19810019777_GPS_REFSAT_Definition_Study_Report_for_Low Cost_Terminals_Nov80 19810019777 GPS REFSAT Definition Study Report For Low Cost Terminals Nov80
19810019777_GPS_REFSAT_Definition_Study_Report_for_Low-Cost_Terminals_Nov80 19810019777_GPS_REFSAT_Definition_Study_Report_for_Low-Cost_Terminals_Nov80
User Manual: 19810019777_GPS_REFSAT_Definition_Study_Report_for_Low-Cost_Terminals_Nov80
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Systematics General Corporation
2922 Tdefitas lltlrarf
Falls Ohurch. Uirqi~lia221142
GPS/REFSAT D e f i n i t i o n Study Report
For
Low-Cost Terminals
November 1980
Prepared for
National Aeronautics and Space Admini stv'rz t i on
Goddard Space f l i - " t Center
Earth Obser*vation Sy. 2 : - ";-!i
sion
User Termilia1 & Location Sya i.e,nr; Branch
Greenbelt, Mary1and 20771
Prepared on NASA/GSFC Contract NAS5-23433
by
SYSTEMATICS GENERAL CORPORATtON
2822 TELLSTAR Cf.
FALLS CHURCH, VA. 12042
TABLE OF CONTENTS
SECT l ON
PAGE
-
Abs t racr
1
1.
NAVSTAR GPS System E GPS/REFSAT System-----------------------
2
1.1
1.2
2
NAVSTAR GPS System--------------------------------------The GPS/REFSAT System----------------------------------1.2.1
I n i t i a l GPS S a t e l l i t e Selection-----------------1 . 2 . 2 Doppler and Delay Acquisition-------------------1.2.3 P o s i t i o n Fixing---------------------------------1 .2.4 Format o f t h e REFSAT D a t a Message----------------
.........................
4
.8
8
8
10
Requi red Frequency AssignmentsS p a c e c r a f t Antenna F o o t p r i n g Coverage Requirement------RF L i n k Budget Computations----------------------------Spacecraft Anterlna System------------.-------------------Transponder System--------------------------------------
14
15
15
3.
A l t e r n a t e Methods F o r Implementing a GPS/REFSAT System-------
17
4.
GPS/REFSAT User Terminal Segment--------------------------.---
22
4 1
22
22
2.1
2.2
2.3
2.4
2.5
4.2
4.3
5.1
5.2
Appendix:
2-Channel Receiver Requi rernen ts------------------------Data Transmitter---~-----------------------------------S i m p l i f i e d B l o c k Diagram, GPS/REFSAT Terminal-----------
Generation o f t h e REFSAT D a t a Message------------------REFSAT Reference C a r r i e r ,lnd Message Timing-------------
Performance Speci f i c a t i o n f o r REFSAT/NAVSTAR C i v i 1
User System-----------------------------------------.--
15
15
22
25
25
A- 1
LlST OF TABLES
-
PAGE
TABLE
1-1.
NAVSTAR GPS Signal Charcteristics---------an-------------------
1-2.
GPS Receiver Functions Comparison-----------------------------
3
22
LlST OF FIGURES
PAGE
-
F l GURE
1-1,
GPS Satellite Ground T r a c k s (20 degree elevation visibility
contour shown around Washington, DC)-------------------------
5
1-1A.
GPS Satellite Elevation Angles at Washington, DC------------
6
1-16.
Dilution-of-Precision (DOP) Values at Washington, DC--------
6
1-4.
REFSAT vs. Conventional Receiver------------------------------
3- 1 ,
GPS/REFSAT Terminal (VHF REFSAT Signal ~ink)
3-2.
GPS/REFSAT System Utilizing the ATS-3 Satellite--------------
19
3
GPS/REFSAT System Using a Local Tower------------------------
21
4-1. Simp1 i f i e d
5-1.
-----------------
Block Diagram, GPS/REFSAT Terminal----------------
Simplified Functional Block Diagram, REFSAT Remote Control
~t~ti~~----------------------------------~-------------------
I1
18
24
26
ABSTRACT
The NAVSTAR G l o b a l P o s i t i o n i n g System (GPs) i s a s a t e l l i t e n a v i g a t i o n system c u r r e n t Zy under development by t h e Department of Defense.
I t w i l l c o n s i s t o f 18 s a t e l l i t e s i n c i r c u l a r , 12-hour o r b i t s a t an
a l t i t u d e o f 11,000 NM, i n c l i n e d 63" t o t l i e e q u a t o r , GPS s a t e l l i t e s
w i l l b r o a d c a s t pseudo-random n o i s e codes and ephemerides on two
L-band s i g n a l s t o users w o r l d w i d e , i n c l u d i n g m i l i t a r y and c i v i l u s e r s .
T h i s r e p o r t d e s c r i b e s a concept t h a t u t i l i z e s a r e l a y transponder,
l o c a t e d e i t h e r on a s a t e l l i t e i n g e o s t a ~ ~ o n a royr b i t o r on a l o c a l
tower t o r e l a y a c q u i s i t i o n - a i d i n g data, ephemerides, e t c , from a
ground-based remote c o n t r o l s t a t i o n t o a GPS c i v i l u s e r t e r m i n a l
l o c a t e d on a s h i p o r l a n d - t r a n s p q r t a t i o n v e h i c l e . Termed REFSAT
( ~ e f e r e n c eS a t e l l i t e ) , t h i s concept reduces s i g n i f i c a n t l y t h e c i r c u i t
c o m p l e x i t y and c o s t o f u s e r t e r m i n a l s . A law-cost, REFSAT user t e r m i n a l
might range i n p r i c e from $1,000 t o $2,000 per t e r m i n a l , i n l o t s o f 3,000.
T h i s r e p o r t d e f i n e s t h e v a r i o u s systems needed t o implement t h e REFSAT
concept f o r low-cost, GPS c i v i l t e r m i n a l s . The GPS/REFSAT system c o m p a t i b l e
w i t h t h e NAVSTAR GPS sys tern c o n s i s t s o f a space segment (gees t a t i o n a r y
r e l a y sa t e l l i t e ) , a ground t e r m i n a l segment ( c i v i 1 u s e r t e r m i n a Is), and
t h e remote c o n t r o l s t a t i o n ( t h e c e n t r a l f a c i l i t y w h i c h performs o p e r a t i o n s
Each segmeat i s
common t o a1 ? users f o r r e l a y v i a t h e spacc segment),
described i n d e t a i l .
A GPS/REFSAT system u t i l i z i n g a l o c a l tower f o r t h e r e l a y transponder
i s a l s o described.
The results o f a s t u d y o f c i v i l user requirements i s presented.
D e t a i l e d s p e c i f i c a t i o n s f o r the GPj/REFSAT system and i t s i n d i v i d u a l
segments a r e presented i n t h e appendix.
1 , IAVSTAR GPS System
& GPS/REFSAT System
The GPS/REFSAT Systemfi augments t h e planned NAVSTAR G l o b a l P o s i t i o n i n g
System, a1 lowing a s i g n i f i c a n t r e d u c t i o n i n t h e c o s t and conrplexi t y
of c i v i l user terminals.
(GPs)
A number o f o p e r a t i o n s common t o a l l c i v i l users w i t h i n a p a r t i c u l a r
geographic a r e a may be performed a t a c e n t r a l remote c o n t r o l s t a t i o n .
Information c o n t a i n i n g t h e r e s u l t o f these o p e r a t i o n s i s broadcast a l o n g w i t h
a p r e c i s i o n frequency r e f e r e n c e t o a l l c i v i l u s e r s t h r o u g h t h e REFSAT/GPS
g e o s t a t ! o n a r y space seyment.
The f o l l o w i n g s e c t i o n s develop t h e d e f i n i t i o n o f t h e GPS/REFSAT
system, d e t a i l i n g i t s method o f o p e r a t i o n and r e l a t i o n t o t h e NAVSTAR
GPS system.
1.1
NAVSTAR GPS System
The NAVSTAR Global P o s i t i o n i n g System (GPS) o f f e r s a c c u r a t e three-dimensions 1
p o s i t i o n (and v c l o c i t y i f d e s i r e d ] i n f o r m a t !on t o u s e r s anywhere i n t h e w o r l d .
A user p o s i t i o n f i x c o n s i s t s o f t h e f o l l o w i n g s t e p s :
Measuring t h e t r a n s i t t i m e o f RF s i g n a l s f r o m t h r e e GPS s a t e l l i t e s o f
a t o t a l c o n s t e l l a t i o n o f 18 sate1 1 i t e s and cornput i n g t h e r e s u l t a n t
( T r a n s i t t i m e i s determined u s i n g t h e speed o f l i g h t ) .
distances.
Computing t h e c u r r e n t p o s i t i o n o f t h r e e GPS s a t e l l i t e s u s i n g t h e
ephemeris d a t a t r a n s m i t t e d w i t h each s a t e l l i t e s i g n a l ,
S o l v i n g t h e r e s u l t a n t l l t r i a n g u l a t i o n l ' e q u a t i o n s t o determine t h e
pos i t i o n o f t h e user t e r m i n a l
.
I f u s e r t e r m i n a l s m a i n t a i n e d p r e c i s i o n c l o c k s s y n c h r o n i z e d w i t h GPS system
time, a p o s i t i o n f i x c o u l d be accomplished i n t h i s s i m p l e manner. The user
t e r m i n a l would then be a t t h e i n t e r s e c t i o n o f t h r e e spheres whose c e n t e r s were
located a t t h e respective s a t e l l i t e s .
I n p r a c t i c e , the requirement f o r user terminals t o contain a p r e c i s i o n
In this
c l o c k i s e l i m i n a t e d by making range measurements t o f o u r s a t e l l i t e s .
case, t h e n a v i g a t i o n e q u a t i o n s c o n t a i n four unkqowns: t h e u s e r p o s i t i o n i n
t h r e e dimensions and t h e e r r o r , o r f i x e d b i a s , i n t h e u s e r ' s i m p r e c i s e c l o c k .
The n a v i g a t i o n s i g n a l t r a n s m i t t e d f r o m each NAVSTAR G P S s a t e l l i t e c o n s i s t s
o f two R F frequencies, L1 and L2. As shown i n T a b l z 1-1, t h e L1 s i g n a l a t
1575.42 MHz i s modulated w i t h b o t h t h e P and C/A pseudo-random n o i s e (PRN) codes
i n phase q u a d r a t u r e . The 12 s i g n a l a t 1227.6 MHz i s modulated w i t h t h e P-code.
Both s i g n a l s a r e a l s o c o n t i n u o u s l y modulated w i t h t h e n a v i g a t i o n message datab i t stream a t 50 bps.
*
J. W. Sennott, A . K. Choudhury, R.E. T a y l o r , "The REFSAT Approach t o LowCost G P S Terminals", NASA Goddard Space F l i g h t Center, TM 79655, A p r i l 1979.'
Table 1 - 1 ,
NAVSTAR GPS S i g n a l C h a r a c t e r i s t i c s ,
L1 RF Frequency
Conta i n s :
= 1575.42 MHz
L2 RF Frequency
Conta i ns
= 1 2 2 7 . 6 0 YHz
PRN P-Code
SO BPS Data Stream
Long (P) Code
Frequency
Epoch
(Reset each
= 10.23 MHz
= 267 Days
7 days)
Short ( c / P ) Code
Frequency
Epoch
PRN P-Code
PRN C/A Code
50 BPS D a t a Stream
= 1.023 MHz
= 1 millisecond
The d a t a stream i n c l u d e s s r : t e l l j t e e p h e r ~ ~ es r i i n f o r m a t i o n , t o a1 low a user
t o compute t h e s a t e l l i t e p o s i t i o n c o o r d i n a t e s needed f o r s o l u t i o n of t h e n a v i g a t i o n e q u a t i o n , The PRN codes serve two f u n c t i o n s :
1)
S a t e l l i t e i d e n t i f i c a t i o n . The code p a t t e r n s a r e unique t o each
s a t e l l i t e and a r e matched w i t h l i k e codes i n t h e user r e c e i v e r and,
2)
The measurement o f n a v i j a t i o n s i g n a l t r a n s i t time, by measuring t h e
phase s h i f t r e q u i r e d t o match t h e codes.
The u s e r t e r m t n a l s d i s c u s s e j i n t h i s document u t i l i z e t h e L l s i g n a l a t
1575,42 MHz and t h e s n o r t C/A code which repeats every m i 11 isecond. More
s o p h i s t i c a t e d users may make use o f b o t h L1 and L2 s i g n a l s t o measure ionos p h e r i c p r o p a g a t i o n delays o r t o u t i l i z e t h e l o r ~ g c rP-code f o r extreme accuracy,
The b a s e l i n e c o n s t e l l a t i o n of I8 NAVSTAR s a t e l l i t e s w i l l be p l a c e d i n
12-hour o r b i t s p r o v i d i n g a t least f i v e s a t e l l f t e s a t 5 degrees o r more above
t h e l o c a l h o r i z o n t o a w o r l d w i d e user.
