Drag-free motion control of satellite for high-precision gravity field mapping

High precision mapping of the geoid and the Earth´s gravity field are of importance to a wide range of ongoing studies in areas like ocean circulation, solid Earth physics and ice sheet dynamics. Using a satellite in orbit around the Earth gives the opportunity to map the Earth´s gravity field in 3 dimensions with much better accuracy and spatial resolution than ever accomplished. To reach the desired quality of measurements, the satellite must fly in a low Earth orbit where disturbances from atmospheric drag and the Earth´s magnetic field will perturb the satellite´s motion. These effects will compromise measurement accuracy, unless they axe accurately compensated by onboard thrusters. The paper concerns the design of a control system to performing such delicate drag compensation. A six degrees-of-freedom model for the satellite is developed with the model including dynamics of the satellite, sensors, actuators and environmental disturbances to the required micro-Newton accuracy. A control system is designed to compensate the nongravitational disturbances on the satellite, in three axes using an H_∞ design. Performance is validated against mission requirements.