Satellite maintenance requirements constellation

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Unformatted text preview: rvice availability performance over the month of June 2000. A-3.1 Effects of Varying Mask Angle on Service Availability Receivers employ a variety of different elevation mask angles, depending on the application. The information presented in Figure A-3-1 and Figure A-3-2 show the effect varying elevation mask angles has on service availability, for an all-in-view receiver and a four-satellite solution receiver, respectively. GPS Service Availability Sensitivity to Mask Angle -- All-in-View Receiver 100% Service Availability ANALYSIS CONDITIONS - Current Operations Nominal 24-Satellite Constellation (as of 19 June 2000) 99% - 5 x 5 degree global grid spacing with one minute sample steps over 24 hours 98% Global Horizontal Availability - All-in-view position solution Global Vertical Availability 97% - 6 meter constellation RMS User Range Error (URE) Worst Site Horizontal Availability Worst Site Vertical Availability 96% 0 2 5 7.5 10 - HORIZONTAL THRESHOLD: 36 meters - VERTICAL THRESHOLD 77 meters Mask Angle (Degrees) Figure A-3-1. Mask Angle Variation Effect -- All-in-View Receiver Page A-13 October 2001 GPS SPS Performance Standard GPS Service Availability Sensitivity to Mask Angle -- Best 4-SV Receiver 100% Service Availability ANALYSIS CONDITIONS 99% - Current Operations Nominal 24-Satellite Constellation (as of 19 June 2000) 98% - 5 x 5 degree global grid spacing with one minute sample steps over 24 hours Global Horizontal Availability - Best 4-SV position solution Global Vertical Availability 97% - 6 meter constellation RMS User Range Error (URE) Worst Site Horizontal Availability Worst Site Vertical Availability 96% 0 2 5 7.5 10 - HORIZONTAL THRESHOLD: 36 meters - VERTICAL THRESHOLD 77 meters Mask Angle (Degrees) Figure A-3-2. Mask Angle Variation Effect -- Best Four Satellite Receiver As shown in the figures, mask angle variations do not have a significant effect on global availability with a fully healthy constellation. Worst-case sites become significantly worse after the five-degree case. In the nominal constellation analysis, no occurrences of less than four satellites were observed, even at a ten-degree elevation mask. However, the number of sites experiencing less than four satellites climbs rapidly with increasing mask angle as individual or pairs of satellites are removed. Note that variations of elevation angle mask do not apparently have a significantly different impact between four-satellite and all-in-view position solutions. This is probably due to the fact that the marginal geometries at the tail of the distribution have very few satellites in view. This indicates that the worst-case four-satellite and all-in-view position solutions are virtually identical. A-3.2 Orbital Variation Effects on Service Availability In Section A-2.1.1, a tolerance of ± 2° was established for each satellite orbit’s GEC/LAN. An analysis of satellite LAN variation within the specified tolerance indicates that satellite movement within the LAN toleran...
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