Appts. for transmitting hydroacoustic pulses with predetermined shape and carrier frequency at specified times determines times of arrival of received hydroacoustic pulses externally transmitted with predetermined shape and carrier frequency. This has transducer for converting hydroacoustic energy received through water into received electrical energy. Synthesiser digitally synthesises electrical pulses of a certain shape and carrier frequency. Electrical transmission path conducts electrical pulses from synthesiser to transducer. An electrical reception path conducts received electrical energy. Converter converts the energy conducted through reception path into a sequence of digital samples representative of amplitude.