The quality of the links between the different space geodetic techniques (VLBI, SLR, GNSS and DORIS) is still one of the major limiting factors for the realization of a unique global terrestrial refer-ence frame (TRF) that is accurate enough to allow the monitoring of the Earth’s system, i.e., of proc-esses like sea level change, postglacial rebound and silent earthquakes. According to the specifica-tions of the Global Geodetic Observing System (GGOS) of the IAG, such a reference frame should be accurate to 1 mm over decades, with rates of change stable at the level of 0.1 mm/y. The deficiencies arise from inaccurate, incomplete or even missing local ties at many fundamental sites as well as from systematic instrumental biases in the individual space geodetic techniques. In order to improve this critical situation, all available information on local ties, on the ground and onboard the geodetic satel-lites, needs to be collected, combined and jointly analyzed. Furthermore all sources for nonreciprocal instrumental effects need to be identified and removed. Innovative concepts like twin telescopes, opti-cal time transfer, dedicated satellite mission, etc. have to be considered to achieve a clear improve-ment in the accuracy and quality of the ties. This project focuses on developing new technologies on the ground and on improving and using the information on co-location onboard satellites.
