A transponder is an electronic device aboard an aircraft that enhances its identity on the air-traffic controllers RADAR screen. It is the airborne portion of the secondary surveillance RADAR system and one in which every pilot is familiar. The air-traffic control RADAR beacon system (ATCRBS) cannot display the secondary information unless and 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 on 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 transponder code, altitude information, to ATC and receive and reports information to other transponders of the same type.
Transponders need to have 4096 capabilities. This means they must the ability to have a 4-digit code including the numbers 1 – 7 entered into them. The number 4096 comes from the amount of different codes you can make out of 7 different numbers in sets of 4. A mode C transponder is required in order to enter class A airspace, go within or into 30 nautical miles of class B airspace primary airports, or go in or above class C airspace. Mode C transponders are also required when flying at or above 10,000ft mean sea-level (MSL) not including any airspace at or below 2,500ft 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 4 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. Also, 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 transponders 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.