DFG Research Unit 5456
Clock Metrology: A Novel Approach to TIME in Geodesy

The goal of this research unit is the development of a new method for a highly accurate long- term stable realization of Geodetic Reference Systems by linking all space geodetic observation techniques to a common time system. Global reference frames are the metrological basis for a multitude of applications reaching from positioning, terrestrial and space navigation, to the quantification of change processes in the system Earth. Highest accuracy and long-term stability of the reference frame are of paramount importance for the identification of long-lasting geodynamic or climate-related processes, such as plate tectonics or sea level change. These global reference frames are realized by the combination of measurements from a diversity of space geodetic observation techniques, tied together at a number of globally distributed core stations like the Geodetic Observatory Wettzell (GOW). Despite all efforts, a reference frame accuracy of 1 mm in position and 0.1 mm/yr in velocity as required by GGOS is still not achieved today, as prevailing variable systematic measurement errors in these techniques still pose a serious limitation. The geometric ties alone do not precisely represent the real physical displacement between the reference points of the different instruments. Neither can the ties keep track of their evolution over time, which exacerbates the problem of creating long-term stable reference frames.

In this research unit we have identified time as a novel observable to resolve this problem. Coherence of time, adequately realized for the space geodetic techniques at a campus, provides an additional new tie between the observing techniques and it offers the solution to overcome the limitation of the remaining variable systematic measurement errors in the diverse techniques, as well as in the geometric ties, by means of the application of closure measurement techniques. Furthermore, when we tie accurate optical clocks to specific geodetic markers at the observatories, namely GOW and Potsdam, we can consistently add physical heights to these geometric reference frames. The now possible combination of space and time with the help of the ACES clocks on the ISS requires a proper treatment within the frame of general relativity and offers a sensitve test of gravitational redshift. The introduction of actively stabilized delay-compensated time in our measurement techniques leads to significant consequences for the current analysis models in space geodesy, which have to be adapted to this new situation. The absence of systematic measurement errors in the different techniques will allow to derive improvements for the atmospheric propagation delay, therefore this is included here.