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

天体测量学的基本问题是如何将观察者接收到的光线追踪到光源上。因此，对太阳系弯曲时空中的光轨迹进行精确的相对论建模，对于正确解释天体测量结果具有决定性的意义。欧洲航天局的天体测量任务“希巴谷”和“盖亚”在角度观测方面分别达到了毫角秒和微角秒的水平。同样，地面干涉测量设备（VLBI）在角度测量方面也达到了微弧秒的精度。鉴于这些发展，亚微弧秒级的高精度天文测量显然成为天文测量科学的战略重点。事实上，下一代天体测量任务正在讨论中。有几个天体测量任务向欧空局提出，目标是达到这样的精度水平，如Gaia-NIR， Theia, Astrod, Lator, Odyssey， Sagas或TIPO的天体测量任务。这些努力不仅是由技术进步引起的，而且主要是由科学目标引起的，例如：探测类地系外行星、寻找引力波的天体测量学、绘制暗物质地图、高度精确的相对论测试、独立于模型的星系外物体测量。这些发展要求光的传播理论有相应的进步。近年来，光传播理论的最新发展主要集中在四个方面：1.光传播理论；对时间延迟测量的高度精确描述。太阳系天体引力场的现实表述。太阳系天体的轨道运动。4. 确定后-后牛顿（2PN）校正项的影响。光的传播理论必须面对这些挑战，而这些挑战对亚微弧秒级的高精度天文测量具有重要的基础意义。应用DFG项目的重点就是这些方面。这个项目的工作是基于我们在之前的DFG项目中已经取得的成果。

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.