Transponder (aeronautics)

A transponder (short for transmitter-responder and sometimes abbreviated to XPDR, XPNDR, TPDR or TP) is an electronic device that produces a response when it receives a radio-frequency interrogation. Aircraft have transponders to assist in identifying them on air traffic control radar. Collision avoidance systems have been developed to use transponder transmissions as a means of detecting aircraft at risk of colliding with each other. A transponder (short for transmitter-responder and sometimes abbreviated to XPDR, XPNDR, TPDR or TP) is an electronic device that produces a response when it receives a radio-frequency interrogation. Aircraft have transponders to assist in identifying them on air traffic control radar. Collision avoidance systems have been developed to use transponder transmissions as a means of detecting aircraft at risk of colliding with each other. Air traffic control units use the term 'squawk' when they are assigning an aircraft a transponder code, e.g., 'Squawk 7421'. Squawk thus can be said to mean 'select transponder code' or 'squawking xxxx' to mean 'I have selected transponder code xxxx'. The transponder receives interrogation from the Secondary Surveillance Radar on 1030 MHz and replies on 1090 MHz. Secondary surveillance radar (SSR) is referred to as 'secondary', to distinguish it from the 'primary radar' that works by passively reflecting a radio signal off the skin of the aircraft. Primary radar determines range and bearing to a target with reasonably high fidelity, but it cannot determine target elevation (altitude) reliably except at close range. SSR uses an active transponder (beacon) to transmit a response to an interrogation by a secondary radar. This response most often includes the aircraft's pressure altitude and a 4-digit octal identifier. A pilot may be requested to squawk a given code by an air traffic controller, via the radio, using a phrase such as 'Cessna 123AB, squawk 0363'. The pilot then selects the 0363 code on their transponder and the track on the air traffic controller's radar screen will become correctly associated with their identity. Because primary radar generally gives bearing and range position information, but lacks altitude information, mode C and mode S transponders also report pressure altitude. Mode C altitude information conventionally comes from the pilot's altimeter, and is transmitted using a modified Gray code, called a Gillham code. Where the pilot's altimeter does not contain a suitable altitude encoder, a blind encoder (which does not directly display altitude) is connected to the transponder. Around busy airspace there is often a regulatory requirement that all aircraft be equipped with altitude-reporting mode C or mode S transponders. In the United States, this is known as a Mode C veil. Mode S transponders are compatible with transmitting the mode C signal, and have the capability to report in 25 foot increments; they receive information from a GPS receiver and also transmit location and speed. Without the pressure altitude reporting, the air traffic controller has no display of accurate altitude information, and must rely on the altitude reported by the pilot via radio. Similarly, the traffic collision avoidance system (TCAS) installed on some aircraft needs the altitude information supplied by transponder signals. All mode A, C, and S transponders include an 'IDENT' switch which activates a special thirteenth bit on the mode A reply known as IDENT, short for 'identify'. When ground-based radar equipment receives the IDENT bit, it results in the aircraft's blip 'blossoming' on the radar scope. This is often used by the controller to locate the aircraft amongst others by requesting the ident function from the pilot, e.g., 'Cessna 123AB, squawk 0363 and ident'. Ident can also be used in case of a reported or suspected radio failure to determine if the failure is only one way and whether the pilot can still transmit or receive, but not both, e.g., 'Cessna 123AB, if you read, squawk ident'. Transponder codes are four digit numbers transmitted by an aircraft transponder in response to a secondary surveillance radar interrogation signal to assist air traffic controllers with traffic separation. A discrete transponder code (often called a squawk code) is assigned by air traffic controllers to identify an aircraft uniquely in a flight information region (FIR). This allows easy identification of aircraft on radar.