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© Henning Bach Andersen
7.2.2.2 Transponder - use and phraseology
The radar system was developed during World War II to prevent enemy aircraft
from entering.
The system is based on the reflection of radio waves when they encounter an
impenetrable mass. The reflected rays were picked up by the radar screen.
On the radar screen, you could see that there was something in the relevant
direction and at a certain distance from the radar that was reflecting the
radiation.
It was impossible to determine whether it was friend or foe. You had a “primary”
track on the radar screen.
By sending back response pulses when affected by radar pulses, it could be
determined whether it was “friend or foe.”
Thus was born the idea for what was called the “Transponder.”
The thick white line
symbolizes the radar signal
towards the aircraft.
The signal is processed in the transponder, which sends a response signal that
can be the set code, the flight altitude and the aircraft's registration letters.
The reflected beam just show that there is something of metal in the air that
maybe is mowing. The radar is only a primary radar.
If you now could code the reflected rays with a response to the question “Who
are you”.
Smart guys found the solution and mounted a secondary radar on top of the big
round radar. They called it an Interrogator.
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