Budgetary constraints the us government reserves the

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Unformatted text preview: d users. One of the potential adjustments includes a redefinition of the baseline constellation slots, as they are represented in Table 3-1. In the event of such a redefinition, the U.S. Government will provide an updated definition to the user community. SPS performance standards are based on signal-in-space performance. Contributions of ionosphere, troposphere, receiver, multipath, or interference are not included. Position and time transfer performance estimates for a single-frequency user equipped with a representative receiver, inclusive of ionosphere, troposphere and receiver noise contributions, are provided in Appendix A, Section A-5.5. SPS performance standards are established in a manner consistent with the following definitions: • Service Volume: The spatial volume supported by SPS performance standards. Specifically, the SPS Performance Standard supports the terrestrial service volume. The terrestrial service volume covers from the surface of the Earth up to an altitude of 3,000 kilometers. • Availability of Position Dilution of Precision (PDOP): The percentage of time over a specified time interval that the Position Dilution of Precision (PDOP) is less than or equal to a specified value. • SPS SIS User Range Error (URE) Statistic: A satellite SPS SIS URE statistic is defined to be the Root Mean Square (RMS) difference between SPS ranging signal measurements (neglecting user clock bias and errors due to propagation environment and receiver), and “true” ranges between the satellite and an SPS user at any point within the service volume over a specified time interval. A constellation SPS SIS URE statistic is defined to be the average of all satellite SPS SIS URE statistics over a specified time interval. • Service Reliability: The percentage of time over a specified time interval that the instantaneous SIS SPS URE is maintained within a specified reliability threshold at any given point within the service volume, for all healthy GPS satellites. The likelihood of the reliability threshold being broken is referred to as the Probability of Hazardously Misleading Information (HMI). • Service Availability: The percentage of time over a specified time interval that the predicted position accuracy is less than a specified value for any point within the service volume. • Positioning Accuracy: The statistical difference between position measurements and a surveyed benchmark for any point within the service volume over a specified time interval. • Time Transfer Accuracy Relative to UTC (USNO): The difference at a specified probability between user UTC time estimates and UTC (USNO) at any point within the service volume over a specified time interval. Page 11 October, 2001 3.1 GPS SPS Performance Standard Constellation Management Standard The current architecture for the constellation identifies nominal orbit slots for a 24-satellite constellation and tolerances for establishing and maintaining satellites within the slots. The baseline...
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