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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 109 exchanging clock and ephemeris information every 15 minutes via UHF communications crosslinks, which will connect each satellite in the constellation to all of the other satellites in view, each satellite will have knowledge of the ephemeris and clock information of all the satellites in the constellation. Based on the 15-minute ranging data exchanged, the Block IIR satellites can autonomously update the navigation message being broadcast to users. The current plan for testing the autonomous ranging capability is initially to download the 15-minute ranging data from each satellite's Kalman Filter once per day to the OCS so that it can be compared with the ground-based data derived from the MCS's Kalman Filter. After successful testing of autonomous satellite ranging capability is completed, clock and ephemeris corrections will be determined with the on-board Kalman Filter, and the satellites will automatically update the navigation message every hour. However, even with autonomous generation of clock and ephemeris corrections, the Air Force plans to continue daily uploads of the satellites' clock offset relative to UTC.49 After 24 hours, the combined clock and ephemeris error for the Block IIR satellite constellation is expected to be 1.9 meters (ls).50 Suggested Improvements Using the Autonomous Ranging and Crosslink Communication Capability Current plans call for the use of the Block IIR satellite crosslink capability only for specific commands related to SA and autonomous navigation. Further improvements in accuracy, system reliability, and integrity could be obtained by exploiting the satellite ranging data obtained during the 15-minute autonomous ranging cycles and by more effectively utilizing the communication crosslinks. These improvements are discussed below. Accuracy Improvements by Incorporating Satellite Ranging Data into Ground Solution Since the satellite ranging data will initially be sent to the OCS for comparison with the ground-based data, the space-based measurements also could be incorporated into the MCS's Kalman Filter. By uploading these integrated corrections to the satellites, an incremental improvement in accuracy can be achieved over the initial planned Block IIR operational procedure, where the satellites will be uploaded with only ground-based clock and ephemeris corrections. When autonomous satellite ranging capability has been activated, further accuracy improvements could be achieved if the integrated corrections were sent to satellites at least once per day. Ideally, one satellite could be sent the corrected data every hour and the crosslinks could be used to relay the information to all the other satellites. These integrated 49 Source: Input provided to the NRC committee by Capt. Christopher Shank and Capt. Earl Pilloud, Air Force Space Command, January and February 1995. 50 Response from Martin Marietta Astro Space Division of Lockheed-Martin, 6 February 1995.
