a system for producing a decoy to enable a target to avoid a homing torpedo that uses a sonar ping signal for homing in on the target comprises a transducer that operates in a receive mode in which it receives sonar signals and produces corresponding electrical signals and in a transmit mode in which it emits a sonar return pulse . a digital signal processor connected to the transducer is arranged to analyze the electrical signals corresponding to sonar signals received by the transducer and to determine whether they were emitted by the homing torpedo . the digital signal . processor is further arranged to switch the transducer to the transmit mode in response to receipt of a sonar ping signal from the homing torpedo and to cause the transducer to transmit a return pulse that acts as a decoy signal to the homing torpedo .