Transponder (aviation)
Encyclopedia
A transponder 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 system
s.
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".
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
.
This concept was adapted in the 1950s by civil air traffic control
using secondary surveillance radar
(beacon radar) systems to provide traffic services for general aviation
and commercial aviation.
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 large aircraft needs the altitude information supplied by transponder signals.
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".
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.
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".
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.
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
and on other aircraft's collision avoidance system
Airborne Collision Avoidance System
An airborne collision avoidance system is an aircraft system that operates independently of ground-based equipment and air traffic control in warning pilots of the presence of other aircraft that may present a threat of collision. If the risk of collision is imminent, the system indicates a...
s.
Air traffic control
Air traffic control
Air traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...
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".
History
The aviation transponder was originally developed during World War IIWorld War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
by the British and American military as an "Identification friend or foe
Identification friend or foe
In telecommunications, identification, friend or foe is an identification system designed for command and control. It is a system that enables military and national interrogation systems to identify aircraft, vehicles, or forces as friendly and to determine their bearing and range from the...
" (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
Cold War
The Cold War was the continuing state from roughly 1946 to 1991 of political conflict, military tension, proxy wars, and economic competition between the Communist World—primarily the Soviet Union and its satellite states and allies—and the powers of the Western world, primarily the United States...
.
This concept was adapted in the 1950s by civil air traffic control
Air traffic control
Air traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...
using secondary surveillance radar
Secondary surveillance radar
Secondary surveillance radar is a radar system used in air traffic control , that not only detects and measures the position of aircraft i.e. range and bearing, but also requests additional information from the aircraft itself such as its identity and altitude...
(beacon radar) systems to provide traffic services for general aviation
General aviation
General aviation is one of the two categories of civil aviation. It refers to all flights other than military and scheduled airline and regular cargo flights, both private and commercial. General aviation flights range from gliders and powered parachutes to large, non-scheduled cargo jet flights...
and commercial aviation.
Secondary Surveillance Radar
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, 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 requires a radio transponder in the aircraft to respond to interrogation signals from the ground station to make the aircraft more visible and to report the aircraft's altitude.Operation
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
Gillham Code
Gillham code is a digital code using an eleven-wire interface that is used to transmit uncorrected barometric altitude between an encoding altimeter or analog air data computer and a transponder...
, is used to pass altitude information to the transponder. Around busy airspace
Airspace
Airspace means the portion of the atmosphere controlled by a country above its territory, including its territorial waters or, more generally, any specific three-dimensional portion of the atmosphere....
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 C veil
Mode C veil refers to a kind of airspace which surrounds most of primary Class B airports within United States. This airspace extends horizontally to a circle of 30 NM radius centered on the airport and extends vertically from the surface up to 10,000 feet MSL. The name refers to the mode of...
. 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
Traffic Collision Avoidance System
A traffic collision avoidance system or traffic alert and collision avoidance system is an aircraft collision avoidance system designed to reduce the incidence of mid-air collisions between aircraft...
(TCAS) installed on large aircraft needs the altitude information supplied by transponder signals.
Ident
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, 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 identity of the aircraft on radar.
Squawk codes are four-digit octal
Octal
The octal numeral system, or oct for short, is the base-8 number system, and uses the digits 0 to 7. Numerals can be made from binary numerals by grouping consecutive binary digits into groups of three...
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.
The use of the word "squawk" comes from the system's origin in the World War II Identification Friend or Foe
Identification friend or foe
In telecommunications, identification, friend or foe is an identification system designed for command and control. It is a system that enables military and national interrogation systems to identify aircraft, vehicles, or forces as friendly and to determine their bearing and range from the...
(IFF) system, which was code-named "Parrot".
Routine codes
- 0000:
- Military intercept code (in the U.S.)
- Mode C or other SSR failure (in the UK).
- Shall not be used — is a non-discrete mode A code (Europe)
- 0021: VFR squawk code for German airspace (5000 feet and below) — from 15 March 2007 replaced by the international 7000 code for VFR traffic.
- 0022: VFR squawk code for German airspace (above 5000 feet) — from 15 March 2007 replaced by the international 7000 code for VFR traffic.
- 0033: Parachute dropping in progress (UK)
- 0041 to 0057: In Belgium assigned for VFR traffic under Flight Information Services (BXL FIC)
- 0100: In Australia: flights operating at aerodromes (in lieu of codes 1200, 2000 or 3000 when assigned by ATC or noted in the Enroute Supplement Australia)
- 1000:
- Instrument Flight RulesInstrument flight rulesInstrument flight rules are one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other are visual flight rules ....
(IFR) flight below 18,000' when no other code has been assigned (Canada) - Non-discrete mode A code reserved use in Mode S radar/ADS-B environment where the aircraft identification will be used to correlate the flight plan instead of the mode A code
- Instrument Flight Rules
- 1200:
- Visual flight rulesVisual flight rulesVisual flight rules are a set of regulations which allow a pilot to operate an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going. Specifically, the weather must be better than basic VFR weather minimums, as specified in the rules of the...
(VFR) flight, this is the standard squawk code used in North American airspace when no other has been assigned. - In Australia civil VFR flights in class E or G airspace.
