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Performance Evaluation of On-Airport Local Area Augmentation System Architectures

Recently there has been growing demand for a Category III precision landing system architecture which is completely contained on airport property. In response, Stanford is exploring different Local Area Augmentation System (LAAS) architectures and evaluating their performance. To maximize this performance, the architectures incorporate information from a variety of sources, including code-centered carrier techniques, satellite motion, and airport pseudolites (APLs). In addition to using APLs as additional ranging sources, a new concept was developed for positioning with pseudolites. The new concept involves using the differential carrier phase between a pair of pseudolites. This differential carrier phase provides an additional observable that can improve the accuracy of GPS positioning. The surfaces of constant differential phase are hyperbolic. In the far field however, the system approximates an angular based system such as the Instrument Landing System (ILS). For example, if the two pseudolites are located at the top and bottom of a tower near the aim point, the phase difference measurement at the aircraft is analogous to the glideslope measurement for ILS. Although there is an integer cycle ambiguity associated with the differential carrier phase measurement, this ambiguity can be resolved when the aircraft is on long final approach. At this distance, the position difference between integer candidates is so large that differential code phase positioning can isolate the correct integer. Several APL configurations have been explored through computer simulation. The effects of both additional ranging sources and APL differential carrier phase measurements are incorporated into the simulation. Also incorporated are the effects of satellite motion and codecentered carrier smoothing. The resulting performance is due to a combination of these effects. In addition to simulation, ground experiments and flight tests have been performed. The results of simulation and flight testing are presented, and the navigation performance of these architectures is assessed.