Hartebeesthoek Radio Astronomy Observatory (HartRAO) antenna axis offset determined by geodetic VLBI analysis and ground survey

Abstract

In the Very Long Baseline Interferometry (VLBI) space geodetic technique, various station-specific error sources corrupt the VLBI delay observable. An antenna axis offset (AO) model is applied in the VLBI data analysis for antennas with non-intersecting rotation axes, such as the Hartebeesthoek Radio Astronomy Observatory (HartRAO) 26-m and 15-m antennas. The a priori AO values recommended by the International VLBI Service for Geodesy & Astrometry (IVS) for use in geodetic VLBI data analysis are taken from values measured in ground surveys where possible, as such values are more accurate than values obtained from the VLBI analysis itself. The a priori AO values used for the HartRAO antennas in geodetic VLBI analysis have been identified as possible sources of error. The 26-m antenna’s a priori AO originates from a 2003 ground survey, conducted before a major bearing repair in 2008, which could have changed the AO. The a priori AO in use for the 15-m antenna was determined in a preliminary GPS survey only. In this study, the AO of the HartRAO 26-m and 15-m antennas were estimated from VLBI analysis using the Vienna VLBI and Satellite Software (VieVS) and compared with measurements from the latest ground survey conducted in 2014. It is found that the VLBI estimated values do not agree within the formal error with ground survey values. As the ground survey values are considered to be more accurate than the VLBI estimated values, further investigation of site-specific error sources that may be contaminating the accuracy of VLBI results is required.