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GPS APPLICATIONS AND REQUIREMENTS 56 The presence of SA, despite the fact that it does not degrade timing accuracy to less than currently acceptable levels, is considered to be another limitation on overall system reliability. The telecommunications industry believes that GPS, being "subject to failures or deliberate denial of signal", cannot and should not be used without being backed up by other technologies able to provide the same information.54 In the future, it is also likely that accuracies in the range of 50 to 100 nanoseconds will be required for some telecommunications applications. It will be difficult for direct GPS timing to meet this requirement, even without the presence of SA. Findings GPS currently meets all accuracy requirements for both GPS time transfer and time synchronization using direct GPS time. Many telecommunications companies are still hesitant to utilize GPS because of concerns about system reliability and the presence of SA. Future accuracy requirements for both time synchronization and time transfer will be difficult to achieve with the current capabilities of GPS. SPACECRAFT USES OF GPS The application of GPS to spacecraft navigation and control has the potential to provide significant savings in spacecraft costs and mission operations and is being introduced into spacecraft systems today in both government and commercial programs. The feasibility of using GPS for satellite navigation was first demonstrated in 1982 by a receiver placed aboard Landsat 4.55 Since then a number of additional missions and satellites have utilized GPS, including the Topex/Poseidon satellite launched in 1992. Other spacecraft programs have flown GPS, but it has been used primarily in an experimental mode. GPS receivers also have been used experimentally for launch vehicle applications. The experimental Ballistic Missile Defense Organization/McDonnell Douglas Delta Clipper (DC-X) utilized a GPS receiver integrated with an inertial navigation unit and flight control avionics during its flight testing. The system has reportedly cut the rocket's development 54 GPS and Global Telecommunications: A Summary Briefing Prepared for the National Research Council Committee on the Future of the Global Positioning System, p. 8. 55 H. Heuberger and L. Church, "Landsat-4 Global Positioning System Navigation Results," (Presentation to the American Astronautical Society/American Association of Aeronautics and Astronautics (AAS/AIAA) Astrodynamics Conference, AAS 83-363, August 1983).
