Global positioning system theory and applications; volume II

Global Positioning System: Theory and Applications Volume U Edited by Bradford W Parkinson Stanford University, Stanford, California James J Spilker Jr Stanford Telecom, Sunnyvale, California Associated Editors: Penina Axelrad University of Colorado, Boulder, Colorado Per Enge Stanford University, Stanford, California Volume 164 PROGRESS IN ASTRONAUTICS AND AERONAUTICS Paul Zarchan, Editor-in-Chief Charles Stark Draper Laboratory, Inc Cambridge, Massachusetts Published by the American Institute of Aeronautics and Astronautics, Inc 370 L'Enfant Promenade, SW, Washington, DC 20024-2518 Progress in Astronautics and Aeronautics Editor-in-Chief Paul Zarchan Charles Stark Draper Laboratory, Inc Editorial Board John J Bertin U.S Air Force Academy Leroy S Fletcher Texas A&M University Richard G Bradley Lockheed Martin Fort Worth Company Allen E Fuhs Carmel, CD.lifornia William Brandon MITRE Corporation Ira D Jacobsen Embry-Riddle Aeronautical University Clarence B Cohen Redondo Beach, California John L Junkins Texas A&M University Luigi De Luca Politechnico di Milano, Italy Pradip M Sagdeo University of Michigan Martin Summerfield Lawrenceville, New Jersey Dedication To Anna Marie, Elaine, Virginia, and Tim 612 D G ABBY datastream that modulo-two multiplies the codes In particular, the ephemeris data must be different for a GT vs an SV RAJPO contracted Stanford Telecommunications, Inc to develop GTs primarily for use with SDIO test programs Ground transmitters are presently used in the Pacific to supplement the SV constellation Their signals can be processed by the RAJPO receivers and the RAJPO TPS-the ground-based portion of the RAJPO GPS translator system Figure is an artist's rendition of a RAJPO model 5502 GT Each GT simulates one GPS SV, and typically several GTs are employed over a test range to augment the GPS constellation or to account for line-of-sight blockage to an Sv GTs perform four basic functions: 1) synchronization to the GPS Lt C/A- and P-code SV time signals-P code is primary; 2) synchronization to the GPS ~ P-code time signal; 3) generation of a master timing reference compensated for first-order ionospheric time delay-using the Lt and ~ signals; and 4) simultaneous transmission of a simulated LI C/A- and P-code signal A typical GT scenario is provided in Fig 10 The figure depicts the standalone operation of each GT for initial GPS system time synchronization with visible SVs and the subsequent transmission of GPS signals Global Positioning Systems Joint Program Office The first differential GPS test was conducted by the JPO at the U.S Army Yuma Proving Ground (YPG) in December 1979.11The Inverted Range Control Center (IRCC) was modified to operate as a differential GPS reference station, to compute the pseudorange corrections, and to transmit them to a test vehicle via the navigation message from a ground transmitter The IRCC continued to operate as a reference station until 1987 and was used to monitor the space and control segments and to continue the development of differential GPS techniques During this same period, tests were conducted to validate the YPG range and to evaluate JPO Phase II GPS receivers as a reference system for range applications In 1985 a study was conducted to replace the IRCC with a dedicated GPS reference station A system was built using TI-4100 GPS receivers and delivered to the JPO in 1987 An identical system was built and delivered to the B-2 Aircraft Combined Test Force (CTF) at Edwards Air Force Base and was used as the reference station to generate TSPI for test support in November 1987 A TI-4100 was used as the aircraft GPS receiver and TSPI was generated for flight test missions The TI-4100 reference receiver was replaced with a Collins 3A receiver in 1990 At that time, General Dynamics Services Company designed 12 and delivered one new reference station to the B-2 CTF and three systems to the JPO for test applications Collins 3A receivers also replaced the TI-4100s in the flight test aircraft In 1991, responsibility for managing the four JPO reference stations called data analysis stations (DAS) was transferred to the 6585th Test Group's guidance test division, also known as the Central Inertial Guidance Test Division (CIGTF) at Holloman, Air Force Base, New Mexico Three of the DGPS systems have been installed at Holloman Air Force Base, New Mexico, Edwards Air Force Base, California, and Melbourne, Florida to support DOD test programs The fourth system is installed in a trailer and supports test programs on a mobile basis 614 D G ABBY a Equipment description Testplatform-The GPS receivers used in the test vehicle are the Collins 3A or the Collins miniaturized airbone GPS receiver (MAGR) Raw measurement data from the RS-422 instrumentation port (IP) is recorded for postprocessing Data are typically recorded on a PC buffer box (PCBB), which is either a 2861 386 PC with a large hard disk or a digital tape recorder If an analog tape recorder is available, the RS-422 digital data can be recorded on one channel and then downloaded after the mission to a PCBB After each mission, the data are