1ORBIT AND ATTITUDE PERTURBATIONS DUE TO AERODYNAMICS AND RADIATION PRESSURE

For satellites which are operated in low-Earth orbits (LEO), gravitational forces due to the non-uniform mass distribution of the Earth are dominating the orbit and at-titude perturbation spectra. Non-gravitational forces are mainly caused by momentum exchange with the space-craft surface, and they are mostly of second order. The most prominent of these forces originate from the interac-tion of the spacecraft surface with molecules and ions 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 space-craft geometry and surface properties, and they also re-quire reliable estimates of the molecule and photon par-ticle 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 time, the season (sun position), and solar and geomagnetic activity levels. The variabil-ity of these environment models will be explained, and mathematical models will be described which allow to use the resulting molecule and photon flux models to com-pute perturbing forces and torques acting on a LEO satel-lite. Examples will be provided for ESA’s ERS-1 and EN-VISAT satellites. Key words: free-molecular aerodynamics, radiation pres-sure, non-gravitational perturbations