Apparently have a significantly different impact

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Unformatted text preview: ces has nil effect on global service availability, and impacts worst site availability by less than one percent in the worst case examined. A-3.3 Satellite Outage Effects on Service Availability The most common reason for variations in service availability is the routine removal of a satellite for maintenance, or less commonly in response to a component failure. Analysis of the singlesatellite out case demonstrates a minimal impact on service availability. Since one of the constellation design objectives was to minimize the effects of removing a single satellite from the constellation, this is a reasonable result. A routine maintenance event coupled with a random failure in another satellite can result in a twosatellite out case. Position service geometry can be degraded significantly when certain pairs of satellites are removed from service, as shown in Figure A-3-3. Note that the impact of removing any pair of satellites is not static, but will vary as the constellation drifts. Page A-14 October 2001 GPS SPS Performance Standard 13 12 11 10 9 8 7 6 5 4 3 2 1 0 ANALYSIS CONDITIONS - Current Operations Nominal 24 Satellite Constellation (as of 19 June 2000) - 5 x 5 degree global grid spacing with one minute sample steps over 24 hours - All-in-view position solution with five degree mask angle 1-20 10-17 5-9 22-31 2-26 25-29 4-7 1-25 22-25 21-29 3-31 17-25 2-11 7-20 2-20 11-20 20-25 11-31 7-11 Global 95% HDOP W orst Site 95% HDOP Global 95% VDOP W orst Site 95% VDOP Global 95% PDOP W orst Site 95% PDOP 11-25 95% Dilution of Precision Value All-in-View DOP Variations 20 Worst Case Combinations of Two Satellites Out GPS PRNs Removed Figure A-3-3. Worst-Case DOP Values with Two Satellites Out Figure A-3-4 and Figure A-3-5 represent the worst-case impacts on horizontal and vertical service availability of the 276 possible two-satellites out permutations. The results vary from almost no impact to an almost 9% reduction in worst-site vertical availability, for the worst-case combination of two satellites removed. Worst 20 Cases of Two Satellites Out -- Horizontal Service Availability Worst Case URE Performance ANALYSIS CONDITIONS 99% - Current Operations Nominal 24 Satellite Constellation (as of 19 June 2000) 98% 97% - 5 x 5 degree global grid spacing with one minute sample steps over 24 hours 96% 95% 94% - All-in-view position solution with five degree mask angle 93% - 6 meter constellation RMS User Range Error (URE) 92% Global Horizontal Availability 91% Worst Site Horizontal Availability 11-29 3-22 3-31 2-27 2-10 22-31 10-26 4-7 21-29 7-20 2-26 2-11 7-26 2-20 1-20 11-31 22-25 1-25 11-25 90% 7-11 Horizontal Service Availability 100% - PREDICTED HORIZONTAL 95% ERROR USED FOR AVAILABILITY THRESHOLD: 36 meters GPS Satellite PRNs Removed Figure A-3-4. GPS Horizontal Availability – Two Satellites Out Page A-15 October 2001 GPS SPS Performance Standard Worst 20 Cases of Two Satellites Out -- Vertical Service Availability Worst Case URE Performance Vertical Service...
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