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Microlight » Flight Radio » SSR Transponders


Ground based radar units send a signal (interrogates) to the transponder unit in the aircraft. 

The signal triggers the transponder which then sends a coded signal back with the information regarding the aircraft which can include the SQUAWK code, altitude and groundspeed.

Different aircraft have different four digit transponder codes to assist air traffic control units with what that dot on their radar screen represents. 

Two examples are Helicopters 1500 and Fixed Wing 1200.


The manipulation and adjustment of the controls of commonly used transponders.

The above transponder has 4 knobs which are selectable from 1 to 7 and together give the desired four digit transponder SQUAWK code.

The IDENT (identify) button when pressed sends a signal that causes the aircraft dot on a radar screen to flash which identifies the aircraft to the Air Traffic Controller.

The left-hand switch has 5 functions used for different purposes.

  • OFF                Turns the transponder OFF
  • SBY                Turns the transponder on but will not reply to interrogation (used after start up and during taxi to the runway)
  • TST                 Self testing function
  • ON                 The transponder will reply to interrogation and send Mode A information. 
  • ALT                The transponder will reply to interrogation and send Mode A and C information (set just before take-off)
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