Out gps prns removed figure a 2 2 example of current

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Unformatted text preview: ge A-8 October 2001 GPS SPS Performance Standard where to launch a spare satellite focus primarily on the average age of the satellites in each plane, the projected health and status of each satellite, and whether or not a spare already exists in a given plane. Retaining a spare satellite in an active state on-orbit has little or no impact on the operational life of the satellite. GPS on-orbit spares are therefore maintained as part of the operational constellation and are useable by the GPS user community. Performance standards are still, however, defined based upon the core 24-satellite constellation. Note that since spare satellites are not considered primary operational assets, the U.S. Government reserves the right to place them and employ them as necessary to support system development, test, and operational objectives. Any improvement in user performance that comes as a result of the placement of a spare satellite is regarded as a fortunate by-product of constellation management practices. A-2.2 Constellation Slot Availability Constellation slot availability is a function of two major factors: • Satellite Reliability, Maintainability, and Availability (RMA) behavior over each satellite’s lifetime, and • Replacement timeline at the end-of-life for a satellite in a primary slot. GPS satellites are designed to provide virtually continuous service over the lifetime of the satellite. Satellites do experience some downtime each year, due to scheduled events such as stationkeeping maneuvers, and due to random component failures. Table A-2-3 provides a comparison of actual versus theoretical RMA attributes for the Block II/IIA satellites since January 1994. Block IIR satellites have not yet had enough on-orbit operational time to support a proper RMA trend analysis, but their theoretical RMA characteristics are somewhat better than those of the Block II/IIA satellite. See Appendix B for definitions of terms used in Table A-2-3. Table A-2-3. GPS Satellite RMA Performance Since IOC GPS Satellite RMA Parameter: Actual Theoretical/Design January 1994 – July 2000 Total Forecast Downtime per SV per year (hrs) 35.6 NA Total Scheduled Downtime per SV per year (hrs) 18.7 24 Total Unscheduled Downtime per SV per year (hrs) 39.3 64 Total Actual Downtime per SV per Year (hrs) 58.0 88 Satellite MTBF (hrs) 10,749.4 2,346.4 Satellite MTTR (hrs) 48.2 17.1 Satellite MTBDE (hrs) 3,255.9 1,528.8 Satellite MDT (hrs) 21.5 15.4 # Unscheduled SV Downing Events per SV per year 0.9 3.7 # Scheduled SV Downing Events per SV per year 1.9 2.0 # Total Average SV Downing Events per SV per year 2.7 5.7 Average SV Availability per year – Scheduled Downtime 99.79% 99.73% Average SV Availability per year - All Downtime 99.34% 99.00% Note that although the total downtime per satellite per year is less than the projected values based on satellite design, the Mean Down Time (MDT) is actually somewhat greater. The MDT statistic is skewed by extended downtimes for satellites th...
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