In the 1991 Gulf War, the U.S. Air Force learned an awkward lesson. Its laser-guided bombs were superb in clear skies, but dust, smoke, fog and cloud could blind them, and most of the bombs American aircraft dropped were still unguided. Air & Space Forces Magazine records that a postwar analysis found those dumb bombs landed, on average, about 200 feet from their targets.

The answer was not a new bomb. It was a kit. The Joint Direct Attack Munition, or JDAM, bolts onto bombs the services already had in their magazines and turns them into weapons that steer themselves to a set of GPS coordinates, day or night, in any weather. Here is how a few thousand dollars of hardware makes an old iron bomb smart.

Three parts bolted onto an iron bomb

A JDAM is not a standalone weapon. Ground crews take an ordinary bomb body, such as the 500-pound Mk 82, the 1,000-pound Mk 83, or the 2,000-pound Mk 84 or BLU-109 penetrator, and add three things. At the back goes a new tail section carrying the guidance unit and movable fins. Along the body goes a strake kit, a set of small fixed fins strapped around the bomb that improve its stability and give it a little lift. Up front, the normal fuze stays where it was.

Once fitted, the weapon gets a new name. A Mk 84 with a JDAM kit becomes a GBU-31, a Mk 83 becomes a GBU-32, and a Mk 82 becomes a GBU-38. The bomb itself is unchanged, which is the whole trick: the services did not need to buy a new warhead, only the brain to steer it.

How it finds its target

The brain in the tail is an inertial navigation system, a package of gyroscopes and accelerometers that measures every movement of the bomb, aided by a GPS receiver. Before release, the aircraft passes its own position and velocity to the weapon, a process called transfer alignment, so the bomb knows exactly where it is at the moment it leaves the rack.

Target coordinates can be loaded before take-off, changed by the crew in flight, or generated by the aircraft's own sensors, such as a targeting pod. After release the JDAM is on its own. It compares where it is with where it needs to be, and the actuators in the tail move the fins to fly it there. There is no seeker in the nose looking for the target, which is why cloud and smoke do not matter.

An animated explainer of how a JDAM kit turns an unguided bomb into a precision-guided munition.

The U.S. Air Force puts the accuracy at a circular error probable of 5 metres or less when GPS is available. If GPS is jammed or lost, the inertial system alone keeps the bomb within 30 metres, provided it has been free-flying for no more than 100 seconds. The aircraft can release it from very low to very high altitude, in level flight, a dive, a toss or a loft, and several weapons can hit several targets in a single pass.

From Desert Storm to Kosovo

Research into an adverse-weather precision weapon began in 1992. In October 1995 McDonnell Douglas, later part of Boeing, was selected to build the kit, and Air & Space Forces Magazine reports a final unit price of about $18,000, under half the $40,000 ceiling the Air Force had set. The first kits were delivered in 1997, and operational testing ran in 1998 and 1999.

The combat debut came in 1999 over Serbia during Operation Allied Force, when Espíritu B-2 bombers flying from Whiteman Air Force Base in Missouri dropped more than 650 JDAMs. Air Force Magazine reported at the time that about 90% of B-2 bombs landed within 40 feet of their targets.

Eighty bombs in one pass

Because a JDAM needs only coordinates, one aircraft can attack many targets at once. On 10 September 2003, a B-2 over the Utah Test and Training Range released 80 inert 500-pound JDAMs in a single pass lasting about 22 seconds, each weapon guiding to its own aimpoint. The project pilot, Maj. William Power of the 419th Flight Test Squadron, called it truly revolutionary.

Footage of the 2003 test in which a B-2 released 80 inert 500-pound JDAMs on 80 separate aimpoints.

The family has kept growing. The Laser JDAM adds a laser seeker for moving targets while keeping GPS and INS as a fallback, and JDAM-ER bolts on a folding wing that greatly extends the glide range. Today the kit flies on almost everything in the U.S. inventory that can carry a bomb, from the F-16 y F-15E Strike Eagle hacia B-52, and Boeing has built hundreds of thousands of kits.

A 2,000-pound JDAM dropped by an F-35 during training, filmed from roughly 4,000 metres away.

The JDAM's real legacy is economic as much as technical. It made precision cheap enough to become normal, and it moved the hard part of bombing from the aircraft to the target list. As the engineer at Eglin put it, the limiting factor is no longer the bomb, it is how good the coordinates are.

Sources: U.S. Air Force JDAM fact sheet, Air & Space Forces Magazine (Grier 2006; Tirpak 1999), Boeing, U.S. Air Force news (2003), Wikipedia