IGS Co-located Station Antenna Tests

GNSS stations of the IGS are very important, especially when they are located at locations with other space techniques such a Satellite Laser Ranging (SLR) and Very Long Based Interferometry (VLBI), which all provide inputs to the establishment of the Earth’s International Reference Frames. The ‘ties’ between all the different antenna’s reference points is essential to be able to merge all the station position data into a consistent and coherent system.

The problem is when GNSS antennas are covered by uncalibrated Radomes, these are enclosures that cover the top part of the antenna to protect them from rain, snow, animal damage, etc. Below you can see some of the Radomes in use around the IGS network

Most of the Radomes used by station operators around the world are calibrated (more on that in a later post!) like the one pictured on the left. But older stations are still using Radomes that are not calibrated (such a pictured on the right) so we cannot know what effect exists in the signal transmission and the effect on the long term position estimations at these sites.

Covering a GNSS antenna with a radome whose phase center variations (PCVs) have not been calibrated will usually cause an unknown bias in the estimated position for that station. This bias is equivalent to a local tie error for those IGS stations colocated with other techniques and is thus a serious concern for ITRF (International Terrestrial Rotation Frame), which aims for tie accuracies at the 1 to 2 mm level. Radome effects can often be much larger, up to several cm.

At the IGS Infrastructure Committee, which I chair, we have been coordinating with the IGS reference Frame Working group, the IGS Antenna Working group and the IGS Network Coordinator since we think that the net position bias can be measured empirically by comparing solutions with and without the radome present. This requires that the radome can be safely removed temporarily without disturbing the antenna construction and then later be reinstalled. The empirical position offset provides a correction for the local tie vector to other techniques.

Essentially we will organize with each station operator an 8 week period of “radome off” operations over the next 18 months. During this period the station position will continue to be included in the IGS processing and it is hoped that the combined station position before , during and after the radome-off campaign will produce empirical tie corrections to be applied to the ITRF tie vectors.

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