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TECHNICAL ENHANCEMENTS FOR FUTURE CONSIDERATION 134 Knowledge of Spacecraft Characteristics Another limitation in the accuracy obtained by precision GPS users is caused by errors in the dynamic model used to predict the behavior of a GPS satellite as it expands and contracts due to the space environment. The current model could be improved if better information was available on the thermal emissivity of the exterior of the spacecraft surfaces, including its solar panels. In addition, telemetry information on temperatures of spacecraft surfaces is needed. For long baseline applications, the limiting error source is usually the GPS orbit error, even when high- accuracy post-processed orbits are utilized. To reach accuracies of 10-9 times baseline length, the orbits must be known to about 10-9 times the distance to the satellites (approximately 20,000 km), or about 0.02 meters. In other terms, an unmodeled acceleration of 10-12 g would accumulate to 0.02 meters after a 12-hour orbit. Unequal radiation of heat from sides of the GPS satellite causes accelerations much larger than 10-12 g. With a minimal amount of effort, the thermal properties of the materials on the exterior surfaces of the Block IIR satellite could be determined. To accomplish this, instruments to measure temperature could be added to the GPS satellites prior to launch, and the data received from these instruments could be transmitted to the ground. This would allow accelerations of the spacecraft surface, which result from uneven heating in space, to be calculated. These accelerations could then be included in models to improve accuracy. In the absence of deterministic models developed through actual measurements, these radiation pressure parameters can only be estimated. Improved L1 Signal Reception at Angles Below the Earth's Horizon In order to support the Block IIR crosslink capability, the specifications related to the UHF antenna on the Block IIR satellites are different from the Block IIA specifications. As a result of these UHF antenna changes, the L-band antenna on the Block IIR satellites will be less symmetrical and will have a narrower pattern. For angles beyond the limb of the Earth as viewed from GPS satellites, this change will probably result in a reduction in the L1 power currently observed with Block IIA spacecraft by approximately 3 dB.14 For spacecraft applications of GPS, this reduction in received L1 power over that currently observed with Block IIA satellites and the narrower antenna pattern could decrease the ability of low-Earth orbit and geosynchronous satellites to receive GPS signals and make GPS-based positioning more difficult from orbit. By increasing the L1 signal power or improving the symmetry of the L-band antenna, spacecraft applications using GPS could be greatly enhanced. 14 Currently, there is no official specification by the Air Force for end-of-life-power requirements beyond the Earth's horizon.
APPENDIX A 135 Appendix A Study Participants ACCQPOINT Communications Corp. Donald Latterman Art Humble Mark MacDonald Ricardo Martinez Aerospace Corporation Al Mason Mohan Ananda Charles Meadows Harold Bernstein John Nauseef Chia-Chun (George) Chao Donna Shipton John Clark Stephen Steiner Bill Feess Earl Vaughn Paul Massatt Michael Wiedemer Prem Munjal David Nelson Air Transport Association of America Bryant Winn, Jr. William Russell Air Force Space Command Allen Osborne Associates John Anton Robert Snow Harrison Freer Richard Koons Alpha Instrumentation/Information Earl Pilloud Management (A12M) Christopher Shank Daniel Alves Aircraft Owners and Pilots Association ANSER Stephen Brown David Martin Air Force Space and Missiles Center Association of American Railroads Lynn Anderson Howard Moody Kim Cirillo Steve Decou Ashtech Incorporated Charlie Golden Jonathan Ladd Bernard Gruber Brian Knitt Aviation Management Associates, Inc. Larry Barnett
APPENDIX A 136 Booz.Allen & Hamilton Federal Highway Administration Michael Dyment Frank Mammano Central Intelligence Agency Lee Simmons Terry McGurn Federal Highway Administration, Turner Crown Communications Fairbanks Highway Research Center Harry Hodges Jim Arnold Deere & Company Garmin International Wayne Smith Gary Kelley Defense Intelligence Agency General Railway Signal Barry Joseph Gordon Quigley Albert Glassman Global Telecommunications & Defense Mapping Agency Information Systems Mike Full Eric Bobinsky Steven Malys Scott True IVHS America William Wooden Jim Costantino Jerry Marsh Department of Transportation Peter Serini INMARSAT George Wiggers Jim Nagle Differential Corrections Inc. Interstate Electronics Ron Haley Peter Canepa Bruce Noel Jim Grace Eagle/Lowrance Electronics ITT Aerospace/Communications Steve Schneider Division Rafi Kedar Peter Brodie Laurence Doyle E-Systems Thomas Ernst Anton Gecan Jon Schnabel Federal Aviation Administration Jansky/Barmat Telecommunications, Paul Drouilhet Inc. Joseph Dorfler Melvin Barmat Dave Peterson Martin Pozesky Jet Propulsion Laboratory William Melbourne George Purcell
APPENDIX A 137 John E. Chance & Associates, Inc. Massachusetts Institute of Technology, Andy Bogle Lincoln Laboratory Philip Stutes William Delaney Jay Sklar Johns Hopkins University, Applied Physics Laboratory MITRE Corporation Lee Pryor Robert Berkowitz Robert Bales Joint Chiefs of Staff Bakry ElArini Jim Burton Thomas Hsiao Pat Carlile Young Lee Joe Lortie Kan Sandoo Tim Meyers Andrew Zeitlin William Owens Motorola Leica, Inc. Robert Denaro Paul Gaylean National Aeronautics and Space Litton Aero Products Administration, Wallops Island Abdul Tahir Bill Krabill Loral Federal Systems National Air Intelligence Center Brian Hemley Scott Feairheller Jay Purvis Magellan Systems Corporation Frank Scenna Randy Hoffman Jim White National Institute of Standards and Technology Magnavox Electronic Systems Company Marc Weiss Walter Airth Vito Calbi National Oceanic and Atmospheric Kenneth Lindenfelser Association Don Pryor Martin Marietta Astro Space Division of Benjamin Remondi Lockheed-Martin Robert Bebee Odetics Precision Time Division Aniruddha Das Don Greenspan Jim Graf John Hrinkevich Offke of the Assistant Secretary of John Mergen Defense for Command, Control Robbin Shultz Communications and Intelligence Noel Longuemare Jules McNeff
APPENDIX A 138 Office of the Assistant Secretary Tampa Bay Vessel Information and of the Air Force for Acquisition Positioning System, Inc. Matthew Brennen John Timmel Lee Carrick Mike Shiro Chad Pillsbury 3S Navigation PlanGraphics, Inc. James Danaher Mike Kevanney Trifed Corporation Rand Corporation Robert Ballew Gerald Frost Louis Decker Irving Lachow Ray Helmering Scott Pace Trimble Navigation Riverside County Flood Control and Ann Ciganer Water Conservation District Walt Melton William Young Charles Trimble Rockwell-Collins True Time Tyler Trickey Bruce M. Penrod Mike Yakos University Navstar Consortium Rockwell International Corporation Randolph Ware Rich Arris Denny Galvin U.S. Coast Guard Steve Fisher George Privon Steven Scott U.S. Department of Agriculture RTCA, Inc. Galen Hart David Watrous U.S. GPS Industry Council Sea River Maritime Michael Swiek Robert Freeman Scripps Institute of Oceanography Jean-Bernard Minster Stanford University Bradford Parkinson TASC Iris Roberts