Cessna ARC RT-359A transponder (beige box), beneath a VHF radio. In this example, the transponder code selected is 1200 for VFR flight. The green IDENT button is marked "ID".
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; and 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" to mean "I have selected transponder code xxxx".
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 the 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 radar screen of the air traffic controller 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. Some lower-end altimeters do not normally have a built in encoder and so a modified Gray code, called a Gillham code, is used to pass altitude information to the transponder. Around busy airspace there is often a regulatory requirement that all aircraft be equipped with an 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. 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" button, 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 the transponder in an aircraft in response to a secondary surveillance radar interrogation signal to assist air traffic controllers in traffic separation. A discrete transponder code (often called a squawk code) is assigned by air traffic controllers to uniquely identify an aircraft. This allows easy identification of aircraft on radar.
Squawk codes are four-digit octal numbers; the dials on a transponder read from zero to seven, inclusive. Thus the lowest possible squawk is 0000 and the highest is 7777. Four octal digits can represent up to 4096 different codes, which is why such transponders are often called "4096 code transponders." Care must be taken not to squawk any emergency code during a code change. For example, when changing from 1200 to 6501 (an assigned ATC squawk), one might turn the second wheel to a 5 (thus 1500), and then rotate the first wheel backwards in the sequence 1-0-7-6 to get to 6. This would momentarily have the transponder squawking a hijack code (7500), which might lead to more attention than one desires. Pilots are instructed not to place the transponder in "standby mode" while changing the codes, as it causes the loss of target information on the ATC radar screen, but instead to carefully change codes to avoid inadvertently selecting an emergency code. Additionally, modern digital transponders are operated by buttons to avoid this problem.
Most codes above can be selected by aircraft if and when the situation requires or allows it, without permission from ATC. Other codes are generally assigned by ATC units. For IFR flights, the squawk code is typically assigned as part of the departure clearance and stays the same throughout the flight.
VFR flights, when in uncontrolled airspace, will "squawk VFR" (or conspicuity code in the UK)(1200 in the U.S., 7000 in Europe). Upon contact with an ATC unit, they will be told to squawk a certain unique code. When changing frequency, for instance because the VFR flight leaves controlled airspace or changes to another ATC unit, the VFR flight will be told to "squawk VFR" again.
In order to avoid confusion over assigned squawk codes, ATC units will typically be allocated blocks of squawk codes, not overlapping with the blocks of nearby ATC units, to assign at their discretion.
Not all ATC units will use radar to identify aircraft, but they assign squawk codes nevertheless. As an example, London Information — the Flight Information Service station that covers the lower half of the UK — does not have access to radar images, but does assign squawk code 1177 to all aircraft that receive a FIS from them. This tells other radar equipped ATC units that that specific aircraft is listening on the London Information radio frequency, in case they need to contact that aircraft.