How does transponder work

A transponder is an electronic device used to wirelessly receive and transmit electrical signals. Fittingly, its name is equally derived from the words "transmitter" and "responder. This device functions by receiving a signal, called an "interrogator" because it is effectively "asking" for information, then automatically conveying a radio wave at a predetermined frequency.

In order to broadcast a signal on a different frequency than the one received, a frequency converter is built in. By receiving and transmitting on different frequencies, the two signals can be detected simultaneously. This is the typical setting for a transponder. This is a useful tool for controllers who can then quickly identify a specific aircraft without having the pilot change the code. Aircraft can still operate with legacy transponders outside of these airspace areas, and they can receive a waiver should they need to operate in an ADS-B required airspace.

The Federal Aviation Regulations spell out exactly where aircraft are required to have a transponder. But most small planes do occasionally venture into these spots, in which case they must have one on board. According to the Aeronautical Information Manual, if you have a functioning transponder onboard the aircraft, it must be switched on during flight.

This makes logical sense, considering it helps air traffic control separate other traffic from you. This is even true if you are VFR and not receiving separation services. Transponders are required above 10, feet MSL , within 30 nautical miles of a Class B airport, and inside and above Class C airspace.

The ability to set the four-digit code provides the ability for an air traffic controller to positively identify a specific aircraft. If a controller needs to track a particular flight, a discrete code is assigned. A discrete transponder code is a code that is only given to one aircraft at a time, so ideally, no one else should be squawking it. All instrument flights have their own discrete codes, as do most aircraft that are in contact with a Class B or C approach controller, or transiting a special flight rules area.

Air traffic controllers work from ground-based stations, such as towers. At these stations are radars that communicate with airplanes and their transponders. Transponders work in conjunction with a squawk code. A squawk code is a unique identifier that associates a transponder with an airplane.

Prior to taking off, pilots must enter the squawk code so that air traffic controllers can identify them. Each flight has a squawk code. Squawk codes are important because they allow air traffic controllers to identify specific flights. Air traffic controllers may receive its location, but the squawk code is ultimately what identifies airplanes in the air.

A transponder is a small device that receives and sends radio signals. The SSR uses that return signal to determine aircraft location much more accurately than the primary system could alone, and it eliminates radar returns from spurious sources, such as birds and terrain.

Responses from a Mode A transponder include a four-digit identification code assigned by a ground controller via radio, which pilots update manually during flight. An improved surveillance radar technology is Mode S, for Mode Select. Each Mode S-equipped aircraft has a unique, permanent identification number that remains during the life of the aircraft. It enables the air traffic control computer to tailor its interrogations, addressing only specified targets.

As a result, to track a target, Mode S needs far fewer interrogations than earlier radars, which translates into more accurate position reporting. This capability enables new kinds of air-to-air communication, such as the automatic signals of TCAS, the Traffic Alert and Collision Avoidance System, which helps prevent midair.