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Antenna phase offsets

NOTE - to coincide with the release of updated OS Net station coordinates ("OS Net v2009") on 26th August 2016 the antenna offsets given on this page have changed.

A Global Navigation Satellite System (GNSS) baseline is measured between the phase centres of the two antennae. To relate this baseline to the station markers of the two end points you need to know the vertical height from the station marker to the antenna phase centre. 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 OS Net data it is very likely that your antenna will be different from the antennae at the OS Net stations. The difference in phase centre offsets between the OS Net antennae and your antenna need to be included in your GNSS data processing.

The antenna heights of all OS Net 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.

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 tables below lists the various antennae and offsets currently used in OS Net:

Official IGS antenna code used in RINEX header
(NAME__RADOME Code)

LEIAR20__LEIM

LEIAR20__NONE
LEIAR25__LEIT
ARP to phase centre offsets (meters)NorthL1= 0.0003
L2= 0.0001
L1= 0.0001
L2= 0.0000
L1= 0.0010
L2= -0.0001
EastL1= -0.0007
L2= -0.0007
L1= -0.0008
L2= -0.0008
L1= 0.0012
L2= 0.0004
UpL1= 0.1267
L2= 0.1352
L1= 0.1242
L2= 0.1337
L1= 0.1551
L2= 0.1631
Official IGS antenna code used in RINEX header
(NAME__RADOME Code)

LEIAR25__NONE

LEIAT504GG__NONETRM55971.00__NONE
ARP to phase centre offsets (meters)NorthL1= 0.0014
L2= -0.0002
L1= 0.0008
L2= -0.0001
L1= 0.0013
L2= 0.0004
EastL1= 0.0010
L2= 0.0003
L1= 0.0009
L2= -0.0001
L1= -0.0002
L2= 0.0006
UpL1= 0.1553
L2= 0.1640
L1= 0.0903
L2= 0.1191
L1= 0.0667
L2= 0.0577

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 tables 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.

Where available, the offsets used by Ordnance Survey are the same as those used by the International GNSS Service (IGS - ftp://ftp.igs.org/pub/station/general/) and the EUREF Permanent Network (EPN - http://www.epncb.oma.be/_documentation/equipment_calibration/), at the time of the computation of the OS Net station coordinates. There are sometimes differences between the IGS/EPN offsets and the calibrations from the NGS. It is more important that Ordnance Survey use antenna calibration consistent with IGS/EPN so that OS Net is in the closest possible agreement with other national networks in Europe.

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