The Tragedy of Flight 655 : Transponders: Airliners’ Electronic ID Cards

Airplane transponders, which have become central to the investigation of the downing of a commercial Iran Air jetliner Sunday by a Navy cruiser, are transmitter/responders that are supposed to identify the type of aircraft carrying them.

For the aircraft, what they say and how they say it can mean the difference between peaceful passage and disaster.

Transponders automatically “squawk,” or broadcast in repeating bursts, an identifying code when they are asked what they are by air traffic controllers, other pilots or ships such as the Vincennes, the cruiser that brought down Iran Air Flight 655.

The transponders on most civilian airliners, when they are asked electronically for identification, are programmed to give the plane’s flight number and its altitude. All but the smallest recreational aircraft are equipped with transponders that operate in a frequency band pilots call Mode C, the civilian channel that Pentagon spokesman Dan Howard referred to Tuesday as Mode 3.

Howard said the Vincennes picked up transmissions from Iran Air Flight 655 in Mode 3. But he said it also received messages in Mode 2, which, like Mode 1, is an exclusively military channel. That helped convince the Vincennes, Howard said, that the plane bearing down on it was hostile.

Civilian airline pilots, when seeking clearance for takeoff, are given a code to program into their transponders. Throughout the flight the transponder responds automatically, often without the pilot’s even noticing, when queried by successive air traffic controllers.

On military channels, identification is more complex. In peacetime, military transponders operate much as their civilian counterparts do--but in Modes 1 and 2, which are radio frequencies that are easily distinguished from those of civilian aircraft.

But in wartime, military aircraft operate on a “password” system designed to reassure friends of their arrival without alerting foes that attackers are on the way. Operating in Mode 4, an encrypted channel, warplanes reveal their identity only to questioners who preface their query with the right electronic password. That protects defenders from giving away their own position.

In combat, when friend and foe are mingled in the skies, anything short of responding correctly to a challenge is considered hostile. But in the skies over the Persian Gulf, where commercial traffic mixes with military warplanes of several nations, the distinction between friend and foe is not so clear, experts say.

In the gulf, all commercial planes willingly respond to electronic challenges so they will not be targeted as hostile, according to both civilian and military pilots.

It is “inexplicable,” said one, that Flight 655 would have broadcast in a mode that requires equipment a civilian airliner should not have.