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In-Depth Information
2. Focused
target isolation
3. Final tracking: Scud
falls outside “range
gate”
3
2
1. Wide area
search
Range
gate area
1
Patriot
radar
system
FIGURE 8.2
A software error caused the Patriot missile system to lose track of incoming
Scud missiles. (1) The radar system doing a wide area search picks up the Scud missile. (2)
The radar system isolates the proposed target. (3) A software error causes the system to
produce a faulty range gate. The system loses track of the missile, because it does not fly
through this gate.
(Figure from
Science
255:1347. Copyright © 1992 by the American Association for
the Advancement of Science. Reprinted with permission.)
Why did the program scan the wrong range gate? The tracking system relied upon
getting signals from the system clock. These values were stored in a floating-point vari-
able with insufficient precision, resulting in a small mathematical error called a
trunca-
tion
. The longer the system ran, the more these truncation errors added up. The Patriot
missile system was designed to operate for only a few hours at a time. However, the sys-
tem at Dhahran had been in continuous operation for 100 hours. The accumulation of
errors led to a difference between the actual time and the computed time of about 0.3433
seconds. Because missiles travel at high speeds, the 0.3433-second error led to a track-
ing error of 687 meters (about half a mile). That was enough of an error to prevent the
missile battery from locating the Scud in the range gate area [24].
The Ariane 5 was a satellite launch vehicle designed by the French space agency, the
Centre National d'Etudes Spatiales, and the European Space Agency. About 40 seconds
into its maiden flight on June 4, 1996, a software error caused the nozzles on the solid
boosters and the main rocket engine to swivel to extreme positions. As a result, the rocket
veered sharply off course. When the links between the solid boosters and the core stage
ruptured, the launch vehicle self-destructed. The rocket carried satellites worth $500
million, which were not insured [25].
A board of inquiry traced the software error to a piece of code that converts a 64-bit
floating-point value into a 16-bit signed integer. The value to be converted exceeded the
maximum value that could be stored in the integer variable, causing an exception to be
