Accurate Prediction of Non-Gravitational Forces for Precise Orbit Determination Part II: Determination of Perturbing Forces and Torques in an Orbital Environment

Precise orbit determination is a prerequisite for Earth-observation and geodetic satellite missions. Gravitational eects are the dominant contribution to the forces acting on an Earth-orbiting satellite, but they can be modelled with high accuracy, particularly when using dedicated geopotential models (e.g. for the CHAMP and GRACE missions). The non-gravitational contributions, mainly originating from the interaction of the spacecraft surface with molecules and atoms of the thermosphere, and from the impact of photons which come directly from the Sun, which are reflected as albedo from the illuminated Earth hemisphere, or which are re-emitted by the whole Earth as delayed infra-red (IR) reradiation are of second order, but they are much more dicult to model since they require a good knowledge of the spacecraft geometry and surface properties, and they also require reliable estimates of the molecule and photon particle flux. The necessary models of the thermosphere, and of the Earth albedo and IR re-radiation distributions are depending on a large set of parameters, including the spacecraft location, the local solar time, the season (Sun position), and solar and geomagnetic activity levels. The present paper describes how the molecular and photon flux levels can be accurately computed for a given spacecraft orbit, taking into account all major environmental parameters. The eective perturbing forces and torques can be determined by interpolation and denormalisation of tabulated coecients, taking into account real environmental conditions and spacecraft orbit and attitude state.