Joint Direct Action Munition

From Citizendium, the Citizens' Compendium

In U.S. inventory, the primary guided bomb is the Joint Direct Action Munition (JDAM), whose basic Go-onto-location-in-space guidance is inertial, with the drop and target coordinates set by the aircraft just before drop. The JDAM proper is a guidance kit consisting of movable tail fins, which attach to a variety of standard Mark 8x series conventional bombs. Newer versions use supplemental GPS or laser guidance. All are unpowered precision-guided munitions.^[1]

While troops on the ground could not directly control a JDAM, they can use a GPS-equipped laser rangefinder to take a precise sighting on the target and radio the data to the bomber. A new laser-guided version, the GPU-54, replaces GPS with a laser seeker, and can home on a target, including a moving target, marked with a ground-based laser designator rather than a rangefinder, considerably increasing precision in good weather, when the ground personnel can see the target.^[2]

Variants

The JDAM guidance kit attaches to the 2000 lb BLU-109 hard-target-penetrator, the 2000-pound MK 84/BLU-117, the 1,000-pound MK 83/BLU-110, the Air Force'sGBU-39 Small Diameter Bomb and ^[3] or the 500-pound MK 82/BLU-111 bombs. There is also a Navy/Marine (BLU)-126/B Low Collateral Damage Bomb ^[4]

GPS supplement to intertial guidance

Once dropped, the JDAM navigates to the target without human input. In its most accurate mode, when GPS data is available, the JDAM system will have an error of less than 13 meters (about 40 feet). Since it does not rely on external commands, it cannot be jammed -- the worst case would be that jamming GPS would reduce its accuracy to that of the inertial-only guidance, 30 meters.

Longshot JDAM torpedo

Another application, called Longshot, combines JDAM guidance with torpedoes. The combination, called Longshot, would allow high-altitude drops both from the P-8 Poseidon and the older P-3 Orion.^[5]

Laser JDAM

In this version, the GPS supplementary guidance is replaced by a laser seeker. Inertial guidance still gets the JDAM to the target area, but it then follows a laser designator spot onto a potentially moving target.

Called the GBU-54, it has been tested on F-16 Fighting Falcon and now B-52 aircraft, and has been used operationally in Iraq in 2008. ^[6]

Future

A recent demonstration showed the power of preprogrammed targeting for the JDAM: a B-2 Spirit stealth heavy bomber released 80 JDAMs in 2 seconds, each aimed at a different target up to 15 miles away.

The higher and faster that a basic guided bomb can be dropped, the farther the dropping aircraft can be from the target -- and safer from its defenses. This is one of the reasons that the attack version of the F-22 Raptor can drop from at least 50,000 feet and at supersonic speed, which gives it a much greater JDAM range than a current fighter-bomber or heavy bomber.

References