Terrestrial reference systems (TRS) are needed for geodetic positioning and navigation on the Earth surface, for representation of the Earth’s gravity field, and for geophysical interpretations. As the geodetic observations include extraterrestrial objects like artificial satellites and quasi-stellar radio sources (quasars), celestial reference systems (CRS) are required, too. The systems are realized by reference frames (CRF, TRF) in terms of positions and velocities of a set of points on the Earth surface or in space, respectively. For the transformation between the frames, besides relativistic time scaling, the relative orientation is needed which is given by the Earth orientation parameters (EOP). For a consistent realization, the coordinates in TRF and CRF and the EOP have to be estimated in a common adjustment including well-defined datum constraints.

Terrestrial reference systems are realized by combining observations of the space-geodetic techniques: VLBI, SLR, GPS, and DORIS. An essential role plays the geodetic datum (coordinate origin, scale and orientation). Its definition (geocentre, light velocity, rotation axis) and the methods of its realization are investigated in detail, making use of the respective strengths of the individual techniques. Another objective is the handling of non-linear station motions for an improved realization of the terrestrial reference system. The investigations also focus on the realization of the celestial reference system. Here, the model of the position of the radio sources can be extended by linear velocities.

The "Fundamentals of geometric reference systems" also include investigations related to the Earth orientation parameters. E.g. the so-called "free core nutation (FCN)" can be considered an insufficiently modeled component of presently available nutation models. On the basis of VLBI observations the nutation models can be extended by taking the FCN-signal (see figure) into account.