Detection and Tracking of Aircraft Using the NORFOX Distributed Acoustic Sensing Array

Distributed Acoustic Sensing (DAS) enables dense, continuous measurements of dynamic strain and is increasingly used to monitor anthropogenic activity. Aircraft generate clear acoustic signals that can be recorded by such arrays, with Doppler frequency shifts providing a direct observable of source motion. Previous seismic and acoustic studies of aircraft have typically relied on sparse sensor networks or single-point measurements, limiting the ability to resolve source–receiver geometry and extract trajectory information. Here we exploit the two-dimensional, multi-arm geometry of the NORFOX fibre array in Norway to track aircraft signals across spatially distributed sensors, providing improved constraints on aircraft kinematics.

We focus on helicopter fly-bys and selected fixed-wing aircraft. For helicopters flying at altitudes of a few hundred metres, we extract Doppler signatures from time–frequency representations of the DAS data and validate these against ADS-B flight records. The observed frequency shifts are consistent with expected aircraft motion, and analysis of the Doppler shifts allows estimation of aircraft speed and source frequency; the latter is related to rotor dynamics and can be used to discriminate between aircraft types. The shape of the Doppler curves, including their slope and temporal extent, is diagnostic of the aircraft's distance at closest approach, providing geometric constraints on the source–receiver configuration. The dense spatial sampling of NORFOX enables these signatures to be tracked across multiple fibre arms, enabling estimation of aircraft trajectories directly from the DAS observations.

For jets, the source frequencies typically exceed the 62.5 Hz Nyquist frequency of the DAS recordings, and Doppler harmonics are therefore not resolved. Nevertheless, coherent lower-frequency arrivals can still be observed, even at cruising altitudes, and analysing their propagation across the array yields time-varying back-azimuth and apparent slowness estimates, providing directional constraints on source motion.

Together, these results demonstrate that array-based DAS deployments can meaningfully enhance aircraft detection and tracking using existing fibre infrastructure. The NORFOX geometry illustrates how fibre layout determines the quality of kinematic information recoverable from such observations.