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Mode C (CHARLI): If the selector is set to the ALT position, information about the

aircraft's pressure altitude is sent with the transponder code.

However, this only happens if the aircraft is equipped with an encoder that
provides the transponder with information about the aircraft's pressure altitude

(altitude in relation to 1013 HPa).

If the aircraft is equipped with a transponder, this must be switched on during

flight. If the transponder is in mode C, it must be in the ALT position.

Mode S (SIERRA): In addition to the transponder code and altitude, the mode

also sends registration information and speed to ATC and to other aircraft

equipped with Mode S transponders within range.

Mode S provides the air traffic controller with this information directly on the

computer screen, so that he is immediately aware of the aircraft's identification

and altitude, and whether the aircraft is ascending or descending in altitude.

In addition, computer systems generate vectors that show the aircraft's position
in time, based on the current flight direction and speed. The air traffic controller

can then see if there is or will be conflicting traffic.

Here the signal from a

single aircraft is seen on a
radar-generated

computer screen.

See animation here

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Flyveteori PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 904