Since, in the case of radar, the receiver is also the transmitter, it knows exactly when a pulse was sent.

ECONOMIST: How to look through walls

In 1994, researchers from America's airforce tested this idea by using the radio transmitter on a space probe called Clementine as a makeshift radar.

ECONOMIST: Creative destruction

In the second world war Mr Pierce worked on radar, in which signals directed at an object are bounced back to the transmitter.

ECONOMIST: John Pierce