I Can Hear You Loud and Clear: GNSS-Less Aircraft Navigation With Terrestrial Cellular Signals of Opportunity

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: Region A: Edwards Air Force Base (rural) and 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 were 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 and 56.8 km in regions A and B, traversed in 455 and 601 s, a 3-D position root mean-squared error of 6.8 and 8.2 m was achieved by exploiting an average of about 22 and 11 eNodeBs, respectively.