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2.6 SSR Transponders

2.6.2 Describe the manipulation and adjustment of the controls of commonly used transponders.

2.6.4 Describe the function of the following terms, and explain the procedures to transmit:

(a) Mode A information; and

(b) Mode C information.


Ground based radar units send a signal (interrogates) 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 may 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 above transponder has 4 knobs which are selectable from 1 to 7 and the combination of the 4 give the desired four digit transponder SQUAWK code.

The IDENT button when pressed sends a signal that causes the aircraft dot on a radar screen to flash which identifies the aircraft.

The right 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)
  • 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. 
  • Mode A            transmits the 4-digit SQUAWK code
  • Mode C            transmits the altitude
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