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NA Digest Monday, June 1, 1992 Volume 92 : Issue 22

Leggi questo documento per una descrizione più estesa (link originale, copia locale).

From: Robert Skeel
Date: Mon, 18 May 92 21:21:48 -0500
Subject: Did Roundoff Cause Patriot Failure?

Article on page 1347 of the 13 March 92 issue of Science
``Fatal Error: How Patriot Overlooked a Scud''
The error in question was a roundoff error, or roundoff errors.
The description of what happened is not clear, to me.
I would be interested in more detail than is offered in the article.
I am hoping that someone out there might be able to offer a plausible explanation.


From: David Keaton
Date: Thu, 28 May 92 11:12 MDT
Subject: Patriot Missle Bug Report

A government report on the failure of the Patriot Missile is available.
The report is
Patriot Missile Defense: Software Problem Led to System Failure
at Dhahran, Saudi Arabia
It is report number GAO/IMTEC-92-26, dated February 1992.

The report is quite well done and includes pictures that show the exact process used to track a Scud.
It contains the level of technical detail needed for us to understand what happened, yet
presents it so that a non-technical person has a fighting chance at understanding it too.

The problem began to manifest itself as roundoff error, but the real problem was precision error instead.
They put the "real time since boot" clock in a single precision floating point number.
To calculate a time interval, they took two snapshots of the clock and subtracted them.
When the system had been up for more than eight hours, the absolute uptime started to get bigger
than the mantissa and calculations got less and less accurate. Eventually, the Patriot
would miss the window for tracking an incoming Scud.

The field fix was just to reboot the Patriots every eight hours until a software fix arrived.
However, the people manning the Patriots didn't get the word.