  Unmanned air vehicles often produce significant noise from their propulsion
systems. Using this broadband signal as "acoustic illumination" for an
auxiliary sensing system could make vehicles more robust at a minimal cost. We
present an acoustic beamforming-based algorithm that estimates object-relative
distance with a small two-microphone array using the generated propulsion
system noise of a vehicle. We demonstrate this approach in several closed-loop
distance feedback control tests with a mounted quad-rotor vehicle in a noisy
environment and show accurate object-relative distance estimates more than 2x
further than the baseline channel-based approach. We conclude that this
approach is robust to several practical vehicle and noise situations and shows
promise for use in more complex operating environments.
