Co-location of Space Geodetic Techniques on Ground and in Space

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According to the Global Geodetic Observing System (GGOS) of the IAG, the terrestrial reference frame, as required to quantify processes like sea level change, postglacial rebound and silent earthquakes, should comply with an accuracy of 1 mm and a stability of 0.1 mm/y over decades. The quality of the ties between the different space geodetic techniques (VLBI, SLR, DORIS, and GNSS) is still one of the major limiting factors for the realization of such a precise and unique terrestrial reference frame. The deficiencies in the ties arise from (a) inaccurate or incomplete local ties at many fundamental sites as well as from (b) systematic instrumental biases in the individual space geodetic techniques. This project focusses on the latter aspect, with the main goal to improve existing and to establish new ties between the space techniques both, on ground and in space, and to assess and reduce technique-specific biases as an essential prerequisite for the realization of a unique high-precision terrestrial reference frame for GGOS. To achieve this goal the project will make use of the Geodetic Observatory Wettzell (3 VLBI and 2 SLR telescopes, >5 GNSS receivers, terrestrial local tie measurements over three decades) as a laboratory that is unique worldwide to study local ties, systematic intra- and inter-technique biases, and local environmental effects. In particular, an actively stabilized two-way compensated optical time and frequency distribution system will be established between all major instruments, allowing the monitoring and evaluation of system delays and realizing an additional tie, a so-called clock tie, between the instruments. This will be exploited in the data processing by combining the clock corrections of different instruments and techniques. Two VLBI telescopes, two SLR telescopes and the GNSS antennas will be used simultaneously to record observations of GLONASS and Galileo satellites, providing a real co-location of the three techniques in space, i.e., at the GNSS satellites, and a unique dataset to study the rigorous combination of all common parameters. In addition, the dataset from extremely short baselines will allow the thorough validation of the VLBI observation of satellites and the forming of single- and double-differences from (quasi-)simultaneous SLR observations to reduce and better understand satellite- and station-specific biases. The co-location in space will be complemented on the ground by the development of a common reference target for calibrating and linking all techniques in a single point on the ground. Local environmental effects will be assessed based on multi-antenna GNSS sites. The outcome of the project is a detailed assessment of the benefits from new ties on ground and in space, new processing strategies and a rigorous common parameterization for the quality of the reference frame realization, that is essential for the project partners and, more importantly, for the monitoring of the changing Earth.