GPS-Based Precision Landing Systems
Precision landing systems guide airplanes to the touchdown point on a runway in inclement weather (low visibility) conditions. The civilian precision approach and landing system in current wide use is based on system adopted in the 1940s and relies on analog signals. In the past decade there has been a significant world-wide effort to develop differential GPS-based precision landing systems. A wide-area variant of these systems known as WAAS has been fielded and is currently providing some level of precision guidance services. Local-area variants of these systems are also under development. In the local-area systems, information from the aircraft’s GPS receiver is compared to measurements from a reference GPS antenna at the touchdown point (or close to it). This allows removing common mode errors and when used with the GPS carrier phase observables can result in accuracies on the order of centimeters. This level of accuracy will be required by the local-area system known as Shipboard Relative GPS (SRGPS). SRGPS is the seaborne variant of the Joint Precision Approach and Landing System (JPALS) being developed by the US Department of Defense. SRGPS will support flight operation to and from naval vessels (Pervan, Fang, Gebre-Egziabher, Pullen & et. al, Navigation, 50:181-191).
GPS/WAAS-Based Precision Landing Systems
In collaboration with S. S. Jan (National Cheng Kung University, Tainan, Taiwan), Todd Walter and Per K. Enge (both Stanford University), we have examined the performance of a WAAS/GPS-based precision landing system which integrates barometric altimeter information in the navigation solution. The barometric altimeter information is used as a range measurement from a virtual satellite directly overhead. It develops a framework by which barometric altimeter information can be incorporated into accepted GPS/WAAS error models. It does this, in part, by developing statistical error models for the barometric altimeter measurement. The models are developed and validated using 30 years worth of atmospheric data collected for observation stations in CONUS.
JPALS/SRGPS Precision Landing System
One of the challenges associated with SRGPS is how to deal with the effects of ship structural flexure on the accuracy of navigation solution. Because of heavy personnel and equipment traffic, a GPS reference antenna cannot be installed at the touchdown point of an aircraft carrier. Thus, measurements taken from various GPS antennas installed at various points on the ship will be used to construct a virtual or synthetic measurement that would be observed by a GPS antenna at the touchdown point. If ships were a rigid bodies, this would be a simple problem of geometry. However, ship flexure can introduce errors on the order of several centimeters. Thus, constructing the virtual measurements becomes a non-linear estimation problem with input noises which have non-stationary statistics. Our research in this area has been to characterize the non-stationary ship flexure statistics, design algorithms which allow constructing the virtual measurement and develop methods for computing precise confidence bounds on the errors of this virtual measurement. Confidence bounds such as these will be required to support SRGPS real-time integrity risk calculations.
Relevant Publication
Jan, S., D. Gebre-Egziabher, T. Walter, P. Enge, 2007, "Altimeter Confidence Bound for Improving GPS Vertical Guidance," to appear in IEEE Transactions on Aerospace Electronic Systems, (Journal Article) More Details, Request PDF
D. Gebre-Egziabher and Y. Shao, “Model for SRGPS Flexure and Attitude Error Allocation," in review by IEEE Transaction on Aerospace Electronic Systems.
Pervan S, F. C. Chan, D. Gebre-Egziabher, 2003, Performance Analysis of Carrier Phase DGPS Navigation for Shipboard Landing of Aircraft, ION Journal of Navigation, Vol. 50, No. 3, pp. 181-191., (Journal Article) More Details, Request PDF
Last Modified: 2007-08-05 at 23:57:18
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