The idea of using two or more radio receivers to find the bearings of a radio transmitter and with the use of simple triangulation find the approximate position of the transmitter had been known and used since the invention of wireless communication. The general principle is to rotate a directional aerial and note where the signal is strongest. With simple aerial design the signal will be strongest when pointing directly towards and directly away from the source, so two bearings from different positions are usually taken, and the intersection plotted. More modern aerials employ uni-directional techniques.