Transponders

Transponders

The Federal Aviation Administration (FAA) uses radar to monitor the position and flow of aircraft in flight. When the radar beam sweeps across an aircraft, some of that radio energy is reflected back to the radar installation. But the reflection is often relatively weak and contains no altitude information.

To help improve the "visibility" of aircraft as radar targets, aircraft are equipped with little boxes called transponders. The transponder detects the radar sweep, and in response, generates its own very powerful return pulse. This 200-watt pulse makes the aircraft much easier to see on radar.

Aircraft operating near major cities, at high altitudes, and in some types of airspace, are required to use altitude encoding transponders. The transponder is connected to a little electronic device on board the aircraft that measures the aircraft's altitude. The transponder encodes the altitude data into the return pulse that it broadcasts to air traffic control. ATC uses the altitude data to help separate different aircraft from each other. Other airplanes with traffic alert and collision avoidance systems (TCAS) can see and use the altitude data. These transponders must be checked for accuracy every 24 months.

When air traffic controllers want to distinguish one airplane from another, they will temporarily assign the pilot a unique four-digit transponder code (some codes are reserved for special purposes).

Once the pilot has set the transponder to the specified code (called "squawking" i.e., squawk 4367), ATC's radar display will then isolate the target aircraft from all the rest. This allows the controller to assign the aircraft's registration number (it's N-number) or flight number to the individual blip on the radar screen.

The information assigned to the radar blip is called a data block. The data block follows the airplane through the ATC system as it's handed from one controller to the next throughout its flight.