Abstract: When existing radar systems fail to detect a target the operator may be unsure whether this is because of some failure in the apparatus or because there is in fact no target there.The invention deals with this problem by providing a testing device for simulating targets at different ranges. The testing device includes a small aerial arranged to sample a transmitted pulse. This sample is fed to a circuit which imposes successive delays on the sample and feeds it back to the aerial at successive different times.The successive delays are produced by a delay line and a feed-back loop containing an amplifier. The circuit also includes a frequency shifter which simulates a Doppler shift.