A Statistical Interpretation of the Presence of Multipath at Ground Reference Sites

Janet S. Booth

Abstract:	In October of 1996, Boeing Flight Test began using a differential GPS system as an autoland verification system. As in any DGPS system, multipath is the primary source of error. We were faced with the problem of siting the reference receiver in a location near the runway but with minimal multipath effects. Multipath can be caused by reflections of the GPS signal off nearby objects as well as by diffuse reflections off the ground or ground plane that usually arrive from below the antenna. It is commonly known that in order to minimize multipath the autenna should be placed to maximize the receiver-to- object distance and to use an antenna with low gain at low elevations. This is difficult to accomplish on or near some airports with a predefined hardware configuration. Therefore, it is important to have some method to evaluate the presence of multipath to assist in choosing the best site possible. This paper discusses a method for evaluating the presence of multipath when siting a reference station. The method discussed uses the GPS signal from the receiver/antenna pair. The code-minus carrier method is used to isolate multipath plus receiver noise. Ionospheric delay is calculated and removed for each data point. The resulting data can be statistically expressed to determine the presence of specular multipath due to stationary objects as well as diffuse multipath from ground reflections. As a result, time and data can be saved and confidence levels in data quality are increased during flight tests using the differential GPS system.
Published in:	Proceedings of the 1998 National Technical Meeting of The Institute of Navigation January 21 - 23, 1998 Westin Long Beach Hotel Long Beach, CA
Pages:	247 - 253
Cite this article:	Booth, Janet S., "A Statistical Interpretation of the Presence of Multipath at Ground Reference Sites," Proceedings of the 1998 National Technical Meeting of The Institute of Navigation, Long Beach, CA, January 1998, pp. 247-253.
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