Transponder (aviation)
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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.
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[edit] 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.
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.
[edit] 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.
[edit] Transponder modes
- Mode 1 - provide 2-digit 5-bit mission code (military only - cockpit selectable).
- Mode 2 - provide 4-digit octal unit code (military only - set on ground for fighters, can be changed in flight by transport aircraft).
- Mode 3/A - provides a 4-digit octal identification code for the aircraft, assigned by the air traffic controller (military and civilian).
- Mode 4 - provide a 3-pulse reply to crypto coded challenge (military only).
- Mode 5 - provide a crypto secure version of Mode S and, ADS-B GPS position (military only).
- Mode C - provide 4-digit octal code for aircraft's pressure altitude (military and civilian).
- Mode S - provide multiple information formats to a selective interrogation. Each aircraft is assigned a fixed 24-bit address. (military and civilian).
[edit] 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 3C", or more commonly "Mode C".
Mode 3A and C are used to help air traffic controllers to identify the aircraft and to maintain separation.
[edit] 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 tranponders are compatible with Modes A & C.
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.
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. The Garmin G1000 glass cockpit is one example of an avionics suite that supports Mode S TIS.
[edit] 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.
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. This has resulted in at least one accident. On August 31, 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[1] 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.
[edit] 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").
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").
[edit] 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.
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 [1]. 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.
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".
[edit] Routine codes
- 1200: Visual flight rules (VFR) flight, this is the standard squawk code used in North American airspace when no other has been assigned.
- 7000: VFR standard squawk code for most European airspace when no other code has been assigned.
- 7004: Aerobatic and display code.
- 0001: Military code for highspeed uncontrolled (non-ATC directed) flight (US)
- 7001: Sudden military climbout from low-level operations (UK)
- 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)
- 0000: military escort (in the US), suspected transponder failure (in the UK).
- 7777: military interception (US/FAA)
"Under no circumstances should a pilot of a civil aircraft operate the transponder on Code 7777. This code is reserved for military interceptor operations."
In Germany the following codes have been used:
- 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[2] these have been replaced by the international 7000 code for VFR traffic.
[edit] Emergency codes
- 7700: General Emergency
- 7600: Lost Communications
- 7500: Unlawful Interference (Hijack)
[edit] 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" (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 not to get 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 may 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 7401 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.
[edit] References
- ^ AOPA.org, Collision over Cerritos
- ^ http://www.skycontrol.net/service-providers/change-of-german-vfr-transponder-codes-ac-7000-replaces-ac-0021-and-ac-0022
- Airman's Information Manual, Section 4-1-19 (as of March 2007)
- Transport Canada Aeronautical Information Manual RAC Section 1-9 Transponders accessed 17 Nov 2006
