Autonomous orbit determination via integration of epoch-differenced gravity gradients and starlight refraction is proposed in this paper for low-Earth-orbiting satellites operating in GPS-denied environments. The starlight refraction can compensate for the significant along-track position error using solely gravity gradients and benefit from the integration in view of accuracy improvement in radial and cross-track position estimates. The betweenepoch differencing of gravity gradients is employed to eliminate slowly varying measurement biases and noises near the orbit revolution frequency. The refraction angle is directly used as measurement and its Jacobian matrix is derived from an implicit observation equation. An information fusion filter based on sequential extended Kalman filter is developed for the orbit determination. Truth-model simulations are used to test the performance of the algorithm and the effects of differencing intervals and orbital heights are analyzed. A semi-simulation study using actual gravity gradient data from the Gravity field and steady-state Ocean Circulation Explorer (GOCE) combined with simulated starlight refraction measurements is further conducted and a three-dimensional position accuracy of better than 100 m is achieved.

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