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. In aviation, aircraft have transponders to assist in identifying them on radar and on other aircraft's collision avoidance systems.
Air traffic control units use the term "squawk" when they are assigning an aircraft a transponder code, e.g. "Squawk 7421". Squawk or squawking thus can be said to mean "select transponder code" or "I have selected transponder code xxxx".
The aviation transponder was originally developed during World War II by the British and American military as an "Identification friend or foe" (IFF) system to differentiate friendly from enemy aircraft on radar. The concept became a core of NORAD technology in the defence of North America during the Cold War.
Secondary Surveillance Radar is referred to as "secondary", to distinguish it from the "primary radar" that works by passively bouncing a radio signal off the skin of the aircraft. Primary radar works best with large all-metal aircraft, but not so well on small, composite aircraft. Its range is also limited by terrain and rain or snow and also detects unwanted objects such as automobiles, hills and trees. Furthermore not all primary radars can estimate the altitude of an aircraft. Secondary radar overcomes these limitations but it depends on a transponder in the aircraft to respond to interrogations from the ground station to make the aircraft more visible and to report the aircraft's altitude.
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. 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 installed on large aircraft as a last resort safety net needs the altitude information supplied by transponder signals.
This limitation has resulted in at least one accident. On 31 August 1986, a Piper Archer with a pilot and two passengers had inadvertently penetrated the 6,000-foot floor of controlled airspace without a clearance and collided with Aeromexico Flight 498, a DC-9 with 58 passengers and 6 crew at an altitude of 6,650 feet. The Archer had only a mode A squawk reporting capability and the air traffic controller assumed it was below the controlled airspace.
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 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 (i.e. "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 (i.e. "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 identity of the 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. There are 4096 permutations of these four digit codes, which is why they are often called "4096 code transponders." Because these squawks are sensitive, 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.
There are other codes known as conspicuity codes which are not necessarily unique to a particular aircraft, but may have their own meaning and are used to convey information about the aircraft to ATC, possibly when the aircraft is not in radio contact. Codes 2000 and 7000 are examples of conspicuity codes.
The use of the word "squawk" comes from the system's origin in the World War II Identification Friend or Foe (IFF) system, which was code-named "Parrot". Parrot today generally refers to IFF only. The parrot check is generally done as part of the last-chance inspection at the runway, or after becoming airborne. Parrot sweet, and parrot sour are given, and the aircraft will have to abort in a real-world mission when sour, or face being attacked by friendly forces. Modern use of the word Parrot refers to a test transponder located at a fixed location off the radar facility. The parrot verifies range and direction accuracy of the radar facility.
In Belgium following codes are assigned for VFR traffic under Flight Information Services (BXL FIC)
In Germany the following codes have been used:
As from 15 March 2007 these have been replaced by the international 7000 code for VFR traffic.
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" (1200 in the US, 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 0027 to all aircraft that receive an 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.