Here's an excerpt from an article from USAToday on the subject:'Strangle my WHAT?'Question: What does it mean when Air Traffic Control calls up a flight and says, for instance "squawk 0-5-4-4"?
— Kim Pauli, Denmark
Answer: We enter these numbers into our "transponder" which allows us to be identified on radar by air traffic control.
Transponders date back to World War II when the British developed a top secret radar transceiver (a transmitter transmits, a receiver receives and a transceiver does both). The British code name for this system was Parrot and it worked by responding to a radar interrogation signal with a coded transmission. This allowed the British to distinguish their own aircraft from German aircraft on radar.
When the Germans trailed the British night bombers back to England to shoot them down, Parrot was able to distinguish the German aircraft as they showed up on radar as "primary" returns, which are just blips. The returns generated by the British aircraft appeared very different because of their distinct codes.
Aircraft with these units would be given instructions such as "squawk your parrot," meaning they should turn them on. "Strangle your parrot" meant the pilots should turn the unit off as it was possible the transponder signal was powerful enough to prevent the radar system from identifying primary enemy targets.
The same system used in the U.S. by the Army Air Forces was called IFF, which stands for "Identification Friend or Foe." The term "squawk" remains in use today when the controllers issue the four digit code for the pilots to display in their transponder.
For the complete article:http://www.usatoday.com/travel/columnist/getline/2006-04-17-ask-the-captain_x.htm
