Abstract: A relative states autonomous determination algorithm is presented for use in the formation flying of two satellites based on the combination of GPS and gyro according to their different characteristics. With the modified Rodrigues parameters (MRPs)as attitude variables, the relative orbit and attitude motion equations are established, and the observation equations of GPS and gyro are set up. Then the unscented Kalman filtering is used to realize the relative states autonomous determination. Simulation results showed the feasibility and validity of the proposed algorithm.