cThe 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 soundas a signal. Instead they use certain kinds of electromagnetic waves called radio waves and microwaves. This is where the name RADAR comesfrom (RAdio Detection And Ranging). Sound is used as a signal to detect objects in devices called SONAR (SOund NAvigation Ranging). Another type of signal used that is relatively new is laser light that is used in devices called LIDAR (you guessed it...LIght Detection And Ranging).

radar
A radio antenna sends out a stream of radio waves (light gray). When they reach the airplane, they bounce off of it and send reflected waves (dark gray) back to a receiver in the antenna. The reflected waves are then processed electronically and analyzed to determine the airplane's distance, speed, and position. An image of the airplane is sometimes also generated.

A basic radar system is spilt up into a transmitter, switch, antenna, receiver, data recorder, processor ands ome sort of output display.

To determine the range of a distant object that reflected a radar signal, the receiver must record the time when the signal was received and compare it to when that signal was transmitted. This time is the time taken for the radio wave to propagate to the object and back to the antenna. Since all EM waves travel at the speed of light in a vacuum, 299,792,458 meters per second (Air is not quite a vacuum butEM waves still travel through it at approximately this speed) it is very easy to determine how far away the object is (just multiply the speed of light by the time for the signal to get received). Another thing the radar does when it receives a signal is determine how strong it is. For ground penetrating radars the strength of the signal can tell how much the beds under the surface have different properties. A higher received power indicates a larger difference between neighboring beds.

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