The U.S. Air Force continues development of its innovative, but infrequently used, 250 pound SDB (Small Diameter Bomb). The latest improvement is an encrypted data link, that will enable the SDB to hit moving targets. The new communications capability enables the SDB movement to be controlled via the air force's airborne Internet (Link 16). This capability is one of the main improvements to be seen in SBD II, which wont enter service for another year or so.
Meanwhile, existing SDBs have received a software upgrade that enables them to be used like a JDAM (GPS guided bomb). That is, it can now, with the new software, be dropped from an aircraft while directly above the target. The SDB was built as a glide bomb, which was dropped ten or more kilometers from the target, then glided for a bit before diving on the target. This resulted in complaints from troops below, who had to wait longer for a SDB to hit. The SDB was often preferred, especially in urban areas, because it had less bang than a 500 pound JDAM. But not when it took so long to arrive.
It was four years ago that the air force finally got the SDB into service, in Iraq. The SDB was supposed to enter service in 2005, in the wake of the 2004 introduction of the 500 pound JDAM. But there were many technical problems with the SDB. That's because this was not just another "dumb bomb" with a GPS guidance kit attached. The SDB had a more effective warhead design and guidance system. It's shape is more like that of a missile than a bomb (nearly two meters, as in 70 inches, long, 190 millimeters in diameter), with the guidance system built in. The smaller blast from the SDB resulted in fewer civilian casualties. Friendly troops can be closer to the target when an SDB explodes. While the 500, 1,000 and 2,000 pound bombs have a spectacular effect when they go off, they are often overkill. The troops on the ground would rather have more, smaller, GPS bombs available. This caused the 500 pound JDAM to get developed quickly and put into service.
The new F-22 and F-35 warplanes normally carry their bombs internally. This limits how many they can carry, but with the SDB, an F-22 can carry eight of them. The non-stealthy Navy F-18 could easily carry 24 SDBs. The SDBs are carried on a special carriage, which holds four of them. The carriage is mounted on a bomber just like a single larger (500, 1,000 or 2,000) pound bomb would be.
The SDB is basically an unpowered missile, which can glide long distances. This makes the SDB even more compact, capable and expensive (about $70,000 each.) JDAM (a guidance kit attached to a dumb bomb) only cost about $26,000. The small wings allow the SDB to glide up to 70-80 kilometers (from high altitude.) SDB also has a hard front end that can punch through several feet of rock or concrete, and a warhead that does more damage than the usual dumb bomb (explosives in a metal casing.) The SDB is thus the next generation of smart bombs.
There was never much use for 250 pound dumb bombs, as they would be too inaccurate to be useful. So it made sense to merge the guidance kit and the bomb itself. But the superiority of guided bombs is such that the next generation of heavier (500-2000 pound) smart bombs will probably be like the SDB. But in the meantime, the air force was reminded that the troops on the ground don't care about the fancy tricks your new bombs can do, but more about how fast you can get the bomb on the target.
Meanwhile, the army has introduced GPS guided rockets and 155mm shells. These, plus the existing Hellfire missiles, have led to fewer calls for air force smart bombs.