The data sheets for some of our antennas such as GPS-702-GG (http://www.novatel.com/assets/Documents/Manuals/om-20000095.pdf) report phase centre offsets with respect to values obtained in absolute and relative antenna calibrations.
What is a relative calibration?
In relative calibration, all antenna offsets (PCO) and phase centre variations (PCV) are computed with respect to a reference antenna which is normally assigned zero PCV values. For NGS relative calibrations, the reference antenna is the Dorne Margolin choke ring antenna, type T (AOAD/M_T). A relative calibration is therefore biased by the phase advance/delay experienced by the reference antenna. Please see http://www.ngs.noaa.gov/CORS/Articles/MaderGPS-Sol-1999.pdf for more information
What is an absolute calibration?
In an absolute calibration, all antenna offsets and phase centre variations are independent of the reference antenna. To conduct an absolute calibration, the antenna being tested is moved via a robot so that a particular satellite is received at different angles by the test and reference antennas. This angular separation enables cancelation of the reference antenna effects, leaving behind only the antenna offsets and phase center variations of the test antenna. Please see http://www.ngs.noaa.gov/CORS/Articles/Bilich-and-Mader_ION2010.pdf for more information
For more information on antenna calibration please visit https://www.ngs.noaa.gov/ANTCAL/FAQ.xhtml