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> From: roeber@vxcrna.cern.ch
> Date: 3 Apr 92 09:47:14 GMT
> Message-ID: <1992Apr3.104714.1@vxcrna.cern.ch>
> Subject: Re: Design question about Patriot
>
> In article <1992Apr3.062551.7306@sequent.com>, jjb@sequent.com (Jeff Berkowitz) writes:
> > [...]
> > The article states "...this particular Patriot battery had been
> > running continuously for about 100 hours...[and]...had built up
> > a timing lag of 0.3433 second."  This timing error was sufficient
> > to shift the "range gate" enough to cause the system to disregard
> > the Scud.
> > 
> > My "comp.realtime" question: what algorithm in the Patriot system
> > would cause an ABSOLUTE time variation like this to change a
> > relative computation, like the predicted course of the incoming
> > missile?
>
> I just received my copy of the US General Accounting Office's "Report
> to the Chairman, Subcommittee on Investigations and Oversight, 
> Committee on Science, Space, and Technology, House of Representatives:
> Patriot Missile Defense - Software Problem Led to System Failure at
> Dhahran, Saudi Arabia" (phew!)
>
> Since I can't find a copyright notice on it anywhere, I'll quote the
> appropriate paragraph:
>
> "The range gate's prediction of where the Scud will next appear is a
> function of the Scud's known velocity and the time of the last radar
> detection.  Velocity is a real number that can be expressed as a
> whole number and a decimal (e.g., 3750.2563...miles per hour).  Time
> is kept continuously by the system's internal clock in tenths of
> seconds but is expressed as an integer or whole number (e.g., 32, 33,
> 34...).  The longer the system has been running, the larger the number
> representing time.  To predict where the Scud will next appear, both
> time and velocity must be expressed as real numbers.  Because of the
> way the Patriot computer performs its calculations and the fact that
> its registers are only 24 bits long, the conversion of time from an
> integer to a real number cannot be any more precise than 24 bits.
> This conversion results in a loss of precision causing a less accurate
> time calculation.  The effect of this inaccuracy on the range gate's
> calculation is directly proportional to the target's velocity and the
> length of the the system has been running.  Consequently, performing
> the conversion after the Patriot has been running continuously for 
> extended periods causes the range gate to shift away from the center
> of the target, making it less likely that the target, in this case a
> Scud, will be successfully intercepted."
>
> Interestingly, when the Israelies were running their Patriot systems,
> they studied the heck out of them, and noticed this problem after
> merely 8 hours of operation.  (8 hours corresponds to a range-gate
> shift of 55 meters; 100 hours corresponds to 678 meters.)  They
> sent this data back, and the Americans responded with something like,
> "This was designed for the European theater, where it would be run
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> for a few hours before being moved.  Who in his right mind would
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> run a system continuously for eight hours?  Or more?"  They did 
> write a software patch to fix the problem, but it took two weeks to
> arrive in Saudi, because of transport difficulties.  It arrived the
> day after the failure.
>
> -- 
> Frederick G. M. Roeber | CERN -- European Center for Nuclear Research
> e-mail: roeber@cern.ch or roeber@caltech.edu | work: +41 22 767 31 80
> r-mail: CERN/PPE, 1211 Geneva 23, Switzerland | home: +33 50 42 19 44
> --  
> "Sorry, baby, I can't take you to the pizza joint tonight, I've got to go
> back to the lab and split the atom." -- Ayn Rand, "What is Romanticism?"

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WASHISAKA Mitsukazu  (wasisaka@wink.ntt.jp)