Abstract: Relative motion states of non-cooperative space target cannot be fully estimated by single satellite range-only measurement. To solve this problem, formation satellites range-only measurement was proposed. System model and measurement model were established. System observability matrix was derived, and the degree of observability was calculated. Relative motion states of non-cooperative space target were estimated through unscented Kalman filter (UKF). Analyses and simulations show that all relative motion states of non-cooperative space target can be estimated by using two-satellite range-only measurement data. The larger the angle of measurement vectors is,  the more accurate the states estimation is. When formation size is far less than the distance of target, more than two measurement satellites cannot improve states estimation accuracy.

Key words: Relative guidanceRange-only measurement, Non-cooperative space target, Unscented Kalman filter, Formation flight, Satellite