Abstract:
Astrometric and geodetic Very Long Baseline Interferometry (VLBI) at the Hartebeesthoek Radio Astronomy Observatory (HartRAO) contribute to the realisation of the International Celestial Reference Frame (ICRF) and determination of the International Terrestrial Reference Frame (ITRF). Geodetic VLBI also provides the reference point of the HartRAO 26 m antenna used in co-location of the 15 m antenna and three other space geodetic techniques and also as reference datum for South Africa’s national surveying system. The VLBI Global Observing System (VGOS) is currently being introduced worldwide to form part of the Global Geodetic Observing System (GGOS). The HartRAO VGOS antenna will soon become operational and will have to be tied accurately to the 26 m and 15 m antennas. If HartRAO is to contribute to GGOS and continue participating in ICRF realisation and ITRF determination, GGOS requirements of 1 mm accuracy and 0.1 mm/yr stability on global baselines will have to be met, and station-specific errors degrading the accuracy of astrometric/geodetic VLBI results will have to be minimised. Station-specific errors addressed in this study relate to meteorological data, antenna calibration and pointing, antenna axis offset (AO) and the local tie between the 26 m and 15 m antennas.
The quality of HartRAO meteorological data used in VLBI and single-dish analysis was investigated by comparing it with meteorological data from a Global Navigation Satellite System (GNSS) station and a test installation. The opacity correction applied to 22 GHz observations on the 26 m antenna was investigated by comparison of precipitable water vapour (PWV) values calculated from HartRAO and GNSS meteorological data as well as Suominet integrated PWV. The 26 m antenna is one of only a few antennas capable of CRF realisation in the Southern Hemisphere at 22 GHz, the latter requiring accurate pointing. The 26 m antenna’s pointing performance and gain at 22 GHz were determined from gain calibration observations. A possible correlation between heating of the antenna structure and degraded pointing was investigated.
The AO of the HartRAO 26 m and 15 m antennas and the baseline length between the antennas were estimated in geodetic VLBI data analysis with the Vienna VLBI and Satellite Software (VieVS) for comparison with the more accurate ground survey values to determine whether it is possible to estimate these values with the required accuracy in VLBI analysis. Possible seasonal variation of the AO and baseline length were also investigated. The antenna axis offset altitude correction (AOAC), which accounts for the effect of the orientation of the equatorially mounted 26 m antenna on the tropospheric path delay, was simulated in VieVS to study its effect on the estimated AO of the 26 m antenna. Short baseline (SBL) experiments between the two HartRAO antennas were also scheduled and analysed with VieVS in order to estimate the local tie vector between the antennas more accurately and to determine whether the GGOS goal of 1 mm accuracy is achievable on such a short baseline at the very least. The results for baseline components and baseline length were compared with the corresponding results from the most recent ground survey.
