, denoting the nominal center frequencies of L1 and L2 as fL1 and fL2 respectively, γ = (fL1/fL2)2 = (1575.42/1227.6)2 = (77/60)2. The value of TGD is not equal to the mean SV group delay differential, but is equal to the delay differential multiplied by 1/(1- γ). That is, 1 TGD = 1- γ (tL1 - tL2) where tLi is the GPS time the ith frequency signal is transmitted from the SV. IRN-200C-004 ICD-GPS-200C 12 April 2000 90 UNCLASSIFIED UNCLASSIFIED 20.3.3.3.3.3 Ionospheric Correction. The two frequency (L1 and L2) user shall correct for the group delay due to ionospheric effects by applying the relationship: PR = PR 2 - γ PR1 1- γ where PR = pseudorange corrected for ionospheric effects, PRi = pseudorange measured on the L-band channel indicated by the subscript. and γ is as defined in paragraph 20.3.3.3.3.2. The clock correction coefficients are based on "two frequency" measurements and therefore account for the effects of mean differential delay in SV instrumentation. 20.3.3.3.3.4 Example Application of Correction Parameters. A typical system application of the correction parameters for a user receiver is shown in Figure 20-3. The ionospheric model referred to in Figure 20-3 is discussed in paragraph 20.3.3.5.2.5 in conjunction with the related data contained in page 18 of subframe 4. The ERD c term referred to in Figure 20-3 is discussed in paragraph 20.3.3.5.2.6 in conjunction with the related data contained in page 13 of subframe 4. 20.3.3.4 Subframes 2 and 3. The contents of words three through ten of subframes 2 and 3 are defined below, followed by material pertinent to the use of the data. IRN-200C-003 ICD-GPS-200C 11 OCT 1999 91 UNCLASSIFIED UNCLASSIFIED af0, af1, af2, toc TGD* ESTIMATE OF SV TRANSMISSION TIME CLOCK CORRECTION POLYNOMIAL ∆ tr ∆ tSV Ttropo Tiono CODE PHASE OFFSET - TRUE SV CLOCK EFFECTS - EQUIPMENT GROUP DELAY DIFFERENTIAL EFFECTS - RELATIVISTIC EFFECTS TROPOSPHERIC MODEL IONOSPHERIC MODEL* αn, βn FILTER AND COORDINATE CONVERTER PSEUDORANGE DIVIDED BY THE SPEED OF LIGHT GPS TIME PATH DELAY - GEOMETRIC - TROPOSHERIC - IONOSPHERIC* USER POSITION, VELOCITY, and TIME (CLOCK BIAS) ERD ** c - RANGE DATA FROM OTHER SATELLITES - CALIBRATION DATA - AUXILIARY SENSOR USER CLOCK BIAS GPS TIME * SINGLE FREQUENCY USER ONLY ** OPTIONAL Figure 20-3. Sample Application of Correction Parameters IRN-200C-003 ICD-GPS-200C 11 OCT 1999 92 UNCLASSIFIED UNCLASSIFIED 20.3.3.4.1 Content of Subframes 2 and 3. The third through tenth words of subframes 2 and 3 shall each contain six parity bits as their LSBs; in addition, two non-information bearing bits shall be provided as bits 23 and 24 of word ten of each subframe for parity computation purposes. Bits 288 through 292 of subframe 2 shall contain the Age of Data Offset (AODO) term for the navigation message correction table (NMCT) contained in subframe 4 (reference paragraph 20.3.3.5.1.12). The remaining 375 bits of those two subframes shall contain the ephemeris representation parameters of the transmitting SV. The ephemeris parameters describe the orbit during the curve fit intervals described in section 20.3.4. Table 20-II gives the definition of the orbital parameters using terminology typical of Keplerian orbital parameters; it shall be noted, however, that the transmitted parameter values are such that they provide the best trajectory fit in EarthCentered, Earth-Fixed (ECEF) coordinates for each specific fit interval. The user shall not interpret intermediate coordinate values as pertaining to any conventional coordinate system. IRN-200C-003 ICD-GPS-200C 11 OCT 1999 93 UNCLASSIFIED UNCLASSIFIED Table 20-II. Ephemeris Data Definitions M0 Mean Anomaly at Reference Time ∆n Mean Motion Difference From Computed Value e Eccentricity 1/2 (A) (OMEGA)0 Square Root of the Semi-Major Axis Longitude of Ascending Node of Orbit Plane at Weekly Epoch i0 Inclination Angle at Reference Time ω Argument of Perigee OMEGADOT Rate of Right Ascension IDOT Rate of Inclination Angle Cuc Amplitude of the Cosine Harmonic Correction Term to the Argument of Latitude Cus Amplitude of the Sine Harmonic Correction Term to the Argument of Latitude Crc Amplitude of the Cosine Harmonic Correction Term to the Orbit Radius Crs Amplitude of the Sine Harmonic Correction Term to the Orbit Radius Cic Amplitude of the Cosine Harmonic Correction Term to the Angle of Inclination Cis Amplitude of the Sine Harmonic Correction Term to the Angle of Inclination toe Reference Time Ephemeris (reference paragraph 20.3.4.5) IODE Issue of Data (Ephemeris) ICD-GPS-200C 10 OCT 1993 94 UNCLASSIFIED UNCLASSIFIED The issue of ephemeris data (IODE) term shall provide the user with a convenient means for detecting any change in the ephemeris representation parameters. The IODE is provided in both subframes 2 and 3 for the purpose of comparison with the 8 LSBs of the IODC term in subframe 1. Whenever these three terms do not match, a data set cutover has occurred and new data must be collected. The timing of the IODE and constraints on the IODC and IODE are defined in paragraph 20.3.4.4. Any change in the subframe 2 and 3 data will be accomplished with a simultaneous change in both IODE words. The CS shall assure that the toe value, for at least the first data set transmitted by an SV after an upload, is different from that transmitted prior to the cutover. A "fit interval" flag is provided in subframe 2 to indicate whether the ephemerides are based on a four-hour fit interval or a fit interval greater than four hours (reference paragraph 20.3.3.4.3.1). The AODO word is provided in subframe 2 to enable the user to determine the validity time for the NMCT data provided in subframe 4 of the transmitting SV. The related algorithm is given in paragraph 20.3.3.4.4. 20.3.3.4.2 Subframe 2 and 3 Parameter Characteristics. For each ephemeris parameter contained in subframes 2 and 3, the number of bits, the scale factor of the LSB (which shall be the last bit received), the range, and the units shall be as specified in Table 20-III. The AODO word (which is not an ephemeris parameter) is a five-bit unsigned term with an LSB scale factor of 900, a range from 0 to 31, and units of seconds. IRN-200C-002 ICD-GPS-200C 25 SEP 1997 95 UNCLASSIFIED UNCLASSIFIED Table 20-III. Ephemeris Parameters Parameter IODE No. of Bits** Scale Factor (LSB) 8 Crs 16* ∆n 16* M0 32* Cuc 16* e 32 Cus 16* 1/2 (see text) meters -43 semi-circles/sec -31 semi-circles -29 radians 2 2 2 2 -33 2 radians meters1/2 toe 16 24 (OMEGA)0 Cis 16* i0 32* Crc 16* ω 32* OMEGADOT * ** *** 32* 24* 14* dimensionless -19 2 2 16* 0.03 -29 32 Cic Units -5 (A) IDOT Effective Range*** 604,784 seconds -29 radians -31 semi-circles -29 radians -31 semi-circles -5 meters -31 semi-circles -43 semi-circles/sec -43 semi-circles/sec 2 2 2 2 2 2 2 2 Parameters so indicated shall be two's complement, with the sign bit (+ or -) occupying the MSB; See Figure 20-1 for complete bit allocation in subframe; Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor. ICD-GPS-200C 10 OCT 1993 96 UNCLASSIFIED UNCLASSIFIED 20.3.3.4.3 User Algorithm for Ephemeris Determination. The user shall compute the ECEF coordinates of position for the phase center of the SVs’ antennas utilizing a variation of the equations shown in Table 20-IV. Subframes 2 and 3 parameters are Keplerian in appearance; the values of these parameters, however, are produced by the CS via a least squares curve fit of the predicted ephemeris of the phase center of the SVs’ antennas (time-position quadruples; t, x, y, z expressed in ECEF coordinates). Particulars concerning the periods of the curve fit, the resultant accuracy, and the applicable coordinate system are given in the following subparagraphs. 