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GPS APPLICATIONS AND REQUIREMENTS 58 Attitude Determination In the last several years, several manufacturers of GPS receivers have started collaborating with spacecraft developers to design GPS receivers for use as attitude sensors on board spacecraft. On-board attitude determination is a requirement for virtually every modern spacecraft, and most also require an automatic attitude control system. The traditional suite of sensors used for attitude determination range from relatively low-cost magnetometers and horizon sensors to precise gyroscopes, sun sensors, and star trackers. GPS may provide a cost-effective complement or even alternative to many of these existing systems. GPS attitude determination is accomplished by observing the carrier phase of an incoming GPS signal at two or more antennas on board the spacecraft. The difference in phase between the antennas can be related to the vehicle orientation and the rate of change of these phase observations is an indication of the attitude rate of change. The accuracy of GPS for this application is limited by multipath, the phase noise in the receiver, the separation of the antennas, and the stability of the structure supporting the antennas. With the best current technology, accuracies as good as 0.1 degrees (2s) can be expected. The accuracy requirements for satellite attitude determination range from 5 degrees for some simple spacecraft to well below 3 x 10-6 degrees (0.01 arc seconds) for a spacecraft like the Hubble Telescope. At this stage GPS cannot replace the high performance of star trackers for this ultimate precision, but may provide a cost-effective alternative for many mission requirements. Launch and Re-entry Vehicle Guidance GPS also has applications to space launch vehicles as a sensor in the vehicle's navigation system and for providing positioning information to ground controllers for range safety purposes. As previously mentioned, an integrated GPS/inertial navigation system has been tested on the experimental BMDO/McDonnell Douglas Delta Clipper (DC-X), and on Orbital Science Corporation's Pegasus launch vehicle. In addition, an experimental space re-entry vehicle called the Spacewedge, designed for re-entry rather than launch, is demonstrating the ability to make an automatic precision landing using a parafoil and a commercial GPS receiver. A full-scale space vehicle, either piloted or unpiloted, may one day use GPS-based technology for emergency crew return or cargo return from Earth's orbit.57 Accuracy requirements have not been provided for these experimental applications. Most range safety tracking for launch vehicles currently is conducted using a rather elaborate and expensive system consisting of ground tracking radars and associated equipment. According to a previously published NRC study, it is conceivable that pending 57 Spacewedge, known formally as the "spacecraft autoland gliding parachute experiment," has been developed by NASA's Dryden Flight Research Facility, Edwards AFB, California for under $100,000 annually. J. R. Asker, "Space Autoland System Shows GPS' Wide Uses," Aviation Week & Space Technology, 18 October 1993, pp. 54-55.
GPS APPLICATIONS AND REQUIREMENTS 59 further study by range safety experts, GPS-derived trajectory data could be used as a more cost-effective alternative.58 The DOD has been considering the use of GPS as the primary time and space position information source for the national ranges ever since the Range Applications Joint Program Office was established approximately 5 years ago, and the Navy has been utilizing GPS trajectory data for Trident missile testing since the early 1980s.59 Accuracy requirements for GPS range safety applications are very mission specific, and have not been generalized. GPS also could be used to improve range safety by sending flight termination commands to missiles and launch vehicles carrying GPS receivers. This could be accomplished using a DGPS datalink or a pseudolite located at the range or, as suggested by one expert in range safety, by using some spare data bits available in the GPS navigation message itself.60 Current flight termination telecommands, which are used to initiate self- destruction, are broadcast in the UHF frequency band. This band is very susceptible to spoofing, jamming, and interference. Integrating a telecommand with other GPS and DGPS equipment and datalinks already under development for time and position range applications could provide a more secure and cost effective means of initiating a flight termination when it is necessary. A consolidated list of available GPS requirements for spacecraft applications is provided in Table 2-10. Table 2-10 Requirements for GPS Spacecraft Applicationsa Application Accuracy Satellites Orbit Determination (Real Time) 50 m (2 drms) Orbit Determination (Post-Process) Â± 0.001 m (2 drms) Attitude Determination 5 degrees to 3 x 10-6 degrees (2Ï)b Launch Vehicles Launch Trajectory and Position Mission Specific Determination a. Accuracy is currently the only specified requirement for GPS spacecraft applications. The values in this table were derived by the committee from input received during the study. b. Accuracy as good as 3 x 10-6 degrees (2Ï) is currently available only from star trackers. GPS is currently capable of 0.1 degree (2Ï) attitude determination accuracy, which is suitable for most spacecraft missions. 58 NRC, Technology For Small Spacecraft, Aeronautics and Space Engineering Board, National Research Council (Washington, D.C.: National Academy Press, 1994), p. 16. 59 Source of Information: Personal conversation with Daniel F. Alves, Jr. of Alpha Instrumentation/Information Management, AI2M, Santa Maria, California, 20 February 1994. 60 Daniel F. Alves, Jr., Global Positioning System Telecommand Link, U.S. Patent number 5,153,598, 6 October 1992.