transported to the GPS reference station for processing and generation of TSPI data Ground station-The ground station configuration is shown in Fig 11 It consists of four principle components: GPS receiver, GPS antenna, computer, and assorted input/output devices The GPS receiver is a Collins 3A, five-channel, two-frequency, P-code receiver modified to allow external control of tracking channels and for an external clock input The raw pseudorange and delta range measurements and other required data from up to 12 satellites are transmitted to the computer for processing and recording The antenna is a Dome-Margolin The computer is a 80386-based system Its real-time functions include control of the receiver, selection of satellites to be tracked, correction of measurements for propagation effects, and computation of the pseudorange corrections In addition, the computer is used in a postprocessing mode to generate the final TSPI product The input/output devices are shown in Fig 11 system is the Bernoulli removable 1/4 disk unit other agencies can be provided on either nine-track printer is used to generate data products, plots, etc The primary data recording The TSPI output for use by tape or Bernoulli disks The for analysis B Commercial Systems The commercial industry has combined the use of differential GPS with lowcost CIA and P-code GPS receiver technology to develop small, lightweight, cost-effective, turn-key systems for range applications The commercial vendors can provide either 1) complete turn-key systems that can be placed into operation immediately; or 2) hardware and software components that enable users to design a system to meet their requirements Because the test and training applications have similar requirements, the training agencies are also taking advantage of the commercial equipment The generic GPS commercial range system block diagram is shown in Fig 12 The GPS reference station tracks all visible satellites and pseudorange (PR) corrections are computed for each visible satellite and transmitted via the radio communications link to the mobile units The mobile unit applies the appropriate PR corrections and performs a real-time computation to derive position and velocity This solution is available for display in the test vehicle if required and is also transmitted back to the master control station for display and recording The availability of the very accurate GPS time and the use of TDMA provides the capability to transmit data from 10-100 players (depending on the amount TEST RANGE INSTRUMENTATION 617 of data) on one frequency The use of multiple frequencies can increase the number of players by the number of channels available The U.S Army YPG is operating a system with a capacity for 24 players to support positioning of aircraft, helicopters, and ground targets to evaluate airborne targeting sensors The critical problem for the design of a DGPS system was to be able to collect data from ground targets in the rough desert terrain Yuma Proving Ground is using a system developed by Trimble Navigation to support these requirements The system uses a Trimble 4000RL differential reference station and six-channel CIA-code receivers for the mobile units In order to meet the requirement to link data from ground vehicles in rough terrain, Trimble used off-the-shelf low-band vlf communications radios GPS corrections are broadcast about every 10 s with mobile unit position reports scheduled or polled during the intervening period In range operations, a base station collects player ID, position, and velocity of each participant and displays this information on a highresolution color display on a digitized map background to support situation status in real time White Sands Missile Range has procured a IO-player GPS range system to support testing of a forward area Air-Defense Command, Control, and Intelligence System The system was developed by SRI International using off-the-shelf GPS and communication radio equipment SRI used a NavStar PLM/XR3 for the GPS reference station, Magnavox 4200 GPS receivers for the mobile GPS receivers, and Motorola VHF rf-Modems for the data link The positioning systems used for training applications are very similar However, the total system is more complex because of the requirements for information on war gaming such as RTCA, probability of kill calculations, weapon system data, etc A generic training system block diagram is shown in Fig 13 Examples of systems currently deployed or being developed are briefly described, and references are provided Training systems currently in development, test, and deployment include the following Simulated Area Weapons Effects-Radio Frequency (SAWE-RF) is a program that addresses indirect fire weapons, training of mounted and dismounted troops using computer simulated weapons, as well as the multiple integrated laser engagement system (MILES) The Phantom Run Instrumented MILES Enhancement (PRIME) system is being developed to enhance training for armored vehicles The Army is working on a system that combines features of PRIME and SAWE-RF called Combat Maneuvering Training Center-Instrumentation System (CMTC-IS), primarily for