20.3.3.4.3.1 Curve Fit Intervals. Bit 17 in word 10 of subframe 2 is a "fit interval" flag which indicates the curvefit interval used by the CS in determining the ephemeris parameters, as follows: 0 = 4 hours, 1 = greater than 4 hours. The relationship of the curve-fit interval to transmission time and the timing of the curve-fit intervals is covered in section 20.3.4. IRN-200C-003 ICD-GPS-200C 11 OCT 1999 97 UNCLASSIFIED UNCLASSIFIED Table 20-IV. Elements of Coordinate Systems (sheet 1 of 3) µ = 3.986005 x 1014 meters3/sec2 • -5 Ω e = 7.2921151467 x 10 rad/sec A= n0 = ( A ) 2 µ A3 WGS 84 value of the earth's universal gravitational parameter for GPS user WGS 84 value of the earth's rotation rate Semi-major axis Computed mean motion (rad/sec) tk = t - toe* Time from ephemeris reference epoch n = n0 + ∆n Corrected mean motion Mk = M0 + ntk Mean anomaly * t is GPS system time at time of transmission, i.e., GPS time corrected for transit time (range/speed of light). Furthermore, tk shall be the actual total time difference between the time t and the epoch time toe, and must account for beginning or end of week crossovers. That is, if tk is greater than 302,400 seconds, subtract 604,800 seconds from t k. If t k is less than -302,400 seconds, add 604,800 seconds to tk. IRN-200C-004 ICD-GPS-200C 12 April 2000 98 UNCLASSIFIED UNCLASSIFIED Table 20-IV. Elements of Coordinate Systems (sheet 2 of 3) Mk = Ek - e sin Ek Kepler's Equation for Eccentric Anomaly (may be solved by iteration)(radians) sin ν k ν k = tan −1 cos ν k True Anomaly 1 − e 2 sin E / (1 − e cos E ) k k = tan −1 − − ( ) ( cos E e / 1 e cos E k k) e + cos ν k E k = cos −1 1 + e cos ν k Eccentric Anomaly Φ k = νk + ω Argument of Latitude δuk = cussin2Φ k + cuccos2Φ k δrk = crssin2Φk + crccos2Φ k δik = cissin2Φk + ciccos2Φ k } Argument of Latitude Correction Radius Correction Inclination Correction Second Harmonic Perturbations uk = Φ k + δuk Corrected Argument of Latitude rk = A(1 - e cosEk) + δrk Corrected Radius ik = i0 + δik + (IDOT) tk Corrected Inclination ICD-GPS-200C 10 OCT 1993 99 UNCLASSIFIED UNCLASSIFIED Table 20-IV. Elements of Coordinate Systems (sheet 3 of 3) xk′ = rkcosuk yk′ = rksinuk } • Positions in orbital plane. • • Ωk = Ω0 + ( Ω - Ω e ) tk - Ω e toe Corrected longitude of ascending node. xk = xk′cosΩk - yk′cosiksinΩk yk = xk′sinΩk + yk′cosikcosΩk Earth-fixed coordinates. zk = yk′sinik } ICD-GPS-200C 10 OCT 1993 100 UNCLASSIFIED UNCLASSIFIED 20.3.3.4.3.2 Parameter Sensitivity. The sensitivity of the SV's antenna phase center position to small perturbations in most ephemeris parameters is extreme. The sensitivity of position to the parameters (A)1/2, Crc and Crs is about one meter/meter. The sensitivity of position to the angular parameters is on the order of 108 meters/semicircle, and to the angular rate parameters is on the order of 1012 meters/semicircle/second. Because of this extreme sensitivity to angular perturbations, the value of π used in the curve fit is given here. π is a mathematical constant, the ratio of a circle's circumference to its diameter. Here π is taken as π = 3.1415926535898. 20.3.3.4.3.3 Coordinate Systems. 20.3.3.4.3.3.1 ECEF Coordinate System. The equations given in Table 20-IV provide the SV's antenna phase center position in the WGS 84 ECEF coordinate system defined as follows: Origin* = Earth's center of mass Z-Axis** = The direction of the IERS (International Earth Rotation Service) Reference Pole (IRP) X-Axis Intersection of the IERS Reference Meridian (IRM) and the plane passing through the = origin and normal to the Z-axis Y-Axis = Completes a right-handed, Earth-Centered, Earth-Fixed orthogonal coordinate system * Geometric center of the WGS 84 Ellipsoid ** Rotational axis of the WGS 84 Ellipsoid IRN-200C-004 ICD-GPS-200C 12 April 2000 101 UNCLASSIFIED UNCLASSIFIED 20.3.3.4.3.3.2 Earth-Centered, Inertial (ECI) Coordinate System. In an ECI coordinate system, GPS signals propagate in straight lines at the constant speed c* (reference paragraph 20.3.4.3). A stable ECI coordinate system of convenience may be defined as being coincident with the ECEF coordinate system at a given time t0. The x, y, z coordinates in the ECEF coordinate system at some other time t can be transformed to the x′, y′, z′ coordinates in the selected ECI coordinate system of convenience by the simple** rotation: x′ = x cos(θ) – y sin(θ) y′ = x sin(θ) + y cos(θ) z′ = z where • θ = Ω e (t – t0) * The propagation speed c is constant only in a vacuum. The gravitational potential also has a small effect on the propagation speed, but may be neglected by most users. ** Neglecting effects due to polar motion, nutation, and precession which may be neglected by most users for small values of (t – t0). 20.3.3.4.3.4 Geometric Range. The user shall account for the geometric range (D) from satellite to receiver in an ECI coordinate system. D may be expressed as, → → D = | r (tR) - R (tT)| where tT and tR are the GPS system times of transmission and reception, respectively, and where, → R (tT) = position vector of the GPS satellite in the selected ECI coordinate system at time tT, → r (tR) = position vector of the receiver in the selected ECI coordinate system at time tR. IRN-200C-003 ICD-GPS-200C 11 OCT 1999 102 UNCLASSIFIED UNCLASSIFIED 20.3.3.4.4 NMCT Validity Time. Users desiring to take advantage of the NMCT data provided in page 13 of subframe 4 shall first examine the AODO term currently provided in subframe 2 of the NAV data from the transmitting SV. If the AODO term is 27900 seconds (i.e., binary 11111), then the NMCT currently available from the transmitting SV is invalid and shall not be used. If the AODO term is less than 27900 seconds, then the user shall compute the validity time for that NMCT (tnmct) using the ephemeris toe parameter and the AODO term from the current subframe 2 as follows: OFFSET = toe [Modulo 7200] if OFFSET = 0, then tnmct = toe - AODO if OFFSET > 0, then tnmct = toe - OFFSET + 7200 - AODO Note that the foregoing computation of tnmct must account for any beginning or end of week crossovers; for example, if t* - tnmct > 302,400 then tnmct = tnmct + 604,800 if t* - tnmct < -302,400 then tnmct = tnmct - 604,800 * t is GPS system time at time of transmission. Users are advised that different SVs will transmit NMCTs with different tnmct and that the best performance will generally be obtained by applying data from the NMCT with the latest (largest) tnmct. As a result, users should compute and examine the tnmct values for all visible and available SVs in order to find and use the NMCT with the latest tnmct. If the same latest (largest) tnmct is provided by two or more visible and available SVs, then the NMCT from any SV with the latest tnmct may be selected and used; however, the estimated range deviation (ERD) value provided by the selected NMCT for the other SVs with the same tnmct shall be set to zero if those SVs are used in the positioning solution. It should be noted that the intended positioning solution accuracy improvement will not be obtained if the