| The largest non-gravitational forces that will be acting on the Topex satellite will be those due to incident and emitted radiation on the spacecraft surfaces. In order to minimize the effects of these forces on orbit determination, a detailed model has been developed so that they may be accurately predicted. This model requires a precise description of the spacecraft shape and orientation, evaluation of the solar and Earth radiation impinging on the surfaces and a determination of the radiation being emitted from the surfaces as they heat and cool throughout the orbit. The TRASYS software system is used to evaluate the solar and Earth radiation (albedo and infrared) striking each surface of the spacecraft. This software has been modified to include a Earth radiation model which follows the seasonal variations in albedo and infrared radiation. The SINDA software system is then used to determine the transient temperatures of the spacecraft surfaces. Orbital thermal histories of significant features are given. These temperatures are used to determine the force exerted on each surface due to thermal emission. Especially unique to this area of research, is the detailed analysis of the thermal emissive force upon the spacecraft. The emission forces are combined with the incident radiation forces to determine the total force acting on the satellite. In addition, since TOPEX will employ active radiators such as the altimeter, an approximation of the recoil force from its transmission of radiation is given. Results are presented which show the accelerations induced on the spacecraft by these forces. The effects of solar radiation, albedo, infrared and surface emissions are given separately so that their relative importance may be clearly evaluated. A error analysis has been performed upon this computationally extensive model by incorporating upper and lower bounds of the various parameter definitions. The extent to which this "Micro" model is being utilized to improve the orbit determination of TOPEX using simple "Macro" models is also discussed. |
