Transponder (aviation)

Transponder (aviation)
Cessna ARC RT-359A transponder (beige box), beneath Bendix/King KY197 VHF communication 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[1] and sometimes abbreviated to XPDR,[2] XPNDR,[3] TPDR[4] or TP[5]) 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.[6][7]

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".[6]



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]

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.

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.[7][8]

Transponder modes


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.[7][6]

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.[7][6] Similarly, the Traffic Collision Avoidance System (TCAS) installed on large 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 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".[7][6]

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".[7]

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.[7][6]

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.[7][6]

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".[9][10]

Routine codes

  • 0000:
    • Military intercept code (in the U.S.)[11]
    • Mode C or other SSR failure (in the UK).[12]
    • Shall not be used — is a non-discrete mode A code (Europe)[13]
  • 0021: VFR squawk code for German airspace (5000 feet and below) — from 15 March 2007 replaced by the international 7000 code for VFR traffic.[14]
  • 0022: VFR squawk code for German airspace (above 5000 feet) — from 15 March 2007 replaced by the international 7000 code for VFR traffic.[14]
  • 0033: Parachute dropping in progress (UK)[12]
  • 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)[15]
  • 1000:
    • Instrument Flight Rules (IFR) flight below 18,000' when no other code has been assigned (Canada)[6]
    • 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[13]
  • 1200:
    • Visual flight rules (VFR) flight, this is the standard squawk code used in North American airspace when no other has been assigned.[6]
    • In Australia civil VFR flights in class E or G airspace.[15]
  • 1202:
    • Visual flight rules (VFR) glider flight, this is the standard squawk code used in United States airspace for transponder equipped gliders when no other has been assigned.[16]
  • 1400: VFR flight above 12,500'ASL when no other code has been assigned (Canada)[6].
  • 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 ICAO countries[13]
    • In Canada for uncontrolled IFR at or above 18,000'[6]
    • In Australia: civil IFR flights in Class G airspace.[15]
  • 2100: Australia: Ground testing by aircraft maintenance staff.[15]
  • 3000: Australia: Civil flights in classes A, C and D airspace, or IFR flights in Class E airspace.[15]
  • 4000:
    • Aircraft on a VFR Military Training Route or requiring frequent or rapid changes in altitude (U.S.)[17]
    • In Australia: civil flights not involved in special operations or SAR, operating in Class G airspace in excess of 15NM offshore.[15]
  • 4400 to 4477: Reserved for use by SR-71, YF-12, U-2 and B-57, pressure suit flights, and aircraft operations above FL600 (USA only).[17]
  • 5000: Aircraft in Military Operations[15]
  • 6000: Australia: Military flights in Class G airspace.[15]
  • 7000:
    • VFR standard squawk code when no other code has been assigned (ICAO)[13].
    • UK: this code does not imply VFR; 7000 is used as a general conspicuity squawk.[12]
  • 7001:
    • Sudden military climb out from low-level operations (UK)[12]
    • Used in some countries to identify VFR traffic (France, ...)
  • 7004: Aerobatic and display code in some countries.[12]
  • 7010: VFR circuit traffic code in the UK
  • 707X: Paradrop activities in France (7070, 7071, 7072...)
  • 7615: Australia: civil flights engaged in littoral surveillance.[15]
  • 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.")[18]
    • Non-discrete code used by fixed test transponders (RABMs) to check correctness of radar stations (BITE). (U.S., Germany, Netherlands, Belgium, ...)

Emergency codes

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.[7][6] For IFR flights, the squawk code is typically assigned as part of the departure clearance and stays the same throughout the flight.[7][6]

VFR flights, when in uncontrolled airspace, will "squawk VFR" (or conspicuity code in the UK)[19](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.[7][6]

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.[19]

See also

Transponder-related accidents


  1. ^ Mangine, Robert (2007). "The Truth About Transponders". Retrieved 2008-12-24. 
  2. ^ Farlex, Inc (2008). "XPDR". Retrieved 2008-12-24. 
  3. ^ Farlex, Inc (2008). "XPNDR". Retrieved 2008-12-24. 
  4. ^ Farlex, Inc (2008). "TPDR". Retrieved 2008-12-24. 
  5. ^ Farlex, Inc (2008). "TP". Retrieved 2008-12-24. 
  6. ^ a b c d e f g h i j k l m n o p q Transport Canada (20 May 2010). "TP 14371 — Transport Canada Aeronautical Information Manual (TC AIM) RAC 1.9 Transponder Operation". Retrieved 21 August 2010. 
  7. ^ a b c d e f g h i j k Peppler, I.L.: From The Ground Up, pages 238–239. Aviation Publishers Co. Limited, Ottawa Ontario, Twenty Seventh Revised Edition, 1996. ISBN 09690054-9-0
  8. ^ Phil Vabre. "Air Traffic Services Surveillance Systems". Retrieved 2010-04-15. 
  9. ^ Getline, Meryl (17 April 2006). "Ask the Captain: Strangle my WHAT?". USA Today. Retrieved 2008-03-13. 
  10. ^ Vabre, Phil. "Air Traffic Services Surveillance Systems, Including An Explanation of Primary and Secondary Radar". The Airways Museum & Civil Aviation Historical Society. Retrieved 2008-03-13. 
  11. ^ "Radio Communications". Cleared for Takeoff: Your Handbook for Becoming a Private Pilot (Revision E ed.). King Schools, Inc.. 2006. p. 4–16. 
  12. ^ a b c d e "UK AIP ENR 1.6.2 — SSR Operating Procedures and UK SSR Code Assignment Plan" (PDF). UK Civil Aviation Authority. 6 November 2007. 
  13. ^ a b c d ICAO doc 4444 & ICAO Annex 10
  14. ^ a b "Change of German VFR transponder codes A/C 7000 replaces A/C 0021 and A/C 0022". SkyControl Aviation & Aerospace News. 6 November 2007. 
  15. ^ a b c d e f g h i Australian AIP ENR 1.6 para 7.1.4
  16. ^ FAA ORDER/PUBLICATION: 7110.65U dated 5/18/2011 signed by Richard Kagehero
  17. ^ a b "FAA Order 7110.65R (Air Traffic Control procedural manual) — chapter 5.2, Beacon Systems". Federal Aviation Authority. 21 January 2008. 
  18. ^ "Aeronautical Information Manual, chapter 4 — Air Traffic Control". Federal Aviation Administration. 6 November 2007. 
  19. ^ a b ENR 1.6.2 — SSR Operating Procedures

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