Harmon Technologies, Inc.

Transponder

The transponder works with air traffic control's radar.  The air traffic control radar on the ground periodically asks the transponder to identify itself.  When this happens, the transponder replies with the code that is dialed in to the window and also adds the airplanes altitude (it gets this from the altimeter).  This helps air traffic control identify the airplane and also to know hoe high it is so they can separate it properly from other planes.

Airplanes who are not communicating with air traffic control or who are on VFR flight plans, normally dial a default value (1200) into the transponder.  Normally, airplanes are assigned a specific code before takeoff or when they start to receive radar services.  Some codes have specifl meanings.  If a pilot dials in 7700 he can declare an emergency even if his communications radios are not working.  7600 means that the airplane is fine but the radios are out.  There is even a special code that pilots can dial in that inform air traffic control that a hijack is in progress.  This allows a crew to tell ATC what is happening even if they are not free to speak.

There is a button called "ident" on the transponder.  This causes the aircarft's image to blossom (get brighter) or flash on the radar screen.  This can be requested by air traffic control when they are first tlking to an airplane to help make it easier to find.  It can also be used by a pilot to acknowledge instructions if the communications radios are not working.

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