Localization of a merchant ship by means of striations in a lofargram

In low frequency passive sonar a lofargram is a common presentation form for time varying spectral data. A target may emit narrow frequency lines and broadband noise. Depending upon the transmission channel, the target noise can show up as multipath interference patterns – striations - in the lofargram. These may be very consistent, and can for underwater waveguides be described mathematically by use of the so-called waveguide invariant β. Under some idealized circumstances, β is a compact characterization of the waveguide. Variable bathymetry can also be handled. During an experiment in the Barents Sea, a passing tanker having a rich acoustical output gave a lofargram with many striations. These are successfully modelled by means of β. Inversely, parts of the lofargram can be used for localization of the target by way of the striations. The speed of the striation patterns along an array can be related to the target speed, taking account of the target’s track with its offset and course in relation to the antenna axis. A method of speed estimation by means of mode filtering in frequency and direction is also treated. Auxiliary measurements with SUS explosive charges and a CW towing support the tanker results.