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From: boone@IDA.ORG (John Boone)
Newsgroups: sci.military
Subject: Re: patriot missile failure
Message-ID: <1991Jun22.041507.2842@cbnews.cb.att.com>
Date: 22 Jun 91 04:15:07 GMT
References: <1991Jun18.051603.8376@amd.com> <1991Jun19.010743.11663@cbnews.cb.att.com>
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From: boone@IDA.ORG (John Boone)
The following is the latest posting to the Risks Digest on the Patriot missile
failure.  (Some information not pertaining to the actual failure has been 
deleted.)  The posting talks about a 0.36 second error - this error is in
reference to the clock-drift which occurred due to extended operation of the
system;  i.e., the clock was not reset by re-initializing the system, and
incremental errors built up over time to cause a significant discrepency.

I think claims that the Patriot software was "buggy" are somewhat of a
knee-jerk reaction, given the overall performance of the system.  The fact is,
any significantly complex system has some bugs.  With our present state of 
technology, it's just a question of how often (and under what conditions) the
bugs manifest themselves.  I would not call a system with a +95% performance
rate (in shooting down extra-supersonic missiles) - a buggy system.

---------------------------------------
Article 494 in comp.risks (moderated):
Date: 19 Jun 91 00:06:15 GMT

RISKS-LIST: RISKS-FORUM Digest  Tuesday 18 June 1991  Volume 11 : Issue 94


Date: Tue, 18 Jun 91 13:48 EST
From: HORN%HYDRA@sdi.polaroid.com
Subject: Re: The Patriot system and the Dharan Scud: Time Warp (RISKS-11.92)
 
A slight correction, the specification for Patriot was 14 hours of
operation.  The 22 hour figure was the actual operational time for
systems that did detect the Scud but were out of range.  The origin of
the 14 hour spec was the original intended use of Patriot in defense of
mobile forces where it was expected that the systems would relocate
several times per day, so the assumption was not tacit.  It was a
deliberate considered decision, balancing procurement and operational
costs against the operational needs.

For those who don't understand how a 0.36 second (or 1 ppm) error can
cause this problem, you have to examine how a machine tracks a target.
At initial acquisition time you have a single location, but no speed or
direction.  So you put a ``box'' around the location and look again a
very short time later.  Based upon the article, the timing error
resulted in a mislocated box.  A slower target (like an airplane) would
still have been within the box.  A Mach 6 missile was outside the box.
So the computer did not associate the second echo with the first echo.

(information about formal proofs for software deleted)
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   :                      :                                                  :
   :      J.M.Boone       :     Tenacity of purpose for a rightful cause     :
   :                      :     is not shaken by a frenzy of opposition.     :
   :   boone @ ida.org    :                                                  :
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