F i g u r e 1-1 shows ground t r a c k s o f t h e f o u r Phase 1 NAVSTAR s a t e l l i t e s
o p e r a t i o n a l a s o f 1 January 1980 ( o r b i t p o s i t i o n s 1 , 3, 5, and 6 ) . A g i v e n
s a t e l l i t e ground t r a c k r e p e a t s each 12 hours. A l s o shown i s t h e 20 degree
v i s i b i l i t y c o n t o u r about Washington, DC. A s a t e l l i t e whose ground t r a c k i s
n o r t h o f t h e v i s i b i l i t y c o n t o u r w i l l appear 20 degrees o r more above t h e h o r i z o n .
The GPS/REFSAT System
The GPS/GEFSAT System i l l u s t r a t e d on F i g u r e 1-2 o f f e r s a s i g n i f i c a n t
r e d u c t i o n i n b o t h t h e c o s t and c o m p l e x i t y o f u s e r ground t e r m i n a l s .
A remote
c o n t r o l s t a t i o n performs many o f t h e t a s k s r e q u i r e d o f a c o n v e n t i o n a l GPS
user t e r m i n a l ,
D a t a from t h e remote c o n t r o l s t a t i o n i s t h e n broadcast over a
w i d e geograph i ca l area v i a a g e o s t a t i o n a r y r e f e r e n c e s a t e l 1 iie (REFSAT) i n
t h e form o f an L-band " a i d i n g s i g n a l " .
GPS/REFSAT user t e r m i n a l s i n t h e s e r v i c e
a r e a of t h e remote ~ o n t r o ls t a t i o n use t h e L-band REFSAT s i g n a l t o a i d GPS
s a t e l l i t e s i g n a l a c q u i s i t i o n and t o s i m p l i f y computation o f the u s e r ' s p o s i t i o n .
Before d i s c u s s i n g t h e GPS/REFSAT system I n d e t a i l , two o b s e r v a t i o n s from
F i g u r e s 1-1 and I-lA/1-IR are p e r t i n e n t . An examination of Phase 1 GPS ground
t r a c k s n o r t h of t h e 20' v i s i b l l i t y c o n t o u r shows t h a t t h e geometry o f t h e
GPS s a t e l l i t e s r e l a t i v e t o a u s e r t e r m i n a l :
(1)
Changes r a p i d l y w i t h time. A p a r t i c u l a r group of 4 s a t e l l i t e s
remains v i s i b l e t o a u s e r t e r m i n a l , above t b s 20' e l e v a t i o n angle,
f o r 2 t o 3 hours. However, the h o r i z o n t a l d i l u t i o n - o f - p r e c i s i o n
(HDoP) f o r t h e same f o u r sate1 l ites i s 5 o r less f o r o n l y 2 hours.
(2)
Changes s l o w l y : 4 l t h u s e r g e o g r a p h i c a l l o c a t i o n . A t a g i v e n i n s t a n t
i n time, users anywhere i n t h e c c n t i n e n t a l U n i t e d S t a t e s c o u l d
u t i l i z e t h e same 4 - s a t e l l i t e c o n s t e l l a t i o n .
20° Elevation Visibility
Contour
Figure 1-1.
GPS Satellite Ground Tracks ( 2 0 degree elevation vis!billty
contour shown around Washington, G.c.)
80
3
Satellfte O r b I t
Posl tlon Number
70 ,
1
60 *
50 .
0
4
2
6
Figure 1-1A,
14 16
Haurs GMT. 1 January 79
B
10
12
10
20
22
24
GPS Satellite Elevation Angles
a t Wash,, D.C,
. . .
9
87.
5.
5
gcornetrlc d i l u t i o n o f prccislon
IW..-UCIIII..I......W--
p o s t t i o n d i l u t f o n o f precision
4.
4
horizontal dilritlon o f precision
3.
2.
1.
O'
'
4
'
6 - 8
'
10
12
14
16
1'8
'
20
22
24
Hours GHT. 1 January 79
Figure 1-15.
Dilution-of-Precision (DOP) Values
a t Wash.,
D.C.
Figure 1-2.
The GPS/REFSAT System
The f i r s t s t e p r e q u i r e d t o p e r f o r m a GPS n a v f g a t i o n f i x i s t h a t of
s e l e c t i n g from t h e v i s i b l e GPS s a t e l l i t e s t h o s e f o u r t h a t p r o v i d e t h e
minimum h o r i z o n t a \ d l lution-of-precision (HDOP). T h i s i n i t i a l s t e p shown
o n F i g u r e 1-3 i s important s i n c e the magnltude of t h e u s e r p o s i t l o n e r r o r s
I n t h e computed GPS n a v i g a t i o n f t x depend n o t o n l y upon t h e u s e r ' s ranging
e r r o r s , b u t on t h e r e l a t i v e geometry o f t h e f o u r s e l e c t e d s a t e l l i t e s .
1.2.1
I n i t i a l GPS S a t e l l i t e S e l e c t i o n
I n g e n e r a l , a l l user t e r m i n a l s w i t h i n a l a r g e g e o g r a p h i c a l area would
s e l e c t the same f o a r GPS s a t e l l i t e s i n o r d e r t o achieve minimum HDOP a t a
p a r t i c u l a r t i m e . T h i s s e l e c t i d n r e q u i r e s t h e maintenance o f c u r r e n t G P S
satel 1 l t e almanac date i n memory and t h c s o l u t i o n o f an I1HDOP1' a l g o r i t h m .
The REFSAT s y s t e m performs t h i s t a s k a t a c e n t r a l l o c a t i o n and then broadcasts
t h e r e s u l t s t o a l l users, relieving i n d i v t d u a l user t e r m i n a l s of t h i s s o f t ware and s t o r a g e f u n c t i o n .
1.2.2
..
Doppler and Delay A c q u i -s, i t i o n
For s i g n a l a c q u i s i t i o n ( 4 GPb s a t e l l i t e s ) , a c o n v e n t i o n a l user t e r m i n a l
may p e r f o r m a frequency a ~ dccdc d e l a y search o r make use o f s t o r e d G P S almanac
d a t a t o compute expected c n ? ; ' c ~ ' n i f t s f o r the f o u r s a t e l l i t e s i g n a l s . A
p r e c i s i o n frequency r e f e r e n c e : s r e q u i r e d a t t h e user t e r m i n a l t o reduce
d o p p l e r a c q u i s i t i o n time.
I n t h e REFSAT System, a m a j o r p o r t i o n o f t h l s f u n c t i o n i s performed by
t h e remote ? c o n t r o l s t a t i o n , A s shown on F i g u r e 1-3, t h e REFSAT p r o v i d e s a
p r e c i 5 !on f r e q u e n c y r e f e r e n c e t o a1 1 users ( t h e REFSAT C a r r i e r Frequenctt S i g n a l )
I n a d d i t i o n , t h e data message c a r r i e d on t h e REFSAT s i g n a l p r o v i d e s IJdoppler
c o e f f i c i e n t 1 ' i n f o r m a t i o n a l l o w i n g a simple computation o f d o p p l e r o f f s e t s a t
t h e user t e r m i n a l .
1.2.3
Position Fixing
Both c o n v e n t i o n a l GPS end GPS/REFSAT u s e r t e r m i n a l s make "pseudo-range''
measurements t o t h e 4 s e l e c t e d GPS s a t e l l i t e s , S o l u t i o n o f t h e n a v i g a t i o n
e q u a t i o n r e q u i r e s a knowledge o f the p o s i t ion c o o r d i n a t e s o f t h e 4 s e l e c t e d
GPS s a t e l l i t e s . These c o o r d i n a t e s must be computed from GPS ephemeris d a t a
c o n t a i n e d i n the GPS s a t e l l i t e s i g n a l s . Dernodulatlan o f t h i s data r e q u i r e s
GPS c a r r i e r phase t r a c k i n g i n t h e c o n v e n t i o n a l GPS t e r m i n a l .
As shown on F i g u r e 1-3, t h i s function i s performed by each c o n v e n t i o n a l
I n c o n t r a s t , t h e REFSAT system p e r f o r m s t h i s f u n c t i o n a t
a c e n t r a l l o c a t i o n . C u r r e c t p o s i t i o n c o o r d i n a t e s f o r the f o u r s e l e c t e d GPS
s a t e l l i t e s a r e t h e n broadcast t o a l l GPS/REFSAT users as p a r t o f t h e REFSAT
s i g n a l . The u s e r may u t i l i z e t h i s i n f o r m a t i o n d i r e c t l y t o s o l v e t h e n a v i g a t i o n
e q u a t i o n o r s i m p l y t r a n s m i t measured pseudo-range values v i a a " r e p o r t back"
l i n k f o r p o s i t i o n computation a t a c e n t r a l l o c a t i o n .
GPS user t e r m i n a l ,
.
I
I
I
I
I
I
k q u i r e 4 GPS
Satellite Slgnr!s
-
I
I
I
I
I
GPS frttllltc
---*--
This concept assme5 rcccptlon of 50 BPS data
s1gnals fm 4 GPS satellftes acquired. b
optlonal conccu; Is to use a 1 x 5 line frm
CPS Raster Contml Stbtlon (RfS1 to PCS.
[see Flgure 1-21
*.
In slternate concept 1s to carputt
l(nr'5
location back a t R t 5 .
I
Figure 1-3.
1
I
User Fosftlm
T h e REFSAT Concept-
I
1
F i g u r e 1-3 and T a b l e 1-2 i l l u s t r a t e the manner i n which t h e above s t e p s
a r e accomplished i n c o n v e n t i o n a l GPS and t h e proposed GPS/REFSAT systems.
F i g u r e 1-4 c o n t r a s t s the rece!ver f u n c t i o n s r e q u i r e d o f each system.
2
4 Format o f the REFSAT D a t a Messagc
F i g u r e 1-5 shows a proposed format f o r t h e REFSAT d a t a message broadcast
w i t h the REFAAT s i g n a l f o r G P S s a t e l l i t e s A, B, C and 0. I n a d d i t i o n t o t h e
p r e c i s i o n REFSAT c a r r i e r w h i c h p r o v i d e s each GPS/REFSAT u s e r t e r m i n a l w i t h a
p r e c i s e frequency r e f e r e n c e , the REFSAT s i g n a l c o n t a i n s a 128 bi t-per-second
FSK s i g n a l .
time.
Each REFSAT data frame i s t r a n s m i t r e d ''on t i m e t ' r e l a t i v e t o G P S system
Each o f the 4 subf rames c o n t a i n s :
(1)
s y n c h r o n i z a t ion
(2)
c u r r e n t X, Y,
(3)
(4)
(3 b y t e s , 1 4 - C i
ts)
and Z e a r t h - c e n t e r e d c o o r d f n a t e s f o r a s e l e c t e d G P S
s a t e l l i t e (9 b y t e s ) .
c u r r e n t d o p p l a r c o e f f i c i e n t d a t a f o r computation o f d o p p l e r g r a d i e n t
(3 b y t e s )
.
code s e l e c t t o identify t h e selected GPS s a t e l l i :e ( 1 b y t e ) .
( a ) CONVENTIONAL NAVSTAR GPS USER EQUIPMENT
GPS SIGNAL
?
RF/lF
I
1
SIGNAL
PROCESSING
'
MICROPROCESSOR
POSITION
t
1-BAND
SYNTHESIZER
AND STABLE
OSCl LLATO R
PRECISION CODE
AND CARRIER
TRACK LOOPS
-
COIIERENT
DEMOD
STORAGE AND
EPHEMERIS
PREDICTION ALG.
(b) REFSAT-AUGMENT L 3 GPS USER EQUlPMENT
GPS SIGNAL
R F/I F
1
I
-
SIGNAL
PROCESSING
REFSAT SIGNAL
Figure
1-4.
REFSAT VS.
Conventional Receiver.
POSITION
Table f - 2 .
GPS Receiver functions Comparison.
'1
Conventional GPS Termina t
Major Function
Sub-Function
Software
Ijardwarc
1. Initial Satellite Sclcction
GPS Almanac
Compute satellites in view
Eliminate this software/
storage function
Compute range rate for
scleclcd satellites
VCXO lo I part in lo6
2. Doppler Acquisition
1 part 108 synthcsizcr
(in oven)
3. Delay Acquisition
PSK spread spcctrum
Filter ernr signals
programable synthesizer Advanccfrctard commands
(in-phase Pr quadrature)
4. Fine Delay Track
Delay-lock loop
Signal Acquisition
Filter error signals
Advance/retard commands
-
Signal Tracking
REFSAT Sirnplifiwfion
Reduce wdc generator
precision
No punctual code,
cmploy interpolation
5. Fine Doppter Acquisition AFC loop
Not essential for all
Filter error signals
VCXO freq. step commands users
6. Carrier Pliasc Track
Eliminatc cnlircly
Fillcr cror signals
VCXO pliasc step commands
7. TcIernctry Acquisition
Costas b o p
Simple non-cohcrent FSK
PSK demodulate using
above phase reference
Position Fixing
.
