With the increasing accuracy requirements on precise orbit prediction (e.g. for geodetic satellites like ERS-1), the accurate modelling of non-gravitational forces has become an important factor. The key features of a skin force modelling software shall be described which has been implemented by MATRA under ESA contract. The software covers free-molecular aerodynamics, direct and indirect solar radiation pressure effects (diffuse and specular earth albedo, and earth IR), and is supported by state-of-the-art environmental models, and spacecraft geometry and thermo-optical surface models.