The motion of a satellite depends on gravitational and non-gravitational accelerations. A major problem in the precise orbit determination (POD) of LEO satellites is modelling the non-gravitational perturbations. Among them, the atmospheric drag acceleration – mainly depending on the thermospheric density – is largest for LEOs with altitudes lower than 1000 km. Consequently, the knowledge of the thermospheric density is of crucial importance for LEO-POD. In many geo-scientific applications, e.g., remote sensing, satellite altimetry and satellite gravity missions, the orbits of LEO satellites should be known with a sub-centimetre accuracy.

In POD, the thermospheric drag is usually described by empirical models such as the Jacchia-Bowman 2008 (JB2008) model, the COSPAR International Reference Atmosphere 86 (CIRA86) model, the Drag Temperature Model 2013 (DTM2013) or the Naval Research Laboratory Mass Spectrometer Incoherent Scatter Radar Extended (NRLMSISE00) model. These models are amongst other quantities, driven by globally defined space weather parameters such as the F10.7 (reflecting solar activity) index and the K-p (reflecting magnetic activity) index. The two panels of the left part of the Figure visualize the high correlation between the thermospheric density computed by the NRLMSIS00 model and the F10.7 index, i.e. the solar activity. The panel on the right of the Figure shows the behaviour of the total density computed from various empirical models during the Bastille Day event (July 14th, 2000), one of the most violent solar flares in the recorded history. Since there are density differences of more than 100% between the three density models, in particular during the storm time (green box), the estimation of unknown parameters – such as the satellite’s Keplerian elements – within a LEO-POD could also be considerably different. The Figure indicates further that even before and after the space weather event significant differences exist between the empirical model results. Additionally, it is well-known that the thermosphere is expanding and contracting along with the solar cycle. A further hypothesis states that the thermospheric density is decreasing by about 5% per decade due to climate change. All these studies demonstrate that an improvement of thermosphere models for POD is absolutely necessary.