8. Ephemeris Update
Rcal-timc predictor
9. Position Computation
Pseudo range to 1at.-long.
conversion
Eliminate entirely
I
Sat. A Data
I
Data
I
I
Data
4
3
2
Data A
I
1
1
1
I
Sat. D
Sat. C
I
0
1I-
Sat. B
Data
5
I
I
Subf rarne
1 Second
-1
I
1
Code Select
1
I
I
1
-I
Dopp l e r
Sync
I
X~
I
MOTE:
1 Byte
#Gradient
I
Y~
I
=A
I
-
I
I
I
I
I
1
I
1
I
-
8 Blts
Flgure 1-5.
I
REFSAT Data Format.
Data B
I
6
Seconds
2.
REFSAT
Space Segment
T h l s s e c t i o n examines the requirements o f the REFSAT space segment i n
o r d e r t o form the performance s p e c i f i c a t i o n s l i s t e d i n Appendix 1 , The
.geostationary reference s a t e l 1 i t e shown on F i g u r e 1-2 must provide:
2.1
(1)
A space-to-earth down1 i n k broadcast o f the REFSAT reference
c a r r i e r , and t h e REFSAT data message (128 bps), relayed over
an earth-to-space up1 ink.
(2)
A user terminal-to-space up1 i n k f o r those users r e q u i r i n g a
report-back c a p a b i l i t y t o t h e REFSAT remote control s t a t i o n v i a
space-to-earth downlink,
Required Frequency Assignments
Before f i r m frequency assign~nents can be made, i t w i l l be necessary t o
study t h e p o s s i b l e mutual i n t e r f e r e n c e e f f e c t s between REFSAT and those users
p r e s e n t l y a l l o c a t e d i n the frequency bands r h a t m a y be considered. The f o l l o w i n g
d i s c u s s i o n serves the purpose o f demonstrating f e a s i b i l i t y and p o i n t i n g o u t
major requirements. The p r e c i s e frequencies t h a t a r e discussed a r e not n e c e s s a r i l y
those t h a t would be employed i n an o p e r a t i o n a l system.
REFSAT Space-towEarth Reference Signal
The frequency band f o r the REFSAT space-to-earth reference s i g n a l should
be chosen t o minimize t h e s i z e , weight, complexity, and c o s t of i n d i v i d u a l user
terminals over those a l r e a d y required t o r e c e i v e GPS s a t e l l i t e s i g n a l s a t
1575.420 MHz (a maximum dappl e r s h i f t o f +4 kHz may be expected).
-
I n o r d e r t o best meet the above c o n s i d e r a t i o n s , t h e REFSAT s i g n a l frequency
should he w i t h i n a few p e r c e n t of the GPS s a t e l l i t e s i g n a l frequency so t h a t
a common antenna and RF f r o n t - e n d may b~ used. For the purposes o f t h i s d e f i n i t i on study , REFSAT down 1 i nk reference c a r r i e r frequencies z t 1555 MHz
and 1560 MHz w i 1 1 be assumed t o cover t h e c o n t i n e n t a l U n i t e d States (CONUS)
and A l a s k a , r e s p e c t i v e l y .
User Rep, rt-Back,
Earth-to-Space
Frequency
Should a report-back f u n c t i o n be r e q u i r e d o f a p a r t i c u l a r user, a t r a n s m i t t e r
and s u i t a b l e antenna must be added t o a b a s i c user t e r m i n a l . The antenna should
have s u f f i c i e n t beamwidth t o a v o i d t h e need for antenna p o i n t i n g . Although a
number o f frequency hands would be s u i t a b l e , t h i s d e f i n i t i o n study assumes
t h a t t h e same user antenna would be used f o r b o t h t r a n s m i s s i o n and r e c e p t i o n ,
p l a c i n g t h e d e s i r e d e a r t h - t o - s p a c e t r a n s m i t frequency n e a r 1600 MHz.
2.2
S p a c e c r a f t Antenna Footprint Coverage Requirement
Using t h e REFSAT System, t h e c h o i c e o f a p a r t i c u l a r 4 - s a t e l l i t e GPS
c o n s t e l l a t i o n ( f o r minimum HDoP) i s made a t the remote c o n t r o l s t a t i o n . As a
r e s u l t , a l l GPS/REFSAT u s e r t e r m i n a l s i n a p a r t i c u l a r REFSAT coverage area
use the same 4-sate1 1 i t e GPS c o n s t e l l a t i o n . The maximum dimensions o f the
REFSAT antenna f o o t p r i n t a r e t h e r e f o r e l i m i t e d .
A l t h o u g h a d d i t i o n a l s t u d y i s r e q u i r e d t o d e f i n e t h e maximum f e a s i b l e
REFSAT coverage area, p r e l i m i n a r y i n v e s t i g a t i o n i n d i c a t e s t h a t g e o g r a p h i c a l
r e g i o n s a s l a r g e as the c o n t i n e n t a l U n i t e d S t a t e s (CONUS) a r e p r a c t i c a l .
Separate beams c o v e r i n g CONUS and Alaska w i l l be assumed f o r t h i s document.
(See s p e c i f i c a t i o n s i n Appendix 1 ) .
2.3
RF ].ink
Budget Ct.nputations
RF l i n k budget computations a r e g i v e n on T a b l e 9.1 o f t h e Appendix.
A nominal REFSAT e f f e c t i v e i s o t r o p i c r a d i a t e d power (EIRP) o f about 40 d e c i b e l s
above one-watt (dBW) and a u s e r t e r m i n a l r e p o r t - b a c k ElRP o f about 10 dBW would be
r e q u i red.
2.4
S p a c e c r a f t Antenna System
The REFSAT system p l a c e s no s p e c i a l d e s i g n c o n s t r a i n t s upon t h e s p a c e c r a f t
antenna system. A CONUS-coverage antenna a r r a y s t r u c t u r e i s assumed f o r
b o t h t r a n s m i s s i o n and r e c e p t i o n .
2.5
Transponder System
The REFSAT r e f e r e n c e s i g n a l imposes v e r y modest requirements on t h e spacec r a f t transponder i n t e r m s o f bandwidth (a few tens o f kHz) and o u t p u t power
( 1 2 ~ B W ) There a r e , however, s t r i ngent 1 1 mi t s upon t h e frequency accuracy
and s t a b i l i t y o f t h e REFSAT c a r r i e r as r e c e i v e d a t a u s e r t e r m i n a l .
.
I n o r d e r t o a v o i d t h e need t o p e r f o r m a frequency s e a r c h to acqui r e t h e
s e l e c t e d GPS s a t e l l i t e s i g n a l s , i t i s d e s i r a b l e t h a t t h e REFSAT c a r r i e r be
m a i n t a i n e d w i t h i n 50 Hz o f i t s nominal v a l u e ( 3 p a r t s i n l o 8 ) as a p r e c i s i o n frequency r e f e r e n c e t o a i d GPS s i g n a l a c q u i s i t i o n a t t h e u s e r
terminal.
The carrier frequency o f the REFSAT transponder, as seen by a user
t e r m i n a l , must remain constant to within 50 Hz by employing a very precise
nraster oscillator i n t h e spacecraft, Doppler s h i f t due to REFSAT satellite
motion must also be h e l d to limits n o t exceeding 50 Hz.
The transponder used f o r user report-back signals may take a number
o f forms depending upon whether FDMA o r TDMA techniques are employed.
Considering the brevity and low data rate (e.g. 128 bps) o f t h e reportback message t h a t would be r e q u i r e d o f any given user, a single TDMA
channel o f 10 kHz bandwidth should be adequate f o r CONUS coverage, A
detailed l i s t of specifications i s g i v e n in the Appendix.
3.
A l t e r n a t e Methods F o r Implementing a GPS/REFSAT System
T h i s s e c t i o n d e s c r i b e s two a l t e r n a t e methods f o r i m p l e m e n t i n g a
GPS/REFSAT system:
3.1
(1)
Use of a VHF 1 i n k ( r a t h e r than L-band) f o r t h e REFSAT
reference signal,
(2)
Usc o f a l o c a l t o w e r ( r a t h e r t h a n a g e o s t a t i o n a r y s a t e l l i t e )
t o b r o a d c a s t t h e REFSAT s i g n a l over a l i m f t e d g e o g r a p h i c a l
area,
Use o f a VHF L i n k f o r t h e REFSAT S i g n a l
The REFSAT concept p r e v i o u s 1
REFSAT s i g n a 1 frequency ( 1 555 MHz
(1575.42
i l l u s t r a t e d on F i g u r e 1-2 assumed a
be low t h e GPS down 1 i n k frequency
i n o r d e r t o m i n i m i z e GPS/REFSAT C i v i 1 User T e r m i n a l corn-
!
MHZ),
plexity.
The narrowband ( 1 0 kHz) REFSAT s i g n a l may, o f c o u r s e , be b r o a d c a s t
a t any d e s i r e d frequency p r o v i d e d t h a t t h e u s e r t e r m i n a l i s equipped w i t h
an antenna and r e c e i v e r channel t o t r a n s l a t e t h e r e c e i v e d REFSAT s i g n a l t o
the t e r m i n a l I F frequency (21.4 MHz).
A b l o c k diagram f o r a GPS/REFSAT c i v i 1 user t e r m i n a l accommodating
a VHF REFSAT b r o a d c a s t s i g n a l i s shown o n F i g u r e 3-1.
Except f o r t h e
s e p a r a t e V H F and L-band antenna and RF f r o n t - e n d components, t h e t e r m i n a l
b l o c k diagram f u n c t i o n s a r e i d e n t i c a l to t h o s e d e s c r i b e d f o r an L-band
REFSAT system d e s c r i b e d i n S e c t i o n 4 o f t h i s document.
S i n c e the REFSAT s i g n a l i s used as the u s e r t e r m i n a l p r e c i s i o n
frequency r e f e r e n c e , t h e f i r s t LO s i g n a l s ( t o M 1 and M 6 on F i g u r e 3-1)
must be s u f f i c i e n t l y s t a b l e i n f r e q u e n c y t o m a i n t a i n t h e p r e c i s e 50-Hz, f r e quency d i f f e r e n c e r e q u i r e d between t h e GPS and REFSAT c h a n n e l s . The recovered
r e f e r e n c e c a r r i e r from t h e R E F S A T s l g n a l i s c o r r e c t e d f o r GPS s a t e l l i t e d o p p l e r
( i n M4) and used wi t h t h e l o c a 1 l y - g e n e r a t e d PRN codes (M5) as t h e l o c a l
r e f e r e n c e for c o r r e l a t i o n w i t h r e c e i v e d GPS s a t e l l i t e s i g n a l s .
Demodulated d a t a f r o m t h e REFSAT s i g n a l p r o v i d e s t h e t e r m i n a l w i t h :
(1)
I d e n t i f i c a t i o n o f t h e 4 GPS s a t e l l i t e s w h i c h c u r r e n t l y p r o v i d e
minimum h o r i z o n t a l d i l u t i o n - o f - p c s c i s i o n
(HDOP),
(2)
Doppler c o e f f i c i e n t data (allows computation o f doppler
shifts for acquisition),
(3)
P o s i t i o n c o o r d i n a t e s f o r the
,
4 s e l e c t e d GPS s a t e l l i t e s .
An tcnnas
VHF
I
L-Band.
Correlator
PRH Code
AcqufsitIon
torrelation
Reference
Corrected
for Doppler
co
and
-
Trackf ng
Oopp l e r
Demod
I
REFSAT Channel
Doppler
H3
Correctlon
11
4b
Doppler Carff sAents
Satellite I d e n t l f l c a t l o n
b
--
Code
Seltctlon
Master
OscFllator
L ~ t c l l l t eP o r l t l o n
Cosd'inates
i
Posg t i o n
Cmputatlon
Hicroprocessor
-
I
Opt lonal Report
Back Xmtr
Figure 3-1.
liser Posltlon
Pseudo Ranges
GPSfREFSAT Terminal (VHF REFSAT Signal
ink),
-
CI,
ATS-3
Satellite
VHF
.'
,'
0
1575 Wz
(!;
Signal)
#
/
I
I
'0
0
.4
.'
Band
0
/
/
f
I
#'
/
/
.'
