The project DEOS Aero deals with the computation of thermospheric density from satellite orbit and accelerometer data. Versatile topics in the field of satellite dynamics are addressed, especially satellite drag and radiative non-gravitational force modeling, as well as Precise Orbit Determination (POD). Several geodetic satellite missions are used for the investigation.

The density of the thermosphere is of high relevance for the propagation and determination of satellite trajectories in Low Earth Orbit (LEO). Hence, it is important for a lot of satellite operations in LEO. The drag acceleration, resulting from the gas-surface interaction of the rarefied atmosphere with satellites and other bodies, results in a change of trajectory and an orbital decay. This has implications for active satellites, where a precise orbit or position knowledge might be crucial for the missions success (e.g. altimetry, gravimetry, remote sensing), for maneuver planning and satellite lifetime prediction, as well as for space debris, where position determination is important for collision avoidance and reentry calculations. Furthermore, in atmospheric sciences, thermospheric density data can help to restrict, develop and validate complete physics-based thermosphere-ionosphere models.

Atmospheric density models used in satellite dynamics calculations are usually semi-analytical models, which are heavily based on measurement data. Thus the quality of such models relies also on the quality of the underlying density data. Prominent models are e.g. NRLMSISE, JB08 or DTM.

Due to the lack of direct measurements of thermospheric density, derived densities from satellite orbit data are currently the best data source. The most accurate atmospheric density estimates are computed from data of geodetic satellite missions, with onboard accelerometers. Nevertheless, differences in published datasets are rather high. Depending on the temporal resolution and space weather conditions (solar and geomagnetic activity), differences between those datasets might be 100% and 25% on short time scales below orbit period (>1.5h) and longer time scales around some orbit periods, respectively.

The density retrieval approach from accelerometer data is based on three separate competences, which are: (1) Precise radiative non-gravitational force modeling, (2) Modeling of the interaction between the rarefied atmospheric gases and the satellite, i.e. modeling of drag coefficients, and (3) the calibration of the accelerometer data by POD.

The modeling of the drag coefficient is the area with the greatest uncertainty. The gas particle-surface interaction in the an extremely thin atmosphere with very high relative velocities is not yet fully understood. The improved understanding of these interactions and the description by suitable models and parameters is one of the objectives of this project.

The ultra-sensitive accelerometers onboard geodetic satellites like GRACE, CHAMP, GOCE, Swarm and GRACE-FO offer the possibility for density retrieval at different altitudes, epochs and at different solar activities. Thus make up a valuable data set. Furthermore different operation conditions allow for the possibility to disentangle parameters in the drag modeling.

Besides the very high temporal resolution of thermometric densities retrieved from satellites with   accelerometer, an approach based on fitting parameterized density models or functions in a POD process with lower temporal resolution is developed and compared. This offers to use a wider data base of satellites.