data from two different NMCTs are applied simultaneously or if the data from a given NMCT is applied to just a subset of the SVs used in the positioning solution (i.e., mixed mode operation results in potentially degraded solution accuracy). It should be noted that the NMCT information shall be supported by the Block IIR SV only when operating in the IIA like mode of operation including the Autonav Test mode. IRN-200C-003 ICD-GPS-200C 11 OCT 1999 102a UNCLASSIFIED UNCLASSIFIED 20.3.3.5 Subframes 4 and 5. Both subframe 4 and 5 are subcommutated 25 times each; the 25 versions of these subframes are referred to as pages 1 through 25 of each subframe. With the possible exception of "reserved for system use" pages and explicit repeats, each page contains different specific data in words three through ten. As shown in Figure 20-1, the pages of subframe 4 utilize seven different formats, while those of subframe 5 use two. The content of words three through ten of each page is described below, followed by algorithms and material pertinent to the use of the data. 20.3.3.5.1 Content of Subframes 4 and 5. Words three through ten of each page contain six parity bits as their LSBs; in addition, two non-information bearing bits are provided as bits 23 and 24 of word ten in each page for parity computation purposes. The data contained in the remaining bits of words three through ten of the various pages in subframes 4 and 5 are described in the following subparagraphs. IRN-200C-003 ICD-GPS-200C 11 OCT 1999 102b UNCLASSIFIED UNCLASSIFIED A brief summary of the various data contained in each page of subframes 4 and 5 is as follows: a. Subframe 4: • Pages 1, 6, 11, 16 and 21: (reserved); • Pages 2, 3, 4, 5, 7, 8, 9 and 10: almanac data for SV 25 through 32 respectively. These pages may be designated for other functions; the format and content for each page is defined by the SV ID of that page. In this case, the six-bit health word of page 25 is set to "6 ones" (Refer to 20.3.3.5.1.3) and the SV ID of the page will not have a value in the range of 25 through 32; • Pages 12, 19, 20, 22, 23 and 24: • Page 13: • Pages 14 and 15: • Page 17: special messages; • Page 18: ionospheric and UTC data; • Page 25: A-S flags/SV configurations for 32 SVs, plus SV health for SV 25 (reserved); NMCT; reserved for system use; through 32. b. Subframe 5: • Pages 1 through 24: • Page 25: almanac data for SV 1 through 24; SV health data for SV 1 through 24, the almanac reference time and the almanac reference week number. IRN-200C-002 ICD-GPS-200C 25 SEP 1997 103 UNCLASSIFIED UNCLASSIFIED 20.3.3.5.1.1 Data ID and SV ID. The two MSBs of word three in each page shall contain the data ID which defines the applicable GPS NAV data structure. Data ID one (denoted by binary code 00) was utilized during Phase I of the GPS program and is no longer in use; data ID two (denoted by binary code 01) is described in this Appendix. Future data IDs will be defined as necessary. As shown in Table 20-V, the data ID is utilized to provide one of two indications: (a) for those pages which are assigned to contain the almanac data of one specific SV, the data ID defines the data structure utilized by that SV whose almanac data are contained in that page; and (b) for all other pages, the data ID denotes the data structure of the transmitting SV. The SV ID is given by bits three through eight of word three in each page as shown in Table 20-V. Specific IDs are reserved for each page of subframes 4 and 5; however, the SV ID of pages 2, 3, 4, 5, 7, 8, 9 and 10 of subframe 4 may change for each page to reflect the alternate contents for that page. The SV IDs are utilized in two different ways: (a) for those pages which contain the almanac data of a given SV, the SV ID is the same number that is assigned to the PRN code phase of that SV (reference Table 3-I), and (b) for all other pages the SV ID assigned in accordance with Table 20-V serves as the "page ID". IDs 1 through 32 are assigned to those pages which contain the almanac data of specific SVs (pages 1-24 of subframe 5 and pages 2-5 and 7-10 of subframe 4). The "0" ID (binary all zeros) is assigned to indicate a dummy SV, while IDs 51 through 63 are utilized for pages containing other than almanac data of a specific SV. The remaining IDs (33 through 50) are unassigned. Pages which contain identical data (for more frequent repetition) carry the same SV ID (e.g., in subframe 4, pages 1, 6, 11, 16 and 21 carry an ID of 57, while pages 12 and 24 are designated by an ID of 62). ICD-GPS-200C 10 OCT 1993 104 UNCLASSIFIED UNCLASSIFIED Table 20-V. Data IDs and SV IDs in Subframes 4 and 5 Subframe 4 Subframe 5 Page 1 Note(3) Note(3) Note(3) Note(3) 6 7 Note(3) 8 Note(3) 9 Note(3) 10 Note(3) 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 2 3 4 5 Data ID SV ID* Data ID SV ID* Note(2) Note(1) Note(1) Note(1) Note(1) Note(2) Note(1) Note(1) Note(1) Note(1) Note(2) Note(2) Note(2) Note(2) Note(2) Note(2) Note(2) Note(2) Note(2) Note(2) Note(2) Note(2) Note(2) Note(2) Note(2) 57 25 26 27 28 57 29 30 31 32 57 62 52 53 54 57 55 56 58 Note(4) 59 Note(4) 57 60 Note(4) 61 Note(4) 62 63 Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(1) Note(2) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 51 * Use "0" to indicate "dummy" SV. When using "0" to indicate dummy SV, use the data ID of the transmitting SV. Note 1: Note 2: Note 3: Note 4: Data ID of that SV whose SV ID appears in that page. Data ID of transmitting SV. Pages 2, 3, 4, 5, 7, 8, 9, and 10 of subframe 4 may contain almanac data for SVs 25 through 32, respectively, or data for other functions as identified by a different SV ID from the value shown. SV ID may vary. ICD-GPS-200C 10 OCT 1993 105 UNCLASSIFIED UNCLASSIFIED 20.3.3.5.1.2 Almanac Data. Pages 1 through 24 of subframe 5, as well as pages 2 through 5 and 7 through 10 of subframe 4 contain the almanac data and a SV health word for up to 32 SVs (the health word is discussed in paragraph 20.3.3.5.1.3). The almanac data are a reduced-precision subset of the clock and ephemeris parameters. The data occupy all bits of words three through ten of each page except the eight MSBs of word three (data ID and SV ID), bits 17 through 24 of word five (SV health), and the 50 bits devoted to parity. The number of bits, the scale factor (LSB), the range, and the units of the almanac parameters are given in Table 20-VI. The algorithms and other material related to the use of the almanac data are given in paragraph 20.3.3.5.2. The almanac message for any dummy SVs shall contain alternating ones and zeros with valid parity. For twelve or fewer SVs, almanacs may be repeated within the 25-cycle subcommutation limit. Whenever this option is exercised, the following constraints shall apply: (a) each page of subframes 4 and 5, which is assigned by Table 20-V to one of the active SVs in orbit, must contain the almanac data of that SV to which it is assigned by Table 20-V, (b) those almanac-type pages which remain unused per the above rule, shall then be re-assigned to carry a duplicate set of almanac data for the active orbiting SV, (c) these page re-assignments shall be in ascending order of page numbers (starting with subframe 5, followed by subframe 4) being used for SVs having an ascending order of SV IDs, and (d) each re-assigned page must carry the SV ID of that SV whose almanac data it