 The minimum range of any radar system is due to its pulse width , the length of time that the transmitter is turned on before it turns off so the receiver can listen for reflections from targets . If the echo from the target is received while the transmitter is still sending , the echo will be swamped by <unk> of the transmitted pulse from local sources . For instance , a radar with a pulse width of 1 µs would not be able to see returns from a target less than 150 m away , because the radar signal travelling at the speed of light would cover the round trip distance of 300 m before that 1 µs interval had passed . 
