Light propagation in the solar system for high-precision astrometry at the sub-micro and nano-arcsecond level of accuracy

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The fundamental problem of astrometry is about how to trace a light ray received by an observer back to the light source. Therefore, a precise relativistic modeling of the light trajectories though the curved space-time of the solar system is of decisive importance to interpret astrometric measurements correctly. The ESA astrometry missions Hipparcos and Gaia have reached the milli-arcsecond and micro-arcsecond level in angular observations, respectively. Similarly, ground-based interferometric facilities (VLBI) have also reached micro-arcsecond accuracies in angular measurements. In view of these developments it is clear that high-precision astrometry on the sub-micro-arcsecond level comes into the strategic focus of astrometric science. In fact, the next-generation of astrometry missions is under discussion. There are several astrometry missions proposed to ESA, aiming at such levels of precision, like the astrometry missions Gaia-NIR, Theia, Astrod, Lator, Odyssey, Sagas, or TIPO. These efforts are not only triggered by technological advancements, but mainly by the scientific targets, as for instance: detection of Earth-like exoplanets, astrometric search for gravitational waves, mapping of dark matter, highly precise tests of relativity, model-independent measurements of extra-galactic objects. These developments necessitate a corresponding advancement in the theory of light propagation. During the recent years the most modern developments in the theory of light propagation are focusing at four primary directions: 1. Highly precise descriptions of time delay measurements.2. Realistic formulation of gravitational fields of the solar system bodies.3. Accounting for the orbital motions of solar system bodies. 4. Determining the impact of post-post-Newtonian (2PN) correction terms. The theory of light propagation has to meet these challenges, which are of fundamental importance for high precision astrometry on the sub-microarcsecond-level. The applied DFG project is focusing on these aspects. The work of this project is based on results which we have already obtained in the previous DFG project.