contains. The almanac parameters shall be updated by the CS at least once every 6 days while the CS is able to upload the SVs. If the CS is unable to upload the SVs, the accuracy of the almanac parameters transmitted by the SVs will degrade over time. ICD-GPS-200C 10 OCT 1993 106 UNCLASSIFIED UNCLASSIFIED For Block II and IIA SVs, three sets of almanac shall be used to span 182 days. The first and second sets will be transmitted for up to six days each; the third set is intended to be transmitted for the duration of the 182 days from the last upload, but the duration of transmission will depend on the individual SV's capability to retain data in memory. All three sets are based on six-day curve fits that correspond to the first six days of the transmission interval. For Block IIR SVs, multiple sets of almanac parameters shall be uploaded to span 210 days. IRN-200C-004 ICD-GPS-200C 12 April 2000 107 UNCLASSIFIED UNCLASSIFIED Table 20-VI. Almanac Parameters Parameter No. of Bits** Scale Factor (LSB) e 16 2-21 toa 8 212 δi**** 16* 2-19 semi-circles OMEGADOT 16* 2-38 semi-circles/sec (A)1/2 24 2-11 meters1/2 (OMEGA)0 24* 2-23 semi-circles ω 24* 2-23 semi-circles M0 24* 2-23 semi-circles af0 11* 2-20 seconds af1 11* 2-38 sec/sec Effective Range*** Units dimensionless 602,112 seconds * Parameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB; ** See Figure 20-1 for complete bit allocation in subframe; *** Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor; **** Relative to i0 = 0.30 semi-circles. ICD-GPS-200C 10 OCT 1993 108 UNCLASSIFIED UNCLASSIFIED 20.3.3.5.1.3 SV Health. Subframes 4 and 5 contain two types of SV health data: (a) each of the 32 pages which contain the clock/ephemeris related almanac data provide an eight-bit SV health status word regarding the SV whose almanac data they carry, and (b) the 25th page of subframe 4 and of subframe 5 jointly contain six-bit health status data for up to 32 SVs. The three MSBs of the eight-bit health words indicate health of the NAV data in accordance with the code given in Table 20-VII. The six-bit words provide a one-bit summary of the NAV data's health status in the MSB position in accordance with paragraph 20.3.3.3.1.4. The five LSBs of both the eight-bit and the six-bit words provide the health status of the SV's signal components in accordance with the code given in Table 20-VIII. A special meaning is assigned, however, to the "6 ones" combination of the six-bit health words in the 25th page of subframes 4 and 5: it indicates that "the SV which has that ID is not available and there may be no data regarding that SV in that page of subframes 4 and 5 that is assigned to normally contain the almanac data of that SV" (NOTE: (a) this special meaning applies to the 25th page of subframes 4 and 5 only; and (b) there may be data regarding another SV in the almanac-page referred to above as defined in paragraph 20.3.3.5.1.1). The health indication shall be given relative to the "as designed" capabilities of each SV (as designated by the configuration code -- see paragraph 20.3.3.5.1.6). Accordingly, any SV which does not have a certain capability will be indicated as "healthy" if the lack of this capability is inherent in its design or it has been configured into a mode which is normal from a user standpoint and does not require that capability. ICD-GPS-200C 10 OCT 1993 109 UNCLASSIFIED UNCLASSIFIED Table 20-VII. NAV Data Health Indications Bit Position in Page Indication 137 138 139 0 0 0 ALL DATA OK 0 0 1 PARITY FAILURE -- some or all parity bad 0 1 0 TLM/HOW FORMAT PROBLEM -- any departure from standard format (e.g., preamble misplaced and/or incorrect, etc.), except for incorrect Z-count, as reported in HOW 0 1 1 Z-COUNT IN HOW BAD -- any problem with Z-count value not reflecting actual code phase 1 0 0 SUBFRAMES 1, 2, 3 -- one or more elements in words three through ten of one or more subframes are bad 1 0 1 SUBFRAMES 4, 5 -- one or more elements in words three through ten of one or more subframes are bad 1 1 0 ALL UPLOADED DATA BAD -- one or more elements in words three through ten of any one (or more) subframes are bad 1 1 1 ALL DATA BAD -- TLM word and/or HOW and one or more elements in any one (or more) subframes are bad ICD-GPS-200C 10 OCT 1993 110 UNCLASSIFIED UNCLASSIFIED Table 20-VIII. Codes for Health of SV Signal Components MSB LSB 00000 00001 00010 00011 00100 00101 00110 00111 01000 01001 01010 01011 01100 01101 01110 01111 10000 10001 10010 10011 10100 10101 10110 10111 11000 11001 11010 11011 11100 11101 11110 11111 Definition All Signals OK All Signals Weak* All Signals Dead All Signals Have No Data Modulation L1 P Signal Weak L1 P Signal Dead L1 P Signal Has No Data Modulation L2 P Signal Weak L2 P Signal Dead L2 P Signal Has No Data Modulation L1 C Signal Weak L1 C Signal Dead L1 C Signal Has No Data Modulation L2 C Signal Weak L2 C Signal Dead L2 C Signal Has No Data Modulation L1 & L2 P Signal Weak L1 & L2 P Signal Dead L1 & L2 P Signal Has No Data Modulation L1 & L2 C Signal Weak L1 & L2 C Signal Dead L1 & L2 C Signal Has No Data Modulation L1 Signal Weak* L1 Signal Dead L1 Signal Has No Data Modulation L2 Signal Weak* L2 Signal Dead L2 Signal Has No Data Modulation SV Is Temporarily Out (Do not use this SV during current pass**) SV Will Be Temporarily Out (Use with caution**) Spare More Than One Combination Would Be Required To Describe Anomalies (Except those marked by **) * 3 to 6 dB below specified power level due to reduced power output, excess phase noise, SV attitude, etc. ICD-GPS-200C 10 OCT 1993 111 UNCLASSIFIED UNCLASSIFIED Additional SV health data are given in subframe 1. The data given in subframes 1, 4, and 5 of the other SVs may differ from that shown in subframes 4 and/or 5 since the latter may be updated at a different time. The eight-bit health status words shall occupy bits 17 through 24 of word five in those 32 pages which contain almanac data for individual SVs. The six-bit health status words shall occupy the 24 MSBs of words four through nine in page 25 of subframe 5 plus bits 19 through 24 of word 8, the 24 MSBs of word 9, and the 18 MSBs of word 10 in page 25 of subframe 4. The predicted health data will be updated at the time of upload when a new almanac has been built by the CS. The transmitted health data may not correspond to the actual health of the transmitting SV or other SVs in the constellation. 20.3.3.5.1.4 (Reserved). 20.3.3.5.1.5 (Reserved). 20.3.3.5.1.6 Anti-Spoof (A-S) Flags and SV Configurations. Page 25 of subframe 4 shall contain a four-bit-long term for each of up to 32 SVs to indicate the A-S status and the configuration code of each SV. The MSB of each four-bit term shall be the A-S flag with a "1" indicating that A-S is ON. The three LSBs shall indicate the configuration of each SV using the following code: Code SV Configuration 001 "Block II/IIA/IIR" SV (A-S capability, plus flags for A-S and "alert" in HOW; memory capacity as described in paragraph 20.3.2). IRN-200C-004 ICD-GPS-200C 12 April 2000 112 UNCLASSIFIED UNCLASSIFIED Additional codes will be assigned in the future, should the need arise. These four-bit terms shall occupy bits 9 through 24 of word three, the 24 MSBs of words four through seven, and the 16 MSBs of word eight, all in page 25 of subframe 4. Since the anti-spoof information is updated by the CS at the time of upload, the anti-spoof data may not correspond to the actual anti-spoof status of the transmitting SV or other SVs in the constellation. 