Antenna Phase Centre

Fundamentals
Title	Antenna Phase Centre
Author(s)	J. Sanz Subirana, J.M. Juan Zornoza and M. Hernández-Pajares, Technical University of Catalonia, Spain.
Level	Basic
Year of Publication	2011

The GNSS measurements are referred to the so-called antenna phase center. The position of the antenna phase center is not necessarily the geometric center of the antenna. Indeed, it is not constant, but it depends on the direction the radio signal coming in.

The phase center is defined as the apparent source of radiation. If the source were ideal it would have a spheric equiphase contour, but the real case is slightly different, because the equiphase contour is irregular and each segment has its own apparent radiation origin.

The initial phase center offset (PCO) for a particular frequency, given in north-east-up components relative to the antenna reference point (ARP). PCO is considered the average point of signal reception if the satellite signal direction is not taken into account.

The position of the electrical antenna phase center of an individual measurement and the position of the mean electrical antenna phase center are not identical. The deviations, as a result of this comparison, are defined as antenna phase center variations (PCV) .

Hence the phase center of an antenna is not only angle dependent (elevation and azimuth) but also it depends on the signal frequency. Typically, the ARP coincides with the axis of attachment of the antenna to a monument or surveying instrument. The manufacturers include technical sheets indicating the phase center offsets.

Figure 1: Layout of a Dorne Margolin T Antenna including the Antenna Reference Point (ARP) to which the phase center correction values in the corresponding IGS antenna phase center model refer.

To model the effect of the Antenna Phase Center, it must be taken into account the contribution of the Receiver Antenna Phase Centre and the Satellite Antenna Phase Centre.

Antenna Phase Center corrections for different models of receivers, relative to the AOA Dorne-Margolin T (AOAD/M_T) antenna , were compiled by IGS.

These PCV files include North East Up (NEU) offsets of the mean phase center location and Phase center Variations (PCVs) as a function of the elevation angle over [math]\displaystyle{ 10 }[/math] degrees. They were determined using short-baseline field measurements taking the AOAD/M_T antenna as reference [Gendt, G. and Schmid, R., 2005] . Nevertheless, as shown by [Mader, 1999], on long baselines, the relative PCVs are not adequate, even for the same antenna model.

On November 5th 2006, IGS switched from relative to absolute antenna phase center corrections, which took place together with the adoption of ITRF2005 . The absolute antenna phase center offsets and PCVs are determined by a robotic system developed by the University of Hanover and the company Geo++. They include azimuthal values and elevations down to [math]\displaystyle{ 0 }[/math] degrees. IGS is compiling a consistent set of absolute antenna phase center corrections for both receivers network and satellites, which are provided in so-called the Antenna Exchange Format (ANTEX) files .

Since November 5th 2006 (GPS week 1400) the IGS products, i.e., SP3 files with the GPS and GLONASS precise orbits and clocks..., and the Solution (Software/technique) INdependent EXchange Format (SINEX) files with the computed precise coordinates of the reference stations network are associates to such ANTEX files with absolute antenna phase center corrections (offsets and PCVs). It must be pointed out that users should not mix absolute and relative PCVs. Moreover, absolute PCVs require corrections for both satellites and receiver antennas.