Cipher Machines





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Aircraft IFF Code Wheels

P-51 Mustang
P-51 Mustang

Identification, Friend or Foe (IFF) radios were invented during WW2 so that pilots would know who to shoot before visual identification could be made. Prior to WW2, this was not an issue since the planes were slower, they generally flew in good weather during daylight hours and most importantly they did not have radar to identify a target before they came into view.

In 1940, the Germans were the first to invent IFF in a radio called FuG-25a Erstling (translation: Debut). This IFF radio had encryption keys to prevent the enemy from building a radio to send back a valid response as a friend. Unfortunately for the Nazis, British intelligence was able to build a device called a Perfectos which was used in the RAF Mosquito to find the position of the German aircraft. This caused the Germans to keep their IFF radios turned off most of the time.

The IFF radio is really a misnomer, as it should have been called IFU for Identification, Friend or Unknown. The transmitter would send out a signal asking for a response from the target aircraft, which was automatically transmitted back without pilot intervention. The problem is that a Foe would not send back a signal and even a Friend may have a defective radio or the wrong encryption keys installed. So the early IFF radios may positively identify a Friend but could not positively identify a Foe.

An important feature of the IFF radio was the encryption capability. The enemy could easily recover an IFF radio from a downed plane and use that radio in their own planes. The ability to encrypt the signal was the only thing that would prevent the enemy from using these recovered radios. Pictured below is a box of 10 wheels, one of which would be selected to be inserted into the dynamotor of the IFF radio to provide a mechanical means of encipherment. This IFF radio was first made in 1941, so it was one of the first IFF radios invented by the Allies.

After the war, IFF capability was incorporated into a device called a transponder. The transponder, short for transmitter responder, is now used in all military and commercial aircraft to send identification to other aircraft and to ground stations. Aircraft call sign, altitude, ground speed, weapons status and other flight information can be automatically sent.

IFF Radio
IFF radio encryption device

Pictured above is the dynamotor for the ABA-1 IFF radio set. This device provides power for the transmitter and receiver and also has a small door to insert a code wheel. One of ten metal code wheels are changed daily, the one pictured above is labeled "MC-348". The code wheel has teeth which engages morse code type contact points that automatically sends the appropriate code to identify the aircraft as a friend. You can see the contact points in the picture just above and to the right of the code wheel.

Detailed pictures will be shown for the ABA-1 IFF transmitter/receiver, the ABA-1 dynamotor and the box of ten code wheels. Also, a KY-65 emergency keyer will be shown. This is a radio control device which will automatically send out the aircraft tail number on a regular basis in the event of an emergency.

Copyright © 2016 by Ralph Simpson - Creative Commons Attribution 4.0 International License