Mode Filtering for Accurate Group Delay Calibration of GNSS Antennas

User antennas play an important role in the achievable positioning performance of Global Navigation Satellite Systems (GNSS) systems. Their importance is particularly relevant for geodetic or reference station applications, where high positioning accuracy up to the mm level is aimed at. In order to remove antenna-induced errors on GNSS measurements of reference stations, antennas are calibrated in terms of phase center and group delay variations (PCV/GDV). While PCV calibration is now state-of-the-art, the calibration of GDV is still posing measurement-based challenges, due to its extreme sensitivity to measurement system inaccuracies. This paper shows a technique to improve such accuracy when using near field spherical anechoic chambers for the antenna calibration. In particular, by making use of mode filtering, an improvement in the accuracy and repeatability of the measurement can be achieved.