ATMOSPHERIC DENSITY ESTIMATION IN LOW-EARTH ORBIT FOR DRAG-MODULATED SPACECRAFT

Drag-modulation has been proposed for trajectory control of spacecraft in low- Earth orbit for uses such as formation keeping, targeted reentry and constellation phasing. However, using changes in aerodynamic drag to control a spacecraft requires knowledge of the atmospheric density in order to predict the drag force being applied. Due to modeling errors and inaccuracies in space weather forecasting, there will be a difference between the predicted density used to plan the trajectory and the true density encountered in-flight. This density prediction error causes the spacecraft to depart from the intended trajectory. This paper proposes the use of an extended Kalman filter along with a linear model of a spacecraft’s motion relative to a planned trajectory to estimate the density prediction error. The estimated density can then be used to improve the performance of a dragmodulated control strategy or as measurements to improve atmosphere models. The proposed method is tested in simulation and found to be capable of estimating the time-varying density error caused by a space weather forecasting error. Estimated values of the density error are then shown to significantly improve the accuracy of state predictions made by the linear model.