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INTRODUCTION
Space agencies in the world have focused on Solar System exploration. China intends to launch a Mars probe in the near future, which will lead China one step further in interplanetary exploration (Fang and Ning, 2010; Wu et al., 2011; Ma et al., 2013). Launch, Cruise, Approach and Orbit Insertion, and Entry Descent and Landing (EDL) are four major phases in exploring a target; the Approach and Orbit Insertion phase is vital for interplanetary missions because its accuracy deeply affects the following EDL phase (Wang et al., 2008) and avoiding a fault in navigation is essential to ensure mission success.
Radio navigation and autonomous navigation are used in interplanetary navigation. Radio navigation measures the signal delay time and the frequency shift to obtain the range and range rate from a probe to a Deep Space Network station (Thornton and Border, 2003). Its accuracy with respect to Earth is excellent, and is dominant for interplanetary missions. However, this is restricted to the visible arcs of the station and Sun conjunction, and its performance with respect to the target mainly suffers from target ephemeris uncertainty (Jordan et al., 1972).
Autonomous navigation can obtain the probe position and velocity without Earth tracking, which is currently only a complementary navigation method despite progress. Three autonomous navigation methods have been proposed. Optical navigation (OpNav) finds the centroids from the optical image of the target and its background stars, and calculates the probe position with respect to the target, which has been used in Mars Reconnaissance Orbiter (MRO), Deep Space 1, and Deep Impact, etc (Martin-Mur et al., 2008). Optical Doppler navigation measures the Doppler-shift spectrum of the Sun or stars to determine the probe velocity (Yim et al., 2000; Guo, 1999). X-ray Pulsar navigation compares the measured arrival time of the pulsar signal with predictions to locate the probe, which has been validated in an on-orbit experiment (Graven et al., 2008; Wood et al., 2001). Among these methods, OpNav is the only method which has been successfully utilised in interplanetary missions.
In previous missions, the navigation results have been provided by direct combination without information fusion, using OpNav results for those with respect to the target and radio...