Using clock parameters for the combination of GNSS and VLBI

The International Terrestrial Reference Frame (ITRF) serves as the foundation for applications in navigation and Earth sciences. It is computed and released by the International Earth Rotation and Reference Systems Service (IERS) and is a combination of the solution time series of four space geodetic techniques: Global Navigation Satellite System (GNSS), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), Satellite Laser Ranging (SLR), and Very Long Baseline Interferometry (VLBI). The latest release, ITRF2020 and its updates, does not yet achieve the Global Geodetic Observing System (GGOS) goal of 1 mm accuracy and 0.1 mm/yr stability.

One of the major accuracy-limiting factors when combining the four techniques are systematic differences between the techniques. With the aim of improving the combination by reducting these systematics, the combination of clock parameters of different instruments referenced to one common stable clock at one site can be considered. We present an approach that introduces a common time frame for both techniques. Therefore, the definition of the reference clock is of the utmost importance. We realize a mean stable reference clock (MSRC), defined as the mean of the clock estimates of all stable clocks (H-masers). This significantly reduces variations in the reference clock and thus minimizes its impact on the estimated station clock parameters. We discuss first results of a combination of VLBI and GNSS clock parameters performed on a basic level, by introducing GNSS clock estimates as a priori values in the VLBI analysis.

A unified clock parameterization is required when combining different space geodetic techniques considering clock parameters. Until now, each technique uses its own parametrization. In the current GNSS strategy, clock parameters are estimated epoch-wise with a high temporal resolution of 5 minutes, whereas the VLBI strategy uses session-wise clock offset, drift, and quadratic terms, along with 1-hourly piecewise linear continuous clock parameters. We discuss initial concepts for the homogenization of the clock parameterization for VLBI and GNSS on normal equation level.