It is not sufficient in the airspace system for only the pilot to have an indication of the aircraft's altitude; the air traffic controller on the ground must also know the altitude of the aircraft. To provide this information, the aircraft may be equipped with an encoding altimeter.

When the ATC transponder is set to Mode C, the encoding altimeter supplies the transponder with a series of pulses identifying the flight level (in increments of 100 feet) at which the aircraft is flying. This series of pulses is transmitted to the ground radar where they appear on the controller's scope as an alphanumeric display around the return for the aircraft. The transponder allows the ground controller to identify the aircraft under his/her control and to know the pressure altitude at which each is flying.

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A computer inside the encoding altimeter measures the pressure referenced from 29.92" Hg and delivers this data to the transponder. When the pilot adjusts the barometric scale to the local altimeter setting, the data sent to the transponder is not affected. 14 CFR part 91 requires the altitude transmitted by the transponder to be within 125 feet of the altitude indicated on the instrument used to maintain flight altitude.

Encoding altimeter: A sensitive altimeter that sends signals to the ATC transponder, showing the pressure altitude the aircraft is flying....read more

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The absolute altimeter, also called a radar or radio altimeter, measures the height of the aircraft above the terrain. It does this by transmitting a radio signal, either a frequency modulated continuous-wave or a pulse to the ground, and accurately measuring the time used by the signal in traveling from the aircraft to the ground and returning. This transit time is modified with a time delay and is converted inside the indicator to distance in feet.
 
Most absolute altimeters have a provision for setting a decision height/decision altitude (DH/DA) or a minimum descent altitude (MDA) so that when the aircraft reaches this height above ground, a light will illuminate and/or an aural warning will sound. Absolute altimeters are incorporated into ground proximity warning systems (GPWS) and into some flight directors.
 
Tag: Flying instrument, instrument flight, aviation, piloting, instrument rating, instrument flying training, instrument flight rating, instrument rating requirement, instrument rating regulation, aircraft, aero plane, airplane, and aeronautical knowledge.
 
Transponder: The airborne portion of the ATC radar beacon system.
 
Encoding altimeter: A sensitive altimeter that sends signals to the ATC transponder, showing the pressure altitude the aircraft is flying.

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