Development of High-precision Thermosphere Models for Improving Precise Orbit Determination of Low-Earth-Orbiting Satellites (TIPOD)

卫星的运动取决于重力加速度和非重力加速度。低地球轨道（LEO）物体（如卫星或空间碎片）的精确轨道确定（POD）的一个主要问题是模拟热层阻力。对于高度低于1000千米的卫星来说，这是最大的非重力加速度，会使卫星的运动减速。例如，阻力对高度约为350公里的低轨道卫星总加速度的贡献大约等于地球变平的贡献。在高度约460公里的蜂群卫星的情况下，不考虑POD内的阻力将导致沿轨道方向每转约3米的误差。因此，对热层密度的了解在许多地球科学应用中至关重要，例如遥感、卫星测高和卫星重力任务，这些应用要求轨道精度达到几毫米。然而，目前使用的热层模式，通常基于在不同太阳条件下收集的数据，可能只能提供有限的POD精度。因此，TIPOD旨在通过应用进一步开发的热层模型来改善LEO卫星的POD。为此目的，应将各种卫星跟踪技术（SLR、GNSS和DORIS）的数据同化为一个热层-电离层物理耦合模型。由于这些数据相当不均匀，在项目的第一步，将在观测和物理模型之间插入一个经验模型。校准经验模型后，其输出将在项目的第二步内被吸收到物理模型中，并校准一些选定的物理关键参数。TIPOD解决了两个主要的科学问题：(1)在最近的卫星任务（如GRACE和Swarm）所调查的时间跨度内，物理模型能多接近地恢复经验模型；(2)两种模型的应用能在多大程度上提高精确的轨道确定？TIPOD的最终目标是将输入的跟踪数据直接吸收到物理模型中。TIPOD的最终用户产品是诸如Swarm卫星等低轨道卫星的POD，将通过国际卫星组织/全球卫星系统重点区域4向用户分发。

The motion of a satellite depends on gravitational and non-gravitational accelerations. A major problem in precise orbit determination (POD) of low-Earth orbiting (LEO) objects such as satellites or space debris is modelling the thermospheric drag. It is the largest non-gravitational acceleration for satellites with altitudes lower than 1000 km and decelerates their movement. For example, the contribution of drag to the total acceleration of a LEO satellite with an altitude of around 350 km equals approximately the contribution of the Earth’s flattening. In case of the Swarm satellites with an altitude of around 460 km a non-consideration of the drag within the POD would cause an error of around 3 meters per revolution in the along-track direction. Therefore, the knowledge of the thermospheric density is of crucial importance in many geo-scientific applications such as remote sensing, satellite altimetry and satellite gravity missions, where orbits with an accuracy of a few millimetres are required. Yet, today’s usage of thermosphere models, often based on data collected at times with different solar conditions, may provide only limited accuracy in POD. Therefore, TIPOD aims on improving the POD of LEO satellites by applying further developed thermosphere models. For this purpose, data from various satellite tracking techniques (SLR, GNSS and DORIS) shall be assimilated into a physical coupled thermosphere-ionosphere model. Since these data are rather heterogeneous, an empirical model will be interposed between the observations and the physical model in the first step of the project. After calibrating the empirical model, its output will be assimilated into the physical model within the second step of the project and a number of selected physical key parameters will be calibrated. TIPOD addresses two main scientific questions: (1) how close can the physical model recover the empirical one within the investigated time span of recent satellite missions such as GRACE and Swarm, and (2) how far can the precise orbit determination be improved by the application of both models? The ultimate goal of TIPOD is to assimilate the input tracking data directly into the physical model. The end-user product of TIPOD is a POD of LEO satellites such as the Swarm satellites, which will be disseminated to users through the IAG/GGOS Focus Area 4.