Transponder (aeronautics)

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^[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. 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.^[6][7]

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".^[6]

Secondary surveillance radar

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.^[7][8]

Transponder modes

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.^[6][7]

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.^[6][7] Similarly, the traffic collision avoidance system (TCAS) installed on some 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 ground-based radar equipment^[9] 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".^[6][7]

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 identification of aircraft on radar.^[6][7]

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 air traffic control 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 air traffic control (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.^[6][7]

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".^[10][11]

Codes assigned by air traffic control

Most codes in the following table 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.^[6][7] For flights on instrument flight rules (IFR), the squawk code is typically assigned as part of the departure clearance and stays the same throughout the flight.^[6][7]

Flights on visual flight rules (VFR), when in uncontrolled airspace, will "squawk VFR" (or conspicuity code in the UK,^[12] 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.^[6][7]

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.^[12]

Code assignments

Beacon code	Allocated use
0000	Shall not be used — is a non-discrete mode A code (Europe)[13] Mode C or other SSR failure (UK)[14] Should never be assigned (USA)[15] Military intercept code (USA)[16] Internal ARTCC subsets assigned by En Route Safety and Operations Support (Discrete codes only except for first primary block to be used as non-discrete if all discrete codes are assigned) (USA)[15]
0021	VFR squawk code for airspace 5000 feet and below — from 15 March 2007 replaced by the international 7000 code for VFR traffic (Germany)[17]
0022	VFR squawk code for airspace (above 5000 feet) — from 15 March 2007 replaced by the international 7000 code for VFR traffic (Germany)[17]
0033	Parachute dropping in progress (UK)[14]
0041-0057	Assigned for VFR traffic under Flight Information Services (BXL FIC) (Belgium)
0100	Flights operating at aerodromes (in lieu of codes 1200, 2000 or 3000 when assigned by ATC or noted in the Enroute Supplement (Australia))[18]
0100-0400	Allocated to Service Area Operations for assignment for use by Terminal/CERAP/Industry/Unique Purpose/Experimental Activities (USA)[15]
0100-0700	Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[15]
0500, 0600, 0700	External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[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 (ICAO)[13] Used exclusively by ADS-B aircraft to inhibit Mode 3A transmit (USA)[15] Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[15] External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
1100	Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[15] External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
1200	Civil VFR flights in class E or G airspace (Australia)[18] Visual flight rules (VFR) flight, this is the standard squawk code used in North American airspace when no other has been assigned (Canada and USA)[6][15]
1201	Visual flight rules (VFR) glider operations for gliders not in contact with ATC, through February 2012 (USA)[19] Assigned via FAR 93.95 for use by VFR aircraft in the immediate vicinity of LAX (USA)[15]
1202	Visual flight rules (VFR) glider operations for gliders not in contact with ATC; effective February 2012 (USA)[15][20]
1203-1272	Discrete 1200 series codes, unless otherwise allocated (for example, 1255), designated for DVFR aircraft and only assigned by FSS (USA)[15]
1255	Aircraft not in contact with an ATC facility while en route to/from or within the designated fire fighting area(s) (USA)[15][21]
1273-1275	Calibration Performance Monitoring Equipment (CPME) “Parrot” transponders (USA)[15]
1276	Air Defense Identification Zone (ADIZ) penetration when unable to establish communication with ATC or aeronautical facility (USA)[15]
1277	VFR aircraft which fly authorized SAR missions for the USAF or USCG while en route to/from or within the designated search area (USA)[15][21]
1300	Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[15] External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
1400	VFR flight above 12,500' ASL when no other code has been assigned (Canada)[6] External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
1500	Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[15] External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
1600, 1700	External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
2000	Civil IFR flights in Class G airspace (Australia)[18] Uncontrolled IFR at or above 18,000' (Canada)[6] 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 (ICAO countries)[13] Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[15] External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
2100	Ground testing by aircraft maintenance staff (Australia)[18] Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[15] External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
2200, 2300, 2400	Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[15] External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
2500, 2600, 2700	External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
3000	Civil flights in classes A, C and D airspace, or IFR flights in Class E airspace (Australia)[18] External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
3100, 3200, 3300, 3400, 3500, 3600, 3700	External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
4000	Civil flights not involved in special operations or SAR, operating in Class G airspace in excess of 15NM offshore (Australia)[18] Aircraft on a VFR Military Training Route or requiring frequent or rapid changes in altitude (USA)[22] Non-discrete code assignments in accordance with FAA Order JO 7110.65, 5-2 *Also for use in oceanic airspace, unless another code is assigned by ATC (USA)[15] External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
4100	External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
4200, 4300	Internal ARTCC subsets assigned by En Route Safety and Operations Support (Discrete codes only except for first primary block to be used as non-discrete if all discrete codes are assigned) (USA)[15]
4400-4477	Reserved for use by SR-71, YF-12, U-2 and B-57, pressure suit flights, and aircraft operations above FL600 (USA)[15][22]
4401-4433	Reserved in accordance with FAA Order JO 7110.67 (Fed Law Enforcement) (USA)[15]
4434-4437	Weather reconnaissance, as appropriate (USA)[15]
4440-4441	Operations above FL600 for Lockheed/NASA from Moffett Field (USA)[15]
4442-4446	Operations above FL600 for Lockheed from Air Force Plant 42 (USA)[15]
4447-4452	Operations above FL600 for SR-71/U-2 operations from Edwards AFB (USA)[15]
4453	High balloon operations – National Scientific Balloon Facility, Palestine TX, and other providers, some in international operations (USA)[15]
4454-4465	Air Force operations above FL600 as designated in FAA Order 7610.4 (USA)[15]
4466-4477	Reserved in accordance with FAA Order JO 7110.67 (Fed Law Enforcement) (USA)[15]
4500, 4600, 4700	Internal ARTCC subsets assigned by En Route Safety and Operations Support (Discrete codes only except for first primary block to be used as non-discrete if all discrete codes are assigned) (USA)[15]
5000	Aircraft flying on military operations (Australia)[18]
5000	Reserved for use by NORAD (USA and Canada)[15]
5061-5062, 5100, 5200	Reserved for special use by Potomac TRACON (USA)[15]
5100, 5200, 5300, 5500	Internal ARTCC subsets assigned by En Route Safety and Operations Support (Discrete codes only except for first primary block to be used as non-discrete if all discrete codes are assigned) (USA)[15]
5100-5300	May be used by DOD aircraft beyond radar coverage but inside US controlled airspace with coordination as appropriate with applicable Area Operations Directorate (USA)[15]
5400	Reserved for use by NORAD (USA and Canada)[15]
5600, 5700	External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
6000	Military flights in Class G airspace (Australia)[18] External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
6100	Reserved for use by NORAD (USA and Canada)[15]
6200, 6300	External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
6400	Reserved for use by NORAD (USA and Canada)[15]
6500, 6600, 6700	External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
7000	VFR standard squawk code when no other code has been assigned (ICAO)[13] This code does not imply VFR; 7000 is used as a general conspicuity squawk (UK)[14] External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
7001	Used in some countries to identify VFR traffic (France) Sudden military climb out from low-level operations (UK)[14]
7004	Aerobatic and display code in some countries (UK)[14]
7100, 7200, 7300, 7400	External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
7500	Aircraft hijacking (ICAO, worldwide)[6][15]
7501-7577	Reserved for use by Continental NORAD Region (CONR) (USA)[15]
7600	Radio Failure (Lost Communications) (ICAO, worldwide)[6][15]
7601-7607	Reserved for special use by FAA (USA)[15]
7610-7676	External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
7615	Civil flights engaged in littoral surveillance (Australia)[18]
7700	Emergency (ICAO, worldwide)[6][15]
7701-7707	Reserved for special use by FAA (USA)[15]
7710-7776	External ARTCC subsets (Discrete codes of blocks only except for first primary block, which is used as the ARTCC’s non-discrete code if all discrete codes are assigned) (USA)[15]
7777	Non-discrete code used by fixed test transponders (RABMs) to check correctness of radar stations (BITE) (Belgium, Germany, Netherlands, USA) DOD interceptor aircraft on active air defense missions and operating without ATC clearance in accordance with FAA Order 7610.4 (USA)[15][23]

