Antenna phase centre offsets

A GPS baseline is measured between the phase centres of the two antennae. To relate this baseline to the station markers of the two end points the vertical height from the station marker to the antenna phase centre must be known. The phase centre is not a physical point that can be measured to, so the offset of the phase centre from a physical point on the antenna must be known.

Using the correct phase centre offsets becomes very important when different antenna types are used in a survey. When using the Ordnance Survey National GPS Network active stations it is very likely that your antennae will be different from the antennae at the active stations. The difference in phase centre offsets between the Ordnance Survey active station antennae and your antennae need to be included in your GPS data processing.

The antenna heights of all Ordnance Survey active stations are measured vertically from the station marker to the Antenna Reference Point (ARP) for each antenna, which is usually the base of the antenna mount. This height is the one given in the RINEX file header. The offset from the ARP to the phase centre is then added to the ARP height (usually in the processing software) to give the height of the phase centre above the ground marker. For all the antennae in the Ordnance Survey National GPS Network the station marker is the ARP, so the height given in the RINEX file is 0.000. However, that does not account for the phase centre offset from the ARP.

The main phase centre offset component is vertical (up) but there are also small horizontal offsets (north and east) that can be applied. There are actually two phase centres in an antenna – one for the L1 frequency and the other for L2, and each phase centre has a different offset.

The table below lists the various antennae and offsets used in the active network.

Phase centre height above station marker = ARP height (from RINEX file) + Up offset of phase centre

Where the processing software only allows for one up phase centre offset to be entered use the following calculated value:-
Phase Centre Offset = (2.545 x L1) - (1.545 x L2), where L1 and L2 are the up offsets from the table above.

The offsets in the table above give the mean position of the L1 and L2 phase centres. The actual position of the phase centres varies for each satellite being tracked, depending on the elevation of the satellite. For the highest geodetic accuracy (particularly in height) this variation must be modelled also. More information on antenna calibration, phase centre offsets and elevation dependent phase centre modelling can be found at the GPS Antenna Calibration page of the US National Geodetic Survey. Click the link to go there now. (http://www.ngs.noaa.gov/ANTCAL/).

The offsets used by Ordnance Survey are the same as those used by the international GPS Service (IGS) and the EUREF Permanent Network (EPN), at the time of the computation of the Active Network coordinates.

There are differences between the IGS/EPN offsets used and what may appear to be a more appropriate calibration from the NGS. It is more important that Ordnance Survey use antenna calibration consistent with IGS/EPN so that the Active network is in the closest possible agreement with others in Europe. The IGS offsets are in the file "igs_01.pcv" available via anonymous ftp from IGS. Click the link to go there now. (ftp://igscb.jpl.nasa.gov/pub/station/general/igs_01.pcv). The table below shows which calibrations in the igs_01.pcv file relate to the antennas in the Active Network.