Humble Pi: When Math Goes Wrong in the Real World

by Matt Parker

Ironically, if the system had consistently been off from the correct time, it could still have worked OK. Tracking a missile requires accurate tracking of time differences, so a consistent error would cancel out. But now different parts of the system were using different levels of precision in their conversion, and a discrepancy slipped in. The incomplete upgrade is why the system could not track the incoming missile. Even more depressing is the fact that the US Army knew about this problem and, on February 16, 1991, it had released a new version of the software to fix it. As this would take a while to distribute to all the Patriot systems, a message was also sent out to warn Patriot users not to let the system run continuously for long periods of time. But what constituted a “long period of time” was not specified. As well as the mathematical problems, those twenty-eight deaths were also the result of poorly fixed code and the lack of a message simply saying to restart once a day. The software fix arrived at the base in Dhahran on February 26. The day after the missile attack.