Performance Improvements Achieved Using Differential Phase Map Compensation in a Direction Finding System

Karl W. Ulmer and Eric Sutton

Abstract:	This paper presents a successful application of GPS based attitude determination to a man portable military system requiring accurate, low cost direction finding capabilities. Carrier phase interferometry involves resolving relative antenna positions to the millimeter level using GPS signals. Maintaining millimeter level accuracy in a field deployable system requires careful attention to the individual error sources and the factors that impact them. This paper focuses on the error sources associated with the differential antenna phase reception pattern. Receiver phase measurement errors are on the order of 1.0 to 2.0 mm. Differential multipath errors typically range from 1.0 to 5.0 mm, depending on the environment. Errors introduced by variations in the differential antenna phase reception pattern (referred to in this paper as antenna errors) can be as large as 2.0 cm. The antenna errors are unique for L1 and L2 and are a function of the angle of arrival of the electromagnetic wave incident on the antenna element (i.e. a function of both azimuth and elevation angle). The differential antenna phase reception pattern is primarily a function of the particular antenna installation and can be measured and then accurately compensated for in software using phase map lookup tables. This paper will present an analysis of the effects of antenna errors on azimuth and elevation accuracy and techniques for software compensation of antenna errors. This paper will also present data regarding the antenna-to-antenna repeatability of these errors and live test data demonstrating the performance improvements achievable using real-time differential phase map compensation in a direction finding system.
Published in:	Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998) September 15 - 18, 1998 Nashville, TN
Pages:	563 - 572
Cite this article:	Ulmer, Karl W., Sutton, Eric, "Performance Improvements Achieved Using Differential Phase Map Compensation in a Direction Finding System," Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998), Nashville, TN, September 1998, pp. 563-572.
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