The motion of a satellite depends on both 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.

TIPOD aims on improving the POD of LEO satellites by applying further developed thermosphere models.

THERMOSPHERIC DRAG

The thermospheric drag provides the largest non-gravitational acceleration for objects in orbit with altitudes lower than 1000 km. In addition, the thermospheric drag also decelerates orbiting objects' movement.

If the thermospheric drag is not considered within the POD it could cause an error of several meters per revolution in the along-track direction. The knowledge of the thermospheric density is, therefore, 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 and altitudes, may provide only limited accuracy in POD.

IMPROVING POD OF LEO SATELLITES

The TIPOD project was initiated for the second phase of the Special Priority Programme (SPP) 1788 Dynamic Earth which was funded by the German Research Foundation.

The project aims on improving the POD of LEO satellites by applying further developed thermosphere models. For this purpose, data from various satellite tracking techniques (SLR, accelerometer, GNSS, DORIS, RADAR and TLE) will be assimilated into a physical coupled thermosphereionosphere model, namely TIE-GCM (Thermosphere-Ionosphere-Electrodynamics General Circulation 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.

Overall the main objectives of the TIPOD project is to:

• Develope high-precision thermosphere models to improve POD of geo-scientific LEO satellites.
• Compose a set of observation techniques to determine appropriate thermospheric key parameters including a complete stochastic model.
• Improve the knowledge of thermospheric density by extending the empirical model and calibrating model predictions by various observation techniques.

Aalborg University contributes in estimating the thermosphere density from on-board accelerometer measurements and in implementing data assimilation frameworks based on empirical and coupled thermosphere-ionosphere models (doi.10.1093/gji/ggaa507 and doi:10.1093/gji/ggz163).