Ties between kinematic and dynamic reference frames (D-VLBI)

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The goal of project D-VLBI is the realization of frame ties between the dynamic reference frames of space probes like satellites or spacecraft, the Moon and other planetary bodies, the kinematically defined International Celestial Reference Frame (ICRF), and the International Terrestrial Reference Frame (ITRF) realized on Earth. This can be done best by using differential Very Long Baseline Interferometry (D-VLBI), which is a precise method for the determination of relative angles between a calibrator and a target source and is typically applied by the space agencies for spacecraft navigation. D-VLBI data are also extensively incorporated into modern ephemeris calculations and D-VLBI has been proven capable for relative astrometry of faint radio sources and for frame tying of optical and radio catalogues. In the second phase of this project we want to perform proofs of concepts of real differential observations to a variety of satellites to test the astrometric performance of modern D-VLBI techniques and hardware, and to identify limitations and realistic uncertainties in the ultimate frame ties that can currently be achieved, for specific satellite and transmitter configurations. We plan new observations of GNSS satellites, the space VLBI satellite RadioAstron, and the Gaia spacecraft. With our GNSS observations, we aim to demonstrate that differential VLBI observations to a variety of GNSS systems, including Galileo and BeiDou, can be successfully carried out using IVS stations, and that we are able to perform the complete analysis chain to parameter estimation. RadioAstron has a VLBI antenna on board and we want to show how well the satellite positions and frame ties can be made for spacecraft that are themselves simultaneously part of the VLBI observing network. And for Gaia, we aim to determine the optimum observing strategy to tie Gaia's orbit to the ICRF, leading in turn to an improvement in the tie to Gaia's own observational frame. Additionally, we want to share the acquired achievements with the partners in the Research Unit, for example, to improve the planetary and Lunar ephemerides. We will provide D-VLBI analysis results for the Lunar SELENE and Chang'e missions, including the Chang'e-3 lander and rover, to projects PN1 and PN3, to improve the Lunar ephemeris and kinematic-Lunar frame tie. For these same groups we will also provide simulations of differential observations for future missions to investigate possible ephemeris and frame-tying improvements. We will cooperate with project PN2 on core-shift affects in the positions of the VLBI calibrator sources. Core-shift effects limit the accuracy with which all astrometric measurements involving ICRF sources can be made at the level of a substantial fraction of a milliarcsecond. Scheduling and executing our observations will be done in cooperation with project PN7a, which will provide technical and observational assistance.