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The L1 or L2 phase center is the theoretical point in space where the carrier phase of the GPS signal is received "on average." The actual location where this signal is received, however, varies as a function of the direction of the incoming GPS signal and the electrical characteristics of the GPS antenna. Failure to account for antenna-phase-center variation can lead to errors of up to 10 cm in height when processing GPS data for a baseline involving two different antenna types.
The National Geodetic Survey (NGS) has calibrated many GPS antennae to determine how the location of the L1 and L2 phase centers vary with respect to the antenna reference point (ARP, a specified physical point on the antenna). The agency makes these calibration results available for public use via the web site: http://www.grdl.noaa.gov/ANTCAL/, a recent paper by Gerry Mader describes the proceedure for determining the antenna phase calibration.