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Unformatted text preview: global grid Date of Month (June 2000) Figure A-3-9. Typical Example of GPS SPS Horizontal Availability – June 2000 GPS Vertical Service Availability -- June 2000 100.0% ANALYSIS CONDITIONS 99.9% 99.8% 99.7% 99.6% 99.5% 99.4% 99.3% 99.2% Global Vertical Availability, 77 meter 95% Threshold 99.1% Worst Site Vertical Availability, 77 meter 95% Threshold 99.0% 0 5 10 15 20 25 30 - Analysis conducted under normal constellation ops conditions - NANUs applied over intervals defined by 2 SOPS - Global 5 x 5 degree grid (1,666 locations) - 24 hour sample intervals - One minute sample steps - All-in-view NAVSOL - SPS signal-in-space UREs, with 50 cm of added noise to represent uncertainty due to Tgd - Five degree mask angle - Statistics are result of empirical sorts for each location over the global grid Date of Month (June 2000) Figure A-3-10. Typical Example of GPS SPS Vertical Availability – June 2000 Page A-18 October 2001 GPS SPS Performance Standard SECTION A-4 Service Reliability Characteristics This section defines conservative expectations for GPS service reliability performance. These expectations are based upon observed accuracy characteristics; the GPS service failure history to date; long-term failure rate projections; and current system failure response capabilities. The user is provided with information that indicates expected failure rates and their effects on a global and regional basis. A-4.1 Reliability Threshold Selection As defined in Section 1.4, service reliability is the measure of how consistently GPS User Range Error (URE) levels can be maintained below a specified reliability threshold. The selection of an appropriate value for this threshold is based upon an assessment of normal accuracy characteristics. A description of normal URE characteristics is provided in Section A-2.3. The Notto-Exceed (NTE) value must be larger than the practical limit on normal GPS URE performance. The largest URE that is typically experienced under normal operating conditions is approximately 10 meters. A value of 30 meters was chosen as the reliability threshold because it is sufficiently outside the normal URE accuracy envelope to avoid a false alarm condition and because it should serve as a usable input to aviation plans for phases of flight down to non-precision approach operations. A-4.2 GPS Service Failure Characteristics A service failure is defined to be a departure from nominal system ranging accuracy that causes the SPS instantaneous ranging error of a healthy satellite to exceed 30 meters while the User Range Accuracy (URA) multiplied out to 4.42 standard deviations indicates less than 30 meters. An occurrence of this behavior is directly due to a failure somewhere in the GPS ranging signal control and generation process. The characteristics of a service failure and the factors that affect service reliability are listed below. Each is discussed in more detail in the following sections. • Ranging signal failure frequency. • Failure duration....
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