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When processing data from different receiver or antenna types together, the antenna phase center offsets are very important. Different antennas may have antenna phase centers differing by several centimeters. In order to achieve results with millimeter accuracy these values must be considered carefully.
The U.S. National Geodetic Survey NGS (Gerald L. Mader) gives the following information at its WWW home page www.ngs.noaa.gov/ANTCAL/images/summary.html:
“A GPS geodetic solution for a baseline provides the vector between the phase centers of the antennas at either end of the baseline. However, a real antenna does not have a single well-defined phase center. Instead, the location of the phase center is a function of the direction from which it receives a signal. If this variation is ignored, the measured baseline will be between the average phase centers of the two antennas. These average phase center locations are a weighted average of all the individual phase centers for each of the measurements included in the solution. When the antennas at opposite ends of relatively short baselines are identical, these variations should cancel out and no effect is seen. However, different antenna types exhibit different phase variations and baselines with different antenna types will show increasing sensitivity to such things as elevation cutoff angle and the distribution of observations within a solution....
Almost all GPS antennas currently in use are azimuthally symmetric and the dominant phase variation occurs with elevation. However, the local environment around the antenna can introduce both azimuth and elevation variations from the ideally measured phase patterns...
Two types of antenna calibrations exist, the relative antenna calibrations which are determined by field measurements relatively with respect to a reference antenna and absolute calibrations.
Note - Trimble 4D Control Server assumes and requires antenna calibrations used in the system to be absolute, because IGS products, such as their Ultra Rapid Orbits, are based on absolute calibrations.