Antenna phase centre offsets

A Global Navigation Satellite System (GNSS) baseline is measured between the phase centres of the two antennas. 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, so you need to know the offset of the phase centre from a physical point on the antenna.

Why antenna phase centre offsets are important

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 antennas at the OS Net stations. The difference in phase centre offsets between the OS Net antennas and your antenna need to be included in your GNSS data processing.

How are OS Net station antennas heights measured?

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.

OS Net antenna offsets

The tables below lists the various antennas and offsets currently used in OS Net.

Please note that to coincide with the release of updated OS Net station coordinates ("OS Net v2009") on 26 August 2016, the antenna offsets given on this page have changed.

Official IGS antenna code used in RINEX header (NAME__RADOME Code)		LEIAR20__LEIM	LEIAR20__NONE	LEIAR25__LEIT
ARP to phase centre offsets (meters)	North	L1= 0.0003 L2= 0.0001	L1= 0.0001 L2= 0.0000	L1= 0.0010 L2= -0.0001
	East	L1= -0.0007 L2= -0.0007	L1= -0.0008 L2= -0.0008	L1= 0.0012 L2= 0.0004
	Up	L1= 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	LEIAR10__NONE	TRM55971.00__NONE
ARP to phase centre offsets (meters)	North	L1= 0.0014 L2= -0.0002	L1= 0.0014 L2= 0.0006	L1= 0.0013 L2= 0.0004
	East	L1= 0.0010 L2= 0.0003	L1= -0.0002 L2= 0.0002	L1= -0.0002 L2= 0.0006
	Up	L1= 0.1553 L2= 0.1640	L1= 0.0880 L2= 0.0812	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.
This formula is for the ionosphere free observable where the apparent phase centre can often be positioned below both L1 and L2 centres and sometimes seem to be outside the physical antenna.

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

Download the full phase centre offsets (ANTEXT format in a zip file) for all antenna types in OS Net.

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 (OS) are the same as those used by the International GNSS Service (IGS) and the EUREF Permanent Network (EPN) 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 OS use antenna calibration consistent with IGS/EPN so that OS Net is in the closest possible agreement with other national networks in Europe.