Ephemeris Protection Level Equations and Monitor Algorithms for GBAS

Abstract

One troublesome failure mode for Ground Based Augmentation Systems (GBAS) is the possibility of large discrepancies between satellite locations in space and the locations derived by the ephemeris data that they broadcast. For the Global Positioning System (GPS), nominal ephemeris errors are typically 10 meters or less, and it would take large errors (typically greater than 1 km) to be hazardous to GBAS users making precision approaches to Category I minima. Although most large errors will be detected by the GBAS ground segment Message Field Range Test, ephemeris errors orthogonal to the line-of-sight between a failed satellite and a GBAS ground station are not detectable by this simple test. To counter this possibility, RTCA has adopted new protection levels to quantify the potential impact of undetected ephemeris failures on user position errors for both precision approach and terminal area applications. These equations define position error bounds as functions of the approximate aircraft location with respect to each satellite and the GBAS ground station as well as the magnitude of the satellite orbit error detectable by the ground station. This Minimum Detectable Error (MDE) determines the "P-value" that is broadcast by the GBAS ground station for each satellite it has approved for use. Several GBAS monitor algorithms have been developed and tested for use in GBAS installations that lack SBAS coverage. One of these is a comparison between satellite positions given by the current satellite ephemeris and the ephemeris broadcast by the same satellite on its previous pass. Variants of this "YE-TE" test have been shown to support GBAS MDE's as low as 1100 meters, which will minimize the resulting impact on Category I user availability due to the ephemeris protection level equations. In addition, means of using raw measurements to initialize this monitor and to separately verify ephemerides in real-time are proposed.