What is the principle of radar?

The basic idea behind radar is very simple: a signal is transmitted, it bounces off an object and it is later received by some type of receiver. This is like the type of thing that happens when sound echo's off a wall. However radars don't use sound as a signal. Instead they use certain kinds of electromagnetic waves called radio waves and microwaves. This is where the name RADAR comes from (Radio Detection And Ranging). Sound is used as a signal to detect objects in devices called SONAR (Sound Navigation Ranging).