Investigation of a concept for a scout satellite for a LEO Drag Free mission

Two major problems in controlling a Drag Free Satellite in low Earth orbit during the experimental phase of the mission have been identified and are addressed in this thesis. The first problem is the air drag induced by the residual atmosphere on the satellites orbit. The air drag is the largest disturbance and the most problematic are the unknown density variations constituting the random part of this disturbance. The second problem is charging of the test masses. As the satellite progresses on its way around the Earth a charge is accumulated by the test masses which will reduce measurement accuracy. This charge has to be measured and dumped from time to time by the charge management system. Under nominal conditions this charge can be calculated fairly accurately but if the satellite passes through the South Atlantic Anomaly the estimation of the accumulated charge is difficult. In order to reduce the problems for the drag free control and charge management system a companion satellite is proposed that will fly in a predefined distance in front of the drag free satellite on the same orbit. Using a single drag free sensor this scout could be used to measure non- gravitational disturbances as well as the charging rate. The measurements of the disturbances could be used in a feed forward control by the drag free control system whereas the charge measurements could be used in a continuous charge control. Problems associated with the measurement of air drag are addressed and an error budget for the relative drag to be used in feed forward is derived. Realistic atmospheric density variations are derived using measurement- based models and through simulations using a simplified one degree of freedom drag free controller an increase in performance of the science measurements on a reference mission using a scout satellite is demonstrated. A method for estimating the charge rate for use in a feed forward charge control system is briefly discussed, but due to lack of information no conclusions can be drawn about the performance of such a system. A very simplified baseline configuration of a scout satellite is finally presented.Validerat; 20101217 (root