Abstract: A method of determining the bearing of a source of a sonar signal comprising determining the phase of a cyclic variation of the output signal of a sonar detector having a directional characteristic which occurs as the orientation of the detector, and hence its characteristic, is rotated. In a preferred method the sonar detector output is spectrum analysed at each of a plurality of different orientations of the detector to provide for each orientation a signal representative of the amplitude of a component of the signal produced by the source of a chosen frequency. The signals so obtained are then arranged to form a time series of samples of said component as the orientation of the detector is rotated at a predetermined frequency and the time series used to analyse the spectrum of the signal defined by the time series and thereby determine the phase of the sinusoidal component of the signal defined by the time series at the predetermined frequency. Apparatus for carrying out the method is also disclosed.