Error and Availability Analysis of CAT IIIb LAAS Augmented by Radar Altimeter

Curtis A. Shively, and Thomas T. Hsiao

Abstract:	The FAA’s Local Area Augmentation System (LAAS), already in a procurement phase for Category I (CAT I) precision approach applications, is being evaluated for extension to Category IIIb (CAT IIIb) autoland applications. In order to ensure sufficient availability for the more stringent CAT IIIb requirements, several augmentations to the basic LAAS CAT I capable system are under consideration. One such augmentation is use of the radar altimeter, which is already onboard aircraft equipped for CAT IIIb operations using the Instrument Landing System (ILS). In the simplest such concept, the radar altimeter is used as another ranging source in the position solution. This paper evaluates the improvement in availability of ranging source geometry for CAT IIIb operations from using the radar altimeter in this manner. A theoretical analysis is performed on the resulting error covariance matrix. New mathematical expressions are derived for the vertical error standard deviation using satellites and radar altimeter together. For the case of three satellites and radar altimeter, the vertical error standard deviation is equal to that of the radar altimeter output. For the case of four or more satellites and radar altimeter, the vertical error standard deviation is no larger than the lesser from using either the satellites alone, or the radar altimeter alone. Consequently, if satellite geometry is poor, this augmentation can considerably limit the vertical error, depending on the quality of the radar altimeter. This limiting is illustrated for the distribution of vertical error standard deviation corresponding to GPS satellite geometries visible at a particular location. Availability analyses are also presented for all ILS CAT III locations. Sensitivity of the results to the radar altimeter error standard deviation is shown. Results indicate that contemplated performance of the radar altimeter (with additional terrain data) could significantly limit the impact of poor satellite geometries and thus significantly improve availability of LAAS for CAT IIIb.
Published in:	Proceedings of the 2004 National Technical Meeting of The Institute of Navigation January 26 - 28, 2004 The Catamaran Resort Hotel San Diego, CA
Pages:	771 - 780
Cite this article:	Shively, Curtis A., Hsiao, Thomas T., "Error and Availability Analysis of CAT IIIb LAAS Augmented by Radar Altimeter," Proceedings of the 2004 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2004, pp. 771-780.
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