Accurate Prediction of Non-Gravitational Forces for Precise Orbit Determination - Part I: Principles of the Computation of Coefficients of Force and Torque

For Earth-orbiting satellites gravitational forces due to the non-uniform mass distribution of the Earth are dominating the orbit and attitude perturbation spectra. Nongravitational forces are mainly caused by momentum exchange with the spacecraft surface, and they are mostly of second order. The most prominent of these forces originate 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) re-radiation. In contrast with gravitational perturbations, the aerodynamic and radiation pressure effects are difficult 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 concept of the approach described in the present paper is as follows: 1) Generate a realistic 3D model of the spacecraft to be considered, 2) Compute the coefficients of force and torque due to momentum exchange with photons and molecules in a way, which is independent of the real environment in orbit, 3) For a given orbit position, interpolate and denormalise the pre-computed coefficients, using local fluxes of photons and molecules computed for the real environment. In the present paper the spacecraft model generation process is described and the methods used for the coefficient calculations are explained. Two methods are available: A statistical test particle Monte Carlo method and an analytical integral method. The computation of forces and torques in a real orbit for sample satellites will be the subject of a follow-up paper.