Teletrac BTLTRC GSM / GPRS Vehicle Tracking Unit User Manual Unit Five

Teletrac Inc GSM / GPRS Vehicle Tracking Unit Unit Five

Users Manual 3

Teletrac, Inc. - Prism TM2 Information and Installation Guide --------------Unit Two-------------- THE GPS AND GPRS SYSTEMS There are two systems that the Prism TM2 uses outside of the Teletrac system. One is the Global Positioning System, more commonly called GPS and the other is the General Packet Radio System otherwise known as GPRS. The following pages will give you a basic introduction to where these systems came from as well as how they work. At the end of this section will be a summary of how Teletrac uses these two systems together to get the location of a vehicle. GLOBAL POSITIONING SYSTEMS (GPS) The following information was taken from information posted to the Teletrac Intranet. Included here is the abridged version of GPS. The full text appears in Appendix A at the end of this Information Guide. AN INTRODUCTION TO GLOBAL POSITIONING SATELLITE SYSTEMS Global Positioning Systems GPS uses "man-made stars" or satellites as reference points to calculate positions on Earth accurate to within meters. In fact, with advanced forms of GPS you can make measurements to better than a centimeter. In a sense, it's like giving every square meter on the planet a unique address. Since GPS receivers have been miniaturized to just a few integrated circuits and have become very economical, the technology has become increasingly accessible. Here's how GPS works in five logical steps: Here is a summary of each of the steps involved with GPS in order to determine a location. This is the first part of Teletrac finding the locations of vehicles using a Prism TM2. Once a location is determined then it is sent via another system. We'll explain each of the following points in the next five sections. 1. The basis of GPS is "triangulation" from satellites. 2. To "triangulate," a GPS receiver measures distance using the travel time of radio signals. 3. To measure travel time GPS needs very accurate timing, which it achieves with some tricks.
Teletrac, Inc. - Prism TM2 Information and Installation Guide 4. Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret. 5. Finally you must correct for any delays the signal experiences as it travels through the aTM2osphere. Step 1: Triangulating from Satellites Improbable as it may seem, the whole idea behind GPS is to use satellites in space as reference points for locations here on earth. That's right, by very, very accurately measuring our distance from three satellites we can "triangulate" our position anywhere on earth. Step 2: Measuring Distance from a Satellite But how can you measure the distance to something that's floating around in space? We do it by timing h arrive at our receiver. a s se the le thing boils down to those "velocity times travel time" math problems we did es 60 miles per hour for two hours, how far does it f GPS we're measuring a radio signal so the velocity is going to be the speed of ng the travel time of a radio signal is the key to GPS, then our stop watches had better because if their timing is off by just a thousandth of a second, at the speed of light, n locate a point in 3-dimensional space, then four ow long it takes for a signal sent from the satellite to THE BIG IDEA, MATHEMATICALLY In en , whoin high school. Remember the old: "If a car gotravel.?" Velocity (60 mph) x Time (2 hours) = Distance (120 miles) In the case olight, or roughly 186,000 miles per second. Step 3: Getting Perfect Timing If measuribe darn goodthat translates into almost 200 miles of error! The secret to perfect timing is to make an extra satellite measurement. That's right, if three perfect measurements caimperfect measurements can do the same thing. 1622-0300 B2-5 05 2006 2
Teletrac, Inc. - Prism TM2 Information and Installation Guide EXTRA MEASUREMENT CURES TIMING OFFSET If everything were perfect (i.e. if our receiver's clocks were perfect) then all of our satellite ranges uld intersect at a sing ur position). But with imperfect clocks, a fourth ct with the first three. ceiver looks for a to all the rest of its measurements, and now we've got wing Where a Satellite is in Space all GPS receivers have an almanac programmed into their computers that tells the sky each satellite is, moment by moment. tes are constantly onitored by the DeparTM2ent of Defense. They use very precise radar to ch s exact altitude, position and speed. wo le point (which is omeasurement, done as a cross-check, will NOT interseSo the receiver's computer says "Uh-oh! There is a discrepancy in my measurements. I must not be perfectly synced with universal time." Since any offset from universal time will affect all of our measurements, the resingle correction factor that it can subtract from all its timing measurements that would cause them all to intersect at a single point. That correction brings the receiver's clock back into sync with universal time, and bingo! - you've got atomic accuracy time right in the palm of your hand. Once it has that correction it appliesprecise positioning. Step 4: Kno On the groundthem where in CONSTANT MONITORING ADDS PRECISION The basic orbits are quite exact but just to make things perfect, the GPS satellim eck each satellite'1622-0300 B2-5 05 2006 3
