A transponder is an electronic device aboard an aircraft that enhances its identity on the air-traffic controller's radar screen. It is the airborne portion of the secondary surveillance radar system, and one with which every pilot is familiar. The air-traffic control radar beacon system (ATCRBS) cannot display the secondary information unless an aircraft is equipped with a transponder. It automatically receives radio signals from interrogators on the ground, and selectively replies with a specific reply pulse or pulse group only to those interrogators being received in the mode in which the transponder is set. There are three main types of transponders: A, C, and S.
- Transponder A: sends back a transponder code to ATC.
- Transponder C: sends a transponder code plus altitude information to ATC.
- Transponder S: sends a transponder code plus altitude information to ATC, and receives and reports information to other transponders of the same type.
Transponders need to have 4096 capabilities. This means they must have the ability to have a 4-digit code including the numbers 1 – 7 entered into them. The number 4096 comes from the number of different codes you can make out of 7 different numbers in sets of 4. A mode C transponder is required to enter class A airspace, fly within 30 nautical miles of primary airports in class B airspace, or fly in or above class C airspace. Mode C transponders are also required when flying at or above 10,000 ft. mean sea level (MSL), not including any airspace at or below 2,500 ft. above ground level (AGL). This is a mandatory requirement for the entire continental United States.
FAA regulations require that the transponder be tested every 24 calendar months for operations in controlled airspace. Each transponder comes equipped with at least four basic functions: On, ALT, SBY, and Off. On turns the transponder on, ALT gives it the ability to report altitude information, SBY puts the transponder on standby, making it act as a mode A transponder, and Off turns the transponder off. Every transponder must have the ability to IDENT, which makes the transponder send a signal, allowing it to blink on the radar screen. The most basic function of every transponder is its ability to SQUAWK. This is the transponder's natural function to send out an encoded signal with the 4-digit number code. A SQUAWK code is assigned to the pilot by ATC when he calls them up for flight clearance.
There are a few basic codes for a transponder that every pilot needs to know:
- 7500: entered if the aircraft has been hijacked.
- 7600: entered if the aircraft has lost radio communication abilities.
- 7700: entered if the aircraft is in any other type of emergency.
- 1200: entered when the aircraft is flying VFR.