Air Weapons: Brainier Bunker Busters


August 31, 2007: The U.S. Air Force has begun upgrades to 279 of its GBU-28 bombs, making their guidance systems harder to defeat. In essence, the U.S. military's best bunker-buster will become even deadlier.

The GBU-28 was developed in less than a month during Operation Desert Storm in 1991, and two prototypes were dropped on a high-value target, destroying it. It is based around a BLU-113 warhead, and can penetrate 100 feet of earth. Originally designed to be dropped by the F-111, it is now primarily dropped from F-15Es and B-2s. A GPS-guided version, the GAM-113 or GBU-37, is also in service.

So why is the Air Force upgrading the laser-guided variant, which is an older (1970s) technology than GPS (from the 1990s)? The answers are simple: It is more accurate than the GPS-guided munitions (which have a circular error probable, or CEP, of 33 feet). Laser-guided bombs often have a CEP measured in inches. The other reason is that the Air Force is able to have more than one way to precisely hit targets. This is because each of the systems that make precision-guided weapons work has its own set of strengths and weaknesses.

Laser-guided weapons are best used in clear weather, or when you need to exactly hit a smaller target - like dropping a bomb down a bunker's ventilation shaft. The seeker locates the laser spot, and a computer steers fins on the tail of the bomb to chase the spot - much like a kitten chases the spot from a laser pointer. Only this chase ends with a boom. There's a problem. Bad weather can make it hard to properly target a laser. Or, you've bombed other stuff, and there's now a lot of smoke in the target zone. All the sudden, the laser can't quite lock onto the target… and you end up missing. Not by much, but it could be enough.

The GPS guidance has no problem. Instead of looking for a spot from an aircraft or a ground team, the latitude and longitude coordinates of the target are entered. The GPS system on the bomb takes the bomb's present position, and then figures out how to get there - like a navigation system in a cell phone or a car. The good news is that it works in all weather. However, GPS jammers are being used - and that forces a GPS-guided bomb to go to a backup inertial navigation system, which results in a 100-foot CEP. If the target moves, then the GPS-guided bomb will miss as well.

Efforts have been made to combine the systems. JDAMs are now receiving laser seekers that will give them a capability against moving targets, and an accurate backup in case someone is trying to jam GPS. The laser-guided bombs are being given GPS so that they will still impact close to the target - when a 2,000-pound bomb goes off in a near-miss, it still can do a lot of damage. In essence, the Air Force is starting to merge their precision munitions, making it more likely that a time-sensitive target will be hit before it is too late. - Harold C. Hutchison (


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