Page 405 - PPL-engelsk 2025

Communication

The thick white line

represents the radar

signal towards the

aircraft.

The radar signal is processed by the transponder, which sends a reply signal

containing the assigned set code, flight altitude, and the aircraft's registration

letters.

4.10.3.2 SSR radar

Hence, the SSR Radar system (Secondary Surveillance Radar) was invented,

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

to the radar when it picks up the primary radar signal.

4.10.3.3 The transponder

The transponder’s reply consists of four digits between 0 and 7. There are 4096

possible responses from the aircraft, depending on the code set by the pilot.

You can see a

picture of a
transponder

here.

If the selector is set to OFF, the transponder is turned off.

We set the selector to SBY (Stand By) when we start the electrical systems, as

per the checklist.

Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 405