/
#
'
.-8and
VHF Band
v v
I
I
Pos l t Ion
-----
1
,,-,-------1
Figure 3-2.
GPS/REFSAT System Utilizing the ATS-3 Satellite.
An o l t e r n a t l v e Implementat i o n o f a GPS/REFSAT system i s shown i n
Figure 3-2 u s i n g the g e o s t a t i o n a r y ATS-3 spacecraft as the REFSAT t r a n s ponder
.
The REFSAT transponder broadcasts t h e REFSAT s i g n a l over the geog r a p h i c a l coverage area.
i n d i v i d u a l GPS/REFSAT user t e r m i n a l s may then
make use o f the REFSAT s i g n a l t o a c q u i r e the f o u r GPS s a t e l l i t e s i g n a l s
which c u r r e n t l y p r o v i d e t h e mlnimurn v a l u e o f h o r l z o n t a l - d i l u t i o n c o f p r e c i s i o n , and measure pseudo-range values.
Once t h e user termina l has measured pseudo-range values t o t h e
selected GPS s a t e l l i t e s , several o p t i o n s a r e a v a i l a b l e :
(1)
(2)
4
The user may make use o f the GPS s a t e l l i t e p o s i t i o n c o o r d i n a t e
d a t a contained i n t h e REFSAT s i g n a l t o compute t h e p o s i t i o n
of the user terminal, and,
(a)
Make use o f pos i t ion informat ion f o r n a v i g a t i o n , o r o t h e r
purposes o r ,
(b)
Report-back computed p o s i t Ion v i a a user report-back
t r a n s m i t t e r and t h e REFSAT transponder t o a Report-back
Receiving Terminal o r ,
The user may s i m p l y report-back measured pseudo-range, timing,
and user I D data. Computation o f user p o s i t i o n may then he
accomplished a t t h e Report-back Receiving Terminal, r e l i e v i n g the
user terminal o f t h i s t a s k .
3.2
Use o f a Local Tower f o r REFSAT Signal Breadcast
There a r e many a p p l i c a t i o n s where a s p e c i a l i z e d group o f users occupy
a l i m i t e d geographical area t h a t may be economically served by p l a c i n g the
REFSAT transponder on a l o c a l tower r a t h e r than on a g e o s t a t i o n a r y s a t e l l i t e .
F i g u r e 3-3 i l l u s t r a t e s t h e a p p l i c a t i o n o f the REFSAT concept t o maritime n a v i g a t i o n and c o n t r o l i n a congested waterway, A REFSAT transponder
placed on an 800 f t . high tower, f o r example, would p r o v i d e coverage over
a t l e a s t 40 m i l e r a d i u s . P r e c i s i o n t r a c k i n g and c o n t r o l would then be
a v a i l a b l e f o r a l l vessels e q u i p p e d w i t h a user terminal such as t h a t
shown p r e v i o u s l y on F i g u r e 3-1.
Local T o m r
Harltime Vess
'*
1
------.--.---
I
I
I
I
I
I
L-Band
I
I
L
I
I
Shlp PosltIon
Report-Back
Transml t t t r
I
I
I
I
I
I
1
YCssel Tracking 6 Control
Center
r'
I
I
I
1
I
Figure 3-3.
REFSAT R m t *
control s t a t i o n
Termlnal
1
I
1
1
I
Band
REFSAT
Terminal
Report-Back
Recc'ving
-t--
I--
VHF or UHF
I
I
-
EPS/REFSAT System Using a Local Tower.
L
GPS/REFSAT U s e r Terminal Segment
T h i s s e c t i o n describes t h e requirements o f t h e GPSi'REFSAT user t e r m i n a l
i s given i n
segment of t h e REFSAT system. A l i s t of s p e c i f i c a t l o r
S e c t l o n 8 i n t h e Appendix.
4.1
2-Channel Receiver Requirement2
Under t h e REFSAT concept d e s c r i b e d i n S e c t i o n 1, the u s e r t e r m i n a l r e c e i v e r
must process b o t h GPS s a t e l l l t e s l g n a l s (1575.420 MHz + a maxlmum 4 kHz d o p p l e r
s h i f t ) and the REFSAT r e f e r e n c e s i g n a l . For t h e most ~ o n o m i c a lr e c e i v e r
design, the REFSAT c a r r i e r should be w l t h i n a few p e r c e n t o f t h e GPS s i g n a l
frequency t o a1 low a common u s e r antenna and RF f r o n t end t o be used.
I n o r d e r t o s a t i s f y t h e r n a j o r i t y o f pro.j+!~.ted u s e r requirements (see
S e c t i o n 6 ) , a maximum time- t o - f i r s t - f i x o f about i00 seconds s h o u l d be a1 lowed
(REFSAT s i g n a l a c q u i s i t i o n + acquisition o f 4 s e l e c t e d GPS s a t e l l i t e s i g n a l s +
o u t p u t o f u s e r p o s i t i o n o r pseudo-ranse v a l u e s ) . A maximum p o s i t i o n update
time o f 10 seconds f o l l o w i n g f n l t i a l a c q u i s i t i o n w i l l meet the needs of most
p o t e n t la 1 users.
A survey o f p o t e n t i a l u s e r requirements i n d i c a t e s t h a t a h o r i z o n t a l
p o s i t i o ~ iaccuracy o f about 100 meters would meet a l l b u t the most s t r i n g e n t
user needs. The C/A code (1.023 ~ b p s ) i s capable o f meeting t h i s accuracy
requi rernent
.
4.2
Data T r a n s m i t t e r
Since a s i g n i f i c a n t p o r t i o n o f t h e p o t e n t i a l c l v i l u s e r market i d e n t i f i e d
i n S e c t i o n 6 c o n s i s t s o f " l o c a t i o n " o r " s ~ r v e i l l a n c e ' t~y p e users, a d a t a t r a n s m i t t e r f o r "report-back" of u s e r p o s i t i o n may be r e q u i r e d as an o p t i o n . The
report-back message may c o n s i s t o f user p o s i t i o n c o o r d i n a t e s computed a t t h e
user t e r m i n a l o r measured pseudo-range values t o a l l o w l a t e r computation o f
user p o s i t i o n a t a c e n t r a l l o c a t i o n .
S p e c i f i c a t i o n s f o r t h e r e p o r t - b a c k rnessage a r e g i v e n i n S e c t i o n 8.5 i n
t h e Appendix,
The c h o i c e o f t h e p a r t i c u l a r frequency band t o be employed f o r t h e user
r e p o r t - b a c k l i n k r e q u i r e s f u r t h e r study (see S e c t i o n 2 ) , b u t should be near t h e
GPS s i g n a l frequency t o a1 low use o f a s i n g l e antenna f o r user t e r m i n a l t r a n s m i s s i o n and r e c e p t i o n . A t r a n s m i t t e r power l e v e l i n the o r d e r o f 10 w a t t s
would be needed (see Appendix f o r l ink budget cornputat ions).
4.3
Simp1 i f ied Block Diagram, GPS/REFSAT User T e r m i n a l
Figure 4-1 i l l u s t r a t e s a s i m p l i f i e d GPS/REFSAT u s e r t e r m i n a l b l o c k
diagram capable of meeting t h e requirements discussed above. The GPS s a t -
e l l i t e and REFSAT reference s i g n a l s a r e sufficiently close i n frequency t o
Separate intermediateshare a common RF f r o n t - e n d and f i r s t mixer {MI).
frequency channels a r e then employed. Should a second frequency conversion
be d e s i r a b l e , the l o c a l o s c i l l a t o r frequencies a p p i i e d t o t h e two channels
must be i d e n t i c a l o r d e r i v e d from the same source i n o r d e r t o maintaln
t h e p r e c f se frequency d l f f e r e n c e between the REFSAT reference c a r r i e r
and the G P S sate1 l i t e c a r r i e r frequencies (nore M2 and M3).
F o l l o w i n g the REFSAT channel on the b l o c k diagram, the REFSAT c a r r i e r
i s recovered f o r l a t e r use i n t h e c o r r e l a t o r . The REFSAT data message
i s demodulated and a p p l i e d t o t h e micro-processor,
The micro-processor makes use o f t h e satellite l d e n t i f i c a t l o n p o r t i o n of
the REFSAT d a t a message t o s e l e c t t h e 4 PRN codes corresponding t o t h e 4 G P S
sate' 1 i t e s c u r r e n t l y v i s i b l e w i t h minimum h o r i z o n t a l d i l u t l o n - o f - p r e c i s i o n (HDoP).
The micro-processor makes use o f the doppler c o e f f i c i e n t p o r t i o n of the
REFSAT data message along w i t h estimated user p o s i t i o n ( w i t h i n 150 Lm) t o compute
doppler c o r r e c t {on. The dopp t e r c o r r e c t ion, when compared t o the recovered
REFSAT r e f e r e n c e c a r r i e r , fnscres a c o r r e c t e d c o r r e l a t l o n reference t h a t i s
w i t h i n 50 t o 100 Hz of the GPS channel s i g n a l . GPS s i g n a l a c q u i s i t i o n may
then be r a p i d l y accomplished w i t h o u t the need t o perform a frequency search
and w i t h o u t employing a p r e c i s i o n o s c i l l a t o r w i t h i n the user t e r m i n a l . A
long-term frequency stab! 1 i t y o f 1 p a r t 106 i s adequate instead o f 1 p a r t l o a .
The d o p p l e r - c o r r e c t i o n s i g n a l from the micro-processor i s a p p l i e d t o a
low-f requency v o l tage-control l e d o s c i 1 l a t o r ( V C O )
The r e s u l t i n g doppler
c o r r e c t i o n i s added t o t h e recovered REFSAT c a r r i e r {mixer ~ 4 ) . The c o r r e c t e d
c o r r e l a t i o n reference i s then modulat2d w i t h t h e a p p r o p r i a t e PRN code (mixer M5)
and a p p l i e d t o t h e c o r r e l a t o r . The micro-processor monitors the c o r r e l a t o r
output w h i l e performing a code delay search u n t i l G P S s i g n a l a c q u i s i t i o n has
been ach i eved
.
.
A f t e r s i g n a l a c q u i s i t i o n (4 GPS sate1 1 i t e s ) , the micro-processor a d j u s t s
both the doppler c o r r e c t i o n and PRN code delay (sequential l y f o r 4 G P S sate1 1 i t e s )
t o minimize c o r r e l a t i o n e r r o r s . The r e l a t i v e code phases when computed u s i n g the
speed c f l i g h t , c o n s t i t u t e the r e q u i r e d pseudo-range measured values,
Various options a r e a v a i l a b l e f o r the user t e r m i n a l design:
(1)
.Pseudo-range v a l ues may be transmi t t e d v i a the report-back
t r a n s m i t t e r , e l i m i n a t i n g the need for the p o s i t i o n computation
f u n c t f o n i n the micro-processor o r ,
(2)
Computed user pos i t i o n l a t i tude/long it u d e coordinates may be
t r a n s m i t t e d v i a t h e report-back t r a n s m i t t e r o r ,
(3)
User p o s i t i o n coordinates may be d i s p l a y e d o r otherwise used d i r e c t l y
by t h e user t e r m i n a l when a report-back t r a n s m i t t e r i s n o t required.
D e t a i l e d s p e c i f i c a t i o n s a r e given i n S e c t i o n 6 . 0 i n the Appendix.
Lwa
A
,
,Preselect
F l l ter
Hi
.
--
I
HZ
GPs Channel
ns
PRH Code
Correlatar
-
Code
Acqulsltlon
and
Correlation
Reference
Corrected
for Doppler
c1
I&,
REFSAT Channel
Tracking
~-
Doppl e r
"3
Doppler
Correction
Doppler CoefFlcIcnts
1st
LO
I
Satellite I d e n t I f l c a t l o n
2nd LO
S a t e t f I t c Pa~ltlon
Coord I na to
t
Code
5elecZlm
-
Poll tton
Cmpu t a t ton
-
Hicroprocesror
User Position
4
/
Optional Report
Back X m t r
Ffgure 4-1.
or
Pseudo Ranges
Simp?if!ed Block DIagrarn, GPS/REFSAT Hobfle Terminal.
-L
CI
5.
GPS/REFSAT Remote Control S t a t i o n
The GPS/REFSAT remote c o n t r o l s t a t i o n ( R c S ) as p a r t o f the REFSAT system i s
i l l u s t r a t e d t n Figure 1-2.
The remote c o n t r o l s t a t i o n performs t h r e e primary
functions:
( 1
Generat i o n and transmission o f the REFSAT reference c a r r i e r and
REFSAT data message f o r r e l a y broadcast by a g e o s t a t i o n a r y reference
satellite,
(2)
Generation and transmission o f a p o l l i n g message t o c o n t r o l those
user terminals having a report-back c a p a b i l i t y and
(3)
Recept i o n and process i n g o f report-back messages,
Figure 5-1
5.1
i l l u s t r a t e s these f u n c t i o n s .