See also

Transponder-related accidents

• 1986 Cerritos mid-air collision — August 31, 1986 (one of the aircraft equipped with a Mode A, but not Mode C, transponder)
• Iran Air Flight 655 — July 3, 1988 (incorrect interpretation of transponder code a factor in mistaken identity and shoot down)
• Korean Air Flight 85 — September 11, 2001 (suspected hijack involving the transponder code, false alarm)
• Gol Transportes Aéreos Flight 1907 — September 29, 2006 (accident where one aircraft had a non-functional transponder)

References

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. Transport Canada (20 May 2010). "TP 14371 — Transport Canada Aeronautical Information Manual (TC AIM) RAC 1.9 Transponder Operation". Retrieved 21 August 2010.
7. Peppler, I.L.: From The Ground Up, pages 238–239. Aviation Publishers Co. Limited, Ottawa Ontario, Twenty Seventh Revised Edition, 1996. ISBN 0-9690054-9-0
8. Phil Vabre. "Air Traffic Services Surveillance Systems". www.airwaysmuseum.com. Retrieved 2010-04-15.
9. Rogers, Tom. "Transponder Basics". AVweb. Retrieved 2014-03-18.
10. Getline, Meryl (17 April 2006). "Ask the Captain: Strangle my WHAT?". USA Today. Retrieved 2008-03-13.
11. 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.
12. ENR 1.6.2 — SSR Operating Procedures
13. ICAO doc 4444 & ICAO Annex 10
14. "UK AIP ENR 1.6.2 — SSR Operating Procedures and UK SSR Code Assignment Plan" (PDF). UK Civil Aviation Authority. 6 November 2007.
15. US Department of Transportation, Federal Aviation Administration. "JO 7110.66D, National Beacon Code Allocation Plan". US Government. Retrieved 26 April 2012.
16. "Radio Communications". Cleared for Takeoff: Your Handbook for Becoming a Private Pilot (Revision E ed.). King Schools, Inc. 2006. pp. 4–16.
17. "Change of German VFR transponder codes A/C 7000 replaces A/C 0021 and A/C 0022". SkyControl Aviation & Aerospace News. 6 November 2007.
18. Australian AIP ENR 1.6 para 7.1.4
19. FAA ORDER/PUBLICATION: 7110.65U dated 5/18/2011
20. FAA ORDER/PUBLICATION 7110.65V dated 02/19/2014
21. FAA order JO 7110.65U
22. "FAA Order 7110.65R (Air Traffic Control procedural manual) — chapter 5.2, Beacon Systems". Federal Aviation Authority. 21 January 2008.
23. "Aeronautical Information Manual, chapter 4 — Air Traffic Control". Federal Aviation Administration. 6 November 2007.