This material excerpted from a report by the United States General Accounting Office, Information Management and Technology Division, in Washington, D.C., February 4, 1992
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On February 25, 1991, a Patriot missile defense system operating at Dhahran, Saudi Arabia, during Operation Desert Storm failed to track and intercept an incoming Scud. This Scud subsequently hit an Army barracks, killing 28 Americans. This report responds to your request that we review the facts associated with this incident and determine if a computer software problem was involved.
The Patriot battery at Dhahran failed to track and intercept the Scud missile because of a software problem in the system's weapons control computer. This problem led to an inaccurate tracking calculation that became worse the longer the system operated. At the time of the incident, the battery had been operating continuously for over 100 hours. By then, the inaccuracy was serious enough to cause the system to look in the wrong place for the incoming Scud.
Two weeks before the incident, Army officials received Israeli data indicating some loss in accuracy after the system had been running for 8 consecutive hours. Consequently, Army officials modified the software to improve the system's accuracy. However, the modified software did not reach Dhahran until February 26, 1991--the day after the Scud incident.
The Patriot system was originally designed to operate in Europe against Soviet medium- to high-altitude aircraft and cruise missiles traveling at speeds up to about MACH 2 (1500 mph). To avoid detection it was designed to be mobile and operate for only a few hours at one location.
The Patriot's weapons control computer used in Operation Desert Storm is based on a 1970s design with relatively limited capability to perform high precision calculations.
The Patriot's radar sends out electronic pulses that scan the air space above it. When the pulses hit a target they are reflected back to the radar system and shown as an object (or plot) on the Patriot's display screens.
After the Patriot's radar detects an airborne object that has the characteristics of a Scud, the range gate--an electronic detection device within the radar system--calculates an area in the air space where the system should next look for it.
The range gate's prediction of where the Scud will next appear is a function of the Scud's velocity and 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 time 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.
On February 11, 1991, the Patriot Project Office received Israeli data identifying a 20 percent shift in the Patriot system's radar range gate after the system had been running for 8 consecutive hours. This shift is significant because it meant that the target (in this case, the Scud) was no longer in the center of the range gate. The target needs to be in the center of the range gate to ensure the highest probability of tracking the target.
Patriot Project Office officials said that the Patriot system will not track a Scud when there is a range gate shift of 50 percent or more. Because the shift is directly proportional to time, extrapolating the Israeli data (which indicated a 20 percent shift after 8 hours) determined that the range gate would shift 50 percent after about 20 hours of continuous use. Specifically, after about 20 hours, the inaccurate time calculation becomes sufficiently large to cause the radar to look in the wrong place for the target.
On February 25, Alpha Battery had been in operation for over 100 consecutive hours. Because the system had been on so long, the resulting inaccuracy in the time calculation caused the range gate to shift so much that the system could not track the incoming Scud. Consequently, Alpha Battery did not engage the Scud, which then struck an Army barracks and killed 28 American soldiers.
Effect of Extended Run Time on Patriot Operation
Hours Seconds Calculated Time (Seconds) Inaccuracy (Seconds) Approximate Shift In Range Gate
(Meters) 0 0 0 0 0 1 3600 3599.9966 .0034 7 8 28800 28799.9725 .0275 55 20(a) 72000 71999.9313 .0687 137 48 172800 172799.8352 .1648 330 72 259200 259199.7528 .2472 494 100(b) 360000 359999.6667 .3433 687
Calculated Time (Seconds)
Approximate Shift In Range Gate (Meters)
a. Continuous operation exceeding about 20 hours target outside range gate
b. Alpha Battery ran continuously for about 100 hours