Detecting Global Positioning Satellite Orbit Errors Using Short-Baseline Carrier-Phase Measurements

Asystem to detectorbit errorsaffecting differential global positioning system integrity isdescribed.Themethod isbased on the use of carrier-phase ranging measurements madeby two or more ground-based referencereceivers separated by short baselines. A dual-frequency, geometry-free widelane approach is used to resolve cycle ambigu- ities present in the carrier-phase measurements. The performance of the proposed integrity monitor is evaluated relative to existing integrity requirements for aircraft landing navigation applications. The results show that real- time protection is achievable against all types of ephemeris errors that can ine uence aircraft precision approach and landing navigation. LTHOUGH initially developed to support military applica- tions, the global positioning system (GPS) has quickly grown into an important and versatile civil navigation utility. The differ- ential GPS (DGPS) technique, in particular, has provided improve- ments in positioning accuracy sufe cient for many demanding ap- plications, including aircraft navigation during precision approach andlandingphasesofe ight.However,forsuchdife cultapplications, highaccuracyisgenerallynotsufe cient.Thenavigationsystemmust also provide a means to ensure its own integrity by reliably detect- ing navigation system failures or anomalies. In this work, detailed consideration is given to the real-time detection of a specie c type of navigation failure: incorrect knowledge of a GPS satellite orbit. Each GPS satellite broadcasts orbit ephemerides so users can compute satellite locations at any time of interest. The satellite lo- cations, together with ranging measurements also obtained from the satellite signals, are used to compute user position. An error in knowledge of satellite position will, therefore, cause a resulting error in the computed user position. Nominally, these errors are negligibly small for DGPS users, but integrity considerations for aircraft precision landing navigation dictate that anomalous con- ditions must be quickly detected. Furthermore, the effects of orbit anomalies differ from those of other navigation failures, for exam- ple, GPS receiver failures, in that orbit errors ultimately cause nav- igation errors that are dependent on the time-varying displacement between the aircraft and ground-based DGPS reference receiver. Therefore, the impact of undetectable orbit errors on navigation must ultimately be assessed separately by each individual aircraft within the DGPS service volume. In this regard, the Federal Avia- tion Administration (FAA), RTCA, Inc., and the International Civil Aviation Organization (ICAO) are developing specie c ephemeris integrity performance standards for DGPS-based aircraft precision landing. The FAA' s local-area augmentation system (LAAS) will serve as the baseline DGPS architecture considered in this paper. Whenever the GPSdata broadcast by the satellites do not contain the correct satellite orbit parameters, an ephemeris anomaly is said to exist. Although there may be a variety of potential causes for such anomalies, for example, unscheduled maneuvers, incorrect or- bit uploads, and faulted data decoding in the receiver, all ephemeris