Calibration of LAAS Reference Antennas

Alfred R. Lopez

Abstract:	The Differential GPS, DGPS, Local Area Augmentation System, LAAS, utilizes reference antennas and receivers to measure the time of arrival of GPS signals at precisely surveyed points. These measurements are then used to broadcast differential corrections to approaching aircraft. A common misconception is to assume that the antenna phase center is the precise point whose position is being measured. The antenna phase center (or equivalent carrier phase-delay center) is a well defined concept: “The location of a point associated with an antenna such that, if it is taken as the center of a sphere whose radius extents into the far-field, the phase of a given field component over the surface of the radiation sphere is essentially constant, at least over that portion of the surface where the radiation is significant,” (IEEE definition). The antenna phase center is defined at one frequency, the carrier frequency. For GPS reference antennas, a new antenna concept, the antenna group phase center (or equivalent code phase center) should be defined. Thus, the antenna has two phase centers, the carrier phase center and the code phase center. These phase centers are not necessarily points and the two phase-delay centers may or may not have the same characteristics. Typically, they do not have the same characteristics. This paper introduces the concepts of code-phase delay and carrier-phase delay as related to the calibration of LAAS reference antennas. It describes the characteristics of one candidate type of reference antenna for LAAS. It discusses the results of some recent field measurements of antenna code-phase-delay minus carrier-phase-delay. It also discusses the measurements of code and carrier phase delays, which may be used to calibrate GPS reference antennas.
Published in:	Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) September 11 - 14, 2001 Salt Palace Convention Center Salt Lake City, UT
Pages:	1209 - 1218
Cite this article:	Lopez, Alfred R., "Calibration of LAAS Reference Antennas," Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 2001, pp. 1209-1218.
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