An aircraft transponder is an electronic device that, when interrogated by radar, transmits identifying information and altitude to give air traffic controllers an accurate picture of an airplane's location in airspace. Installed in the cockpit panel and linked to an antenna normally mounted on the belly, the unit listens for radio-frequency interrogations and instantly replies with a coded reply, enabling controllers to distinguish and track the aircraft among many returns.
Introduced by the British during World War II to identify friendly aircraft, the transponder has evolved into a regulatory requirement for almost every flight in busy airspace, where mode C or mode S capability is now mandatory.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is an aircraft transponder?
An aircraft transponder is a radio or radar set that emits a radio signal of its own upon receiving a designated signal. It transmits transponder codes in response to a secondary surveillance radar interrogation signal. A transponder - short for transmitter-responder - is the system on an aircraft that sends out radio signals in response to radar interrogation. This avionic instrument combines the ability to transmit information and to respond to inquiry from Air Traffic Control radar sites.
What is the purpose of a transponder in aviation?
Transponders serve the main purpose of safety in air travel. Aircraft have transponders to assist in identifying them on ATC radar. Transponders produce a response when they receive a radio-frequency interrogation and make aircraft visible on ATC radar screens. A transponder transmits identification and barometric altitude to ATC, and provides altitude information to TCAS on other aircraft. A transponder aids ATC in traffic separation and helps keep airplanes separated. Transponders distinguish aircraft on secondary surveillance radar for ATC purposes, aid collision avoidance systems and help with tracking and safety. Transponders provide information to ATC andATC uses transponder signals to track aircraft.
Transponder provides altitude reporting for ADS-B Out and gives altitude information to ATC. Transponders enable runway incursion detection systems like ASDE-X and A-SMGCS. A transponder integrates traffic into the ASDE-X system and is used for advanced surface movement guidance and control. Mode S transponder transmits position, speed, and heading, and offers more advanced communication capabilities and can transmit Downlink Airborne Parameters. Secondary surveillance radar receives response from transponders and works by interrogating transponders and is referred to as secondary to distinguish it from primary radar. Transponder codes assist air traffic controllers with traffic separation. ATC uses transponder signals to guarantee safe separation between aircraft.
How does an airplane transponder work?
A transponder receives a radio-frequency interrogation, replying only when the ground interrogator transmits an interrogation on 1030 MHz. Secondary surveillance radar sends interrogation on 1030 MHz in a pulse-amplitude-modulated sequence. The airborne unit decodes this burst and, within microseconds, assembles its own pulse train that it re-radiates on 1090 MHz. Mode A inserts the squawk code selected by the pilot, Mode C inserts pressure altitude in 100-foot increments supplied by an altitude encoder preset to 29.92, while Mode S provides position, speed and heading plus selective addressing to one aircraft at a time. Because the altimeter encoder is preset at standard atmospheric pressure, the wrong altimeter setting does not affect the altitude readout. Instead, local altimeter correction is applied by the surveillance facility before altitude information is presented to ATC. All ADS-B Out systems are capable of automatic altitude reporting, and when ATC requires positive confirmation, it requests the pilot to IDENT, causing a distinctive bright flash on the controller's scope that pinpoints the aircraft among surrounding returns.
An aircraft's transponder automatically responds to each radar sweep from the aviation traffic authority with a four-digit number that carries the selected squawk code. Elevation reporting automatically broadcasts our pressure altitude, letting the ground display show both identity and vertical separation. When the regulator sought it, I pushed the IDENT switch. The little display lighted and temporarily spotlighted our aircraft on their screen. I recall choosing designated command 1200 for visual flying laws.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
Where is the transponder located on an aircraft?
The transponder unit is installed into the dashboard of an aircraft's cockpit, while the transponder antenna is usually on the bottom of the aircraft. Some aircraft place the transponder antenna inside a fin blade. Electronic equipment is distributed at several locations known as electrical compartments or avionics bays and both ATC electronic units are in rack E1. The altitude encoder is a separate piece of equipment that is typically installed in the aircraft's avionics system.
How is an aircraft's transponder labeled?
The aircraft transponder is labeled on the front panel with several rotary or toggle switches. Common nameplates read XPDR, XPNDR, TPDR, or simply TP. Mode selector positions are stenciled SBY, ON, ALT, and OFF. OFF disconnects power, SBY applies power but keeps the transmitter silent, ON activates Mode 3/A identification only, ALT enables both Mode 3/A and Mode C altitude replies.
Independent of the mode switch, a four-window display shows the assigned squawk code. The code is a four-digit octal number whose digits range from 0 to 7, giving 4096 possible combinations. Controllers issue the code by saying, for example, Squawk 4703, and the pilot enters 4703 into the transponder with the numeric knobs. To distinguish aircraft on the radar scope, ATC allocates discrete codes. Non-discrete codes ending in 00 are alternatively permitted for traffic management.
Two additional labels complete the faceplate. The IDENT push-button is pressed when the controller says IDENT, adding a special thirteenth bit to the Mode A reply. Emergency situations are conveyed by reserved squawk codes: 7700 for general emergency, 7600 for radio failure, and 7500 for hijack and the same three labels appear in controller phraseology so the crew knows which numeric label to dial.
