![]() Controllers needed a way to identify friend or foe (IFF), so a basic transponder was developed. Today’s transponder technology was first designed in World War II when radar was first implemented. If you think the word squawk sounds like a bird squawking, you’ve got a good idea of how the transponder came to be. History of the Transponder and the Word “Squawk” With that information, the controller can separate flights safely in three dimensions. The controller calibrates their scope with the current altimeter setting, and all aircraft then show their altitudes. What’s more, modern transponders have “Mode-C.” A Mode-C transponder has the ability to send out the plane’s pressure altitude. This more detailed target is called a “secondary return.” For example, if the air traffic controller tells one plane to “Squawk 1234,” that number will appear on the radar scope. First, the transponder sends a signal to the radar that contains a four-digit squawk code. The transponder fixes all of these problems. Plus, all primary returns look the same, so there’s no way to tell which return belongs to which plane, beyond its location on a map. But it gives no indication of the target’s altitude. This basic radar blip is called a “primary return.” If the controller watches the primary return they can get an idea of speed and direction. Those returns on the scope show that there’s a plane there, but it can’t tell much about it. When the radar sweeps the sky, radio waves bounce off the aircraft and return to the station. The primary purpose of a transponder is to help controllers pick apart all the targets on their radar scopes. Instead, it creates a signal that only air traffic control (ATC) radar systems can see. What is a Transponder? How does it Work?Ī transponder is a radio in your avionics stack, but it isn’t used for voice communications. So this funny word is not just for the birds–pilots of all types of planes get to squawk, too. What says “squawk” and flies in the sky? Is it a parrot, a Cessna, or a 787? Or all three? Squawk is a common term used in air traffic control to describe a specific type of radio signal that comes from the plane’s transponder.
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