20.3.3.5.1.7 Almanac Reference Week. Bits 17 through 24 of word three in page 25 of subframe 5 shall indicate the number of the week (WNa) to which the almanac reference time (toa) is referenced (see paragraphs 20.3.3.5.1.2 and 20.3.3.5.2.2). The WNa term consists of eight bits which shall be a Modulo 256 binary representation of the GPS week number (see paragraph 6.2.4) to which the toa is referenced. Bits 9 through 16 of word three in page 25 of subframe 5 shall contain the value of toa which is referenced to this WNa. 20.3.3.5.1.8 Universal Coordinated Time (UTC) Parameters. The 24 MSBs of words six through nine plus the eight MSBs of word ten in page 18 of subframe 4 shall contain the parameters related to correlating UTC time with GPS time. The bit length, scale factors, ranges, and units of these parameters are given in Table 20-IX. The related algorithms are described in paragraph 20.3.3.5.2.4. IRN-200C-004 ICD-GPS-200C 12 April 2000 113 UNCLASSIFIED UNCLASSIFIED The UTC parameters shall be updated by the CS at least once every six days while the CS is able to upload the SVs. If the CS is unable to upload the SVs, the accuracy of the UTC parameters transmitted by the SVs will degrade over time. 20.3.3.5.1.9 Ionospheric Data. The ionospheric parameters which allow the "L1 only" or "L2 only" user to utilize the ionospheric model (reference paragraph 20.3.3.5.2.5) for computation of the ionospheric delay are contained in page 18 of subframe 4. They occupy bits 9 through 24 of word three plus the 24 MSBs of words four and five. The bit lengths, scale factors, ranges, and units of these parameters are given in Table 20-X. The ionospheric data shall be updated by the CS at least once every six days while the CS is able to upload the SVs. If the CS is unable to upload the SVs, the ionospheric data transmitted by the SVs may not be accurate. ICD-GPS-200C 10 OCT 1993 114 UNCLASSIFIED UNCLASSIFIED Table 20-IX. UTC Parameters Parameter *** **** Effective Range*** Units 32* 2-30 Seconds A1 24* 2-50 sec/sec ∆ tLS 8* 1 seconds tot 8 212 WNt 8 1 weeks WNLSF 8 1 weeks 8**** 1 8* 1 ∆ tLSF ** Scale Factor (LSB) A0 DN * No. of Bits** 602,112 7 seconds days seconds Parameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB; See Figure 20-1 for complete bit allocation in subframe; Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor; Right justified. ICD-GPS-200C 10 OCT 1993 115 UNCLASSIFIED UNCLASSIFIED Table 20-X. Ionospheric Parameters * ** *** Parameter No. of Bits** Scale Factor (LSB) α0 8* 2-30 Seconds α1 8* 2-27 sec/semi-circle α2 8* 2-24 sec/(semi-circle)2 α3 8* 2-24 sec/(semi-circle)3 β0 8* 211 seconds β1 8* 214 sec/semi-circle β2 8* 216 sec/(semi-circle)2 β3 8* 216 sec/(semi-circle)3 Effective Range*** Units Parameters so indicated shall be two's complement with the sign bit (+ or -) occupying the MSB; See Figure 20-1 for complete bit allocation in subframe; Unless otherwise indicated in this column, effective range is the maximum range attainable with indicated bit allocation and scale factor. ICD-GPS-200C 10 OCT 1993 116 UNCLASSIFIED UNCLASSIFIED 20.3.3.5.1.10 Special Messages. Page 17 of subframe 4 shall be reserved for special messages with the specific contents at the discretion of the Operating Command. It shall accommodate the transmission of 22 eight-bit ASCII characters. The requisite 176 bits shall occupy bits 9 through 24 of word three, the 24 MSBs of words four through nine, plus the 16 MSBs of word ten. The eight MSBs of word three shall contain the data ID and SV ID, while bits 17 through 22 of word ten shall be reserved for system use. The remaining 50 bits of words three through ten are used for parity (six bits/word) and parity computation (two bits in word ten). The eight-bit ASCII characters shall be limited to the following set: Alphanumeric Character ASCII Character Code (Octal) A-Z A-Z 101 - 132 0-9 0-9 060 - 071 + + 053 - - 055 . (Decimal point) . 056 ' (Minute mark) ' 047 ° (Degree sign) ° 370 / / 057 Blank Space 040 : : 072 " (Second mark) " 042 IRN-200C-002 ICD-GPS-200C 25 SEP 1997 117 UNCLASSIFIED UNCLASSIFIED 20.3.3.5.1.11 (Deleted) 20.3.3.5.1.12 NMCT. Page 13 of subframe 4 shall contain the NMCT data appropriate to the transmitting SV. Each NMCT contains a two-bit availability indicator (AI) followed by 30 slots which may contain up to 30 valid six-bit ERD values. The layout of these 31 data items is as shown in Figure 20-1. The two-bit AI in bits 9 and 10 of word three of page 13 of subframe 4 provide the user with the following information. AI Navigation Message Correction Table Availability 00 The correction table is unencrypted and is available to both authorized and unauthorized users. 01 The correction table is encrypted and is available only to authorized users (normal mode). 10 No correction table available for either authorized or unauthorized users. 11 Reserved. Each one of the 30 six-bit ERD slots in bits 11 through 24 of word three, bits 1 through 24 of words four through nine, and bits 1 through 22 of word ten of page 13 of subframe 4 will correspond to an ERD value for a particular SV ID. There are 31 possible SV IDs that these ERD slots may correspond to, ranging from SV ID 1 to SV ID 31. SV ID 32 is not a valid SV ID for any of the slots in an NMCT. The correspondence between the 30 ERD slots and the 31 possible SV IDs depends on the SV ID of the particular transmitting SV in accordance with the following two rules: 1) the CS shall ensure via upload that no SV shall transmit an NMCT containing an ERD value which applies to its own SV ID, and 2) the CS shall ensure via upload that all ERD values shall be transmitted in ascending numerical slot order of the corresponding SV ID. To illustrate: the SV operating as SV ID 1 will transmit (in order) ERD values which correspond to SV ID 2 through SV ID 31 in ERD slots 1 through 30 respectively, while the SV operating as SV ID 31 will transmit ERD values which correspond to SV ID 1 through SV ID 30 in ERD slots 1 through 30 respectively. IRN-200C-002 ICD-GPS-200C 25 SEP 1997 118 UNCLASSIFIED UNCLASSIFIED Each ERD value contained in an NMCT ERD slot shall be represented as a six-bit two’s complement field with the sign bit occupying the MSB and an LSB of 0.3 meters for an effective range of ±9.3 m. A binary value of “100000” shall indicate that no valid ERD for the corresponding SV ID is present in that slot. 20.3.3.5.2 Algorithms Related to Subframe 4 and 5 Data. The following algorithms shall apply when interpreting Almanac, Universal Coordinated Time, Ionospheric Model, and NMCT data in the NAV message. 20.3.3.5.2.1 Almanac. The almanac is a subset of the clock and ephemeris data, with reduced precision. The user algorithm is essentially the same as the user algorithm used for computing the precise ephemeris from the one subframe 1, 2, and 3 parameters (see Table 20-IV). The almanac content for one SV is given in Table 20-VI. A close inspection of Table 20-VI will reveal that a nominal inclination angle of 0.30 semicircles is implicit and that the parameter δ i (correction to inclination) is transmitted, as opposed to the value computed by the user. All other parameters appearing in the equations of Tables 20-IV, but not included in the content of the almanac, are set to zero for SV position determination. In these respects, the application of the Table 20-IV equations differs between the almanac and the ephemeris computations. IRN-200C-002 ICD-GPS-200C 25 SEP 1997 118a UNCLASSIFIED UNCLASSIFIED (This page intentionally left blank.) