ANTENNA PHASE CENTRE OFFSET |
All GPS measurements relate to the distance from the electrical centre of
the satellite's transmitter to the electrical centre of the receiving antenna.
Ideally the physical centre should coincide with the electrical
centre, however there may be a constant offset.
This is an antenna manufacturing problem, but if the antenna is always oriented
in the same direction, the impact on the groundmark-to-groundmark solution
will be a systematic shortening or lengthening of the baseline. Because
the electrical centre varies with the direction and strength of the incoming
signal, a variation in the satellite-receiver geometry will cause the position of the electrical centre to also vary.
Tests have shown that this effect is comparatively insignificant for microstrip
antennas (well below 1cm), though it is larger in the case of quadrifilar
and helical antennas (section 4.1.2).
For high precision applications care has to be taken not to mix antenna types, or to swap antennas between sites and receivers during a survey.
Models of the antenna phase centre variations for various geodetic antennas are available from the IGS.
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© Chris Rizos, SNAP-UNSW, 1999