Passive sonar signature estimation using bispectral techniques

An important task in underwater passive sonar signal processing is the determination of target signatures based on the narrow-band signal content in the received signal. To achieve good classification performance it is important to be able to separate the different sources (e.g. engine, hull and drive) present in the signature, and to determine the distinct frequency coupling pattern of each of these sources. In this work we demonstrate how this can be done using bispectral techniques applied to data recorded at a sea trial in the Baltic Sea. As a target we used a 23 ft fiberglass motor boat powered by a 4-cylinder, 4-stroke, turbo-charged diesel engine connected to a stern drive with two counter rotating propellers. Data was recorded with a bottom mounted hydrophone array as well as with accelerometers mounted on the engine and the hull. It was found that the harmonics that propagated through water are engine related at low speeds and drive related at high speeds. The hull vibrations are only present at very low speeds. Moreover, we found that normalized bispectrum measures (skewness) could provide additional coupling information not visible in the standard bispectrum.