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 118b UNCLASSIFIED UNCLASSIFIED Table 20-XI. (Deleted) IRN-200C-002 ICD-GPS-200C 25 SEP 1997 119 UNCLASSIFIED UNCLASSIFIED The user is cautioned that the sensitivity to small perturbations in the parameters is even greater for the almanac than for the ephemeris, with the sensitivity of the angular rate terms over the interval of applicability on the order of 1014 meters/(semicircle/second). An indication of the URE provided by a given almanac during each of the operational intervals is as follows: Almanac Ephemeris URE (estimated by analysis) * Operational Interval 1 sigma (meters) Normal 900* Short-term Extended 900 - 3,600* Long-term Extended 3600 - 300,000* ,† URE values generally tend to degrade quadratically over time. Larger errors may be encountered during eclipse seasons and whenever a propulsive event has occurred. † After the CS is unable to upload the SVs, URE values for the Block IIR SVs operating in the Autonav mode tend to degrade quadratically such that the URE will approach 300,000 meters 1 sigma at 180 days. ICD-GPS-200C 10 OCT 1993 120 UNCLASSIFIED UNCLASSIFIED 20.3.3.5.2.2 Almanac Reference Time. Normal and Short-term Extended Operations. The almanac reference time, toa, is some multiple of 212 seconds occurring approximately 70 hours after the first valid transmission time for this almanac data set (reference 20.3.4.5). The almanac is updated often enough to ensure that GPS time, t, shall differ from toa by less than 3.5 days during the transmission period. The time from epoch tk shall be computed as described in Table 20-IV, except that toe shall be replaced with toa. Long-term Extended Operations. During long-term extended operations or if the user wishes to extend the use time of the almanac beyond the time span that it is being transmitted, one must account for crossovers into time spans where these computations of tk are not valid. This may be accomplished without time ambiguity by recognizing that the almanac reference time (toa) is referenced to the almanac reference week (WNa), both of which are given in word three of page 25 of subframe 5 (see paragraph 20.3.3.5.1.7). All toa values in subframes 4 and 5 shall be the same for a given almanac data set and shall differ for successive data sets which contain changes in almanac parameters or SV health. Note that cutover to a new upload may occur between the almanac pages of interest and page 25 of subframe 5 (reference paragraph 20.3.4.1), and thus there may be a temporary inconsistency between toa, in the almanac page of interest, and in word 3 of page 25 of subframe 5. The toa mismatch signifies that this WNa may not apply to the almanac of interest and that the user must not apply almanac data until the pages with identical values of toa are obtained. 20.3.3.5.2.3 Almanac Time Parameters. The almanac time parameters shall consist of an 11-bit constant term (af0) and an 11-bit first order term (af1). The applicable first order polynomial, which shall provide time to within 2 microseconds of GPS time (t) during the interval of applicability, is given by IRN-200C-001 ICD-GPS-200C 13 OCT 1995 121 UNCLASSIFIED UNCLASSIFIED t = tsv - ∆tsv t = GPS system time (seconds), tsv = effective SV PRN code phase time at message transmission time where (seconds), ∆tsv = SV PRN code phase time offset (seconds). The SV PRN code phase offset is given by ∆tsv = af0 + af1 tk where the computation of tk is described in paragraph 20.3.3.5.2.2, and the polynomial coefficients af0 and af1 are given in the almanac. Since the periodic relativistic effect is less than 25 meters, it need not be included in the time scale used for almanac evaluation. Over the span of applicability, it is expected that the almanac time parameters will provide a statistical URE component of less than 135 meters, one sigma. This is partially due to the fact that the error caused by the truncation of af0 and af1 may be as large as 150 meters plus 50 meters/day relative to the toa reference time. During extended operations (short-term and long-term) the almanac time parameter may not provide the specified time accuracy or URE component. 20.3.3.5.2.4 Universal Coordinated Time (UTC). Page 18 of subframe 4 includes: (1) the parameters needed to relate GPS time to UTC, and (2) notice to the user regarding the scheduled future or recent past (relative to NAV message upload) value of the delta time due to leap seconds (∆tLSF), together with the week number (WNLSF) and the day number (DN) at the end of which the leap second becomes effective. "Day one" is the first day relative to the end/start of week and the WNLSF value consists of eight bits which shall be a Modulo 256 binary representation of the GPS week number (see paragraph 6.2.4) to which the DN is referenced. The user must account for the truncated nature of this parameter as well as truncation of WN, WNt, and WNLSF due to rollover of full week number (see paragraph 3.3.4(b)). The CS shall manage these parameters such that the absolute value of the difference between the untruncated WN and WNLSF values shall not exceed 127. IRN-200C-004 ICD-GPS-200C 12 April 2000 122 UNCLASSIFIED UNCLASSIFIED Depending upon the relationship of the effectivity date to the user's current GPS time, the following three different UTC/GPS-time relationships exist: a. Whenever the effectivity time indicated by the WNLSF and the DN values is not in the past (relative to the user's present time), and the user's present time does not fall in the time span which starts at DN + 3/4 and ends at DN + 5/4, the UTC/GPS-time relationship is given by tUTC = (tE - ∆tUTC) [Modulo 86400 seconds] where tUTC is in seconds and ∆tUTC = ∆tLS + A0 + A1 (tE - tot + 604800 (WN - WNt)), seconds; tE = GPS time as estimated by the user on the basis of correcting tSV for factors described in paragraph 20.3.3.3.3 as well as for ionospheric and SA (dither) effects; ∆tLS = delta time due to leap seconds; A0 and A1 = constant and first order terms of polynomial; tot = reference time for UTC data (reference 20.3.4.5); WN = current week number (derived from subframe 1); WNt = UTC reference week number. The estimated GPS time (tE) shall be in seconds relative to end/start of week. The reference time for UTC data (tot) shall be referenced to the start of that week whose number (WNt) is given in word eight of page 18 in subframe 4. The WNt value consists of eight bits which shall be a Modulo 256 binary representation of the GPS week number (see paragraph 6.2.4) to which the tot is referenced. The user must account for the truncated nature of this parameter as well as truncation of WN, WNt, and WNLSF due to rollover of full week number (see paragraph 3.3.4(b)). The CS shall manage these parameters such that the absolute value of the difference between the untruncated WN and WNt values shall not exceed 127. IRN-200C-004 ICD-GPS-200C 12 April 2000 123 UNCLASSIFIED UNCLASSIFIED b. Whenever the user's current time falls within the time span of DN + 3/4 to DN + 5/4, proper accommodation of the leap second event with a possible