Teletrac, Inc. - Prism TM2 Information and Installation Guide GETTING THE MESSAGE OUT Once the DoD has measured a satellite's exact position, they relay that information back up to the satellite itself. The satellite then includes this new corrected position information in the timing signals it's broadcasting. Step 5: Correcting Errors ROUGH TRIP THROUGH THE ATM2OSPHERE First, one of the basic assumptions we've been using throughout this tutorial is not exactly true. We've been saying that you calculate distance to a satellite by multiplying a signal's travel time by the speed of light. But the speed of light is only constant in a vacuum. As a GPS signal passes through the charged particles of the ionosphere and then through the water vapor in the troposphere it gets slowed down a bit, and this creates the same kind of error as bad clocks. 1622-0300 B2-5 05 2006 4
Teletrac, Inc. - Prism TM2 Information and Installation Guide ROUGH TRIP ON THE GROUND Trouble for the GPS signal doesn't end when it gets down to the ground. The signal may bounce off various local obstructions before it gets to our receiver. This is called multipath error and is similar to the ghosting you might see on a TV. Good receivers use sophisticated signal rejection techniques to minimize this problem. PROBLEMS AT THE SATELLITE The atomic clocks they use are very, very precise but they're not perfect. Minute discrepancies can occur, and these translate into travel time measurement errors. SOME ANGLES ARE BETTER THAN OTHERS There are usually more satellites available than a receiver needs to fix a position, so the receiver picks a few and ignores the rest. If it picks satellites that are close together in the sky the intersecting circles that define a position will cross at very shallow angles. That increases the gray area, or error margin, around a position. Commonly refered to as HDOP. If it picks satellites that are widely separated, the circles intersect at almost right angles and that minimizes the error region. 1622-0300 B2-5 05 2006 5
Teletrac, Inc. - Prism TM2 Information and Installation Guide Intentional Errors! As hard , the same government that spent $12 billion to develop the most accurate he world can cause errors by intentionally degrading its accuracy. htly erroneous orbital e a decryption key to remove the SA errors and so they're much more accurate. as it may be to believe navigation system in tThe policy is called "Selective Availability" or "SA" and the idea behind it is to make sure that no hostile force or terrorist group can use GPS to make accurate weapons. Basically the DoD introduces some "noise" into the satellite's clock data which, in turn, adds noise (or inaccuracy) into position calculations. The DoD may also be sending sligdata to the satellites which they transmit back to receivers on the ground as part of a status message. Together these factors make SA the biggest single source of inaccuracy in the system. Military receivers usNote: As of Spring 2000, the DoD eliminated the intentional error in the calculation, however, this may come back at any time. The Bottom Line Fortuna uracies still don't add up to much of an error, and a form of GPS called "Differential GPS" can significantly reduce these problems. PRS nd system used by the Prism TM2 is the General Packet Radio System, more called GPRS. This system is meant to be an invisible link from a mobile unit, such as M2ltely, all of these inacc GThe secocommonlya wireless modem, to land line systems. The next few pages will give you an introduction to GPRS and how it works to transmit information. The following information was taken from information posted to http://www.rysavy.com/Articles/GPRS2/gprs2.hT and http://www.geocities.com/mobile4g/gprs.hTM2l. AN INTRODUCTION TO GENERAL PACKET RADIO SERVICE What is GPRS? GPRS o d connections to data networks via mobile technology. It is designed to allow ternet access with continuous connectivity, and enables applications How does GPRS work? GPRS t bile devices and packet networks. Packets can be IP or X.25, though with the Internet's popularity, operators and device vendors will probably emphasize P ffers packet-switche faster and easier Inincluding multimedia messaging, wireless corporate intranet, remote control and maintenance of appliances. It is also considered part of the migration to third generation (3G) mobile networks. The advantages of GPRS technology allows users to stay connected to the Internet by using packet switching technology, providing faster downloads as no time is spent attempting to access a dial-up connection. ransports packets between moIP. Mobile devices will have an IP address, either static or dynamic, and, once on the network, Ipackets can originate from mobile devices and travel to external networks, such as the Internet or privately connected intranets. IP packets from external networks will reach mobile devices, even 1622-0300 B2-5 05 2006 6
