A Cessna ARC RT-359A Transponder (the beige box) mounted beneath a Bendix/King KY197 VHF communication radio mounted in the instrument panel of an 1970 model American Aviation AA-1 Yankee.

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 systems.^[1][2] Image File history File linksMetadata CessnaARC-RT-359ATransponder04. ... Image File history File linksMetadata CessnaARC-RT-359ATransponder04. ... The Bendix Aviation Corporation, a manufacturer of aircraft parts, was started by inventor Vincent Bendix in 1929 as a continuation of his auto parts company. ... A Bendix/King KY197 Airband VHF communication radio mounted above a Cessna ARC RT-359A Transponder (the beige box) in a light airplane instrument panel. ... 1970 Model American Aviation AA-1 Yankee Bottom view of an American Aviation AA-1 Yankee, showing its square fuselage construction The Grumman American AA-1 series is a family of light, 2-seat aircraft. ... For other uses, see Radar (disambiguation). ... Airborne Collision Avoidance System (ACAS) is an ICAO (International Civil Aviation Organization) standard specified in ICAO Annex 10 Vol IV which provides pilots with a system independent of air traffic control to detect the presence of other aircraft which may present a threat of collision. ...

ATC 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.^[1] ATC is an abbreviation of: Adaptive Transform Coding, a method of coding for speech signals Agreement on Textiles and Clothing Air traffic control Air Training Command, the predecessor to Air Education and Training Command of the United States Air Force Air Training Corps Air Training Corps (New Zealand) ALCOA Technical...

History

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.^{[citation needed]} Combatants Allied powers: China France Great Britain Soviet Union United States and others Axis powers: Germany Italy Japan and others Commanders Chiang Kai-shek Charles de Gaulle Winston Churchill Joseph Stalin Franklin Roosevelt Adolf Hitler Benito Mussolini Hideki Tōjō Casualties Military dead: 17,000,000 Civilian dead: 33,000... In telecommunications, identification, friend or foe (IFF) is a crypto identification system designed for command and control. ... NORAD is short for: North American Aerospace Defense Command Norwegian Agency for Development Cooperation This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ... For other uses, see Cold War (disambiguation). ...

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. For the Canadian musical group, see Air Traffic Control (band). ... A Secondary Surveillance Radar (SSR) is a radar device installed in air traffic control facilities to allow the precise identification of aircraft. ... General aviation (abbr. ...

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 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 plane more visible and to report the aircraft's altitude.^[2]

Transponder modes

• Mode 1 - provide 2-digit 5-bit mission code (military only - cockpit selectable).^{[citation needed]}
• Mode 2 - provide 4-digit octal unit code (military only - set on ground for fighters, can be changed in flight by transport aircraft).^{[citation needed]}
• Mode 3/A - provides a 4-digit octal identification code for the aircraft, assigned by the air traffic controller (military and civilian).^[2]
• Mode 4 - provide a 3-pulse reply to crypto coded challenge (military only).^{[citation needed]}
• Mode 5 - provide a crypto secure version of Mode S and, ADS-B GPS position (military only).^{[citation needed]}
• Mode C - provide 4-digit octal code for aircraft's pressure altitude (military and civilian).^[2]
• Mode S - provide multiple information formats to a selective interrogation. Each aircraft is assigned a fixed 24-bit address. (military and civilian).^[2]

Automatic Dependent Surveillance-Broadcast (also called ADS-B) is a system by which airplanes constantly broadcast their current position and altitude, category of aircraft, airspeed, identification, and whether the aircraft is turning, climbing or descending over a dedicated radio datalink. ...

Mode A and Mode C

When the transponder receives a radar signal it sends back a transponder code (or "squawk code"). This is referred to as "Mode 3A" or more commonly "Mode A". A transponder code can be paired with pressure altitude information, which is called "Mode C".^[2]

Mode 3A and C are used to help air traffic controllers to identify the aircraft and to maintain separation.^[2]

Mode S

Another mode called Mode S (Selective) is designed to help air traffic control in busy areas and allow automatic collision avoidance. Mode S transponders are compatible with Modes A & C.^[2]

Mode S is mandatory in controlled airspace in many countries. Some countries require that all aircraft be equipped with Mode S, even in uncontrolled airspace. However in the field of general aviation, there have been objections to these moves, because of the cost, size, limited benefit to the users in uncontrolled airspace, and, in the case of balloons and gliders, the power requirements for these aircraft that do not have electrical systems.^{[citation needed]}

Mode S transponders can relay additional information, including the permanent identity of the aircraft. Mode S TIS, or Traffic Information Service, allows a radar installation to send information about nearby traffic back to aircraft, which then displays it on the moving map. Mode S TIS is only available when the aircraft is within radar range of a radar installation that supports it. A Mode S TIS installation combines a Mode S transponder that sends TIS data to a display device – usually a GPS device or Multi Function Display (MFD). Examples of such pairings are a Garmin GTX330D transponder and a GNS530 GPS, and the Garmin G1000 avionics suite.^{[citation needed]} The Garmin G1000 is an integrated cockpit manufactured by Garmin typically composed of two display units, one serving as a primary flight display, and one as a multi-function display. ...

