Modelling of space weather effects on satellite drag

For satellites on low Earth orbits, aerodynamic drag provides an important contribution to the spectrum of perturbing forces. Aerodynamic drag is only of second order magnitude or less, as compared to the first order Earth oblateness perturbation to the orbit. However, due to its energy dissipating nature, it is the driving effect for altitude decay and associated along-track dispersions. Determining parameters for the drag force are the local air densities, the projected cross-section, and a drag coefficient which describes the interaction between impinging molecules and the spacecraft surface in the regime of free-molecular flow. The local and exospheric temperature, the atmospheric composition, and the resulting densities are strongly driven by space weather effects from solar extreme ultraviolet radiation, and from coronal mass ejections which may lead to geomagnetic storms. Thermospheric models are currently the limiting factor in the accuracy of orbit determination and prediction. Any improvement in these models would greatly aid in applications such as re-entry prediction, ground-track maintenance and gravity field and geodetic science missions. This paper gives an overview of many aspects of satellite drag modelling for orbit determination. The performance of current thermosphere models is analysed using tracking data, and recent developments such as model calibration are described.