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Air Law
1.5.3 Regulation of air traffic
1.5.3.1 Primary radar
The radar system was developed during World War II to ensure protection
against the intrusion of enemy aircraft.
The system is based on the reflection of radio waves when they encounter an
impenetrable mass.
The reflected waves were captured on the radar screen, allowing the detection
of something in the specified direction and at a certain distance from the radar,
which reflected the radiation.
However, it was impossible to determine whether it was a friend or an enemy.
A "primary" trace appeared on the radar screen.
1.5.3.2 SSR Radar
If the reflected waves could be coded with responses, progress would be made.
Thus, the SSR Radar (Secondary Surveillance Radar) system was invented, in
which the aircraft is equipped with a transponder that sends a response back
to the radar when it detects the primary radar signal.
The thick white line
in the illustration
represents the radar
signal toward the
aircraft.
The signal is processed by the aircraft’s transponder, which sends a reply signal,
such as the set code, altitude, and the aircraft’s registration letters.
Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 69