week number transition is provided by the following expression for UTC: tUTC = W[Modulo (86400 + ∆tLSF - ∆tLS)], seconds; W = (tE - ∆tUTC - 43200)[Modulo 86400] + 43200, seconds; where and the definition of ∆tUTC (as given in 20.3.3.5.2.4a above) applies throughout the transition period. Note that when a leap second is added, unconventional time values of the form 23:59:60.xxx are encountered. Some user equipment may be designed to approximate UTC by decrementing the running count of time within several seconds after the event, thereby promptly returning to a proper time indication. Whenever a leap second event is encountered, the user equipment must consistently implement carries or borrows into any year/week/day counts. c. Whenever the effectivity time of the leap second event, as indicated by the WNLSF and DN values, is in the "past" (relative to the user's current time), the relationship previously given for tUTC in 20.3.3.5.2.4a above is valid except that the value of ∆tLSF is substituted for ∆tLS. The CS will coordinate the update of UTC parameters at a future upload so as to maintain a proper continuity of the tUTC time scale. ICD-GPS-200C 10 OCT 1993 124 UNCLASSIFIED UNCLASSIFIED 20.3.3.5.2.5 Ionospheric Model. The "two frequency" (L1 and L2) user shall correct the time received from the SV for ionospheric effect by utilizing the time delay differential between L1 and L2 (reference paragraph 20.3.3.3.3.3). The "one frequency" user, however, may use the model given in Figure 20-4 to make this correction. It is estimated that the use of this model will provide at least a 50 percent reduction in the single - frequency user's RMS error due to ionospheric propagation effects. During extended operations, or for the Block IIR SVs in the Autonav mode if the CS is unable to upload the SVs, the use of this model will yield unpredictable results. 20.3.3.5.2.6 NMCT Data. For each SV, the ERD value in the NMCT is an estimated pseudorange error. Each ERD value is computed by the CS and represents the radial component of the satellite ephemeris error minus the speed of light times the satellite clock error. The satellite ephemeris and clock errors are computed by subtracting the broadcast from current estimates. Therefore, the ERD value may be used as follows to correct the user's measured pseudorange: PRc = PR – ERD where, PRc = pseudorange corrected with the ERD value from the NMCT PR = measured pseudorange Note that as described above, the ERD values are actually error estimates rather than differential corrections and so are subtracted rather than added in the above equation. IRN-200C-003 ICD-GPS-200C 11 OCT 1999 125 UNCLASSIFIED UNCLASSIFIED The ionospheric correction model is given by Tiono x 2 x 4 F ∗ 5.0 ∗ 10− 9 + (AMP)1 − , x < 1 . 57 + 2 24 = (sec) −9 , x ≥ 1.57 F ∗ 5.0 ∗ 10 ( ) where Tiono is referred to the L1 frequency; if the user is operating on the L2 frequency, the correction term must be multiplied by γ (reference paragraph 20.3.3.3.3.2), 3 α n φnm , AMP ≥ 0 AMP = n = 0 if AMP < 0, AMP = 0 ∑ x= 2π (t - 50400) PER (sec) (radians) 3 β n φnm , PER ≥ 72,000 PER = n = 0 (sec) if PER < 72,000, PER = 72,000 ∑ F = 1.0 + 16.0 [0.53 - E]3 and αn and βn are the satellite transmitted data words with n = 0, 1, 2, and 3. Figure 20-4. Ionospheric Model (Sheet 1 of 3) ICD-GPS-200C 10 OCT 1993 126 UNCLASSIFIED UNCLASSIFIED Other equations that must be solved are φm = φi + 0.064cos(λi - 1.617) λ i = λu + ψ sinA cos φ i (semi-circles) (semi-circles) φ u + ψ cosA(semi − circles), φi ≤ 0.416 φi = if φi > +0.416, then φi = +0.416 if φ < −0.416, then φ = −0.416 i i ψ= 0.0137 - 0.022 E + 0.11 (semi-circles) (semi-circles) t = 4.32 * 104λi + GPS time (sec) where 0 ≤ t < 86400: therefore, if t ≥ 86400 seconds, subtract 86400 seconds; if t < 0 seconds, add 86400 seconds. Figure 20-4. Ionospheric Model (Sheet 2 of 3) ICD-GPS-200C 10 OCT 1993 127 UNCLASSIFIED UNCLASSIFIED The terms used in computation of ionospheric delay are as follows: • Satellite Transmitted Terms αn - the coefficients of a cubic equation representing the amplitude of the vertical delay (4 coefficients - 8 bits each) βn - the coefficients of a cubic equation representing the period of the model (4 coefficients - 8 bits each) • Receiver Generated Terms E - elevation angle between the user and satellite (semi-circles) A - azimuth angle between the user and satellite, measured clockwise positive from the true North (semi-circles) φu - user geodetic latitude (semi-circles) WGS-84 λu - user geodetic longitude (semi-circles) WGS-84 GPS time - receiver computed system time X - phase (radians) F - obliquity factor (dimensionless) t - local time (sec) φm - geomagnetic latitude of the earth projection of the ionospheric intersection • Computed Terms point (mean ionospheric height assumed 350 km) (semi-circles) λi - geodetic longitude of the earth projection of the ionospheric intersection point (semi-circles) φi - geodetic latitude of the earth projection of the ionospheric intersection point (semi-circles) ψ - earth's central angle between the user position and the earth projection of ionospheric intersection point (semi-circles) Figure 20-4. Ionospheric Model (Sheet 3 of 3) ICD-GPS-200C 10 OCT 1993 128 UNCLASSIFIED UNCLASSIFIED 20.3.4 Timing Relationships. The following conventions shall apply. 20.3.4.1 Paging and Cutovers. At end/start of week (a) the cyclic paging of subframes 1 through 5 shall restart with subframe 1 regardless of which subframe was last transmitted prior to end/start of week, and (b) the cycling of the 25 pages of subframes 4 and 5 shall restart with page 1 of each of the subframes, regardless of which page was the last to be transmitted prior to the end/start of week. Cutovers to newly updated data for subframes 1, 2, and 3 occur on frame boundaries (i.e., Modulo 30 seconds relative to end/start of week). Newly updated data for subframes 4 and 5 may start to be transmitted with any of the 25 pages of these subframes. 20.3.4.2 SV Time vs. GPS Time. In controlling the SVs and uploading of data, the CS shall allow for the following timing relationships: a. Each SV operates on its own SV time; b. All time-related data in the TLM word and the HOW shall be in SV-time; c. All other data in the NAV message shall be relative to GPS time; d. The acts of transmitting the NAV message shall be executed by the SV on SV time. 20.3.4.3 Speed of Light. The speed of light used by the CS for generating the data described in the above paragraphs is c = 2.99792458 x 108 meters per second which is the official WGS-84 speed of light. The user shall use the same value for the speed of light in all computations. IRN-200C-001 ICD-GPS-200C 13 OCT 1995 129 UNCLASSIFIED UNCLASSIFIED 20.3.4.4 Data Sets. The IODE is an 8 bit number equal to the 8 LSBs of the 10 bit IODC of the same data set. The following rules govern the transmission of IODC and IODE values in different data sets: (1) The transmitted IODC will be different from any value transmitted by the SV during the preceding seven days; (2) The transmitted IODE will be different from any value transmitted by the SV during the preceding six hours. The range of IODC will be as given in Table 20-XII. Cutovers to new data sets will occur only on hour boundaries except for the first data set of a new upload. The first data set may be cut-in (reference paragraph 20.3.4.1) at