Bernese GPS Software Version 5.0 - Bernese GNSS Software

16. Antenna Phase Center Offsets and
Variations
16.1 Introduction
When introducing the observation equations in Section 2.3 the term ̺i k was denoted as the
“geometric distance between satellite i (at signal emission time t − τ) and receiver k (at
signal reception time t)”. The points with respect to which the geometrical distance ̺i k
should be measured have of course to be specified precisely. These points are called antenna
phase centers.
GNSS signals from individual satellites arrive at the receiver antenna from different directions.
The position of the antenna phase center depends on this direction. This directiondependence
is what we call antenna phase center variations. We also have to take into
account that the antenna phase center offset (with respect to the antenna reference point)
and the antenna phase center variations (with respect to the mean antenna phase center)
are not identical for L1 and L2 carriers. Furthermore, radomes that are used at many stations
to protect the GNSS receiver antennas from multipath and environmental effects have
an impact on the antenna phase center variations.
If highest accuracy is required each individual antenna has to be calibrated to obtain its
phase center corrections which then have to be applied in the analysis. For most applications
it can be assumed that antenna phase center variations depend only on antenna
and radome type. Nevertheless the effect has to be carefully modeled, especially if different
antenna/radome combinations with individual characteristics are used simultaneously. Ignoring
the antenna phase center corrections relative station height error may reach values
of up to 10 cm – independent of the baseline length. If only antennas of the same type
are used, the main effect is a scale factor in the network of up to about 0.015 ppm due
to the fact that antennas ”see” the same satellite under different elevation angles for long
distances.
For the transmitting antennas of the satellites of course a similar antenna phase center
variation is expected. The nadir angles of the lines of sight from one satellite to the individual
stations in a large scale network are different. Analogous to receiver antennas a scale factor
for the network solution is thus expected.
Relative antenna phase center models have been used within the IGS for a long time to
account for this effect (model name: IGS 01). One antenna type (AOAD/M T without
radome) has been assumed to represent a “perfect” antenna without any direction dependence
of the antenna phase center. All other antenna types were calibrated relative to this
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