Page 902 - PPL-engelsk 2025
P. 902
Navigation
9.10.4.2 SSR Radar and transponder
Primary radar is not suitable for radar control. That is the SSR Radar.
If the reflected rays could be coded with answers, one would have come a long
way.
The SSR Radar system was invented, a further development of the primary radar,
where the aircraft is equipped with a transponder, which sends answers back to
the radar when it picks up the primary radar signal.
The ground station system is called an interrogator.
Together with the primary radar signal, an interrogation pulse is transmitted.
The thick white line
in the drawing here
symbolizes the
radar signal towards
the aircraft.
9.10.4.3 The transponder
The signal is processed in the aircraft's transponder, which sends a response
signal that can be the set code, the flight altitude and the aircraft's registration
letters.
The response from the
transponder in the aircraft consists
of 4 digits between 0 and 7.
There are 4096 different possible responses from the aircraft, depending on
which digits the pilot has set the transponder to.
If the selector switch is set to OFF, the transponder is switched off.
Flyveteori PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 902
