The International Terrestrial Reference System (ITRS) is the fundamental terrestrial reference system and serves as the basis for describing the figure of the Earth and for referencing processes at the Earth's surface, in the Earth's immediate space environment, and in the Earth's interior. The ITRS is realized by positions and velocities of more than 1700 globally distributed geodetic observing stations. Station positions and velocities are derived from a combination of the space-geodetic techniques Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Global Satellite Navigation Systems (GNSS, e.g., GPS and GLONASS), and the Doppler measurement system DORIS (see Figure). Each of the techniques offers individual strengths in terms of determining geodetic parameters - in particular the parameters of the geodetic datum (origin, orientation, and scale of the reference frame). By combining the different techniques, teir potentials cn be optimally exploited.

The calculation of the reference frame is performed as shown in the flowchart. In a first step, the time series of normal equation systems of the individual techniques are analyzed and combined into one normal equation system per technique (intra-technique combination). In a second step, the normal equation systems of the different techniques are combined (inter-technique combination).

In its role as an ITRS Combination Centre within the International Earth Rotation and Reference Systems Service (IERS), DGFI-TUM has taken responsibility for the continuous realization of the ITRS with best possible accuracy and long-term stability. DGFI-TUM's latest realization of the ITRS is the DTRF2014.

The main goal of DGFI-TUM's research activities in this context is to further develop the global reference frame by improving its accuracy, stability, timeliness and consistency. In particular, DGFI-TUM investigates the potential and optimal strategy for the consistent realization of the terrestrial and celestial reference system according to Resolution 3 (2011) of the International Union of Geodesy and Geophysics (IUGG) as well as the integration of geometry and gravity in the sense of GGOS.