Multipath Considerations for Ground Based Ranging Sources

Chris G. Bartone

Abstract:	This paper discusses fielding of an airport pseudolite (APL) for the Local Area Augmented System (LAAS), which is a differential GPS (DGPS) based precision approach system. The addition of an airport pseudolite (APL) is being investigated to increase the availability of the LAAS. One of the major error sources for the APL link is the ground-to-ground multipath at the LAAS Ground Facility (LGF). This paper investigates the ground multipath performance of the APL link using a prototype LGF. Using a Multipath Limiting Antenna (MLA) for APL transmission, typical double difference (DD) error terms are calculated using a pair of High Zenith Antennas (HZA)s or MLAs for GPS and APL signal reception. Using GPS DD Error analysis as a baseline for comparison, the C/A code biases were in the 2.2-2.4 meter range regardless of whether HZA or MLA antennas were used. These biases were present without performing special antenna siting or radio frequency (RF) gain control and blanking. For tests where the HZAs were used for APL reception, a modest level of desired/undesired (D/U) ratio of 13 dB (11 dB for the MLA and 2 dB for the HZA) was obtained for the APL link geometry. multipath error term on the order of ½ meter is evident while using the HZA for reception, even with the APL Wideband (WB) code format. When two MLAs were used for APL and GPS reception, essentially no bias term was present in the APL DD Error. The non-existence of any real bias in the WB APL DD Error is an indication that the standing-wave multipath in the APL ground-to-ground link can be eliminated by the use of MLAs for APL transmission as well as reception, along with proper antenna siting. This corresponds to a total D/U of 22 dB (11 dB for each antenna) for the APL link geometry.
Published in:	Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999) September 14 - 17, 1999 Nashville, TN
Pages:	1491 - 1498
Cite this article:	Bartone, Chris G., "Multipath Considerations for Ground Based Ranging Sources," Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999), Nashville, TN, September 1999, pp. 1491-1498.
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