Generation of the R E F S A T Data Message
The REFSAT d a t a message provldes I n f o r m a t i o n t o a l l users t o a i d i n
r a p i d a c q u i s i t i o n of the 4 GPS satellites v i s i b l e w i t h minimum h o r i z o n t a l
d i l u t i o n - o f - p r e c i s i o n (HDoP).
I n o r d e r t o accomplish t h i s f u n c t i o n , the
remote c o n t r o l s t a t i o n must have access t o the weekly navigation message
upload data f o r the e n t i r e c o n s t e l l a t i o n o f GPS s a t e l l i t e s .
The remote c o n t r o l s t a t i o n s e l e c t s those 4 GPS sate1 1 i t e s which a r e
v i s i b l e and p r o v i d e the minimum HOOP over t h e REFSAT user area. The s e l e c t i o n
should be updated a t 15-minute i n t e r v a l s .
I n a d d i t i o n , t h e remote c o n t r o l s t a t i o n computes doppler c o e f f i c i e n t s
f o r the four selected GPS s a t e l l i t e s t o allow computation o f GPS s i g n a l
doppler s h i f t s by the i n d i v i d u a l user t e r m i n a l s . This p o r t i o n o f the
REFSAT data message should be updated a t 2-minute i n t e r v a l s .
For n a v i g a t i o n , and o t h e r asers wishing t o r e p o r t t h e i r p o s i t i o n s ,
t h e remote c o n t r o l s t a t i o n computes p o s i t i o n coordinates f o r t h e four
s e l e c t e d GPS s a t e l l i t e s f o r i n c l u s i o n i n the REFSAT data message. A
4-second update i n t e r v a l i s a v a i l a b l e and provided i n the R E F S A T data
message (see F i g u r e 5-1).
5.2
REFSAT Reference C a r r i e r and Message Timing
The REFSAT reference c a r r i e r provides each user t e r m i n a l w i t h a p r e c i s i o n
frequency reference, a l l o w i n g r a p i d GPS s a t e l l i t e s i g n a l a c q u i s i t i o n w i t h o u t
the n e c e s s i t y t o perform a frequency search. The reference c a r r i e r must be
maintained w i t h i n about 50 Hz o f I t s nominal value as r e c e i v e d a t a user
t e r m i na 1 .
GPS Navlgat Ion Message
Upload fran SAnSO
Select 4 CPS
Satcllftcs for
Compute Doppler
Hin. HDOP
Satellites
for
4 Selected
Compute Posit Ion
TO
Coordinates For
Selected Satel l ites
(4-5ec.
REFSAT Relay
Satel 11 te
Update)
b.
REFSAT Reference
Carrier (* 50 Hz
a t User Terminal)
i
d
REFSAT
Transmi
Earth SttIng
tation
Reference Clock
(GPS Time f 200 us)
Figure
5-1.
Simp1 if ied Funct Ional Block Diagram, REFSAT Remote Control S t a t i o n .
In o r d e r t o a l l o w user t e r m i n a l s t o p r o p e r l y a s s o c i a t e measured pseudorange values w i t h the corresponding G P S s a t e l l i t e p o s i t i o n c o o r d i n a t e s , i t i s
necessary t h a t t h e REFSAT d a t a message be synchronized w i t h GPS system t l m e
t o ari accuracy o f about 200 microseconds,
6.
C i v i 1-User Requirements
An e x a m i n a t i o n o f c i v i l - u s e r requirements i s i m p o r t a n t i n t h e GPS/
REFSAT d e f i n i t i o n phase t o i d e n t i f y market p o t e n t i a l s w h i c h i n t u r n
determine:
a
Geographical areas t o be covered
a
Pos i t ion accuracy r e q u i rements
a
Acceptable u n i t c o s t s f o r m o b i l e t e r m i n a l s
P o s i t i o n update r a t e requirements
The m a j o r f i n d i n g s o f a user requirements s t u d y performed f o r
NASA a r e summarized on T a b l e 6-15:,
"Estimated User Volume1' (number o f
u s e r s ) and "Estimated Do1 l a r Volume" were computed f r o m t h e s t u d y d a t a ,
w e i g h t i n g each by t h e t a b u l a t e d "Estimated P r o b a b i l i t y o f Use".
Two d i s t i n c t u s e r f u n c t i o n s a r e i d e n t i f i e d :
(I) M a r i t i m e N a v i g a t i o n F u n c t i o n :
P e r m i t s a u s e r to determine i t s
own p o s i t i o n a t i n t e r v a l s t o compute p r e s e n t p o s i t i o n , p r e s e n t
eled. User t e r m i n a l makes d i r e c t use
heading, and d i s t a n c e t r ~
o f measured p o s i t i o n data.
(2)
*
L o c a t i o n Report-back F u n c t i o n : A l l o w s c u r r e n t p o s i t i o n o f
a remote u s e r t o be determined. The remote user t e r m i n a l
t r a n s m i t s measured p o s i t i o n d a t a t o some c e n t r a l l o c a t i o n ,
making no d i r e c t use o f t h e d a t a i t s e l f .
" D e f i n i t i o n Study o f Land/Sea C i v i l User N a v i g a t i o n a l L o c a t i o n M o n i t o r i n g
Sys terns F o r NAVSTAR GPS", Task 1, User Requi rements, September 1978,
Prepared by Magnavox F o r NAsA/GSFC C o n t r a c t NAS5-23425.
USER
R
CI
IC1
IR
I f
Clt 1 11
MI12
a IV
V
R VI
CI
B
B
fa
IB
I3
IC
R
B
ID!
ID2
B
IE
B
1141
B 1102
B IIP
BI IA
BIIIBl
BItIB2
RIflC
B IVA1
P 1V02
B 1Vn3
F IVFI4
B IVR5
B 1
x
1
B IVCZ
B
V
8 VI
BVI I
NDTES*
USCO VESSELS-LLIU ENFORCEMENT
11550 SCIR-PEOPLE
US VERCHnNT FLEET
M TOMINO VESSELS
US VESSELS k FORGES
L G PaSSERGER SHIPS
US RECREc'.VQN0L BOnTS
US Don. FZWIKS VESSELS
US mPf. F I S H I N G VESSELS
FOREIGN F I S H I N O V E S E L S
O I L TONKERS IFOREION REGISTRY)
LG TOWING VESSELS
US PFIROES
OENEROL CCIRDD VESSELS
US TRUCKS-REOILnR
LIS TRUCKS-SPECfhL
RR CIIRS-SENSITIVE CFIWO
MTEOR. PUOYS
HETUO PCILLL?(?HS
PFILLODNS-R~UT080NE
OTHER DCITR COLL. PLCITFfMHS
PCtLICE CFlRS
EUERO. VEHICLES
PLlPtIC TRANSPCKIT
U T I L I T Y VEHICLES
P M K POiICE
FQRESTERY SEWICE
STClTE POLICE CnRS
GEOLOr)Y
GEODESY
ConnrncInL TWCKINCI (LWSCMRIEO)LU
SLt
5U
SU
SU
BU
C;U
Sib
SU
SW
SU
SU
W
SU
SU
LU
LU
LCI
su
PU
E?J
LU
LU
LU
LU
LU
Llr
LV
LU
LU
LU
+ I MECZ/EEC. V n O C I T Y KASCREHEMT REQUIRED.
ESTIHATED M L L R R VfiLWE
(ESTltlFITEn II O F TERtllN;ILS)*(ESTfll~TED FWOB. @FllS€)*LICCEPTflPLE COST PER 7-INCIL)
ESTtPInTED USER VOLUME = (EBTIMFITED I OF TERUIH&LS)*(ESTIHnTED PROF. O F 'tSEl
USER CODES
13 = HdRITIHE N F I V l t O t f @ N FLWCTION
8 = LOC'clT1ON REWRT-PQCK FUNCTION
RREQ CODES
SU
SER. US ONLY
Ul = LAND. US ONLY
SU = SEQ. WORLD-UIDE USE
L U t LClND. WRLD-WIDE USE
m-1 a LAND k SEFI. US ONLY
PW = LFIND k SEA* UOliL@-WIDE USE
Table 6-1.
Survey o f User Requirements.
Append i x
Performance S p e c i f i c a t i o n
For
REFSAT/NAVSTAR C i v i l User System
J u l y 1980
National A e r o n a u t i c s a n d Space A d m i n i s t r a t i o n
Goddard Space F l i g h t Center
G reenbe 1 t
,
Mary land
2077 1
TABLE OF CONTENTS
PAGE
-
SECT l ON
6.0
Remote C o n t r o l S t a t i o n C h a r a c t e r i s t i c s - - - - - - - - - - - - - - - - - - - - - -
A-11
7.0
REFSAT S a t e l l i t e Characteristics-----------------------------
A- 15
7.1
A- 15
A-I6
7.2
7.3
7.4
7.5
8.0
Typ i ca 1 Ope r a t iona 1 Requi rernents----------------------Trans~~nder-------------------------------------------Antenna Characteristics-------------------------------Locat i o n Moni t o r REFSAT Transponder-------------------P h y s i c a l Characteristics-------------------------------
A- 18
A- 19
A-21
REFSAT M o b i l e Terminal Characteristics----------------------
A-23
8.1
General-------c----------------------------------------
8.2
User Area coverage------------------------------------Performance Requirements------------------------------Design Parameters-------------------------------------REFSAT Mobi 1 o T e r m i na 1 Transmi t t e r (MTT)
F u n c t i o n a l and P h y s i c a l C h a r a c t e r i s t i c s - - - - - - - - - - - - - - - -
A-23
A-24
A-24
A-25
A-26
A-29
8.3
8.4
8.5
8.6
---------------
LlST OF FIGURES
PAGE
-
F l GURE
4-1.
REFSAT/NAVSTAR System Elements-----------------------------
A-6
6-1.
RCS s i g n a l ~~rmat-------------~----------~----------------
A-13
7-1.
REFSAT S a t e l l i t e Antenna C h a r a c t e r i s t i c s - - - - - - - - - - - - - - - - - - -
A-20
LlST OF TABLES
PAGE
-
TABLE
5-1.
RMT Perfornlance Speci fications-----------------------------
A- 10
5-2.
S a t e l l i t e RF S i g n a l C h a r a c t e r i s t i c s - - - - - - - - - - - - - - - - - - - - - - - -
A- 10
9-1.
L i n k Budgets i n t h e REFSAT/NAVSTAR System-------------------
A- 32
Performance Speci f l c a t ion
for
REFSAT/NAVSTAR CIvil User System
1 * 0 scope
T h i s s p e c i f i c a t i o n d e f i n e s t h e performance requirements f o r a
REFSAT/NAVSTAR GPS C i v i l User System i n c l u d i n g a geos t a t i o n a r y reference
satel 1 i t e (REFSAT),
Termi na 1 s (RMT)
.
a Remote Control Station ( R C S ) and REFSAT Mobi l e
2.0
Applicable Documents
a)
" D e f i n i t i o n Study o f Land/cea C i v i l User N a v i g a t i o n L o c a t i o n
Mon i t o r i ng Sy s terns f o r NAVSTAR/GPSIL p r e p a r e d ' b y Magnavox Government and l n d u s t r i a l E l e c t r o n i c s Company, September 1978.
b)
"The REFSAT Approach t o Low-Cos t GPS Termi na 1 s i t NASA/GSFC Techn l ca 1
Memorandum No, TM
c)
79655 October 1978,
"System Segment Specif l c a t i o n f o r t h e U s e r System Segment o f t h e
N A V S T A R Global P o s i t i o n i n g System" S S - U S - 1 0 1 B
30 Sep
74, USAF
Space and M i s s i l e System O r g a n i z a t i o n .
d)
"Space V e h i c l e N a v i g a t i o n Subsystem and NTS PRN N a v i g a t i o n Assembly/
User S y s t e m Segment and M o n i t o r S t a t i o n " ,
Rockwell I n t e r n a t i o n a l
C o r p , I n t e r f a c e C o n t r o l Document No. ~H08-00002-400, 13 Apr
e)
Ii
7'.
REFSAT Low-Cos t GPS T e r m i n a l Hardware Design Reporti1, p r e p a r e d
by Systematics General C o r p o r a t i o n , N a t i o n a l S c i e n t i f i c L a b o r a t o r i e s ,
f o r NASA/Goddard Space F l i g h t Center, Greenbel t , Mary land, May 1980.
3.0
Background
NAVSTAR i s a major, space-based n a v l g a t i o n system being impleniented t c s a t [ sfy the m i
l l t a r y requI rements of the Department of Defense.
