High Accuracy Characterization of Geodetic GPS Antennas Using Anechoic Chamber and Field Tests

Bruce R. Schupler and Thomas A. Clark

Abstract:	Since 1987 we have been characterizing GPS antennas using two anechoic chambers at the NASA Goddard Space Flight Center (GSFC) plus the GPS testing facility at the Goddard Geophysical and Astronomical Observatory (GGAO). Our measurement program has concentrated on determining the radiometric phase center, and the phase, amplitude and polarization response of the geodetic GPS antennas used throughout the IGS network, at DGPS base stations, and by commercial surveyors. Recent measurements have stressed the variations of these parameters over the entire range frequencies of GPS and GLONASS from below L5 to above L1. We have also measured a number of other types of antennas including low cost L1 antennas for timing applications, and specialized antennas for spacecraft and aircraft applications. Measurements have been made to characterize subtle mm-level effects of a number of types of radomes, different types of mountings, structural materials in the near-field of the antenna, and differences between similar antennas made by different manufacturers. During the course of each measurement session we have remeasured one particular antenna to provide a consistency check on our procedures and processing. The phase centers determined from this repeated re-measurement of a single antenna have been consistent at the few millimeter level. While almost all of the geodetic chokering antennas that we have measured in recent years have very similar phase centers, phase patterns, and amplitude patterns, there are some variants of this design that perform quite differently. In addition, the presence of different radomes, mounting configurations, and surrounding material has a significant effect on these parameters. Also, the antenna amplifier has, in some cases, caused a very significant effect on the frequency domain performance. This last factor is extremely important for combined GPS+GLONASS measurements, for time synchronization, and when the new L5 frequency is activated. In this paper we will present a summary of our measurement results for the most common types of geodetic GPS antennas presently in use. These results will focus on the following points: a) The systematic change with frequency in the phase and amplitude characteristics of these antennas b) A comparison of the measured performance of several similar antennas by different manufacturers c) The effect of material near the antenna d) A comparison of the measured performance of several different designs of geodetic GPS antennas e) The frequency range over which these antennas may be expected to operate effectively
Published in:	Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000) September 19 - 22, 2000 Salt Palace Convention Center Salt Lake City, UT
Pages:	2499 - 2505
Cite this article:	Schupler, Bruce R., Clark, Thomas A., "High Accuracy Characterization of Geodetic GPS Antennas Using Anechoic Chamber and Field Tests," Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000), Salt Lake City, UT, September 2000, pp. 2499-2505.
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