Compiling the air situation picture from a submerged submarine

The transmission of sound through the air-water interface produces an underwater sound field that can have as many as four separate contributions from an airborne acoustic source. These contributions are identified as: direct refraction, one or more seafloor reflections, the evanescent wave, and sound scattered from a rough sea surface. The relative importance of each contribution depends on the horizontal distance of the source from the receiver, the water depth, the depth of the receiver in relation to the wavelength of the signal radiated by the source, and the roughness of the sea surface. This paper demonstrates how a submerged submarine towing an array of hydrophones is able to detect, classify, localize and track maritime patrol aircraft and helicopters, whilst at depth, by exploiting the direct and indirect sound propagation paths of the aircraft’s radiated noise (acoustic signature). As the airborne source is in relative motion with respect to the array, the received signal at each hydrophone is Doppler-shifted in frequency. It is shown that the variation with angle of arrival of the observed Doppler-shifted propeller blade passing frequency is in close agreement with that predicted for the direct refraction path. Similar observations are also made for the seafloor reflected propagation paths. Early warning/long range detection of threat aircraft is enabled by one or more reflections from the sea floor.