Calibrating the L1 and L2 Phase Centers of a Block IIA Antenna

Gerald L. Mader and Frank Czopek

Abstract:	Estimates of the L1, L2 and LC (ionosphere-free linear combination) phase centers for the BLK IIA GPS satellite antenna have been determined. These results indicate that the L1, L2, and LC phase centers have z-axis (toward the earth) offsets of 118, 87, 166 cm from the satellite center of mass. These offsets differ significantly from the currently assumed offset values. This calibration used a BLK IIA antenna as a receiving, rather than a transmitting antenna. The BLK IIA antenna was pointed at the zenith from the roof of the Boeing facility in Seal Beach, CA and was connected to a standard dual-frequency, geodetic-quality GPS receiver. By computing baseline vectors from a nearby reference antenna, first to a standard geodetic-quality GPS antenna, and then to the BLK IIA, the phase centers of the BLK IIA relative to the standard antenna could be determined. This calibration procedure is essentially identical to that used by NGS numerous times on previous antenna calibrations. The primary difference for this calibration was to limit the data used to satellites above 60º elevation due to the highly directive antenna beam and to include only those periods when at least two satellites were in view. Graphs of the phase residuals versus elevation from these solutions show an azimuthal variation for L1 but not for L2. These effects may be unique for this particular antenna and to each of the antennas currently in orbit, depending on the tuning done to produce the desired amplitude response. The relation of satellite antenna offsets to position and timing measurements and importance of accurate antenna calibration will also be discussed.
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:	1979 - 1984
Cite this article:	Mader, Gerald L., Czopek, Frank, "Calibrating the L1 and L2 Phase Centers of a Block IIA Antenna," 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. 1979-1984.
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