During WWII the British developed a top secret 10" x 10" x 10" radar transceiver. It would respond to a radar interrogating signal by responding with a coded transmission. A code would allow the land based radar station to distinguish British from German aircraft on their radar screen. The radio also contained an internal thermite bomb which, when triggered by an inertial switch (crash), would destroy the interior of the set. This was supposed to prevent German discovery of the codes. (A reverse ELT?) The British code named the system Parrot. The United States Army Air Forces version of the system was called IFF, for Identification Friend or Foe.
As with many WWII developments, the IFF system was designed to prevent a clever German ruse. The Germans were following the night bombers back to England. German aircraft would join in the stream of returning British bombers. They would wait until the bombers were most vulnerable, just prior to landing, and then shoot them down. Parrot allowed detection of these German aircraft since their (primary) return would not have a distinctive code.
To control the operation of the airborne coded set to the best advantage, the ground based radar station would radio instructions regarding the operation of "Parrot". The aircraft would be directed to "squawk your parrot", meaning to turn on the set for identification; or to "strangle (not kill) your parrot" as a directive for turning the set off. The power of the transponder signal would often hide other targets.
The only vestige of this that remains today, other than the entire ATC system itself, is the term "Squawk", as an ATC directive for operation or code for the transponder. Old time ATC controllers may still have you "strangle" your parrot (x-ponder)
Today the transponder usually has a four position switchoff, stby (standby), on (mode A), and alt (altitude Mode C), a test button, and ident (identification) button, a response light, and four selector switches with numbers from 0 to 7. Certain aircraft letters and numbers cannot be reproduced but frequently the discrete code can be seen to represent a specific aircraft due to their similarity.
ATC has a system by which the code used on the transponder shows a specific type of operation. Operations such as VFR without advisory, VFR with advisory, IFR, specific airport operation, TCA, ARSA, Local IFR, Tower enroute IFR, X-country IFR, emergency, hijack, and radio failure all have differing first two digit codes which tell ATC controllers your situation.
There are 4096 possible code selections on a transponder from 0000 to 7777. This is a Base 8 number system which is used by computers as a short method of storing Base 2. Base 2 is the number system of computers.
The four places of the transponder from right to left are 1's, 8's, 64's, and 512"s. We know it is a base 8 because the highest digit is 7. The eight possible digits are 0, 1, 2, 3, 4, 5, 6, and 7.
Written by Gene Whitt