Abstract: A process and apparatus for detecting underwater objects comprises an elongate array of ultrasound transmitter elements mounted substantially vertically and forward facing on the bow of the ship from which is transmitted a sequence of ultrasound wave pulses, with each pulse having a frequency and vertical beam angle which are different to the frequency and vertical beam angle of the preceding pulse in the sequence, the frequency being characteristic of the vertical beam angle and the vertical beam angle being controlled by using time delays between different transmitter elements, and an elongate array of ultrasound receiver elements mounted substantially horizontally on the bow of said ship with the receiver elements forward facing which receive any ultrasound waves reflected from objects in front of the ship, the output from the receiver elements being steered horizontally using time delays between different receiver element outputs and for each horizontal position of the beam of outputs, the outputs from th