The c h a r a c t e r i s t i c s
and capabilities o f NAVSTAR a r e such t h a t i t can p r o v i d e f o r a wide v a r i e t y
of c i v i l user needs f o r n a v i g a t i o n and p o s i t i o n l o c a t i o n .
The Dspartment o f Defense (DOD)
c u r r e n t l y plans t o make the n a v i g a t i o n s i g -
nals from the NAVSTAR System a v a i l a b l e t o c l v i l users.
Inherent i n the NAVSTAR
design i s t h e a b i l i t y t o p r o v i d e a spectrum o f l o c a t i o n accuracies (from 10
t o 100 meters) depending on t h e complexity of the user r e c e i v i n g t e r m i n a l s .
DOD i s c u r r e n t l y c o n s i d e r i n g a r e d u c t i o n i n the C/A code accuracy, when
NilVSTAR GPS becomes o p e r a t i o n a l i n 1986 o r 1987, t o p r o v i d e a CEP ( c i r c u l a r
probable e r r o r ) o f 200 meters (50% o f the time) w i t h an upper accuracy 1 i m i t
o f 500 meters.
This would s t i l l meet many c i v i l user needs (see Table 6-1).
M i l i t a r y r e c e i v e r t e r m i n a l s , due t o t h e i r sophistication, a r e q u i t e
costly.
To p r o v i d e a n a v i g a t i o n system w i t h moderate performance t o a broad
base o f c i v i l users,
lower c o s t ,
i t i s proposed t o use the m i l i t a r y NAVSTAR system w i t h
l e s s s o p h i s t i c a t e d r e c e i v e r s and s t i l l o b t a i n n a v i g a t i o n f e r -
forrnance komparable t o Loran C b u t b e t t e r than Omega.
This i s t o be accomp-
p l i s h e d by p r o v i d i n g n a v i g a t i o n - a i d i n g s i g n a l s from a reference s a t e l l i t e
(REFSAT)
i n g e o s t a t i o n a r y o r b i t f o r CONUS, Alaska, and m a r i t i m e reception.
This o b v i a t e s the n e c e s s i t y f o r t h e ground r e c e i v e r s t o c o n t a i n a p r e c i s i o n
o s c i l l a t o r and t o perform c e r t a i n computations and thus p e r m i t s a less
complex, lower cost mobile t e r m i n a l design.
T h i s document provldes the system d e f i n i t i o n and performance r e q u i r e ments f o r the REFSAT/NAVSTAR S y s tern a s we1 1 as subsys tern requf rements
for the Remote Control S t a t l o n , REFSAT S a t e l l i t e , and Mobile Terminal.
4.0
System Definition
The REFSAT/NAVSTAR System Is a radionavigat Ion aid designed f o r civi 1
and maritime users.
REFSAT provides a simplified method for deterrnlnation o f
horizon ta 1 geograph i ca 1 cocrd i na t e s ( X - Y pos t t l on) by augmenting the s igna l
structure of an existing military radio navigation system, NAVSTAR/GPS.
Figure
-NAVSTAR
r
4-1
depicts the major elements of the REFSAT/NAVSTAR System.
S y s t e m Elements
NAVSTAR Satellites:
A constellation of radionavigation satellites ( 1 8 for
Phase I I i ) , part of the military global positioning system
Master Control Station:
(GPS).
The central control f a c i l l t y which uploads
navigation-aiding data and timing information to t h e Individual NAVSTAR
satellites.
Monitor Station:
Monitors the individual NAVSTAR satellites and provides
updated ephemeris and other d a t a to the Master Control Station.
REFSAT System Elements
-.
0
Remote Control Statlon:
The earth transmitting station originates
the navigation-aTding REFSAT signal.
Ephemeris and other data i s accepted
from the NAVSTAR Master Control Station.
The Remote Control Station
selects those f o u r NAVSTAR satellites which currently provide maximum
position fix accuracy to mobile terminals within the REFSAT coverage area
and computes navigation-aiding data for broadcast v i a the REFSAT satellite.
Figure 4-1
.
REFSAT/NAVSTAR System E l m e n t s .
REFSAT Satelitte:
A geostattonary relay satellite that accepts upllnk slgnals
from the Remote Control Station and broadcasts those signals over broad
g e o ~ r a p h l c a l areas.
The REFSAT satellite may Include a locatlon monitor
trz:isponder for selectrd users to "report-back" thei r geographical posit ion.
REFSAT Mobile Terminal:
a
Individual user which makes use of t h e navigation-
aiding signal broadcast from the REFSAT to select, acqulre, and process
four
NAVSTAR satellite signals to determine the posltion
terminal.
of the user
Certain user terminals may Include a "report back" function,
wherein the user's position i s transmitted to a centrol location monitor
station via t h e REFSAT satellite.
Location Monitor Station:
Receiving earth stations accepting user
"report-back" s i g n a l s re1 ayed by locat ion moni tor transponder contained
in the REFSAT satellite.
4.1
Principle of Operation
User terminal position coordinates, ljsing the NAVSTAR/GPS are determined
by 4-dimens ional
(3 pos i t ion coordinates and time) "triangulation".
Pseudo-
ranges to 4 selected NAVSTAR satellites a r e measured at the user terminal.
T h i s data is then combined with known (computed from ephemeris data)
NAVSTAR
satellite locations.
The navigation-aiding s i g n a l broadcast by the REFSAT consists of:
a
A precision reference carrier,
Identification o f the f o u r NAVSTAR satellites which currently provide
t h e most favorable "triangulation" geometry,
a
Doppler c o e f f i c i e n t d a t a , a l l o w i n g a user t e r r n l n a l t o compute
the d o p p l e r s h i f t o f each s e l e c t e d NAVSTAR s a t e l l i t e s i g n a l . T h i s
i n f o r m a t i o n , i n c o n j u n c t i o n wi tli t h e p r e c i s i o n r e f e r e n c e c a r r i e r ,
a l l o w s t h e u s e r t e r m i n a l t o b e g i n NAVSTAR s1gnal acquisition "on
frequency",
a
P o s i t i o n c o o r d i n a t e s f o r the f o u r s e l e c t e d NAVSTAR s a t e l l i t e s ,
r e l i e v i n g t h e u s e r t e r m i n a l o f t h e t a s k o f demodulating t h e NAVSTAK
d a t a message and computation o f s a t e l l i t e p o s i t i o n f r o m ephemeris
data c o n t a i n e d i n t h a t message,
A user t e r m i n a l , w i t h che a i d o f t h e REFSA; s i g n a l , may t h e n r a p i d l y
a c q u i r e t h e f o u r s e l e c t e d NAVSTAR s a t e l l i t e s i g n a l s by p e r f o r m i n g a code d e l a y
search, matchi ng t h e t i m i n g o f l o c a l l y generated pseudo-random-no1 se
w i t h the a p p r o p r i a t e NAVSTAR s i g n a l s .
(PRN)
codes
Pseudo-ranges t o the s e l e c t e d s a t e l l i t e s
a r e then determined from t h e r e l a t i v e t i m i n g o f l o c a l PRN codes.
System Performance Requirements-
The performance s p e c i f i c a t i o n s f o r t h e REFSAT/NAVSTAR System a r e shown
i n Table 5-1.
The n a v i g a t i o n s i g n a l s processed by t h e REFSAT M o b i l e Terminal
r e c e i v e d f r o m REFSAT and NAVSTAR s a t e l l i t e s s h a l l have t h e c h a r a c t e r i s t i c s
shown I n T a b l e 5-2.
Under t h e s p e c i f i c a t i o n s o f T a b l e s 5-1 and 5-2,
REFSATINAVSTAR system s h a I l comply w i t h t h e f o l l o w i n g perform;nce
the
requirements:
TTFF i s d e f i n e d as t h e amount o f t i m e r e q u i r e d t o produce a s i n g l e
n a v i g a t i o n f i x f r o m t h e s t a r t of t h e acquisition mode.
With a p r o b a b i l i t y
of success o f 0.95, eacrt REFSAT Mobi l e Terminal s h a l l a c h i e v e a f i r s t f i x
w i t h i n 100 seconds.
TTSF i s the t i m e necessary t o r e a c q u i r e the n a v i g a t i o n s i g n a l s and
execute a n a v i g a t i o n s o l u t i o n .
W i t h a p r o b a b i l i t y o f 0.95, TTSF s h a l l
be l e s s t h a n 10 seconds.
5 . 3 Pos i t ion Accuracy
The 1 0 h o r i z o n t a l p o s i t i o n e r r o r s h a l l n o t exceed 100 meters.
The Department o f Defense (DOD)
i s c u r r e n t l y considering a reduction i n the
C/A code accuracy, when NAVSTAR GPS becomes o p e r a t i o n a l i n 1986 o r 1987, t o p r o v i d e a CEP ( c i r c u l a r p r o b a b l e e r r o r ) o f 200 meters w i t h an upper l i m i t o f 500
meters.
I n any event, t h e C/A code accuracy i s s u f f i c i e n t t o meet t h e m a j o r i t y
o f c i v i l user a p p l i c a t i o n s .
TABLE 5-1
RMT PERFORMANCE SPECIFICATIONS
c/No
a t RMT r e c e l v e r I n p u t
C/N
a t RMT r e c e i v e r i n p u t (GPs
0
(REFSAT 1 Ink)
43 dB-Hz, m i n imum
38 dB-Hz,minimum
1 inks)
User Velocity
35
I n t e r f e r e n c e t o Signal R a t i o (GPS l i n k s )
25 dB,
maximum
150 km,
maximum
User P o s i t i o n Estimate U n c e r t a i n t y f o r
m/sec, maximum
Acqui s i t i o n
TABLE 5-2
SATELLITE RF SIGNAL CHARACTERISTICS
NAVSTAR
Center Frequency
om)
1575.42 MHz
Bandwidth
20 MHz
1555
1560 MHz
fAl aska) (CONUS)
10 kHz, each signal
Modulation
QPSK
FSK (-10 dB reference c a r r i e r )
PRN C/A Code
1023 b ' r Gold Code
C/A Code Rate
1023 kbps
Carrier Frequency S t a b i l i t y
( 1 p a r t i n 10
In-Band Spurious Emissions
< -40 dB below
unmodulated
c a r r i e r level
8
< 2
-
p a r t s i n 108
< -40 dB below
unmodulated
carrier level
6,O
Remote C o n t r o l S t a t i o n C h a r a c t e r i s t i c s
The f u n c t i o n o f t h e REFSAT Remote C o n t r o l S t a t i o n (RCS) i s t o :
a c c e p t NAVSTAR s a t e l l i t e ephemeris, almanac, and o t h e r d a t a f r o m
t h e GPS Master C o n t r o l S t a t i o n ,
e
s e l e c t t h e f o u r NAVSTAR s a t e l l i t e s w h i c h c u r r e n t l y p r o v i d e the most
f a v o r a b l e " t r i a n g u l a t i o n " geometry f o r u s e r s i n t h e
REFSAT coverage
area ,
a
compute dappler c o e f f i c i e n t d a t a f o r each s e l e c t e d NAVSTAR s a t e l l i t e ,
compute c u r r e n t p o s i t i o n coordinates f o r each of t h e s e l e c t e d
NAVSTAR s a t e l l i t e s ,
t r a n s m i t t h i s d a t a along w i t h a p r e c i s i o n r e f e r e n c e c a r r i e r s i g n a l
t o t h e REFSAT s a t e l l i t e f o r broadcast o v e r t h e REFSAT s e r v i c e
a r e a (C-Band u p l i n k ) .
6.1
T r a n s m i t t e d Data
The message data t r a n s m i t t e d t o REFSAT by t h e R C S s h a l l i n c l u d e :
a ) i d e n t i f i c a t i o n f o r t h e f o u r s e l e c t e d NAVSTAR s a t e l 1 i t e s
b) NAVSTAR s a t e l 1 i t e d o p p l e r s h i f t c o e f f i c i e n t s
c) NAVSTAR sate1 1 ite pos i t i o n c o o r d i n a t e s
d ) t i m i n g and s y n c h r o n i z a t i o n as r e q u i red.
The e f f e c t i v e i s o t r o p i c r a d i a t e d power
s t a t i o n s h a l l be 39 dBW, minimum.
(EIRP) o f
t h e remote c o n t r o l
6.3
RF Siqnal C h a r a c t e r i s t i c s
1
Uplink Frequencies
D u a l up1 i n k frequencies i n C band (6 Gtlz n o m i n a l ) , separated by
5 MHz,
s h a l l be transmi t t e d by t h e RCS (separate s Tgnal s f a r Alaska and CONUS
coverage)
.
6,3.2 RF Signal Bandwidth
Each u p l i n k RF s i g n a l s h a l l have an RF bandwidth o f 10 kHz, minimum.
