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Independent URA Monitor for GPS IIIC
April 2011
Ronald Braff, The MITRE Corporation
Brian Bian, The MITRE Corporation
Curtis A. Shively, The MITRE Corporation
ABSTRACT
To assure the integrity of critical navigation operations, the user range accuracy (URA) is envisioned to provide future users the means to rigorously bound the fault-free and fault-induced errors in the signal-in-space (SIS) from each GPS IIIC space vehicle (SV). This paper examines an independent URA monitor (IUM) that could be incorporated into the next generation GPS Control Segment (OCX) to assure that the broadcast URA bounds any errors in the broadcast SV ephemeris and clock correction parameters. The IUM operates on single or multiple snapshot data in order to maintain an independent and timely integrity assured URA for each OCX upload to an SV. It thus generates larger URAs than would be obtained when they are generated from continuous tracking data and an orbital model. A performance assessment, based on an example GPS IIIC constellation and covariance analysis, is used to estimate worldwide minimum monitorable URA values (MMUs) generated by the IUM.
To examine operational feasibility of the IUM, an analysis is presented for an application of GPS IIIC with IUM to the stringent integrity requirements of the Localizer Performance with Vertical guidance (LPV) aircraft approach operation down to a 200 ft decision altitude (LPV200). Although the IUM may produce MMUs that are somewhat larger than the URAs previously envisioned for GPS IIIC, the analysis of the LPV200 operation indicates sufficiently high availability at U.S. locations, including Alaska and Hawaii. Analysis of several non-U.S. locations indicates availability values less than 0.99 and significant loss of availability in the Southern Hemisphere.
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