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How JDAM Bombs Work

We've heard a great deal about those precision JDAM bombs, but how do they actually work? JDAM stands for Joint Direct Attack Munition -- the joint referring to the Air Force and Navy, the two branches of service that use it.

The kits, built by Boeing Integrated Defense Systems, consist of a tail section that attaches to the rear of either a 2,000 or 1,000 pound bomb. A kit for a 500 pound bomb will be released later this year. The kit has four fins -- one is fixed and the other can be moved to position the bomb. There is also a small structure that wraps around the mid section of the bomb to provide stability. The kit itself costs about $20,000 plus the cost of the bomb.

The JDAM is controlled by a small computer that uses a Motorola PowerPC processor -- the same processor used by Apple in the Power Macintosh. The software is loaded by Boeing before the device is shipped but the devices are tested and, if necessary, updated before they are loaded onto the aircraft, according to a Boeing spokesman.

That onboard computer is attached to an Inertial Measurement Unit or IMU. The IMU tracks the movement of the bomb after it leaves the aircraft and updates the computer as to its exact whereabouts. To assure accuracy there is also a Global Positioning System or GPS unit that uses satellites to track the location of the bomb which is used to update the Inertial Measurement Unit.

Prior to launch the bomb's computer is in sync with navigation systems onboard the aircraft. During this period the bomb's Inertial Navigation system is tested against the aircraft's navigation systems to be sure it's tracking accurately. Airborne computers also check the status of the bomb's batteries and other systems. Any failures will disable the bomb, requiring the crew to replace it with another unit.

Once the bomb is launched it takes about 30 seconds for the Global Position System to get a fix on its exact location. From there, the GPS, Inertial Measurement Unit and the onboard computer send signals to bomb's three movable fins to make any necessary adjustment to guide the bomb to its target.

A syndicated technology columnist for nearly two decades, Larry Magid serves as on air Technology Analyst for CBS Radio News. His technology reports can be heard several times a week on the CBS Radio Network. Magid is the author of several books including "The Little PC Book."

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