6.3.3
Frequency S t a b i l i t y
The RCS s h a l l m a i n t a i n r e f e r e n c e frequency s t a b l e w i t h i n 2 p a r t s i n
8
10
r e l a t i v e t o t h e NAVSTAR s a t e l l i t e signals, f o r z e r o doppler.
6.3.4
Spurious Emissions
In-band spurious e m i s s i o n s s h a l l be less than -40 dB r e f e r r e d t o t h e
unmodulated c a r r i e r l e v e l .
6.3.5
Signal Structure
Each u p l i n k s i g n a l s h a l l c o n s i s t o f a r e f e r e n c e c a r r i e r and associated
FSK modulated (128 bps d i g i t a l d a t a stream) s i g n a l .
The power contained i n
t h e FSK s i g n a l s h a l l be a p p r o x i m a t e l y - T O dB r e l a t i v e t o the r e f e r e n c e c a r r i e r .
The f o r m a t o f the RCS u p l i n k data s i g n a l i s shown on F i g u r e 6-1.
frame c o n s i s t s o f
4
One d a t a
subframes each o f 1 second d u r a t i o n f o r each o f the f o u r
s e l e c t e d NAVSTAR satellites.
Each subframe s h a l l contain sync and t i m i n g data,
NAVSTAR s a t e l 1 i t e p o s i t i o n c o o r d i n a t e s
(x,
Y, 2 e a r t h - c e n t e r e d )
, doppler
I--
ONE DATA
FRAME
Sat. B
DATA
Sat. A
t
DATA
0
1
I
Sat.
Sat. A
DATA
Sat. D
DATA
C
DATA
3
2
I
4
I
SYNC
0
I
I
1
3
6
YA
I
9
-
DOPPLER
GRADlENT
A Z
i r
I
1
I
12
15
f
L
15
BYTES
fRs
EPOCH
Figure 6-1.
Sat. B
DATA
5
I
1 Second
I
I
I
(4 seconds)
1
I
-r
\
RCS Signal Format.
t SECONDS
6
gradient d a t a , and NAVSTAR satellite code identification.
Doppler gradlent
d a t a shall allow initial estimates of the doppler shift assaciatsd with each
GPS satellfte signal to an accuracy of 2
100 Hz p r i o r to acqulsltion.
Timing of the data signal as broadcast from the REFSAT shall be rnaintained synchronous with GPS system time within
-+ 200 microseconds.
System modulation shall be non-return-to-zero frequency-shift keyed
at 128 bps.
Doppler g r a d i e n t d a t a s h a l l be updated at intervals not to
exceed 2 minutes.
m a j o r frame,
Satellite position coordinates whall be updated each
7.0
REFSAT Satellite Characteristics
7.1
Typical Operational Requi rernents
7.1.1
Orbit,
REFSAT shall be placed
in geostationary orbit at a particular West
Longitude location for optimum CONUS and Alaska coverage.
Typical Geostationary L o c a t i o n
REFSAT shall be centered
over the equator at 130 degrees
West Longi tude
-
within + 0.5 degrees.
Drift
7.1.3
Correction
Orbital position location shall be maintained by control t h r u s t e r s .
7.1.4
Attitude Control
Three a x i s attitude control shall be maintained to a n accuracy of
-+ 0.1
degrees per axis.
Primary Power
j,1.5
Sun tracking extensible solar arrays shall provide primary spacecraft
power.
These arrays shall be designed to fit folded within a Delta launch
vehicle shroud.
Initial overdesign shall be 259 to compensate for deter-
ioration over the satellite l i f e t i m e .
10 square meters.
The estimated area of solar cells i s
7 16
Power Storage
Power storage shall be accomplished by dual n i c k e l cadmium batteries.
These batteries shall be rechargeable singly through the solar arrays.
7.1,7
Solar Eclipse
During solar eclipses
(88 days per year, 65 minutes maximum duration)
REFSAT shall be capable of operating only on battery power.
7,1.8
Temperature Control
+
Electronics bay temperature shall be maintained to 20°C -
15OC,
7.1.9 Reliability
REFSAT shall be designed to meet a 5-year rnissi~n requirement with a
probability of not less than 0.995.
7.2
Transponder
The REFSAT transponder shall receive upltnk C band signals from the RCS
containing user navigation coding information.
Two channels shall be provided
by the transponder, ane f o r the signal intended for users located i n t h e
contiguous
48 states
located i n A l a s k a .
( C O N U S ) , 2nd the other for the signal intended f o r users
The transponder shal l down convert these signals to L-band,
provide amplification and transmit t h e converted signals on the CONUS and
Alaska antenna beams, respectively.
The f r e q u e n c i e s o f t h e CONUS and Alaska s i g n a l s r e c e i v e d by REFSAT
f r.om RCS sha 1 1 be nomi na l 1y a t 6 GHz and separated by 5 MHz,
7.2.2
Output Frequencies
T r a n s m i t t e d f r e q u e n c i e s s h a l l be nominal l y 1555 and 1560 MHz, each w i t h
a bandwidth o f
10
kHz, minimum.
7.2.3 Spurious Emissions
Spurious emissions s h a l l be a t l e a s t 40 dB below t h e unmodulated c a r r i e r
1 eve1
7.2.4
.
Frequency Convsrsion
The transponder s h a l l c o n v e r t t h e dual C band c o n s t a n t a m p l i t u d e s i g n a l s
8
t o dual L band s i g n a l s w i t h a l o c a l o s c i l l a t o r s t a b i l i t y of 2 p a r t s i n 10
r e l a t i v e t o t h e NAVSTAR s a t e l l i t e s i g n a l c a r r i e r frequency.
D u r i n c frequency
c o n v e r s i o n , baseband data s h a l l be preserved.
7.2.5
C a r r i e r Phase Noise
The phase n o i s e s p e c t r a l d e n s i t y o f an unmodulated c a r r i e r r e l a y e d by
REFSAT s h a l l be such t h a t a phase locked l o o p o f 10
Hz
one-sided n o i s e band-
w i d t h s h a l l be a b l e t o t r a c k t h e c a r r i e r t o an accuracy o f 0.1 r a d i a n s RMS.
7.2.6
Power I n p u t t o Antenna
The t r a n s m i t t e r power i n p u t t o t h e antenna s h a l l be 10 Watts average
f o r each o f t h e two channels.
No power b a c k o f f s h a l l be u t i l i z e d .
RF Power S t a b i l i t y
7.2.7
RF power s h a l l be s t a b l e w i t h i n
7.2.8
-+ 0.5
dB per y e a r .
N o i s e Temperature
Transponder r e c e i v e r system noise temperature s h a l l be 1000 K, o r l e s s .
7.2.9
Phase L i n e a r i t y Over O p e r a t i n g Bandwidth
D e p a r t u r e from l i n e a r ' p h a s e versus frequency s h a l l be l e s s rhan
-+ 0.32
r a d i a n over the o p e r a t i n g bandwidth.
Redundant Design
Redundant t r ~ n s p o n d e r ss h a l l be u t i l i z e d I n REFSAT.
malfunction,
i n case o f
transponders s h a l l be s w i t c h a b l e by ground command.
Antenna C h a r a c t e r i s t i c s
The REFSAT s a t e l l i t e antennas s h a l l c o n s i s t o f an u p l i n k r e c e i v e
antenna and downlink a r e a coverage t r a n s m i t antennas.
T e r m i n a l ' s "report-back"
I f t h e REFSAT M o b i l e
t r a n s m i t t e r i s ~ ~ t i l i z e dthe
, d o w n l i n k and u p l i n k
s a t e l l i t e antennas s h a l l a l s o a c t as r e c e i v e and t r a n s m i t antennas, respecti v e l y , f o r t h e REFSAT M o b i l e Terminal l o c a t i o n m o n i t o r s i g n a l s .
U p l i n k REFSAT Antenna
The RCS u p l i n k r e c e i v e antenna s h a l l have t h e f o l l o w i n g characteristics:
O p e r a t i n g Frequency
Form
Gain
Beam p a t t e r n
Polarization
Bores l g h t p o i n t
C band
TBD
23 dB;, m i n i m u m
Elliptical
R i g h t hand c i r c u l a r l y p o l a r i z e d
TBD
7.3.2 Down1 i n k Area Coverage REFSkT S a t e l 1 i t e Antennas
The downlink area coverage t r a n s m i t t i n g antennas s h a l l have t h e f o l l o w i n g
characteristics:
Opera t I ng Frequency
Fo rrn
Gain
Polarization
Beam p a t t e r n s
L band
Planar array
3 1 . 0 d B I , minimum f o r CONUS
and A l a s k a beams.
R i g h t hand c i r c u l a r l y p o l a r i z e d
Elliptical
The beam s i z e s and o r i e n t a t i o n s , b o r e s i g h t p o i n t s , and f o o t p r i n t s a r e
shown i n F i g u r e
7.4
7- 1 .
Locat i o n Monl t o r REFSAT Transponder
The REFSAT l o c a t i o n m o n i t o r t r a n s p o n d e r i n t h e s a t e l l i t e which r e l a y s
l o c a t i o n i n f o r m a t i o n t o t h e L o c a t i o n M o n i t o r S t a t i o n s s h a l l have t h e following characteristics,
3 d~ [ L o i f power)
F m t p r l n t Contours
1 )
REFSAT
0 Deg.; W 130 Deg. Long.
BORESIGHT
65 Deg. Lat. ; W 170 Deg. Long.
Major A x l s Rotated 20 Ceg. CCW from East
(2 )
3
ANTENNA
7 Deg. M a j o r A x i s
3 Deg. Minor A x i s
BORES l GHT
37 Deg. Lat. ; W 98 Deg. Long.
Major A x i s Rotated I7 DLJ. CW from E a s t
ANTENtjA
9Deg.MajorAxis
3 Der Minor A x i s
Figure
.
7-1.
3
dB points
3depoints
REFSAT Ante,,r.ra Characteristics
7.4.1
Power Output
The power o u t p u t o f t h e l o c a t l o n m o n i t o r power a m p l i f i e r s h a l l be
10 w a t t s (dt3W), minimum,
7.4.2
Limt t e r
Power o u t p u t s h a l l be h a r d l i m i t e d such t h a t s y s t e m n o i s e a l o n e w 1 l l p r o v i d e
f u l l power o u t p u t .
7.4.3
l n p u t and Output Frequencies
I n p u t and o u t p u t f r e q u e n c i e s s h a l l be i n t h e L and C bands, r e s p e c t i v e l y .
7.4.4
Bandwidth
The Location M o n i t o r channel bandwidth s h a l l be 1C kHz, minimum.
The L o c a t i o n M o n i t o r transponder shall u t i l i z e t h e REFSAT CONUS and
A l a s k a L band antennas f o r r e c e p t i o n and t h e C band antenna f o r t r a n s m i s s i o n .
7.5
7.5.1
Physical Characteristics
Transponder
The REFSAT franspander s h a l l have the f o l l o w i n g c h a r a c t e r i s t i c s :
Size
Weight
Design o p e r a t i n g t e m p e r a t u r e
l e s s t h a n 0.018m 3 volume
not more t h a n 16 kg
20°C + 1S0C
-
7.5.2
Downlink Antenna
The d o w n l i n k antenna s h a l l have t h e follawing c h a r a c t e r i s t i c s :
Size
Weight
7.5.3
U n f o l ded
n o t t o exceed 2.4m L x 2 . 4 m W x 0.15m D
Fo 1 ded
t o f i t w i t h i n t h e Delta launch v e h i c l e
shroud. Dimension n o t t o exceed
2 . 4 m I x 1.2m W
n o t t o exceed 6 kg
Solar Arrays
Dual s o l a r t r a c k i n g a r r a y s s h a l l p r o v i d e t h e p r i m a r y s p a c e c r a f t pDwer
(except d u r i n g s o l a r e c l i p s e . )
t h e D e l t a launch v e h i c l e shroud.
The a r r a y s s h a l l be i n t e g r a t e d t o f i t w i t h i n
2
Estimated s a i l area Is torn
.
8.0
REFSAT Mobi le T e r m i n a l C h a r a c t e r i s t i c s
8.1
General
T h i s s e c t i o n d e f i n e s s p e c i f i c a t i o n s f o r t h e d e s i g n o f a low-cost
r e c e i v e r t e r m i n a l f o r c i v i l users o f t h e GPS system.
tasks involved i n performing a n a v i g a t i o n " f i x "
A number o f t h e
a r e common t o a l l u s e r s a t
a g i v e n i n s t a n t , w h i l e o t h e r s a r e unique ( l o c a t i o n dependent) t o each
user,
C e n t r a l t o t h e d e s i g n i s t h e concept o f p e r f o r m i n g common t a s k s
a t a c e n t r a l l o c a t i o n and r e l a y i n g t h e r e s u l t s i n t h e form o f a c q u i s i t i o n
a i d i n g and r e f e r e n c e s i g n a l s v i s a g e o s t a t i o n a r y r e f e r e n c e s a t e l l i t e
(REFSAT) t o each user t e r m i n a l .
