Drag-free satellite control

A drag-free satellite cancels the effect of external disturbances. Although the forces may be small, a satellite is disturbed by residual air drag, radiation pressure, micrometeorite impact, and other small forces that act on its surface disturbing its orbit, which is principally determined by the gravity field. In some missions, these small perturbations that make the satellite deviate from its purely gravitational orbit are limiting. An internal unsupported proof mass is shielded by the satellite from the external disturbances. The position of the shield (or the main part of the satellite) is measured with respect to the internal proof mass, and this information is used to actuate a propulsion system which moves the satellite to follow the proof mass. A drag-free control system is illustrated. Since the proof mass is shielded it follows a purely gravitational orbit - as does the satellite following it - hence the name drag-free satellite. The idea was conceived by Lange (1964) and has been applied to many mission studies since. In some cases, it is not necessary to cancel the disturbances, only to measure them so they may be taken into account. In such cases, an accelerometer may be a more suitable solution (for example, using the ONERA Cactus or the Bell Aerosystems MESA).