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What Are Aircraft Transponders and How Do They Work?

An aircraft transponder, short for transmitter-responder, is an electronic device that transmits a four-digit code, or squawk code, enabling aircraft to be identified by Air Traffic Control (ATC). In the cockpit, pilots enter this code, which is assigned to them by ATC, into the transponder. Once this has been done, the aircraft is displayed on the Air Traffic Control radar screen.

There are two basic types of ATC radar, those of which are primary and secondary radar. Primary radar will typically display a return cue on the controller’s screen when the radar signal is reflected back to the radar by an aircraft. However, it does not provide any other data to the controller apart from the distance and bearing of the aircraft.

Secondary Surveillance Radar (SSR), on the other hand, transmits an “interrogation,” which is essentially a “request for information” signal on 1030 MHz. When the transponder receives this signal, they reply on 1090 MHz and it includes the squawk code, allowing the aircraft callsign or registration to be displayed. In some instances, this exchange may also include the aircraft’s pressure altitude.

Generally, ATC assigns a squawk code with the first clearance the flight receives on the ground or in the air. With VFR flights, pilots set a squawk of 1200 in the US or 7000 in most other places in the world. Squawk codes are unique to each aircraft and will show an aircraft’s flight number or registration on the controller’s screen.

On a transponder, there is an indent button that tells the pilot “squawk ident,” which instructs pilots to press this button. When ATC radar registers the “ident” signal, it makes the aircraft stand out, or “blossom,” on their screens, allowing air traffic controllers to easily identify aircraft. Now that you are familiar with important terms regarding transponders, we will cover their varying types.

Mode “A,” “C,” and “S” Transponders

Modern transponders operate in certain “modes,” and there are three common modes in aviation. A mode “A” transponder, or mode alpha, is the most basic mode and only shows the selected squawk code on the controller’s screen. Meanwhile, a mode “C” transponder, or mode charlie, includes the capabilities of a mode “A” transponder and also provides data about pressure altitude. Lastly, a mode “S” transponder, or mode Sierra, is most common on commercial aircraft and is incredibly advanced. Mode “S” transponders also allow ACAS/TCAS and ADS-B (Automatic Dependent Surveillance-Broadcast).

The ACAS (Airborne Collision and Avoidance System) and TCAS (Traffic Collision and Avoidance System) are systems that receive transponder signals from nearby aircraft via a series of transponder antennas on aircraft. When the ACAS/TCAS detects two aircraft in close proximity to one another, it will sound an alarm in the cockpit and provide action instructions to pilots.

The ADS-B is a system that provides an aircraft’s position through satellite navigation, and it broadcasts its position to ground stations. ADS-B systems can be used instead of ATC radar surveillance in locations where no radars are present. Moreover, it also enables other aircraft to receive its position to provide better separation between aircraft. The transponder panel is located on the center console and has a number of selectors and buttons. On the left, “ATC” signifies the transponder section, while “TCAS” on the right indicates the TCAS mode selectors, alongside the “ident” button.


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