By t h i s method, the REFSAT system
reduces t h e c o s t and c o m p l e x i t y of c i v i l u s e r r e c e i v e r s .
Reference i n f o r m a t i o n r e l a y e d by t h e REFSAT i n c l u d e s GPS s a t e l l i t e
s e l e c t i o n f o r optimum " t r i a n g u l a t i o n "
geometry, a p r i o r i d o p p l e r i n f o r m a t i o n
and precomputed GPS s a t e l l i t e p o s i t i o n c o o r d i n a t e d a t a .
The REFSAT M o b i l e
Terminal (RMT) s h a l l be designed t o r e c e i v e these mu1 t i p l e n a v i g a t i o n s i g n a I s
and d a t a and, by s o l u t i o n o f t h e a p p r o p r i a t e n a v i g a t i o n e q u a t i o n s , p e r m i t
users t o determine t h e i r l a t i t u d e and l o n g i t u d e ( x , Y )
p o s i t i o n s w i t h i n the
REFSAT s e r v i c e coverage a r e a o n t h e e a r t h .
An o p t i o n a l add-on package t o t h e RMT r e c s 3 v e r s h a l l c o n s i s t of a
L o c a t i o n M o n i t o r T r a n s m i t t e r (LMT) t o t r a n s m i t user l o c a t i o n t o a t e r r e s t r i a l r e c e i v e r v i a REFSAT.
8.2
User Area Coverage
The baseline system operating areas for the civil user terminals
are t h e contiguous U . S . s t a t e s (CONUS) and A l a s k a , i n c l u d i n g the 200-naut ical
m i t e territorial
Figure
(e,g.
Typical antenna footprint patterns are shown in
limits.
7-1. Users in Alaska and
CONUS will operate on different frequencies
1555 and 1560 M H ~ )to receive navigation signals o p t i m i z e d for the; r area.
Performance Requirements
The REFSAT Mobi le Terrni nal receivers shal l be capable o f the fol lowing
performance while operating under the conditions g i v e n in Tables
8.3.1
5-1 and
5-2.
Time-to-First F i x
TTFF i s defined a s the amount o f time required to
produce a single
navigation fix from the start of the acquisition n~ode. With a probability
o f success o f 0.95, each M o b i l e Terminal shall achieve
a first f i x within
100 seconds.
TTSF i s t h e time necessary to reaquire t h e navigation signals and execute
a navigation solution.
W i t h a probability o f 0.95, TTSF shall be
less than 10 seconds.
8.3.3
Horizontal Position Accuracy
The
10 error in calculated X-Y user horizontal position shall not exceed
100meters.
(see Section 5.3).
8.3.4
D a t a Recovery Performance
The
REFSAT M o b i l e Terminal r e c e i v e r s s h a l l r e c o v e r the d a t a from the REFSAT
6
-
s i g n a l w i t h a B i t E r r o r R a t e (BER) n o t g r e a t e r t h a n 1 x 10
8.4
,
Design Parameters
Parameters used i n t h e design o f a user REFSAT M o b i l e Terminal r e c e i v e r a r e
g i v e n i n the f o l l o w i n g paragraphs.
8.4.1
Received Power Levels
Received power l e v e l s f o r the C/A code ( L ) s i g n a l , a t elevation
1
a n g l e s above f i v e degrees, s h a l l be -160 dBW, minimum, f r o m any NAVSTAR
GPS s a t e l 1 i t e and
-147 dBW, minimum, f r o m REFSAT a t t h e o u t p u t t e r m i n a l s
o f a 0 d B i g a i n , r i g h t hand c i r c u l a r l y p o l a r i z e d antenna.
8.4.2
Noise Temperature
The REFSAT Mobi l e Terminal sys tern n o i s e temperature ( i n c l u d i n g antenna)
s h a l l be 500 K o r l e s s .
RF S i g n a l S t r u c t u r e
Received RF n a v i g a t i o n s i g n a l c h a r a c t e r i s t i c s from NAVSTAR and REFSAT
s a t e l l i t e s a r e l i s t e d i n Table 5-2.
d e f i n e d i n Section 6.3.
RF s i g n a l c h a r a c t e r i s t i c s have been
The RMT does n o t need t o demodulate t h e
n a v i g a t i o n d a t a c o n t a i n e d i n t h e NAVSTAR s i g n a l s .
GPS
8.4.4
REFSAT M o b i l e T e r m i n a l R e c e i v e r Antenna
The REFSAT M o b i l e T e r m i n a l r e c e i v e r antenna s h a l l have t h e f o l l o w i n g
characteristics:
Hemispherical
Ca i n
-
Polarization
-
R i g h t Hand C i r c u l a r
Frequency coverage
Pattern
8.5
L band
3 dB a t z e n i t h , 0 dBi
at
5'
e l e v a t i o n angle
REFSAT Mobi:e Terminal T r a n s m i t t e r (MTT)
The MTT t r a n s m i t t e r s h a l l t r a n s m i t digltal d a t a p o s i t i o n c o o r d i n a t e s
i n c l u d i n g an i d e n t i f i c a t i o n code t o Lhe REFSAT t r a n s p o n d e r f o r r e l a y t o
a ground m o n i t o r s t a t i o n .
8.5.1
Power Output
The MTT t r a n s m i t t e r power o u t p u t s h a l l be a t l e a s t 10 Watts (10 ~ B W ) ,
average.
8.5.2
T r a n s m i t Antenna
The MTT s h a l l u t i l i z e the REFSAT M o b i l e Terminal r e c e i v e r antenna.
t r a n m i t antenna g a i n s h a l l exceed 0 d 0 i f o r a
The
5" e l e v a t i o n a n g l e o r g r e a t e r .
Transmission Frequency
C a r r i e r s i g n a l frequency s h a l l be 1600 MHz, nominal.
-
815.4
Baseband C h a r a c t e r i s t i c s
The baseband m o d u l a t i o n s i g n a l s h a l l be a b i n a r y , n o n - r e t u r n - t o - z e r o
waveform.
8.5.4.1
Modulation
M o d u l a t i o n s h a l l be c o h e r e n t b i n a r y p h a s e - s h i f t keyed a t a r a t e n o t
exceeding 0.5 k i l o b i t s p e r second (kbps).
T h i s i s e q u i v a l e n t t o an RF
bandwidth o f 2.0 kHz.
8.5.4.2
Bandwidth
D i g i t a l d a t a s h a l l be t r a n s m i t t e d i n a bandwidth o f 10 kHz.
8.5.4.3
Data Content
T r a n s m i t t e d d a t a s h a l l i n c l u d e user i d e n t i f i c a t i o n ( I D )and l a t i t u d e
and l o n g i t u d e p o s i t i o n c o o r d i n a t e s , p l u s preamble.
8.5.4.4
Data Access
The t r a n s m i s s i o n o f p o s i t i o n d a t a t o l o c a t i o n m o n i t o r s v i a REFSAT
s h a l l bo by t i r n e - d i v i s i o n m u l t i p l e access w i t h p ~ e a s s i g n e dt r a n s m i s s i o n
time slots.
The t i m e s l o t s s h a l l be r e f e r e n c e d t o system t i m e as c o r r e c t e d
by user p o s i t i o n ,
D u r i n g t h e preassigned u s e r time slot, t h e t r a n s m i t t e r
s h a l l send a 25 m i 1 1 isecond (ms) b u r s t o f i n f o r m a t i o n a t 4.lcbps.
Each
message s h a l l c o n s i s t o f a preample t o g e n e r a t e c a r r i e r reference and
e s t a b l i s h s y n c h r o n i z a t i o n , a user i d e n t i f i c a t i o n word, and l a t i t u d e and
longitude position.
A-27
Estimated number of bits
15
P reamb l e
-
50 blts
StatlonID,
-
17blts
Latitude
Long i tude
Total
-
as
follows:
15 b i t s
15 b i t s
97 b i t s
Electromagnetic Compatibility
Out-of-band
f i e l d s t r e n g t h shall be l e s s than 23 d B uv/m/kHz when
measured a t a d i s t a n c e of 10 meters from the transmitter/receiver.
8.6
Functional and Physical Characteristics
User REFSAT Mobile Terminals shall simultaneously receive the n a v l -
g a t i o n signals transmitted by the NAVSTAR satellites and the REFSAT trans-
ponder, providing the user with a two-dimensional horizontal position-fix
capability.
8.6l
Equipment Operation
User receivers shall perform the follor~ing functions:
a)
a.
Acquire the navigation-aiding signal generated by REFSAT,
b.
Demodulate the REFSAT data message Including:
1)
2)
3)
8.G.2
NAVSTAR satellite identification
doppler coefficient data
NAVSTAR satel l i te posit ion coordinates
c,
Utilizing b 2) above in conjunction with the precisii.~r,REFSAT
carrier, compensate tuning of the receiver's GPS e h a n ~ e l fcr
doppler s h i f t associated with each of the 4 selected hAVSTAR
signals. NAVSTAR signal acquisition may then be accomplished
w i thnut a f r e q u e n c y search.
d,
Utilizing b 1 ) above, generate the 4 PRN codes needed to acquire
the selected NAVSTAR signals.
e.
Perform the requi red code delay search to acqui re the
selected NAVSTAR signals,
f.
Track the 4 selected NAVSTAR signals ( i n frequency and code delay)
to allow measurement of pseudo-ranges.
g.
Use measured pseudo-range values, and NAVSTAR position coordinates
(from b 3 1 1 , ro compute the position o f the user terminal.
h,
D i s p l a y the user position (optional),
4
Required Ski 11s
No s p e c i a l training or skills s h a l l be required to operate the user
REFSAT Mobile Terminals.
memories.
The microprocessor s h a l l
o p e r a t e from preprogrammed
8.6.3
Physical ,Spectfications-
The f o l l o w i n g a r e des lgn goals f o r t h e REFSAT Mabl l e Terrni n a l r e c e i v e r s :
a.
Weight
2.5 k g , maximum ( e x c l u d i ng antenna)
b,
Volume
5000 cm',
c.
Power r e q u i r e d
8.6.4
maximum (exclus 1 ve antenna)
40 Watts, maximum
Envi ronmenta l Performance
The user REFSAT Mobi le Terminals ( r e c e i v e r and transmi t t e r ) s h a l l o p e r a t e
o v e r the f o l l o w i n g ambient environment c o n d i t i o n s :
8.6.5
-
a.
Temperature range
b,
Humidity
c.
Precipitation
d.
Vibration
e.
Shock
f.
Operating a l t i t u d e
g.
I c i n g an Antenna
-
h,
Wind
-
-
-2QDC t o
+55*~
10% t o 100% R.H.
t o 16mm/hr.
15 g ' s random
300 g ' s maximum
Sea l e v e l t o 5000 meters
up t o 20 mm t h i c k n e s s
up t o 100 k n o t s , w i t h i c i n g , on antenna.
Re1 i a b i 1 i ty
The Mean-Time-Between-Fa i l u r e
(MTBF) f o r t h e
s h a l l not be l e s s than 10,000 h o u r s .
user REFSAT Mobi le Terminal
9.0
REFSA7/NAVSTAR Link Budaets
L i n k budget surnrnarles f o r l i n k s i n t h e NAVSTAR/REFSAT system a r e shown
in Table 9-1.
The l i n k s i n c l u d e the R C S t o REFSAT u p l i n k a t
REFSAT t o RMT down 1 1 nks a t 1555 and 1560 MHz, the GPS t o
1575 MHz, t h e REZT to REFSAT u p l i n k a t 1600 MHz, and
Monitor Station downlivk a t
4 GHz.
6
GHz, t h e
RMT downl l nk
at
t h e REFSAT t o L o c a t i o n
The l i n k c a l c u l a t i o n s r e f l e c t t h e d e s i g n
values assumed i n t h i s s p e c i f i c a t i o n f o r a worst-case RMT location.
Table 9.1
LINK BUDGETS I N THE REFSAT/NAVSTAR SYSTEM
REFSAT
Polarization
Loss, dB
Boltzmann's
Constant
Data-to-reference c a r r i e r
ratio
Symbol
Rate
Eb/No,
data, bps
code, Mbps
dB
data
code
Reference Signal
RCS/REFSAT
6 GHz
REFSAT/RMT
1555 and 1560 MHz
GPS/RMT
1575 t4Hz
up 1 ink
down l
down
ink
l Ink
RHT/REFSAT
1600 MHz
4 GHz
up link
dawn 1 ink
REFSAT/RCS
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