The Global Positioning System A Shared National Asset (1995) / Chapter Skim
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Pages 99-105
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From page 99... ...
With the current GPS constellation, the clock and ephemeris errors contribute approximately 3.4 and {.4 meters (la) , respectively, to the SPS and PPS error budget, for a combined error of 3.6 meters (~32, as shown in Table 3-~.33 Once SA, the atmospheric, receiver noise, and multipath errors have been eliminated or reduced, ephemeris and clock errors become the largest contributors to the WERE.
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From page 101... ...
Based on recent improvements to the DMA's Kalman Filter, which originally had a configuration similar to the GPS Kalman Filter, use of an updated, non-partitioned GPS Kalman Filter is expected to reduce the combined clock and ephemeris error by 15 percent.38 Furthermore, an additional 5 percent improvement can be achieved by using improved dynamic models in the Kalman Filler, which would allow better predictions of satellite behavior 1 day ahead.39 An estimated cost of $7.5 for upgrading the Kalman Filter and improving its dynamic models was provided to the committee.40 Me operational control segment Kaiman Fiber should be improved to solve for ad GINS satellites' clock and ephemeris errors s~mukaneously through the elimir~aiion of partitioning and the inclusion of more accurate dynamic models. These changes should be implemented in the 1995 OCS upgrade request for proposal.
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From page 102... ...
Table 3-9 Reduction of Combined Clock and Ephemeris Errors Enhancement T Anticipated Combined Clock and Ephemeris Error Improvement over Existing Combined Error of 3.6 meters (:1~) Correction Updates (50% ~I.8 meters reduction)
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From page 103... ...
120 Cal 100 80 o : 8 o 60 40 20 o 100 Meters ~; 30 Meters 25 Nleters 12 Meters 6 Meters With SA No SA No SA Typical Receiver Two Frequencies Typical Receiver No SA Two Frequencies Improved Receiver No SA Two Frequencies Improved Receiver Improved Clock and Ephemeris Figure 3-5 Approximate stand-alone horizontal SPS accuracy, 2 arms, resulting from recommended improvements and enhancements.
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From page 105... ...
For example, for a stand-alone horizontal accuracy of 100 meters, the availability of four satellites would increase from the previous value of 99.96 percent to 99.97 percent. The improved RAIM availability is shown in Table 3 12.42 Table 3-12 Effect of SA Removal, Dual-Frequency Capability and Reduced Clock and Ephemeris Errors on RAIM Availability for Aviation Applicationsa Aviation Application Availability W-'th SA Turned Availability With SA Turned to to Zero and L4 Added Zero, L4 Added, and Reduced Clock and Ephemeris Error base of Flight | Protection | 1 SateHitesb | 24 SatellitesC | 2 Satellites | 24 Satellites Limit ~ En Route1 2.0 nautical I 76.80% 1 100.00%1 9 .08% 1 100.00% miles Terminal Area1.0 nautical 95.19% 99.98%95.70% 100.00% miles Non-Precision | 0.3 nautical | 3.12% | 100.00%d | 9 .36% | 100.00%d Approach miles 42 Based on analysis conducted by the MITRE Corporation for the NRC committee, 7 February 1995.
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