Abstract: Piecewise correlators can be used in sonars transmitting continuous narrow-band random signals to avoid decorrelation caused by phase fluctuation of received signals.Unlike a conventional heterodyne correlator,the piecewise correlator does not include a bandpass filter,making it particularly suitable to be used in a digital system.This paper derives the expressions of output signal-to-noise ratio for both analogue and polarity piecewise-correlators and gives some fundamental principles in choosing the number of pieces and their length.A polarity piecewise-correlator designed in accordance with these principles has been used in a practical system and produced satisfactory results.