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).
Absolute chamber calibrations are another possibility for the determination of GPS antenna's PCV. The test antenna will be precisely moved within an anechoic chamber. An artificial GPS signal is used. Difficulties for this type of calibration may be the artificial signal, the precise definition of the reference point, the mechanical precision of the whole mechanical setup and, possibly, remaining multipath. It is hard to achieve a very high number of observations for a well covered antenna hemisphere.
The absolute field calibration
is another calibration procedure for the determination of PCV. The technical
constraints for this calibration are quite high. But through a special
field setup and measurement program (rotation AND tilts of the antenna,
elimination of multipath), the results are absolute and not site dependent.
Azimuthal PCV can be resolved and results down to elevation zero can be
determined. More detailed information on absolute PCV field calibration
can be found in the next paragraph and in various
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