Acquisition, Doppler Tracking, and Differential LEO-Aided IMU Navigation With Uncooperative Satellites

This article presents a navigation framework, complemented with real-world experiments, for exploiting uncooperative low Earth orbit (LEO) satellites. To deal with the unknown nature of the satellite's signals, a receiver capable of acquiring and tracking the satellite's Doppler frequency is developed. A sequential matched subspace detector is derived to estimate the number of active LEO satellites. A generalized likelihood ratio-based algorithm is developed to estimate the Doppler frequency of LEO satellites with and without integer constraint modulation. A Kalman filter-based tracking loop is designed to track the Doppler. To deal with the poorly known satellites’ ephemerides, a differential Doppler LEO-aided inertial measurement unit (IMU) framework is developed, comprising a stationary base with a known position and a mobile rover. Experimental results are presented demonstrating the proposed framework, where the base and the rover acquired and tracked five LEO satellites (one OneWeb, one Starlink, one Iridium, and two Orbcomm). The rover (ground vehicle) was equipped with an altimeter, an industrial-grade IMU, a Global Navigation Satellite Systems (GNSS) receiver, and the proposed LEO receiver. The vehicle traversed a trajectory of 540 m in 60 s. GNSS signals were available for the first 20 s but were fictitiously cutoff for the last 40 s, during which the vehicle traveled 492 m. The differential LEO-aided IMU navigation system yielded a 3D position root-mean-squared error of 7.6 m, compared to 87.7 m with the GNSS-aided IMU.