Teletrac, Inc. - Prism TM2 Information and Installation Guide when moving. GPRS doesn't care what protocols operate above IP. This indifference enables all standard Internet protocols to operate, including TCP, UDP, HTTP, Secure Sockets Layer (SSL), and IPSec. GPRS uses two essential new infrastructure elements, the Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). The SGSN, which connects to base-station ay to s o users actually connect to the network and send data, and how does the network keep track of users as they move around? When users turn on s requests using a packet random-access channel. Channels are logical data paths consisting of predefined time slots in select GPRS radio s w base station's coverage range. If the user travels out of one SGSN's coverage to another, then the old SGSN ill HE OVERALL TELETRAC PICTURE GPS and GPRS, let’s talk about how Teletrac uses these d messaging services to our customers. In Unit One we ent controllers, tracks the mobile station's location and sends data packets to and from the mobile station. It forwards packets using a tunneling protocol to the GGSN, which acts as a gatewexternal networks, such as the Internet or private intranets. An operator will have multiple SGSNfor different service areas, but needs only one GGSN for each external network it interconnects with. The GGSN assigns IP addresses to mobile stations, and IP packets from external networks route to the GGSN, which tunnels them to the appropriate SGSN for delivery to the mobile station. Architecture and protocols are fine, but how dthe GPRS device (GPRS PC Card modem) in a GPRS coverage area, the device first registers with the network and then requests a Packet Data Protocol (PDP) context. The PDP context activates an IP address for the device, generally a dynamic address assigned by the GGSN. At this stage the device can send and receive data. To actually send a packet of data, the device makechannels, and are the primary mechanism in the MAC layer. The network responds by assigning a data-traffic channel for a temporary period sufficient to send the data packet. GPRS networks use 200KHz radio channels, with each channel divided into eight time slots. Each time slot can support 13Kbits/sec of throughput in today's networks (though options exist to increase data rateto over 20Kbits/sec), and so actual user throughput will depend on the number of time slots a user's device can handle and the particular service options from the carrier. To support mobility, the GPRS device informs the SGSN when it's within a neand the new SGSN must collaborate and inform the GGSN of the user's new location. Users walso be able to roam into networks operated by other GPRS carriers. TNow that you have an understanding of systems in order to provide location antalked about the components that make up the Prism TM2, now let’s talk about how those components work together. As shown in the diagram, a computer running eCliconnects to the Teletrac NCC via the Internet. The NCC is where all the customer databases are stored and where customer location requests are processed. From there, the NCC contacts the customer’s vehicles via the Internet. The vehicles that use GPS SatelliteGPRS SiteeClienteClientCDPD SiteRadio TowerTeletrac NCCInternet Internet1622-0300 B2-5 05 2006 7 The three types of location systems Teletrac usesVLUplus or VLU-GRF VLU
Teletrac, Inc. - Prism TM2 Information and Installation Guide GPS to determine their location send that information directly to the NCC and it is in turn sent back to the eClient workstation. The GPS receiver built into the Prism TM2 works to determine the location of itself. As long as the receiver is able to see enough satellites it can tell the Prism TM2 where it is. If a vehicle drives into an underground garage, inside a warehouse or even under an overpass, the receiver may not be able to see enough, if any, satellites to determine it’s location. Since the signals coming from the satellites to the receiver are very low they can easily be blocked, even dense cloud cover can reduce the actual signal. The GPS receiver will determine its location every few seconds and store the information. When the Prism TM2 Controller is contacted through the GPRS modem, the Controller contacts the GPS receiver and a request is made for its location at a certain time. Once the Controller receives the location information from the GPS receiver, it relays the locate to the Teletrac NCC via the GPRS modem. Even if the GPRS modem cannot be contacted by the NCC, the GPS receiver is still collecting the information on where it is located. When the GPRS modem is able to communicate with the NCC, the Prism TM2 will download the location information that the GPS receiver has been providing. GPRS Communication to NCCSatelliteGPRS ModemControl BoardGPS ReceiverLocation UnitNow, lets say your driver is taking a lunch break under the awning of a drive-up restaurant. In this location the GPS receiver probably cannot see enough satellites to determine it’s location. In this event, when the controller requests a locate from the GPS receiver, the last known location will be used. Since the receiver takes it’s own readings every few seconds the last known location is probably just outside the restaurant awning. When it’s time to send in a locate to the NCC, the Prism TM2 can still “pick up” the GPRS modem and contact the NCC. But, the only location that will be returned is the last known location reported to the Prism TM2 Controller, which was probably just outside the awning. This location will be reported as a poor quality locate and display as the last known location. Even though the GPS receiver is blocked, a dispatcher can still send messages to a driver. Since the messages travel over the GPRS system they will be sent to the Prism TM2 and simply a poor locate (last known location) will be returned to the dispatcher. Lastly, the Prism TM2 can be set up to store events such as ignition on/off, messages and location information when the GPRS modem is out of it’s coverage area. The events, messages and locations can be stored in a memory buffer and later transmitted once the modem is able to communicate. See the following chart to help explain what happens when each system is able to operate or is blocked. 1622-0300 B2-5 05 2006 8

Navigation menu