How much accuracy does a smart bomb need? It depends on what you are trying to hit. If it's a small target, like the upright supports of a bridge or the mouth of a cave, you need a bomb that will land within ten feet of where you aim it. If it's just a large building or a bunch of troops or tanks on the ground, landing within a hundred feet of the "aiming point" will do it. 

During the 1991 Gulf War, the most accurate guided bombs were the GBU-15s, that could hit within ten feet of where they were aimed. This was done by putting a TV camera in the nose of the bomb and having a weapons officer in an aircraft overhead literally fly the bomb into the target. Often this would be an open window, or mouth of a cave. Unfortunately, these weapons were expensive (nearly $300,000 each) and only 2800 were built. Since then, 1200 have had GPS added so that the bomb can be dropped through clouds and automatically glide close enough to the target so the operator can see it and guide the bomb the rest of the way. The next most accurate are the laser guided bombs, which will land within 30 feet of where they are aimed. But lasers don't operate well in mist, fog, smoke or sand storms. Depending on the size of the bomb (and extra features), laser guided bombs cost from $70-150,000 each. Most are under $100,000. In the 1990s, GPS guided bombs (JDAM) were introduced. These cost about $20,000 each had would land within 40 feet of their target (or 100 feet if GPS is jammed and the bomb has to rely on the back up inertial guidance system.) The U.S. Navy has developed a terminal guidance device (Damask) for JDAM which uses a picture of the target (taken previously with a digital camera and copied into the memory of Damask) and literally looks for anything on the ground that looks like that picture and heads right for it. This allows the smart bomb to land within ten feet of the target. Damask has been successfully tested, but not yet put into production. It would add $12,000 to the cost of a JDAM. There are also several similar terminal guidance systems competing for inclusion in JDAM bombs,

Most of the smart bombs being built right now (over 3,000 a month) are JDAMS. There are thousands of laser guided bombs still available, and these are being used up as frequently as the situation allows. But bombing accuracy may become a major issue if U.S. forces find themselves involved fighting Iraqis inside a large city like Baghdad. While the troops have had a lot of training in urban fighting, and have received a lot of special equipment (battering rams, special explosive charges for blowing holes in the sides of buildings and 62 ton armored bulldozers) smart bombs will also play a role. The smaller JDAMs (500 pounds) can be used to take down parts of buildings containing enemy troops, and the larger JDAMS (2000 pounds) can be used to flatten buildings that are away from other structures. But accuracy is important. Miss the target and hit the hospital or apartment house next door, and you have a major public relations problem with all the dead civilians. Plus, the bad guys are still shooting at you. The Iraqis, following the advice of their Serb friends (who we bombed in 1999) have dug trenches around Baghdad, filled them with oil and set them afire. This creates a wall of black smoke that blocks the laser beams needed by laser guided bombs. The Iraqis may also have GPS jammers. We may also have GPS guidance systems that can defeat some forms of GPS jamming. For obvious reasons, the U.S. Air Force won't talk about this. But if GPS is jammed, the hundred foot accuracy of JDAMs back up inertial guidance systems may not be good enough for supporting troops fighting inside the city.