Chiawei Lee

Aircraft navigation without global navigation satellite system (GNSS) signals is considered. To this end, a high sensitivity receiver design is presented that could exploit terrestrial cellular signals of opportunity. The receiver operates on downlink orthogonal frequency division multiplexing (OFDM) signals transmitted by long-term evolution (LTE) eNodeBs. Two challenges encountered on high altitude aircraft are addressed: weak signal power and insufficiently accurate initial Doppler estimation. A so-called ultimate reference signal is proposed, which exploits all available LTE resources transmitted by eNodeB's multiple antenna ports. A time-domain-based receiver design is presented to extract carrier phase observables without the need to reconstruct the received OFDM frame. The proposed approach is shown to significantly improve the receiver's sensitivity, amplifying the received power by nearly 21 dB, while also improving the carrier phase estimation accuracy. To evaluate the efficacy of the proposed receiver, experimental results are presented of an aircraft flying over two regions in California, USA: (i) Region A: Edwards Air Force Base (rural) and (ii) Region B: Palmdale (semi-urban). For three aircraft maneuvers (climbing teardrop, descending teardrop, and grid), the results were consistent: more than 100 eNodeBs were trackable, some of which more than 100 km away, at altitudes as high as about 11,000 ft with carrier-to-noise ratio over 40 dB-Hz. Upon fusing the carrier phase observables with altimeter data via an extended Kalman filter, a sustained accurate navigation solution was achieved. Over trajectories of 43.6 km and 56.8 km in regions A and B, traversed in 455 s and 601 s, a three-dimensional position root mean-squared error (RMSE) of 6.8 m and 8.2 was achieved by exploiting an average of about 22 and 11 eNodeBs, respectively

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.

This article presents the first demonstration of navigation with cellular signals of opportunity (SOPs) on a high-altitude aircraft. An extensive flight campaign was conducted by the Autonomous Systems Perception, Intelligence, and Navigation Laboratory in collaboration with the U.S. Air Force to sample ambient downlink cellular SOPs in different regions in Southern California, USA. Carrier phase measurements were produced from these signals, which were subsequently fused in an extended Kalman filter along with altimeter measurements to estimate the aircraft’s state (position, velocity, and time). Three flights are performed in three different regions: 1) rural, 2) semiurban, and 3) urban. A multitude of flight trajectories and altitudes above ground level (AGL) was exercised in the three flights: 1) a 51-km trajectory of grid maneuvers with banking and straight segments at about 5,000 ft AGL, 2) a 57-km trajectory of a teardrop descent from 7,000 ft AGL down to touchdown at the runway, and 3) a 55-km trajectory of a holding pattern at about 15,000 ft AGL. The estimated aircraft trajectory is computed for each flight and compared with the trajectory from the aircraft’s onboard navigation system, which utilized a GPS receiver coupled with an inertial navigation system and an altimeter. The cellular SOPs produced remarkable sustained navigation accuracy over the entire flight trajectories in all three flights, achieving a 3D position root mean-squared error of 10.53 m, 4.96 m, and 15.44 m, respectively.

Opportunistic navigation with cellular signals is studied and demonstrated on a high-altitude aircraft. An extensive campaign was conducted by the Autonomous Systems Perception, Intelligence, and Navigation Laboratory (ASPIN) in collaboration with the U.S. Air Force (USAF) to sample ambient cellular signals of opportunity (SOPs) at different altitudes in different regions in Southern California, USA. The carrier-to-noise ratio is characterized as a function of altitude and horizontal distance. It was found that signals from cellular transmitters can be reliably acquired and tracked for navigation purposes by receivers mounted on high dynamics aircraft flying at altitudes as high as 23,000 ft above ground level and horizontal distances as far as 100 km. The multipath channel is also characterized as a function of altitude, showing clean channels with a dominant line-of-sight component for altitudes up to 23,000 ft. To further assess the potential of cellular SOPs for aircraft navigation, a 51-km trajectory traversed over a period of 9 minutes was estimated exclusively using cellular SOPs (i.e., with no GPS or other sensors, except for barometric altimeter measurements), resulting in a three-dimensional position (3-D) 10.5-m position root-mean-squared error.

I am not afraid of the GPS jammer, as long as there are ambient signals of opportunity (SOPs) to exploit in the environment. In environments where GPS signals are challenged (e.g., indoors and deep urban canyons) or denied (e.g., under jamming and spoofing attacks), SOPs could serve as an alternative positioning, navigation, and timing (PNT) source to GPS, and more generally, to global navigation satellite systems (GNSS). This article presents a radio simultaneous localization and mapping (radio SLAM) approach that enables the exploitation of SOPs for resilient and accurate PNT. Radio SLAM estimates the states of the navigator-mounted receiver simultaneously with the SOPs’ states. Radio SLAM could produce an SOP-derived navigation solution in a standalone fashion or by fusing SOPs with sensors [e.g., inertial measurement unit (IMU), lidar, etc.], digital maps, and/or other signals (e.g., GNSS). This article presents the first published experimental results evaluating the efficacy of radio SLAM in a real GPS-denied environment. These experiments took place at Edwards Air Force Base, California, USA, during which GPS was intentionally jammed with jamming-to-signal ratio as high as 90 dB. This article evaluates the timing of two cellular long-term evolution (LTE) SOPs located in the jammed environment, showing timing stability over 95 min of GPS jamming. Moreover, the article presents navigation results showcasing a ground vehicle traversing a trajectory of about 5 km in 180 s in the GPS-jammed environment. The vehicle's GPS-IMU system drifted from the vehicle's ground truth trajectory, resulting in a position root mean-squared error (RMSE) of 238 m. In contrast, the radio SLAM approach with a single cellular LTE SOP whose position was poorly known (an initial uncertainty on the order of several kilometers) achieved a position RMSE of 32 m.