Accuracy Analysis of Clock Offset Compensation of Shanghai Sheshan VLBI Station

  Objectives  The compensation error of clock offset for a very long baseline interferometry (VLBI) station leads to the shift of delay observables in reference epochs, and further affects the estimation of universal time (UT1). The station clock offset includes the formatter clock offset and the instrumental delay, and the instrumental delay is essential for clock offset compensation. Since the Sheshan VLBI station is the fiducial station of the global geodetic reference frame, it is important to evaluate the compensation accuracy of the clock offset in the Sheshan station.

  Methods  We used the data of Sheshan station from all international geodetic observations from 2002 to 2017. In terms of different international VIBL service (IVS) correlators, data acquisition systems and recorded bandwidth, the compensation values of clock offsets were computed and analyzed by extracting clock parameters from correlator reports and formatter clock offsets from station log files.

  Results  Different VIBL correlators show good agreement and stability in long term for the instrument delay compensation of the analog terminal in the Sheshan station. However, a systematic bias of clock offset compensation of more than 1 μs is revealed in Sheshan station.

  Conclusions  In most cases, the compensation accuracy of clock offsets for Sheshan VLBI station can meet the requirement of UT1 measurements. However, it is necessary to improve the searching algorithm of the clock offset and the accuracy of clock offset compensation due to the system bias.