any time during the hour and therefore may be transmitted by the SV for less than one hour. During short-term operations, cutover to 4-hour sets and subsequent cutovers to succeeding 4-hour data sets will always occur Modulo 4 hours relative to end/start of week. Cutover from 4-hour data sets to 6-hour data sets shall occur Modulo 12 hours relative to end/start of week. Cutover from 12-hour data sets to 24-hour data sets shall occur Modulo 24 hours relative to end/start of week. Cutover from a data set transmitted 24 hours or more occurs on a Modulo 24-hour boundary relative to end/start of week. The start of the transmission interval for each data set corresponds to the beginning of the curve fit interval for the data set. Each data set remains valid for the duration of its curve fit interval. IRN-200C-004 ICD-GPS-200C 12 April 2000 130 UNCLASSIFIED UNCLASSIFIED Table 20-XII. IODC Values and Data Set Lengths Days Spanned 1 2-14 15-16 17-20 21-27 28-41 42-59 60-87 88-122 123-182 Transmission Interval (hours) (Note 5) Curve Fit Interval (hours) IODC Range (Note 1) 4 6 8 14 26 50 74 98 122 146 (Note 2) (Note 2) 240-247 248-255, 496 (Note 3) 497-503 504-510 511, 752-756 757-763 764-767, 1008-1010 1011-1020 2 (Note 4) 4 6 12 24 48 72 96 120 144 Note 1: For transmission intervals of 6 hours or greater, the IODC values shown will be transmitted in increasing order. Note 2: IODC values for blocks with 2- or 4-hour transmission intervals (at least the first 14 days after upload) shall be any numbers in the range 0 to 1023 excluding those values of IODC that correspond to IODE values in the range 240-255, subject to the constraints on re-transmission given in paragraph 20.3.4.4. Note 3: The ninth 12-hour data set may not be transmitted. Note 4: Some SVs will have transmission intervals of 1 hour per paragraph 20.3.4.4. Note 5: The first data set of a new upload may be cut-in at any time and therefore the transmission interval may be less than the specified value. IRN-200C-004 ICD-GPS-200C 12 April 2000 131 UNCLASSIFIED UNCLASSIFIED Normal Operations. The subframe 1, 2, and 3 data sets are transmitted by the SV for periods of two hours. The corresponding curve fit interval is four hours. Block IIR SVs operating in the Autonav mode will deviate. They will transmit subframe 1, 2, and 3 data sets for periods of one hour. The corresponding curve-fit interval will be four hours. Short-term and Long-term Extended Operations. The transmission intervals and curve fit intervals with the applicable IODC ranges are given in Table 20-XII. 20.3.4.5 Reference Times. Many of the parameters which describe the SV state vary with true time, and must therefore be expressed as time functions with coefficients provided by the Navigation Message so as to be evaluatable by the user equipment. These include the following parameters as functions of GPS time: a. SV time, b. Mean anomaly, c. Longitude of ascending node, d. UTC, e. Inclination. Each of these parameters is formulated as a polynomial in time. The specific time scale of expansion can be arbitrary. Due to the short data field lengths available in the Navigation Message format, the nominal epoch of the polynomial is chosen near the midpoint of the expansion range so that quantization error is small. This results in time epoch values which can be different for each data set. Time epochs contained in the Navigation Message and the different algorithms which utilize them are related as follows: IRN-200C-004 ICD-GPS-200C 12 April 2000 132 UNCLASSIFIED UNCLASSIFIED Epoch Application Algorithm Reference toc 20.3.3.3.3.1 toe 20.3.3.4.3 toa 20.3.3.5.2.2 and 20.3.3.5.2.3 tot 20.3.3.5.2.4 Table 20-XIII describes the nominal selection which will be expressed Modulo 604,800 seconds in the Navigation Message. The coefficients of expansion are obviously dependent upon choice of epoch, and thus the epoch time and expansion coefficients must be treated as an inseparable parameter set. Note that a user applying current navigation data will normally be working with negative values of (t-toc) and (t-toe) in evaluating the expansions. The CS will introduce small deviations from the nominal if necessary to preclude possible data set transition ambiguity when a new upload is cut over for transmission. A change from the reference time is used to indicate a change of values in the data set. IRN-200C-004 ICD-GPS-200C 12 April 2000 133 UNCLASSIFIED UNCLASSIFIED Table 20-XIII. Reference Times Hours After First Valid Transmission Time Fit Interval (hours) Transmission Interval (hours) toc (clock) toe (ephemeris) 4 2* 2 2 6 4 3 3 8 6 4 4 14 12 7 7 26 24 13 13 50 48 25 25 74 72 37 37 98 96 49 49 122 120 61 61 146 144 73 73 144 (6 days) 144 (6 days) * toa (almanac) tot (UTC) 144 70 70 4080 70 70 Some SVs will have transmission intervals of 1 hour per paragraph 20.3.4.4. IRN-200C-004 ICD-GPS-200C 12 April 2000 134 UNCLASSIFIED UNCLASSIFIED 20.3.5 Data Frame Parity. The data signal shall contain parity coding according to the following conventions. 30.3.5.1 SV/CS Parity Algorithm. This algorithm links 30-bit words within and across subframes of ten words using the (32.26) Hamming Code described in Table 20-XIV. 20.3.5.2 User Parity Algorithm. As far as the user is concerned, several options are available for performing data decoding and error detection. Figure 20-5 presents an example flow chart that defines one way of recovering data (dn) and checking parity. The parity bit D30* is used for recovering raw data. The parity bits D29* and D30*, along with the recovered raw data (dn) are Modulo-2 added in accordance with the equations appearing in Table 20-XIV for D25 . . . D30, which provide parity to compare with transmitted parity D25 . . . D30. ICD-GPS-200C 10 OCT 1993 135 UNCLASSIFIED UNCLASSIFIED Table 20-XIV. Parity Encoding Equations D1 = d1 ⊕ D30« D2 = d2 ⊕ D30« D3 = d3 ⊕ D30« • • • • • • • • D24 = d24 ⊕ D30« D25 = D29« ⊕ d1 ⊕ d2 ⊕ d3 ⊕ d5 ⊕ d6 ⊕ d10 ⊕ d11 ⊕ d12 ⊕ d13 ⊕ d14 ⊕ d17 ⊕ d18 ⊕ d20 ⊕ d23 D26 = D30« ⊕ d2 ⊕ d3 ⊕ d4 ⊕ d6 ⊕ d7 ⊕ d11 ⊕ d12 ⊕ d13 ⊕ d14 ⊕ d15 ⊕ d18 ⊕ d19 ⊕ d21 ⊕ d24 D27 = D29« ⊕ d1 ⊕ d3 ⊕ d4 ⊕ d5 ⊕ d7 ⊕ d8 ⊕ d12 ⊕ d13 ⊕ d14 ⊕ d15 ⊕ d16⊕ d19 ⊕ d20 ⊕ d22 D28 = D30« ⊕ d2 ⊕ d4 ⊕ d5 ⊕ d6 ⊕ d8 ⊕ d9 ⊕ d13 ⊕ d14 ⊕ d15 ⊕ d16 ⊕ d17 ⊕ d20 ⊕ d21 ⊕ d23 D29 = D30« ⊕ d1⊕ d3⊕ d5⊕ d6⊕ d7⊕ d9 ⊕ d10 ⊕ d14 ⊕ d15 ⊕ d16 ⊕ d17 ⊕ d18 ⊕ d21 ⊕ d22⊕ d24 D30 = D29« ⊕ d3 ⊕ d5 ⊕ d6 ⊕ d8 ⊕ d9 ⊕ d10 ⊕ d11 ⊕ d13 ⊕ d15 ⊕ d19 ⊕ d22 ⊕ d23 ⊕ d24 Where d1, d2, ..., d24 are the source data bits; the symbol « is used to identify the last 2 bits of the previous word of the subframe; D25, D26, ..., D30 are the computed parity bits; D1, D2, ..., D29, D30 are the bits transmitted by the SV; ⊕ is the "Modulo-2" or "Exclusive-Or" operation. ICD-GPS-200C 10 OCT 1993 136 UNCLASSIFIED UNCLASSIFIED ENTER COMPLEMENT D1 . . . D24 TO OBTAIN d1 . . . d24 YES * = 1? IS D30 NO DO NOT COMPLEMENT D1 . . . D24 TO OBTAIN d1 . . . d24 SUBSTITUTE d1 . . . d24, * & D30 * INTO D29 PARITY EQUATIONS (TABLE 20-XIV) NO ARE COMPUTED D25 . . . D30 EQUAL TO CORRESPONDING RECEIVED D25 . . . D30? YES PARITY CHECK FAILS PARITY CHECK PASSES FAIL EXIT PASS EXIT Figure 20-5. Example Flow Chart for User Implementation of Parity Algorithm ICD-GPS-200C 10 OCT 1993 137 UNCLASSIFIED UNCLASSIFIED (This page intentionally left blank.) ICD-GPS-200C 10 OCT 1993 138 UNCLASSIFIED
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