Independent Control Segment URA Monitor for GPS IIIC with Application to LPV200

To assure the integrity of critical navigation operations, the user range accuracy (URA) is envisioned to provide future GPS IIIC users the means to rigorously bound the fault-free and fault-induced errors that may be contained in the signal-in-space (SIS) from each 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 proposed IUM estimates these errors based on the difference between the SV position computed using the broadcast parameters and the SV position computed using range measurements from receivers located at the worldwide GPS monitor stations (MSs). The IUM applies a threshold test to the projection of the estimated errors onto the SV line-of-sight for each assumed user located at a set of grid points in the SV footprint. The IUM output is either a computed minimum monitorable URA (MMU) or an alert if there is a threshold violation at any assumed user grid point. The magnitude of the MMU is based on the quality of the IUM measurements. 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 the worldwide magnitudes of the 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). In this application, for the position error bounding to be feasible, the distribution of the errors contained in each range measurement (GPS IIIC SIS + airborne) used in the position solution has to be bounded by a normal error distribution. This requirement is addressed as part of the LPV200 application analysis. 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.