Watson-Watt designed the first workable 'radio
direction finding,' or RDF, device for locating moving objects
by bouncing radio waves off them and calculating the range by
transmitted pulses. Independently, Rudolph Kühnold
was working on a similar system, but only realized that pulse
transmission was necessary some months later. It began to
be called 'radar,' for 'radio detection and ranging,' after 1940.