Sensing the underwater sound field produced by a moving airborne signal source

Sound emitted by a turboprop aircraft in level flight is received by each of the uniformly spaced hydrophones of a horizontal line array located below the sea surface. The primary signal emitted by the source is an acoustic tone corresponding to the propeller blade rate of the aircraft. As the airborne source is in relative motion with respect to the array, the received signal at each hydrophone is Doppler-shifted in frequency. For each transit of the aircraft, the variation with time of the observed Doppler-shifted blade rate is found to match the theoretical curve predicted by a basic ray path propagation model. This model assumes that the sound propagates from the airborne acoustic source to a subsurface sensor through two separate isospeed media (air and water) that are separated by a planar boundary (the air-sea interface). More importantly, as a result of beamforming the hydrophone array data, it is shown that the variations with angle of arrival of the observed Doppler-shifted blade rate are in close agreement with the variations predicted for both the direct refraction and bottom bounce propagation paths. In addition, the observed temporal variation of the differential time-of-arrival of the signal at two spatially separated sensors is shown to agree with the predicted variation. This paper also reviews other contributions made by the authors to the underwater sensing of transiting aircraft using different passive acoustic signal processing methods.