What is a Transponder? How does it Work?
A transponder is a radio in your avionics stack, but it isn’t used for voice communications. Instead, it creates a signal that only air traffic control (ATC) radar systems can see.
The primary purpose of a transponder is to help controllers pick apart all the targets on their radar scopes. When the radar sweeps the sky, radio waves bounce off the aircraft and return to the station. Those returns on the scope show that there’s a plane there, but it can’t tell much about it.
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. But it gives no indication of the target’s altitude. 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.
The transponder fixes all of these problems. First, the transponder sends a signal to the radar that contains a four-digit squawk code. For example, if the air traffic controller tells one plane to “Squawk 1234,” that number will appear on the radar scope. This more detailed target is called a “secondary return.”
What’s more, modern transponders have “Mode-C.” A Mode-C transponder has the ability to send out the plane’s pressure altitude. The controller calibrates their scope with the current altimeter setting, and all aircraft then show their altitudes. With that information, the controller can separate flights safely in three dimensions.
History of the Transponder and the Word “Squawk”
If you think the word squawk sounds like a bird squawking, you’ve got a good idea of how the transponder came to be.
Today’s transponder technology was first designed in World War II when radar was first implemented. Controllers needed a way to identify friend or foe (IFF), so a basic transponder was developed. If the radar hit a plane and it returned a friendly transponder code, it was a friend. If it returned a primary only–or something else–it was treated as a foe. The codename for the IFF system was “Parrot,” and the signals the planes sent out became known as squawks.
Common Transponder Terms
The transponder has been a foundational technology in air traffic control for more than half a century. ADS-B and NextGen technologies are starting to phase out traditional radar transponders in favor of satellite-based identification systems. But until the entire National Airspace System has completely transitioned, transponders will be around for the foreseeable future.
Here are a few of the most common controller requests that involve the transponder. On the pilot side, there’s not much to this little radio. But it’s a vital part of dealing with ATC that you should be familiar with.
Squawk XXXX, as in “Squawk 4356”
The most basic function of the transponder is to send a discreet four-number code to ATC. This positively identifies you as you.
In instrument flying, your flight is assigned the code right at the beginning before you even get in the air. The radar system knows it, and instead of showing the four-digit code, it will display your tail number as indicated on your flight plan.
VFR pilots are assigned codes when they request transitions through controlled airspaces, like Class B or Class C. They also get codes when they request VFR flight following. And finally, discrete transponder codes are used to identify flights crossing the ADIZ (air defense identification zone) on the national borders.
“Squawk VFR” or “Squawk 1200”
Of course, VFR pilots aren’t always talking to controllers. When in Class G or E airspace, they can operate independently without contacting anyone. In these cases, if the plane has a transponder, they set it to the standard VFR code, which is 1200.
“Squawk IDENT” or “IDENT”
The IDENT button on the face of the transponder allows the pilot to send out a particular signal that is picked up on the controller’s radar. It doesn’t change the code or anything, but it makes your radar return flash–which draws the controller’s attention. This is handy when there are many radar returns in one area, all squawking 1200. The IDENTing aircraft will flash on the radar screen.
You might be asked to “Squawk IDENT” when you first call a control tower inbound for landing. Many Class D airports have a radar screen, but it’s not worth assigning you a code. Instead, they just want to know which radar return they’re talking to.
Another time you’ll hear it is when you first call for an IFR flight plan in the air or want to request VFR flight following. The IDENT helps the controller verify that you’re in their airspace.
Only press the IDENT button when asked, and only press it momentarily–there’s no reason to hold it down.
“Squawk Altitude” or “Stop Altitude Squawk”
Transponders have a few settings that allow them to override equipment issues. There are usually two modes–ON and ALT. The standard setting is “ALT,” which means that altitude reporting (Mode C) is active. If you accidentally set the transponder to ON, you might get asked by ATC to “squawk altitude.”
Controllers will likely start asking you to verify your altitude if your altitude reporting starts going haywire and reporting inaccurate numbers. If they discover that the transponder is in error, they may request that you “stop altitude squawk.”
Transponder Squawk Codes You Should Know
There are a handful of four-digit codes that every pilot should know. These are used every day in the National Airspace System. Never forget your transponder can communicate with ATC even when you think you can’t.
0000 — A generic code that is not assigned and should not be used.
1200 — VFR aircraft. The default code for all flights–if you aren’t asked to set anything else, you should set 1200.
7500 — Hijacking
7600 — Voice radio failure
7700 — Mayday or emergency
These last three codes, 7500, 7600, and 7700, display special notes for air traffic controllers. Controllers can set up their scopes to filter some planes off. For example, if a controller only worries about the approach area for a Class C airport, they don’t need to see the en-route traffic controlled by Center at 35,000 feet.
However, if a plane at 35,000 squawks 7700, it will light up on every controller’s scope.
A radio failure is less dramatic, but the radar computers note it. For example, a 7600 code will show as “NORAD” on a controller’s scope. It’s important to note that a voice radio failure does not necessarily mean a complete communication failure for the pilot.For example, it’s possible you can still hear ATC but not transmit, in which case you could respond to controller requests and acknowledge with an IDENT.
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Liz Brassaw is a first officer for a regional airline and the former Chief Pilot and Chief Flight Operations Officer for Thrust Flight. She holds an ATP, CFI, CFII, MEI, AMEL, ASES with over 2,500 hours of flight instruction given. She earned her Bachelor of Science degree from the Utah Valley University School of Aviation Sciences. She’s passionate about flying and enjoys instilling that love in the instructors on her team and the new students she trains.