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追踪观测者接收到的光线回到光源。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具有决定性的重要意义。欧空局的Hipparcos和Gaia天体测量任务在角度观测方面分别达到了毫角秒和微角秒级别。同样，地面干涉设施(VLBI)在角度测量方面也达到了微弧秒的精度。鉴于这些发展，很明显，亚微角秒级的高精度天体测量成为天体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。有几项天体测量任务向欧空局提出，目标是达到这样的精度水平，比如盖亚-近红外、泰亚、阿斯特罗德、拉托、奥德赛、萨加斯或蒂波。这些努力不仅是由技术进步引发的，而且主要是由科学目标触发的，例如：探测类地系外行星、天体测量搜索引力波、绘制暗物质图、高精度相对论测试、与模型无关的银河系外天体测量。这些发展需要光传播理论的相应进步。近年来，光传播理论的最新发展主要集中在四个方面：1.时间延迟测量的高精度描述。太阳系物体引力场的逼真公式。解释了太阳系天体的轨道运动。4.确定后牛顿(2PN)校正项的影响。光传播理论必须应对这些挑战，这些挑战对于亚微弧秒级别的高精度天体测量至关重要。应用DFG项目就是围绕这些方面展开的。这个项目的工作是基于我们在上一个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.