Aircraft Navigation in GNSS-Denied Environments via Radio SLAM With Terrestrial Signals of Opportunity

A radio simultaneous localization and mapping (radio SLAM) framework enabling aircraft navigation with terrestrial signals of opportunity (SOPs) is presented and experimentally validated. The framework does not assume availability of global navigation satellite system (GNSS) signals. Instead, it assumes the aircraft to have an initial estimate of its own states, after which it navigates by exploiting pseudorange measurements extracted from terrestrial SOPs, while estimating the states of the aircraft simultaneously with the SOPs' states. Two radio SLAM frameworks are presented: (i) tightly-coupled SOP-aided inertial navigation system (INS) and (ii) utilizing a Wiener process acceleration (WPA) dynamical model for the aircraft's dynamics instead of the INS. Results from four flight runs on a US Air Force C-12 aircraft, equipped with an altimeter and an industrial-grade inertial measurement unit (IMU), are presented. The flight runs took place over semi-urban (SU), urban (U), and rural (R) regions in California, USA; while exercising different aircraft maneuvers: holding (H), descending (D), and grid (G). Different a priori conditions of the SOPs' positions were studied: from all unknown, to some known, to all known. In all cases, the SOPs' clock error states (bias and drift) were unknown and estimated alongside the aircraft's states. The results consistently demonstrated the promise of real-world aircraft navigation via radio SLAM, yielding bounded errors along trajectories of tens of kilometers. The three-dimensional (3-D) position root-mean squared errors (RMSEs) are summarized next, where $N$ denotes the number of SOPs exploited along the trajectory: (1) SU, H, INS-SOP, $N=6$ , 56.7 km in 8.5 minutes, maximum altitude of 5,577 ft: 43.27 m with all unknown and 10.14 m with all known; (2) U, H, INS-SOP, $N=6$ , 72.7 km in 12.9 minutes, maximum altitude of 5,906 ft: 89.82 m with all unknown and 16.97 m with all known; (3) SU, D, WPA-SOP, $N=18$ , 111.9 km in 20.0 minutes, maximum altitude of 6,234 ft: 36.42 m with all unknown and 18.62 m with all known; and (4) R, G, WPA-SOP, $N=32$ , 78.4 km in 13.8 minutes, maximum altitude of 7,546 ft: 67.01 m with all unknown and 25.65 m with all known.