What Is A Transponder?
A transponder (also called transmitter-responder), used as SSR (Secondary Surveillance Radar), is an electronic device that is mandatory on every engine airplane since 2006.
The transponder makes it possible for Air Traffic Controllers to see you and your aircraft on their screen with information regarding Altitude, Squawk Code, Call Sign, registration, and more if equipped with a Mode Charlie Transponder, more about that later ;)
How does a Transponder work?
A Transponder is a device onboard an aircraft that receives and transmits signals to an SSR(Secondary Surveillance Radar) Antenna. It is used to measure the aircraft's position and bearing on the screen, but can also show additional information. The Position and bearing can be calculated with time and frequency. The Transponder receives a signal and then transmits an encrypted code to the SSR antenna. With help of this system, Air traffic controllers can identify air traffic. A transponder receives on 1030MHz and transmits on 1090MHz.
Different Transponder Modes (interrogation modes)
Mode A: transmits only a Squawk, a Squawk is a transponder code, an Air Traffic Controler may give to a pilot to set into his transponder. A squawk is a 4 number code with numbers between 0 and 7. There are 4096 combinations possible.
Mode C: transmits a Squawk, but also the Aircraft's Pressure Altitude.
A Pressure Altitude is an aiecraft's Altitude set to a standard pressure setting: 1013,25hPa or 29.92 inHg.
Mode S: Most aircrafts have built-in mode Sierra (S) transponders. Mode A and C Transponders are disappearing more and more these days. There are airspaces, where Mode Sierra Transponders are obligatory.
A mode S Transponder has the same features as Mode A and C however, it is also compatible with TCAS (Traffic Collision Avoidance System) and ADS-B (Automatic Depended Surveillance-broadcast) it also has a third feature, which sends a unique 24-bit address with the callsign. This system features 16'700'000 codes, which are available to ATC (Air Traffic Control). Mode S Transponders transmit every second. It should be noticed that mode S is also compatible with mode A & C SSR systems.
Operation of a Transponder
- OFF — Transponder is off
- SBY, STBY (= Standby) — Transponder is on but doesn’t transmit information
- ON — Transponder transmits current squawk
- ALT (= Altitude), NORM (normal mode) — Transponder transmits current squawk and the current altitude (Mode C) if it also features mode S it will operate in mode Sierra.
- Squawk VFR — 7000 in Europe & 1200 in the US
When commencing a flight under VFR rules a pilot should start using the standard VFR code (EU: 7000 US: 1200, check with your national authority…). As soon as advised to change your transponder code you should comply and set the appropriate code. For pilots flying under IFR rules you often get your squawk code in your IFR clearance before departure.
If a controller ask you for example “Please identify”, this means you should press the IDENT button on your transponder → your aircraft will then be highlighted on a controller's screen this is used to identify new traffic in an airspace and should only be made when advised by ATC!
International Emergency Codes
There are different squawk codes (7500, 7600, 7700) which should be used in cases of an emergency. Every pilot must know these codes because they tell the ATC that something is going wrong…
- 7500 — Hijacking (= man with a knife)
- 7600 — Radio failure (= hear nix / radio nix / need a radio fix)
- 7700 — Emergency (= going to heaven / falling from heaven / pray to heaven / close to heaven)
If a plane enters one of these codes into his transponder, they will get immediate attention! Only use these codes when really necessary ;)
Transponder Failure
Maybe you ask yourself now, what happens if an aircraft has a transponder failure… This is a difficult and dangerous situation for everyone, the pilots and the air traffic controllers who are responsible to safely land the airplane. The ATC can’t see you on his radar anymore and doesn’t know where you are, what’s your altitude, your speed, etc. What is clear is, that the airplane has to land immediately.
These are some reasons why an aircraft should land immediately:
- The aircraft may enter controlled airspace without obtaining a clearance
- Loss of separation between the aircrafts in this airspace
- The controllers might forget to transfer the aircraft to the next sector
This book helped me get a better understanding of the transponder. I can only recommend it:
