Aviation Transponders Information

aviation transponder

Aviation transponders identify the location of aircraft and provide other flight information via a radio-frequency signal. These devices, also known as beacons, allow aircraft location to be plotted on the air traffic control (ATC) radar. Traffic collision avoidance systems (TCAS) use data from a transponder to reduce the risk of aircraft collision.


Air traffic control radar facilities employ two sets of radar-primary and secondary. Primary radar refers to traditional technology that emits microwave signals and detects signals reflected from the. Primary radar has several weaknesses and disadvantages. It does not distinguish between flying targets and ground objects such as trucks and automobiles. Also, its effective range is reduced due to hills and trees, along with interfering terrain and weather conditions. Primary radars operate well with all-metal objects, yet have difficulties detecting composite-skinned structures, such as small planes. These radars give the range and azimuth of an identified object, but they fail to provide the altitude information.


Secondary radar is referred to as secondary surveillance radar (SSR). It was developed along with the implementation of transponders to address the deficiencies of primary radars. The system sends out pulses or interrogations that trigger a high-energy response from an aircraft transponder. This increases the effective radar range and supplies ATC systems with critical information, such as aircraft identity and altitude. 


All transponder models share a broad set of similar characteristics due to regulations that exist. However, functionality varies based on the supported modes, also known as interrogation types, which produce a unique response from an aircraft. The majority of models respond to Mode A and Mode C interrogations. Transponders with Mode C capability are mandatory in Class A, B, and C airspace, as well as for operating above 10,000 feet.


In addition to supporting Mode A and Mode C systems, Mode S transponders respond to Mode S interrogations that are based on a single reply. These signals assist air traffic control in congested airspaces and enable the operation of Traffic Collision Avoidance Systems.


ATC systems obtain further information using Mode S transponders, including the fixed 24-bit code combining the aircraft address and parity. Mode S TIS (traffic information service) activates when a plane comes within range of a radar station that supports it. The radar station shares nearby air traffic information with an aircraft using the service. Mode S is required in controlled airspace in Europe but is not mandatory equipment in the United States for planes not equipped with TCAS. 


Due to the limitations inherent in primary radars, SSRs linked to aviation transponders are applied worldwide to facilitate air traffic management. The secondary radar sends out pulses to aircrafts within range to determine their position, speed, altitude, and direction. The transponder presents this information in the form of a "squawk code" when interrogated. This unique 4-digit code marks the plane on the radar and is entered by the pilot at the flight's outset. The digits making up transponder codes are limited to ranges between 0-7.


The primary objective behind the development of SSRs was to compensate for the inability of primary radars to report altitude. As a result, Mode C and Mode S transponders transmit information related to pressure altitude. These indicators are critical for TCAS. The transponder must be linked to an altitude encoder to generate a digitized altitude reading. The device exists as a separate unit, known as a "blind encoder" or is incorporated into an altimeter, called an encoding altimeter.


Transponders feature the following mode switch positions:


  • OFF: fully disables communication
  • STBY: set when the engine starts; the transponder warms up yet does not respond to interrogations
  • ON: If a transponder encounters a malfunction and sends out inaccurate altitude information, ATC may request to switch the mode to ON. The transponder will respond to Mode A, but not to Mode C interrogations.
  • ALT: Set when an aircraft proceeds down the runway for takeoff. It enables the transponder to reply to Mode A and C interrogations.

Transponders have a button labeled IDENT along with a reply light and four switches for code selection. A standard transponder assigns up to 4096 distinct identification code sequences. Code 1200 informs air traffic control when the aircraft operates under visual flight rules. Other special purpose transponder codes reserved for exclusive use of the military include 0000, 4000, and 7777. Code 7600 indicates communication radio failure. Broadcasting code 7700 is reserved for emergencies, and 7500 for hijacking situations.




Selecting an aviation transponder relies on crucial information such as the type of aircraft and airspace the aircraft may enter. Commercial airliners must be equipped with transponders supporting specific modes to traverse a controlled and crowded airspace. Small personal or experimental aircraft not intended for flight in such areas do not require the same functionality. Validate the selected transponder functionality is in full compliance with rules and regulations of the relevant regulatory authorities of the applicable zone.




EUROCAE ED 115 - Minimum operational performance specification for light aviation secondary surveillance radar transponders


RTCA DO-144 - Minimum operational characteristics - airborne ATC transponder systems


TSO C74 - Airborne ATC transponder equipment


Image Credits:


Rochelle Avonics, Inc.


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