Observation of GNSS satellites with VLBI antennas : from observation planning to practical implementation

Abstract

Within this master thesis options for an operational path of carrying out VLBI observations of satellites are investigated. From scheduling, over satellite tracking, to actual data acquisition, the complete process chain is considered. One concept for VLBI satellite observations has been exemplarily realized with the radio antennas at the Geodetic Observatory Wettzell (GOW), Germany. It is based on a newly developed satellite scheduling module of the Vienna VLBI Software (VieVS) in combination with dedicated satellite tracking features provided directly by the Antenna Control Units (ACU), which were activated in the NASA Field System (FS). Observing satellites with VLBI is a promising topic and has been discussed vividly in the VLBI community for the last few years. Despite several successful experiments, a clear strategy has not been shown so far, demonstrating a way of realizing such observations operationally. The challenges already start at the observation planning level, because the common scheduling software packages are currently not able to schedule moving satellites as targets. Additionally, the current standard data-formats used for VLBI schedule files do not provide the possibility to define satellite orbits in a suitable way. Finally, the most recent version of the FS does not yet support the generation of appropriate local control-files (SNAP files) for satellite observations, which would be required to carry out such VLBI sessions automatized. These restrictions are the reason that previous satellite observations had to be done with hand-written schedules and numerous manual interaction at the stations. Therefore, an operational path for satellite observations with VLBI is still missing. On that account a joint project with the GOW was started to investigate concepts for an operational path for VLBI observations of satellites. VieVS was extended with a new module providing the possibility of scheduling VLBI satellite observations. Considering several observation conditions, such as the common visibility from a selected station network, the program determines a selection of satellites being potentially available for observations and presents this information to the user, who is asked to assemble an observation plan. The schedule is then saved in the VEX format, with the changing satellite positions converted to sequences of corresponding topocentric celestial coordinates. Based on these VEX files satellites can be tracked - virtually stepwise - by consecutively re-positioning the antennas in a defined time interval. Alternatively, preparations were made at Wettzell to be able to track satellites in a truly continuous fashion by making use of the satellite tracking mode provided by the ACU of Wettzell's antennas. Therefore, modifications in the station-specific code of the FS were required to enable this feature. Several successful observations of GLONASS satellites were carried out in January 2014 on the baseline Wettzell-Onsala based on the implemented concept, validating that all developments worked properly. These experiments showed that VLBI satellite observations can already be carried out nearly operationally, which is important to promote further developments and research activities in this field of VLBI.