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May 16, 2003

During the Iraq campaign, only four coalition jets were shot down, But two of them were brought down by American Patriot anti-aircraft missiles. Patriot batteries fired 22 missiles during the war, and the attacks on friendly aircraft appear to have been caused by flaws in the Patriot fire control software. When Patriot radar detects an aircraft, it immediately sends out an electronic signal to the aircraft's IFF (Identify, Friend or Foe) radio. This device is designed to listen for those "interrogation" signals, and immediately respond with a coded signal indicating who the aircraft belongs to. The IFF is used in training, and constantly during the Iraq fighting. So how did the Patriot not recognize the IFF signal, and launch missiles at a British and a U.S. Navy aircraft? Well, it appears that if an aircraft is flying in a certain way (altitude, speed), the Patriot identifies it as a missile, not an aircraft. When the Patriot thinks it has detected missile, it does not waste a few seconds looking for a friendly IFF signal, but promptly fires (if the missile is set on "automatic," which it often is if enemy missiles are expected.) Some types of missiles do go at about the same speed as fast jets, but in retrospect it seems false economy to not send out the IFF interrogation signal unless the radar has spotted a real fast moving (faster than any aircraft) target. If this was indeed the cause of the two friendly fire shoot-downs, changes in the software will no doubt be made.