Mode C Transponder Testingby Myles Stetch

*All content in this document is designed for training purposes only. Do not attempt any testing in real situations unless authorized by a certified technician or equivalent.*


The aviation transponder was originally developed during the second World War by the British and American military as an "Identification Friend or Foe" (IFF) system to identify friendly aircraft from enemy aircraft on radar. During the Cold War, North America utilized this concept as a core technology for the NORAD defence system.
General and commercial aviation received traffic services from this concept after it was adopted by civil air traffic control using "Secondary Surveillance Radar" (SSR) (beacon radar) in the 1950s.


When the transponder receives a radar signal, it replies with a squack code. This is referred to as Mode A.
A transponder code can thereafter be paired with pressure altitude information, which is called Mode C.

Pressure altitude is obtained from an altitude encoder, a separate self-contained unit mounted in the aircraft (image displayed below).
The altitude information is passed to the transponder using Gillham Code, a modified version of Gray code.

external image gtx327.jpg
GTX 327 Altitude EncoderReference: Google Images
Mode C is used to help air traffic controllers to identify other aircraft and to maintain separation.


When testing is required for Mode C on a transponder, remember to keep the following factors in mind:
  • Mode C Interrogates on 1030Mhz and receives replies on 1090Mhz
  • Compatible Mode C will give a response if interrogated by a Mode S ground station (or compatible ramp test equipment)
  • Mode C codes must always be checked for sensitivity
  • Sensitivity checks can either be done by doing a correlation test(a pitot-static test to correlate the altimeter to the encoder), or a test box can be used to monitor the encoder and transmitter

Mode C replies can be determined by the use of Gray code as described in the Description section.

Mode C interrogations contain 3 pulse responses labeled P1, P2, and P3. The pulse spacing for Mode C from the start of P1 to the start of P2 is measured to be 2 microseconds long, whereas the pulse spacing from P1 to P3 is 21 microseconds long.
(image displayed below).
external image Interrogation_m.jpg
Mode A/C Pulse SpacingReference: Google images
All references were acquired from the following sources:
Sait Avionics Technology program