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Per sv per year 19 20 total average sv downing events

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Unformatted text preview: at have reached their end-of-life, and by efforts from the constellation operators to get as much service as possible before declaring the satellite dead. The replacement timeline for a failed satellite in a primary slot depends on whether or not a spare satellite is available in the appropriate plane. The current nominal timelines for replacing a failed satellite in a primary slot with a fully operational satellite are as follows: Page A-9 October 2001 GPS SPS Performance Standard • 30 days given an available on-orbit spare • 120 days when a launch call is required The intent of the U.S. Government is to minimize the probability that such a failure event occurs. If the system operator considers the prognosis on a satellite in a core slot marginal, a scheduled launch is targeted for that satellite’s node, and the marginal satellite has historically been phased to a spare slot. The intent of the U.S. Government is to manage the constellation such that it never drops below 22 healthy satellites in nominal slots. Figure A-2-3 illustrates the percentage of time historically the constellation has been comprised of a given number of healthy satellites. Core GPS Constellation Availability -- Cumulative Percentages GPS Constellation Availability -- Cumulative Percentages ASSESSMENT INTERVAL: 1 Jan 95 - 30 Apr 99 Availability Assessment 100% 90% - Analysis conducted every hour over assessment interval, based on NANUs - Analysis removes satellites unhealthy for whatever reason, to include routine maintenance 80% 70% 60% 50% AVERAGE NUMBER OF HEALTHY SATELLITES AT ANY INSTANT: 40% 25+ 99% 100.0% 99.9% 98.9% 98% 98.3% 97% 96% 95% Historical Availability 94% Minimum Standard Probability 93% 30% 92% 20% 10% HISTORICAL ASSESSMENT INTERVAL: 1 Jan 95 - 30 Apr 99 100% 100.0% 99.6% 99.0% 58.4% 28.0% 21.3% 22+ 23+ 24+ 25+ 26+ 91% 27+ 0% 90% # Healthy Satellites over Time Interval 99.7% 21+ 99.2% 22+ 98.0% 95.0% 23+ 24 Number of Healthy Primary Slot Satellites Figure A-2-3. GPS Constellation Size Distribution In an effort to mitigate the potential impacts of satellite outages on constellation geometry and service availability, projections are made every month of global Dilution of Precision (DOP) patterns for the following month. These patterns are based on evaluating all permutations of two satellites removed from the constellation to ensure the system operator knows where its sensitivities lie in scheduling satellite maintenance. More refined projections are run in support of actual maintenance a few days ahead of the event as a further safeguard against unnecessary impacts on user service. If future impacts are identified, options are defined to mitigate the problem if possible. If mitigation is not possible, it is the intent of the U.S. Government to notify GPS users a minimum of 48 hours in advance of an anticipated service degradation. A-2.3 SPS Signal-in-Space User Range Error SPS Signal-in-Space (SIS) User Range Errors (UREs) are comprised of two basic...
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