Aerodynamics in Low LEO: A Novel Approach to Modeling Air Density Based on IGS TEC Maps

Abstract

Here we present some theoretical aspects of the modeling of aerodynamic acceleration in the precise orbit determination of a LEO satellite. We have included this section because of the great importance of the role that aerodynamic drag plays in all gravity field missions, as they are typically placed in a very low LEO orbit. Thus, here we look at the geometrical properties of this effect. We show that the accuracy of the velocity in the calculation of the aerodynamic drag for a LEO satellite, in particular the velocity of thermospheric horizontal winds, is as important as the atmospheric density. We then give a geographical representation of the models used to calculate atmospheric density and thermospheric horizontal winds, with an emphasis on the GOCE (Sun-synchronous) orbit, and compare this with the orbits of altimetry satellites in high LEO. In addition, we present the prospects of investigating atmospheric density and thermospheric winds using the GOCE mission at 220–250 km altitude. Models of neutral horizontal winds show that thermospheric winds mainly occur around the geomagnetic poles where they are driven by the perturbations in the geomagnetic field. The highest thermospheric wind velocities may be expected along the dawn-dusk regions, and from that point of view, the GOCE orbit is the perfect candidate to provide unique information on the neutral horizontal winds in the lower thermosphere. Section 10.3 of this thesis triggered an ESA study that demonstrated the retrieval of thermospheric wind parameters from GOCE data. At the end of this section, we demonstrate a novel approach to calculating and predicting air density in the thermosphere based on the global TEC maps provided by IGS. This approach could be used to predict solar activity in an alternative way, independent of the number of Sun spots or the solar flux index at a wavelength of 10.7 cm (F_10.7). We also show that information on the ionization of the thermospheric part of the ionosphere, as provided in IGS TEC maps, can be used to calculate the LEO mission duration (as was done for GOCE). This opens up new applications for the global IGS TEC maps in monitoring air density in the thermosphere, including spatial and temporal variations. In addition, we show that variations in air density driven by variations in solar activity (heating) are empirically proportional to the ionization of the ionosphere. Thermospheric density and TEC can be related by an empirical linear model as shown here.