
Roundoff Error and the Patriot Missile (1992) - auslander
https://web.archive.org/web/20100702180720/http://mate.uprh.edu/~pnm/notas4061/patriot.htm
======
matchagaucho
The computer science behind TBM interception is a fascinating problem with
very high stakes consequences, as occurred on February 25, 1991.

I was an active duty Patriot Technician and Systems Mechanic (24T) during that
time; and even though we were rebooting our systems regularly to diminish the
impact of this roundoff error, there were other critical timing issues during
the intercept stage.

TBM interception involves the science and math behind a "bullet hitting a
bullet". These are extremely high velocities converging on each other.

To Raytheon's credit, they were iterating rapidly and released patches almost
daily as new data was collected.

Even when we managed to launch a Patriot to engage an oncoming Scud, success
was dependent on the proximity fuzed warhead detonating at just the perfect
predictive moment ahead of the projectile. This timing was perfected over a
decade in White Sands NM using lower velocity drones. But Patriot's software
was not optimized for TBM-scale velocity (it is now).

A "perfect" hit typically resulted in a shower of hot metal and undetonated
debris raining down on civilian populations.

From a game theory perspective, this is basically a no-win situation. You're
just trying to minimize collateral damage once a theater of operations
escalates to using TBMs.

~~~
tyingq
Is TBM "theatre ballistic missile", "tactical ballistic missile" or something
else?

~~~
matchagaucho
Good question. The terms seem to be used interchangeably.

But Wikipedia classifies Scuds as "Tactical" in this case, because of their
short range.

------
sixstringtheory
I’ll never forget my Computational Methods professor discussing exactly this
project, and how a lack of knowledge or care about things like the
conditioning of your routines, or compounding rounding errors in floating
point, could literally get people killed.

Most of the students laughed at this idea.

------
auslander
Also: "As a stopgap measure, the Israelis had recommended rebooting the
system's computers regularly."

[https://en.wikipedia.org/wiki/MIM-104_Patriot#Failure_at_Dha...](https://en.wikipedia.org/wiki/MIM-104_Patriot#Failure_at_Dhahran)

~~~
auslander
I read further down, and it got more interesting. Success rate is claimed from
95% by ones to below 10% by others. "standard firing doctrine on average four
Patriots were launched at each incoming Scud .. suggests low confidence in
individual missiles".

~~~
kjs3
_suggests low confidence in individual missiles_

Or the high consequence of an individual miss. Not that I disagree that
Patriot took a long time to evolve to effectiveness, but it's probably more
complicated. Take for example the Phalanx CIWS, which fires hundreds of rounds
at each target, even though theoretically a single round should be enough,
because missing a single incoming target would be very, very bad.

~~~
auslander
Yeah, but bullets a dumb and cheap. The multi $M rocket with radar, guidance
should have at least 90% hit rate against SCUD like big target. Agree, not an
easy task for Mach 5 SCUD.

~~~
kjs3
_should have at least 90% hit rate_

Why? Just because it's expensive?

