An Update to TU Graz Thermospheric Density Estimates and Fine-Tuning Attempts

Over the years, TU Graz has processed thermospheric densities for several satellite missions. Thanks to recent adaptations of our GROOPS software package, we have been able to estimate such a time series for TerraSAR-X, a LEO satellite in sun-synchronous orbit, up to the present day. In order to make these data available to researchers who are interested in space weather related studies, we are switching to a new publishing scheme to provide the data with less latency.

Estimating these thermospheric densities from accelerometer measurements or GNSS observations is a tedious process that depends on several parameter choices. For example, the Sentman model is often used to calculate drag coefficients for satellites. In addition to the geometry of the satellite, the drag coefficient of this model depends on the surrounding gas (temperature, composition) as well as the surface properties of the satellite (energy accommodation coefficient). While it is easy to compute solutions for different parameter choices, it is usually not trivial to decide which result is best.

In this pre-study, we investigate the possibility of fine-tuning the choice of aerodynamic parameters by evaluating the quality of the resulting gravity field. This may be a feasible approach for GRACE-FO due to the specific design of the mission: two identical satellites following each other in essentially the same orbit while measuring their precise distance. The failure of the second satellite's accelerometer requires a "transplant" of the measurements from the first satellite to the second. This procedure includes the estimation of the thermospheric density, and thus the gravity field is sensitive to the chosen parameters.