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PCV Calibration of GNSS Antennas

General Overview

Robot of the Institut für Erdmessung (IfE) in use for absolute Antenna Field Calibration. The picture shows the robot with the calibrated Allan Osborne Dorne Margolin (AOA_DM_T) Antenna (former Null Antenna for relative calibrations).
Robot of the Institut für Erdmessung (IfE) in use for absolute Antenna Field Calibration. The picture shows the robot with the calibrated Allan Osborne Dorne Margolin (AOA_DM_T) Antenna (former Null Antenna for relative calibrations).

The point of reception of the GPS signal at the antenna is not homogenous. This phase error is a function of the signal's direction and varies with azimuth and elevation. The magnitude of the phase center variations (PCV) typically spans a range of mm to cm. The neglection of this antenna behavior can lead to baseline errors between mm and cm. It can even reach up to 10 cm for the height component. The different error magnitudes of the effects are due to differences of measurement setups and following GPS processings (antenna types, mixed baselines, used linear combination, estimation of tropospheric parameters, network type, kinematic applications, used elevation angle, measurement time, engineering applications, rotated/tilted antennas, type and quality of PCV correction ...).

A widely used correction for the PCV is a pure offset. But the offset estimation depends on the elevation mask and the location/constellation/multipath. Furthermore, there are different definitions of the offset (3D offsets relativ to a reference antenna, 2D horizontal absolute offsets + vertical relative offset, minimum conditions). An absolute 3D offset can only be determined with the help of rotations AND tilts of the antenna. Therefore, offsets are only a rudimentary and approximate correction. A complete correction modell including PCV should be used in precise applications.

The most common calibration type is the relative field calibration. The setup is quite simple. Reference coordinates are in most cases necessary. The results for the PCV of the tested antenna are relative to a reference antenna. Per definition, the PCV of this reference antenna are set to zero and the offsets are also fixed. The typical reference antenna for the relative field calibration is the AOAD/M_T choke ring antenna (e.g. IGS/NGS). Because of the influence of site multipath and the insufficient covering of the antenna hemisphere with observations, typical results for the PCV corrections are only elevation dependent and have a minimum elevation mask of 10 degrees. Due to the "lack" of absolute PCV information, the corrections are not sufficient for networks with differently orientated antennas (large networks, engineering applications).