Mode S capable transponders are also a building block for next generation air traffic control systems, as they can be used to transmit location information for ADS-B and potentially other air traffic control communications. Currently the FAA is deactivating several Mode S TIS equipped stations^{[citation needed]} Automatic Dependent Surveillance-Broadcast (also called ADS-B) is a system by which airplanes constantly broadcast their current location, type of aircraft, airspeed, flight number, and whether the aircraft is turning, climbing or descending. ...

Operation

In flight a pilot is told to squawk a given code by air traffic control over the radio, such as in the phrase "Cessna 123AB, squawk 0363". The pilot inputs these digits and his blip on the radar becomes correctly associated with his identity.^[2][1]

Because primary radar gives position information but lacks altitude information, mode C and mode S transponders report altitude. Around busy airspace there is often a regulatory requirement that all aircraft be equipped with an altitude-reporting mode C or mode S transponder. In the United States, this is known as a Mode C veil. Mode S transponders are compatible with transmitting the mode C signal, hence no need for a separate designation. Without the altitude reporting, the controller cannot see any altitude information, and the controller must rely on the altitude as reported by the pilot.^[2][1] This 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^[3] with Aeromexico Flight 498, a DC-9 with 58 passengers and 6 crew at an altitude of 6,650 feet. The Archer had only mode A reporting capability and the controller assumed it was below the controlled airspace. is the 243rd day of the year (244th in leap years) in the Gregorian calendar. ... The Cerritos air disaster on August 31, 1986. ...

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 (i.e. "Cessna 123AB, squawk 0363 and ident").^[2][1]

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 can hear me, please ident").^[2]

Transponder codes

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.^[2][1]

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. 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.^[2][1] The octal numeral system, or oct for short, is the base-8 number system, and uses the digits 0 to 7. ...

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.^{[citation needed]} For the Canadian musical group, see Air Traffic Control (band). ...

The use of the word "squawk" comes from the system's origin in the World War II Identification Friend or Foe system, which was code-named "Parrot".^{[citation needed]} Combatants Allied powers: China France Great Britain Soviet Union United States and others Axis powers: Germany Italy Japan and others Commanders Chiang Kai-shek Charles de Gaulle Winston Churchill Joseph Stalin Franklin Roosevelt Adolf Hitler Benito Mussolini Hideki Tōjō Casualties Military dead: 17,000,000 Civilian dead: 33,000... In telecommunications, identification, friend or foe (IFF) is a crypto identification system designed for command and control. ...

Routine codes

• 0000: military escort (in the US)^{[citation needed]}; mode C or other SSR failure (in the UK).^[4]
• 0001: Military code for highspeed uncontrolled (non-ATC directed) flight (US)^{[citation needed]}
• 0033: Parachute dropping in progress (UK)^[4]
• 1000: Instrument Flight Rules (IFR) flight below 18,000' when no other code has been assigned (Canada)^[1]
• 1200: Visual flight rules (VFR) flight, this is the standard squawk code used in North American airspace when no other has been assigned.^[1]
• 1400: VFR flight above 12,500' when no other code has been assigned (Canada) ^[1].
• 2000: The code to be squawked when entering a secondary surveillance radar (SSR) area from a non-SSR area (used as Uncontrolled IFR flight squawk code in some European countries, and in Canada for uncontrolled IFR at or above 18,000')^[1]
• 4000: Aircraft on a VFR Military Training Route or requiring frequent or rapid changes in altitude (US) ^[5]
• 7000: VFR standard squawk code for most European airspace when no other code has been assigned. (However, in at least the UK, this code does not imply VFR; 7000 is used as a general conspicuity squawk.)^[4]
• 7001: Sudden military climbout from low-level operations (UK)^[4]
• 7004: Aerobatic and display code in some countries.^[4]
• 7777: military interception (US) ("Under no circumstances should a pilot of a civil aircraft operate the transponder on Code 7777. This code is reserved for military interceptor operations.")^[6]

In Germany the following codes have been used: For the Canadian musical group, see Air Traffic Control (band). ... It has been suggested that Air traffic control#Instrument Flight Rules (IFR) be merged into this article or section. ... Visual flight rules (VFR) are a set of aviation regulations under which a pilot may operate an aircraft in weather conditions sufficient to allow the pilot, by visual reference to the environment outside the cockpit, to control the aircrafts attitude, navigate, and maintain safe separation from obstacles such as... A Secondary Surveillance Radar (SSR) is a radar device installed in air traffic control facilities to allow the precise identification of aircraft. ... The Frecce Tricolori aerobatic team of the Italian Air Force, flying at the Royal International Air Tattoo, Fairford, England, in 2005 The UK Utterly Butterly display team perform an aerobatic maneuvre with their Boeing Stearmans Red Arrows Hawks in Concorde formation Indian Air Forces Surya Kiran during an aerobatic...

• 0021: VFR squawk code for German airspace (5000 feet and below)
• 0022: VFR squawk code for German airspace (above 5000 feet)

As from 15 March 2007 these have been replaced by the international 7000 code for VFR traffic.^[7]

Emergency codes

• 7700: General Emergency^[1]
• 7600: Lost Communications^[1]
• 7500: Unlawful Interference (Hijack)^[1]

NORDO, short for No Radio, is an aviation term for flights that have experienced a radio communications failure. ...

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.^[2][1] For IFR flights, the squawk code is typically assigned as part of the departure clearance and stays the same throughout the flight.^[2][1] 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.^[2][1]

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 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 get hold of that aircraft. ^[4]

References