- Visual flight rules
- 1202:
- Visual flight rulesVisual flight rulesVisual flight rules are a set of regulations which allow a pilot to operate an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going. Specifically, the weather must be better than basic VFR weather minimums, as specified in the rules of the...
(VFR) glider flight, this is the standard squawk code used in United States airspace for transponder equipped gliders when no other has been assigned.
- Visual flight rules
- 1400: VFR flight above 12,500'ASL when no other code has been assigned (Canada).
- 2000:
- The code to be squawked when entering a secondary surveillance radarSecondary surveillance radarSecondary surveillance radar is a radar system used in air traffic control , that not only detects and measures the position of aircraft i.e. range and bearing, but also requests additional information from the aircraft itself such as its identity and altitude...
(SSR) area from a non-SSR area used as Uncontrolled IFR flight squawk code in ICAO countries - In Canada for uncontrolled IFR at or above 18,000'
- In Australia: civil IFR flights in Class G airspace.
- The code to be squawked when entering a secondary surveillance radar
- 2100: Australia: Ground testing by aircraft maintenance staff.
- 3000: Australia: Civil flights in classes A, C and D airspace, or IFR flights in Class E airspace.
- 4000:
- Aircraft on a VFR Military Training Route or requiring frequent or rapid changes in altitude (U.S.)
- In Australia: civil flights not involved in special operations or SAR, operating in Class G airspace in excess of 15NM offshore.
- 4400 to 4477: Reserved for use by SR-71, YF-12, U-2Lockheed U-2The Lockheed U-2, nicknamed "Dragon Lady", is a single-engine, very high-altitude reconnaissance aircraft operated by the United States Air Force and previously flown by the Central Intelligence Agency . It provides day and night, very high-altitude , all-weather intelligence gathering...
and B-57B-57 CanberraThe Martin B-57 Canberra was a United States-built, twin jet engine light bomber and reconnaissance aircraft, which entered service with the United States Air Force in 1953. The B-57 was initially a version of the English Electric Canberra built under license. However, the Glenn L...
, pressure suit flights, and aircraft operations above FL600 (USA only). - 5000: Aircraft in Military Operations
- 6000: Australia: Military flights in Class G airspace.
- 7000:
- VFR standard squawk code when no other code has been assigned (ICAO).
- UK: this code does not imply VFR; 7000 is used as a general conspicuity squawk.
- 7001:
- Sudden military climb out from low-level operations (UK)
- Used in some countries to identify VFR traffic (France, ...)
- 7004: AerobaticAerobaticsAerobatics is the practice of flying maneuvers involving aircraft attitudes that are not used in normal flight. Aerobatics are performed in airplanes and gliders for training, recreation, entertainment and sport...
and display code in some countries. - 7010: VFR circuit traffic code in the UK
- 707X: Paradrop activities in France (7070, 7071, 7072...)
- 7615: Australia: civil flights engaged in littoral surveillance.
- 7777:
- Military interception (U.S.) ("Under no circumstances should a pilot of a civil aircraft operate the transponder on Code 7777. This code is reserved for military interceptor operations.")
- Non-discrete code used by fixed test transponders (RABMs) to check correctness of radar stations (BITE). (U.S., Germany, Netherlands, Belgium, ...)
- 7500: Unlawful Interference, i.e., Aircraft hijackingAircraft hijackingAircraft hijacking is the unlawful seizure of an aircraft by an individual or a group. In most cases, the pilot is forced to fly according to the orders of the hijackers. Occasionally, however, the hijackers have flown the aircraft themselves, such as the September 11 attacks of 2001...
- 7600: Lost CommunicationsNORDONORDO, short for "No Radio", is a North American aviation term for aircraft flying without a radio. The term originates from the 5-character uppercase abbreviated notation "NORDO" displayed on controllers' radar scopes when an aircraft transmits the "radio failure" code on its transponder...
- 7700: General Emergency
Codes assigned by ATC
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.
See also
Transponder-related accidents- Aeroméxico Flight 498Aeroméxico Flight 498Aeroméxico Flight 498, registration , was a Douglas DC-9-32 on route from Mexico City, Mexico to Los Angeles International Airport, Los Angeles, California, United States on August 31, 1986. N4891F was a privately-operated Piper PA-28-181 Archer owned by the Kramer family en route from Torrance to...
— August 31, 1986 (Aircraft equipped with a Mode A, but not Mode C transponder) - Iran Air Flight 655Iran Air Flight 655Iran Air Flight 655 was a civilian jet airliner shot down by U.S. missiles on 3 July 1988, over the Strait of Hormuz, toward the end of the Iran–Iraq War...
— July 3, 1988 (Incorrect interpretation of transponder code a factor in mistaken identity and shoot down) - Gol Transportes Aéreos Flight 1907Gol Transportes Aéreos Flight 1907Gol Transportes Aéreos Flight 1907 was a Boeing 737-8EH, registration PR-GTD, on a scheduled passenger flight from Manaus, Brazil, to Rio de Janeiro. On 29 September 2006, just before 17:00 BRT, it collided in midair with an Embraer Legacy business jet over the Brazilian state of Mato Grosso...
— September 29, 2006 (Accident where one aircraft had a non-functional transponder)