Did Roundoff Cause Patriot Failure?

[David Keaton]

     I have read the technical report describing the Patriot failure.
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.

     The report I read belonged to somebody else, but he said it was
easy for anyone to call in and get it.  I'll find out how and post the
answer.

					David Keaton
					dmkadmk.com


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