Applying Clock Ties in GNSS and VLBI Integrated Solution

The global high-precision Terrestrial Reference Frame (TRF) is determined through the combination of the four space geodetic techniques: VLBI, SLR, GNSS, and DORIS. The multi-technique combination heavily relies on the common objects or parameters, i.e., the ties, which link the different techniques. Current realizations of the International Terrestrial Reference Frame (ITRF) employ local ties and Earth Orientation Parameter (EOP). Additional ties such as tropospheric ties and satellite orbit ties are being explored. Another promising approach is using clock ties, where co-located instruments connected to a common clock enable intra- and inter-technique clock tie modeling.

Clock ties between GNSS and VLBI are of particular interest as (1) GNSS provides dense observations at every epoch, allowing the epoch-wise estimation of clock parameters, and (2) VLBI typically relies on clock modeling due to the insufficient number of observations per epoch. Using the GNSS and VLBI co-location stations during VLBI CONT campaigns, we demonstrate inter-technique clock agreements within 0.1 and 0.2 ns for short (24-hour) and long (15-day) periods, respectively. We further explore the application of clock ties in the GNSS and VLBI integrated solution, focusing on the impact on station coordinates and tropospheric delays. We also address the challenges of modeling deterministic and stochastic components of the clock ties, highlighting their potential to improve the precision of TRF realizations.