Page 57 - Radio BEG-engelsk- engelsk tekst

© Henning Bach Andersen

The SSR Radar (Secondary Surveillance Radar) system was then invented, where

the aircraft is equipped with a transponder that sends a response back to the

interrogating radar when it picks up the secondary radar signal.

The response from the transponder in the aircraft consists of 4 digits between 0

and 7. There are 4096 possible responses from the aircraft, depending on which
digits the pilot has set the transponder to.

You see a picture of a

transponder here.

If the selector switch is set to OFF, the transponder is switched off.

We set the selector switch to SBY (Stand By) when we reach the point in the

checklist during the start-up of the electrical equipment. In this position, the
transponder warms up so that it is ready when we set the transponder to ON on

the runway, just before take-off.

In the ON position, the transponder sends the 4 digits turned into the 4 windows.
The transponder sends a secondary response signal in mode A (ALFA).

If the selector is set to the ALT position, information about the aircraft's altitude

is sent with the response signal. 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).

When the transponder sends information about altitude, we say that it is in

mode C (CHARLI).

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

flight.

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

BEG-english radio marts 2026 - Midtjysk Flyveskole 57