armored vehicle training at the Hohenfels Training area in Germany In addition, the Army is planning to develop a transportable system that combines all aspects of modern army warfare, including close air support and defense This system, called Mobile Automated Instrumentation Suite (MAIS) will be designed to be deployed at any location worldwide and to be operational within days Magnavox MX 7100 and MX 4200 6channel CIA-code receivers are used by most of these training systems as the differential GPS equipment Details on these programs can be found in Refs 13-15 TEST RANGE INSTRUMENTATION 619 C Data Links The area that most limits the use of differential GPS in range applications is linking of corrections and/or TSPI to where it is required The factors must be considered are the following: 1) data rate; 2) test vehicle dynamics; 3) size of area to be covered; 4) cost; 5) number of participants; and 6) DGPS method Data link requirements will be addressed by area size progressively Diameters of areas considered will be 25-50 miles, 50-200 miles, 200-1000 miles, and greater than 1000 miles or what is termed wide-area differential For the first case, the design issues are minimal, and as previously discussed, off-the-shelf communication radio equipment along with TDMA and use of multiple frequencies can accommodate hundreds of participants at a fairly reasonable cost For the 50-200 mile case, the rf line-of-sight limitations become a problem The RAJPO is using ground relays to transmit data bidirectional from the ground station The RAJPO data link system is a custom design to handle up to 200 players over these distances The key word is custom, which results in a highcost solution to the problem For limited numbers of low dynamic players, a potential solution would be cellular telephones where coverage is available Satellite communications is a solution described in the following paragraphs The cases of 200-1000 miles and over 1000 miles have the same data link problem but the potential need for additional reference stations comes into play The most effective data link solution is satellite communications.I6-'9 Cost, however, at this point is a limiting factor The radius of coverage for one differential station depends on several factors A P-code reference station can provide coverage over a larger area than C/Acode systems because ofthe dual-frequency code-tracking capability The 6585th Test Group at Holloman has verified differential GPS accuracies less than m on test aircraft at distances of up to 600 miles The coverage for CIA-code reference stations, however, is limited to approximately 50-100 miles [A concept useful for large or nationwide test beds is wide area DGPS (WADGPS) as described in detail in Chapter of this volume.] A "network" concept for linking reference stations and generating differential corrections over relatively broad areas is also being studied Pseudorange corrections (PRC) are measured at each reference station and then processed at a central location to generate corrections as a function of user location The resultant is an "iso-PRC" contour map for each satellite Because of the slowly changing error sources and change of the line-of sight vector to each satellite, the contour maps would have to be updated frequently.20 A series of tests to evaluate the use of a network of GPS reference stations as a source of differential corrections was conducted for the Burlington Northern Railroad The tests were conducted over networks of 100, 200, and 300 miles The results showed that the network concept can be used to cover large areas and achieve accuracy requirements required by the test range community.21 VI Accuracy Performance A Position Accuracy Test results from evaluation of CIA-code range systems against an accepted truth reference are very limited The Joint Program Office has conducted limited TEST RANGE INSTRUMENTATION testing of CIA-code receivers at YPG, and differential processing and analysis was perfonned on these receivers A summary of those results are given in Table The JPO P-code differential GPS test support capability has undergone extensive testing at YPG under a variety of conditions The YPG laser system was the truth reference for all tests The results are summarized in Table The difference in the position accuracies between P code and CIA code seem to be approximately m P-code accuracies range from 2-4 m 3dnns and C/Acode accuracies range from 6-8 m 3dnns If there is a conclusion to be made, it is that high-accuracy, high-dynamic test and training requirements shown in Table I will most likely require P-code systems, which requires more investment and complexity On the other hand, the low-cost CIA-code systems can meet many of the land and low-dynamic requirements very cost effectively B Velocity Accuracy Validation of GPS velocity accuracies is even more of a problem because of the lack of accurate truth reference systems The laser tracker velocity accuracy 621 is only around 0.2-0.3 mis, which is inadequate to evaluate the GPS specification accuracy of 0.1 mls Spot checks have been perfonned with specialized systems that have verified GPS velocity accuracies of