The transponder itself carries no singular, conspicuous symbol. Instead, the aircraft's identity is unified into the registration number painted on the tail. That alphanumeric reference N12345 on our example becomes the main tag once the four-digit code is keyed in. I situated the transponder board within the primary communication system pile, then pushed the switch handle to enter 0412. The action shifted our generic radar echo to a blip specifically labeled with the allotted code. On the controller's scope, the number is the important bit of data that the air traffic authority links with my trip schedule
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What are the types of transponders used in aviation?
There are three main modes: Mode A, Mode C, and Mode S. Mode A is the oldest and most basic civilian transponder mode and transmits only the four-digit squawk code selected by the pilot. Mode C adds automatic altitude reporting in 100-foot increments, so the combination of Mode A identification and Mode C altitude is normally just referred to as Mode C. Mode S, short for Mode Select, offers more advanced communication capabilities than Mode A or Mode C transponders. It can report altitude in 25-foot increments, broadcast unique 24-bit aircraft addresses, and exchange data with TCAS receivers and the ADS-B system. Many Mode S transponders include ADS-B Out, the gold standard of transponders, and some can transmit the selected heading on the autopilot or receive information from a GPS receiver. Military transponders correspond to these civil modes: military Mode 3 matches civilian Mode A, and the combined Mode 3/A plus altitude equals civilian Mode C. Modes B and D are not in use.
When is an aircraft transponder required?
An operable transponder is required in all Class A, B, and C airspace and in Class E airspace at or above 10,000 feet MSL (3,048 meters). It is also required within the 30-nautical-mile (55.56 kilometers) Mode C veil surrounding Class B primary airports. For flight at or above 10,000 feet MSL (3,048 meters), a transponder with altitude-reporting Mode C or Mode S capability is mandatory. Aircraft operating at and above FL 180 (18,000 feet MSL) must be equipped with a Mode-S transponder-based ADS-B transmitter.
Any airspace that requires a transponder also requires the aircraft to carry a Version 2 ADS-B Out system. Pilots must operate the transponder, including Mode C if installed, on the appropriate ATC-assigned code while in controlled airspace. Transponders must be inspected by an FAA-certified repair station every 24 calendar months in accordance with FAR 91.413. Balloons, gliders, and aircraft not equipped with an engine-driven electrical system are excepted from transponder and ADS-B requirements within the Mode C veil and certain other designated airspaces.
Do all planes have a transponder?
No. Gliders, balloons, and aircraft without an engine-driven electrical system can operate within the Mode C veil without a Mode C transponder, so they routinely fly without one.
Can a plane fly without a transponder? Yes, provided it stays outside the airspace where the equipment is mandatory. Airplanes whose transponders cannot squawk Mode C lack permission to fly close to class B airports, and you must be equipped with ADS-B Out to enter most controlled airspace. If the unit fails you continue with an inoperative transponder only under specific exceptions and after coordination with ATC. Military aircraft operate in dedicated Mode 1 to Mode 5 transponder modes. The US Air Force can switch off all transponders across its entire fleet on certain missions, so none of its aircraft show up on public tracking systems. Low-flying military airplanes therefore do not get picked up by trackers, and a US military tanker with its transponder off once forced JetBlue pilots to take evasive action.
When should an aircraft with an SSR transponder turn it on?
An aircraft with an SSR transponder must operate it at all times during flight, regardless of whether it is within or outside airspace where SSR is used for air traffic service purposes. Civil and military pilots set transponders to ON or the normal altitude-reporting position prior to moving on the airport surface, and they set them to STANDBY or OFF as soon as practicable after landing. Before take-off, pilots are encouraged to set the transponder to ON as late as practicable. Once airborne, the transponder remains active for the entire flight. Pilots entering Madrid, Barcelona, or Canarias FIR/UIR must turn the transponder on before entering that airspace until otherwise instructed.
What is the history of the aircraft transponder?
Transponders on aircraft date back to the IFF systems of World War II, crude devices invented so Allied forces separated friendly airplanes from foes. British operators called the box ‘Parrot’ and told pilots to ‘squawk your Parrot’, while Americans used the formal label Identification Friend or Foe. Those first sets gave radar a digital back-channel and extended the usefulness of early scopes, yet replies produced a hard ceiling on how many aircraft could be tracked.
Refinements were employed on military jets in the mid-1950s, and when civil airspace grew crowded the CAA experimented with a radar-equipped tower in 1946. Approach and departure control employed radar by 1952. Mode A transponders started in the early 1960s, offering a basic identity code only, and Mode C transponders started in the early 1960s to add automatic altitude. The FAA installed a computerized system from 1965 to 1975 that produced alphanumeric screen readouts of a plane's position, speed and altitude, making conflict-alert software possible.
Garble and reply overlap still created an issue, so Mode S was introduced in the late 1970s, adding selective interrogation and solving the overlap problem. Mode S provided advanced features including TCAS coordination. From crude wartime boxes that let pilots squawk their identity over radio, the transponder transformed the cathode-ray curiosity into the modern information network that today transmits every transponder code.
