Use of Standalone GPS for Approach with Vertical Guidance.
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Use of Standalone GPS for Approach with Vertical Guidance.

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      The accuracy of GPS has improved dramatically over the past year with the removal of Selective Availability. The largest error source now is the ionosphere which can be removed in the future when the additional civil frequencies become available. Presentations at ION GPS 2000 have suggested that clock and ephemeris errors will be able to predicted to within 25-30 cm each in the future, resulting in a User Equivalent Range Error (UERE) as low as 50 cm for both the GPS and Galileo constellations. This UERE would depend on very low receiver noise and multipath mitigation. However, if achieved, this accuracy will result in horizontal position errors on the order of 1.5m 95% and vertical 2.5m 95%. Although these position errors are small enough to satisfy Category I precision approach operations, could GPS satisfy the more stringent integrity requirements as well? Integrity for standalone GPS generally is provided by algorithms within the receiver known as Receiver Autonomous Integrity Monitoring (RAIM) and Fault Detection and Exclusion (FDE), although these algorithms usually are focused on meeting horizontal navigation requirements. For operations involving the approach phase of flight, it is likely that only the detection capability provided by RAIM is required, assuming that the exclusion function is available in the terminal phase of flight if an anomaly is detected. However, one of the limitations of the RAIM and FDE algorithms typically is not having enough ranging sources to form a solution. Therefore, in this paper the availability of standalone GPS, as well as a combined GPS/Galileo constellation, will be evaluated. Tradeoffs between accuracy and availability are examined in this analysis. For example, in practice a UERE of 50 cm may not be achievable. This paper will examine the resulting availability if higher UEREs are used. The paper evaluates vertical alert limits for both categories of approach with vertical guidance (APV) which range from 50m for APV-I to 20m for APV-II.
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