Reply #7: "US Looking for suckers in S. Asia"

[bemildred]

A reality check on doings in the High-Tech wonderland at Raytheon.

The Patriot Missile System is an example of how a software project can change over time. It was proposed as a way to protect against Soviet planes, then soviet missiles, and twenty years later, was used against Iraqi missiles. The system was meant to be temporary, but was used in permanent settlements. If a system can be patched to perfection, then the Patriot was an attempt to prove this point.



The Patriot Missile project began in the late sixties as a portable system meant to protect local airspace against intrusion by enemy aircraft. One of the selling points of the project was that although it would not be programmed to shoot down missiles, the system could later be altered to serve as an Anti-Ballistic Missile System. The system was first tested against a drone plane in 1974, and later, in 1986, the system’s developers at Raytheon Labs modified it to be used as a portable, short-term defense against Soviet Missiles and Aircraft. <1>

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The reason this bug occurred is because of a problem with storing time in a 24-bit register. The problem is that time is stored to an accuracy of 1/10th of a second, but a 24-bit register does not have enough precision to store 0.1, so a small fraction of each second is lost. The result is that the register used to keep track of time is off by 0.0001% of the amount of time that the system has been in operation. The problem is that computers do not store information as a standard decimal. Instead, they use binary code, which can not accurately store 1/10th of a second.

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Part of the reason this error was not found sooner was that the program was written in assembly language, 15 years earlier. Over time, it was patched and new things were added. In short, because the system was written in assembly code, it was difficult to understand and maintain. And because the system was fifteen years old and had been patched several times, the very people who had written the code were not as familiar with the code as they would be if it were written more recently. Then, during the gulf war, the system had to be modified to handle the SCUD missiles, and time was a critical factor. The developers could have been influenced by the fact that prolonged testing could have caused a disaster by keeping a necessary system out of the hands of soldiers in a time of war.

http://seeri.etsu.edu/SECodeCases/ethicsC/PatriotMissile.htm