This project is funded by the German Research Foundation (DFG) within the framework of the research unit on „Space-time reference systems for monitoring global change and for precise navigation in space“ (FOR 1503). The goal of the project is to deliver consistent terrestrial and quasi-inertial celestial reference frames based on a common set of parameters. The reference frames will be generated by combining time series of homogeneously processed VLBI, SLR and GNSS observations.

A main focus is on an improved handling of non-linear station motions by an extended parameterization (e.g., for seasonal signals). In order to allow for the estimation of periodic station signals besides the linear station velocities, the amplitudes of sine and cosine waves were implemented as additional parameters into DOGS-CS, DGFI's combination software. This extended parameterization was applied to a global GNSS solution computed over a time span of 5 years. The results were compared with the standard approach (linear station motion model) and with the BKG results obtained from improved geophysical fluid models. As an example, the figure shows the height time series for the GNSS station Zwenigorod (Russia) in comparison with the geophysical model results and the estimated annual signal.

The main tasks are:

• Development of refined strategies for the combination at the normal equation level
• Parameterization of non-linear station motions
• Generation of homogeneously processed time series
• Computation of long-term and epoch reference frames