Global positioning system (GPS) data is so accurate that weapons using it for guidance normally hit within three feet of a target, meaning that even a small boom can ruin your day. So why would the Air Force be adding older technology in the form of a laser seeker to an already GPS-equipped bomb?
The answer is simple: The LJDAM is more accurate than the GPS-only guided munitions, which have a circular error probable (CEP) of 33 feet. Laser-guided bombs often have a CEP measured in inches. The other reason is redundancy—the ability to strike targets more precisely with more than one guidance system. Each of the guidance systems that make precision-guided weapons precisely that—precision guided, has its own set of strengths and weaknesses.
Laser-guided weapons are best used in clear weather, or when you need to strike 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. But there’s a problem. Bad weather can make it difficult to properly target a laser. Or, you’ve bombed other targets, and there’s now a lot of smoke in the target area. All of a 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 with target obscuration. Instead of looking for a laser spot from an aircraft or a ground team, the air crew enters the target’s latitude and longitude coordinates in their on-board computer. The GPS system on the bomb knows the bomb’s present global position, and then figures out how to get to the target’s global position – 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.
Until the GBU-54 entered service, American warplanes carried a mix of laser-guided or GPS-guided bombs. This was a bit of a problem, since some of these weapons would be dead weight. Bad weather (or the smoke from a target that has already been bombed) can defeat attempts to guide bombs with a laser. GPS jammers, or a moving target, render GPS-guided bombs useless.
By using both guidance systems, the GBU-54 brings the best of both worlds to bear against terrorists and other malefactors.