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9 Things You Didn't Know About The History Of The Transponder

During the end of WWII, England developed a system to identify their own military aircraft: the transponder. Here are 9 cool facts about where the transponder came from, and how it became what it is today.

1) German infiltration

British bombers returning from missions in Europe had a problem: German aircraft would join with the returning aircraft formations and sneak into England for surprise attacks. The solution? A radio transponder system the British called the "Parrot". The Americans had a name for it too, "Identification Friend or Foe", or IFF.


2) Too powerful for radar

In the mid 1940s, radar was pretty unsophisticated, which caused a problem for the Parrot. The original Parrot transponder was so powerful, it masked radar screens, covering everything up on radio operators screens.

RAF Museum

3) "Squawk your parrot"

To solve the problem, radio operators would have Allied aircraft turn their Parrot transponder on at certain times to identify themselves. When they wanted pilots to turn their transponder on, they would say "squawk your Parrot". And that is where the term "squawk" originated from.


4) "Strangle your parrot"

When radio operators wanted pilots to turn their transponders off, they would say "strangle your Parrot".


5) Is it really a foe?

The Parrot/IFF system didn't tell you if an aircraft was a foe. It only told you if it was a friend, or if the aircraft's transponder wasn't working.

Warbirds Resource Group

6) It's all 1s and 0s

Transponders back then (and still today) have the option of numbers 0-7. That's because each number can be represented by only three bits, or three 1s or 0s.


7) Not much has changed

In 70 years, very little has changed for the transponder. The original system had two numbers (0-7), for a total of 64 different code combinations. Today's system has 4 numbers, for a total of 4096 different code combinations. Basically, the only thing that's changed are two more knobs stuck in the middle of the transponder.


8) Altitude information included

Today's Mode-C transponders translate altitude into a 4096 code, starting at -1,200' pressure altitude (0400), all the way up to 126,700' pressure altitude (0042). So no matter what altitude you're flying (almost), your transponder can tell ATC what altitude you're at, within a 100 foot increment.


9) It takes 3.35 NM

Your transponder radio signal travels at the speed of light (didn't you wish your airplane did as well???). A transmission only takes 20.75 microseconds, which means your transponder signal is spread across 3.35 NM.


So the next time ATC tells you to 'squawk' your transponder, you'll know where the term originated from.

Colin Cutler

Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at

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