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The three letters IFF were, until a week ago, just part of the government and military alphabet soup. Since the downing of the Iranian Airbus by the American cruiser Vincennes, the signals received from IFF equipment supposedly aboard the Iranian airliner have made IFF almost a household acronym.

In an earlier news story, the midair crash of a Sky West commuter plane and a light aircraft over Kearns in 1986 led to calls for a variant of IFF on all aircraft flying near major airports.But just what is this apparently arcane science? IFF stands for Identification, Friend or Foe, and it is an acronym that has been used by military fliers since World War II, when the British developed radar that could detect attacking German bombers. Since all radar blips look much alike, the defenders in the Battle of Britain needed a means for telling their Spitfire fighters from enemy Dornier and Heinkel bombers.

The device English scientists came up with was a receiver that picked up the radar signal sent from the ground and returned its own signal, containing a code, that told the radar operators they were seeing a friendly aircraft.

The U.S. Army Air Corps and Navy had been working on a similar device under the initials RR, for Radio Recognition. The Navy had an additional need for a homing device so carrier planes could find their way back to their ship, which might have moved dozens of miles since its planes took off for a mission.

Both types of system needed to identify the transmitting point correctly to friendly forces without giving away its location or identity to the enemy, and even more important, without offering the enemy a chance to mimic the "friendly" code and play an electronic Trojan horse trick on the radar.

The American RR set first built was known as SCR-515, and it was succeeded by the British IFF Mark II. Under the pressure of time, the Mark II was adopted by the U.S. as SCR-535 for use in the invasion of North Africa (Operation Bolero), and 38,000 were hurriedly built by the Philco Radio Corp. and installed in all American aircraft used in North Africa.

The SCR-535 threw back an intense radar echo which flashed prominently on the oscilloscopes of ground search radar sets.

A more advanced Mark III British IFF design was adopted as the universal Allied IFF set for the remainder of the war, with American SCR-533 recognition sets as a backup in case the Germans or Japanese learned enough from equipment in downed planes to develop a means of fooling Allied radar.

So seriously did the Allies take the security of IFF that the Mark III sets were built with a thermite self-destruct mechanism that would burn the set beyond recognition if it were downed.