Unformatted text preview: e State Estimation/Prediction
Navigation Data Generation Figure 1-2. The GPS Control Segment 1.5 Current SPS Performance Assessment Concept and Limitations The U.S. Government does not currently monitor and assess SPS performance in real time. The
U.S. Government does monitor PPS SIS UREs for all satellites in view of Operational Control
System (OCS) monitor stations in near-real time, to ensure they are meeting performance
objectives. This monitoring is obviously constrained by holes in the current monitor station
visibility patterns, and by monitor station and communications maintenance requirements. The
U.S. Government intends to assess ways for improving its GPS monitoring and measurement
capabilities in accordance with program plans, subject to resource limitations.
Performance summaries based on estimates of SPS SIS UREs are periodically generated, to
provide feedback concerning GPS support for the SPS SIS URE performance standard.
Additionally, the U.S. Government periodically assesses global constellation geometry behavior,
and performs projections to anticipate potential Dilution of Precision (DOP) problems anywhere in
the world. The U.S. Government uses monitor station measurements, constellation geometry and
estimates of SPS SIS UREs to evaluate on a regular basis the ability of GPS to support SPS
position accuracy and availability standards. However, these assessments may be several days
after the fact.
GPS satellites automatically remove themselves from service when they experience any of a
number of specified failure modes. When a service failure occurs that is not covered by the
automatic removal capability, the U.S. Government will respond to the failure by removing the
satellite from service in a prompt manner, subject to current monitor station and ground antenna
Page 4 October 2001 GPS SPS Performance Standard visibility and reliability constraints. In order to detect satellite component failures unique to C/A
code, the OCS monitor stations are configured to perform C/A to P(Y) code handover for each
satellite track session when the satellite comes into view. However, this does not provide
continuous, real-time monitoring of C/A performance.
Due to the reasons stated above, the risks inherent in implementing planned major modifications
to the control segment and satellite block changes, as well as considerations for ensuring safetyof-life, there is a substantial margin introduced between the committed standards in Section 3
and the performance observed in Appendix A.
The U.S. Government does not directly monitor range rate or range acceleration error. URE rateof-change is used internally by the OCS to estimate when a satellite may exceed service
reliability limits, but the parameter is not tracked or reported within the OCS. Page 5 October 2001 GPS SPS Performance Standard (INTENTIONALLY BLANK) Page 6 October 2001 GPS SPS Performance Standard SECTION 2 SPS Ranging Signal Characteristics and
Minimum Usage Assumptions
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