Monitoring global positioning system satellite orbit errors for aircraft landing systems

Abstract

Ground-based augmentations of the global positioning system (GPS) demand the greatest safety and reliability to support aircraft precision approach and landing navigation. One troublesome failure mode for these systems is the possibility of large orbit errors; discrepancies between the locations of GPS satellites in space and the locations derived by the ephemeris data that they broadcast. To counter this possibility, several ephemeris monitor algorithms detecting orbit errors are described. The meth( c is based on a comparison between satellite positions given by the current satellite ephemeris [today's ephemeris (TE)] and the ephemeris broadcast by the same satellite on its preceding pass [yesterday's ephemeris (YE)]. Variants of this YE-TE test are shown to provide protection against ephemeris errors and also to support minimum detectable errors as low as 1145 m, which will minimize the resulting impact on ground-based augmentation system us-availability. In addition, to initialize these monitors when no earlier validated ephemerides are available, mean!)fusing raw measurements are proposed. The results show that the YE-TE and the measurement-based methods together are adequate to meet navigation integrity and availability requirements for category 1 precision approaches.