Impact of erroneous station coordinates on the estimation of UT1-UTC with VLBI Intensive sessions

So-called Intensives are one-hour-long VLBI sessions including mostly two stations, which are routinely observed to derive the Earth's phase of rotation expressed through the parameter UT1-UTC. Due to the limitation in time and participating stations, only a few parameters of interest can be estimated during the analysis, whereas others are fixed to their a priori values, such as the remaining Earth orientation parameters, as well as station and source coordinates. 

It is common knowledge that the impact of errors in the a priori station coordinates on the UT1-UTC results changes depending on the location, orientation and length of the baseline. In this presentation, we examine these effects for the first time in a systematic way covering the whole Earth. We performed Monte-Carlo simulations (MCS) with realistic noise models for a global 10° grid of artificial VGOS stations. The grid covers latitudes of -80° to 80° and longitudes of 0° to 180°. All possible and unambiguous baselines between these artificial telescopes are investigated. For every baseline, monthly schedules were generated over one year to eliminate source selection effects. In the MCS, the station coordinates are compromised with an error of 5 mm in either North-South, East-West or Up-Down direction. 

Thereby, we demonstrate that errors along the East-West direction tend to be less critical for long East-West baselines compared to errors in North-South direction. Furthermore, we show that errors in the station height are less critical compared to errors in North-South or East-West direction. The simulation results show that investigations of suitable locations for additional radio telescopes for UT1-UTC Intensive sessions cannot be selected by investigations in analytical equations such as the partial derivatives of the parameters alone but need more sophisticated analyses of error propagation.