Modifications to GPS Reference Station Antennas to Reduce Multipath

A. Kerkhoff, R.B. Harris, C.P. Petersen, A. Pickard

Abstract:	The National Geospatial Intelligence Agency’s (NGA) GPS Monitor Station Network (MSN), consisting of 11 ground reference stations distributed throughout the world, is used collaboratively by NGA and the Air Force Operational Control Segment (OCS) to monitor the health of the GPS constellation, and to generate both broadcast and precise ephemeris products. Multipath caused by signal scattering off objects in the vicinity of the antennas at these stations continues to be a dominant error source in MSN measurement observables. Both hardware-based and processing-based techniques are implemented in the MSN to mitigate this effect, however further suppression of multipath is sought in order to improve system performance. This paper considers two different approaches to modifying the basic choke ring antenna design used in the MSN in order to reduce its reception of multipath. One approach consists of placing a large metallic ground plane directly beneath the antenna. This has the effect of shaping the antenna radiation patterns near and below the horizon so as to reduce reception of multipath signals from those directions. Another approach is to place a specific arrangement of RF absorbing foam around and beneath the base of the antenna in order to attenuate incoming multipath signals before they reach the antenna. Live-sky GPS receiver measurements were performed with a ground plane and then a novel arrangement of RF absorbing foam applied to a choke ring antenna. The results of a comprehensive analysis are presented to demonstrate the degree to which each of these approaches reduce multipath error in raw code and carrier phase observables as well as in carrier phase smoothed observables used in the GPS precise ephemeris production process.
Published in:	Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010) September 21 - 24, 2010 Oregon Convention Center, Portland, Oregon Portland, OR
Pages:	866 - 878
Cite this article:	Kerkhoff, A., Harris, R.B., Petersen, C.P., Pickard, A., "Modifications to GPS Reference Station Antennas to Reduce Multipath," Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010), Portland, OR, September 2010, pp. 866-878.
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