Joint Direct Attack Munition
The Joint Direct Attack Munition, usually abbreviated to JDAM, is an add-on guidance kit used by the United States Air Force and United States Navy to enhance the precision of conventional bombs. The JDAM components, made by Boeing, convert unguided (so-called "dumb") bombs into guided ("smart") bombs whose preliminary drop and target coordinates are set immediately before they are dropped from the aircraft. Guidance kits exist for various warheads; for standard Mark 8x bombs and compatible BLU-series warheads, for example, attached movable tail fins are used. Initially guidance was based on inertial navigation alone, but newer models also use GPS and may be laser guided. In the 2003 Iraq War, the majority of bombs dropped by the U.S. were JDAMs. Pure unguided gravity bombs are used less and less.
One of the classic principles of war is the utilization of "mass". Originally, this meant the number of soldiers that could be used in attacking a single point. Later on, it referred to the weight of artillery shells that could be delivered to a target. Precision-guided munitions, however, introduce a new way of regarding mass: many soldiers or shells are not needed, if an adequately destructive munition can be delivered to the exact place where it will do the most damage. Precision is a force multiplier.
|“||Admiral James O. Ellis, former commander, U.S. Strategic Command, said "We've migrated the number of sorties it will take to hit a target to the number of targets that one sortie can strike. ||”|
The basic JDAM uses an example of the go-onto-location-in-space guidance paradigm. Initially, it used only inertial guidance, but the GPS was added for additional precision. If GPS is not available, the JDAM still can use inertial guidance. In the GBU-53 and GBU-54 variants, a go-onto-target capability, adding terminal guidance, is optional.
While troops on the ground could not, at first, 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. Since the laser signals are encoded and each JDAM is told which code to follow, several JDAMs at a time can be released from a single aircraft. The designation signals can come from different aircraft or ground observers.
Yet another approach uses imaging radar to determine the target coordinates to be loaded into the bomb. This appears to be one of the functions of the AN/APS-149 Littoral Surveillance Radar System on the P-3 Orion and P-8 Poseidon maritime and overland surveillance aircraft.
Uses and economics
In Fiscal Year 2010, a typical JDAM kit cost approximately $30,000 exclusive of reusable aircraft equipment and the bomb itself, with expected price reductions into the $25,000 range. A high probability of destroying one target with one bomb, as well as other operational advantages, indicate it saves money in comparison to traditional bombs. There is an economic benefit in keeping the carrier aircraft further away from ground defenses, because the JDAM, a glide bomb, can be released further from the target than pure gravity (i.e., fee-fall) bombs.
Limits of unguided bombs
|War||Number of bombs||Number of aircraft||Circular error probability (CEP) (in feet)|
|World War II||9,070||3,024||3,300|
By the Gulf War, accuracy from medium altitude improved somewhat, but the inherent inaccuracies of "dumb" munitions obviated the "smarter" delivery platform. Dumb bombs had reached their limit of utility against point targets. No matter how accurate the release, for example, an unguided bomb, once released, could not compensate for sudden winds over the target. The PGMs available there used laser and television guidance, which had operational limitations, especially in the weather and land characteristics in Saudi Arbia and Iraq.
At first, JDAMs were used purely against targets predetermined by air planners, but there was an increasing need, critical in the Afghanistan War (2001-), to be directly responsive to the needs of forward air controller personnel with ground troops, to provide close air support. It had long been assumed that close air support needed direct target marking by a ground or air observer, typically with a laser. Another approach was to specify the target in relation to a beacon. Groiund personnel, had had the capability to use laser designators for the GOT model, but that required they stay in line of sight of the target, possibly exposing themselves.
Another model, more precise than the smoke grenade, was to place a radio or radar offset beacon near the target, but the troops still face the problem of precise angular and distance measurement from the beacon to the target. In the Afghanistan campaign of 2001, a new technique was adopted, only recently believed possible: ground-aided precision strike (GAPS). To put GAPS in practice, MG Daniel Leaf, USAF Director of Operational Requirements for Air and Space Operations said, in 2002, "If you had offered the B-1 with JDAMs in direct support of ground forces as a solution 10 years ago, I would have laughed heartily because it’s not what we envisioned." The JDAM's principal guidance mechanism is inertial, with a GPS correction option: a GOLIS model.
"CAS and GAPS operations do not care what color of airpower is delivering the weapons. Certain segments of the USAF wanted to break out the use of heavy bombers and term it “bomber CAS. However, at the joint CAS symposium held at Eglin, the Navy and Marine Corps were successful in not letting the Air Force call this by a different name.
"If heavy bombers are supporting ground troops in the traditional CAS role, then a name change for that aspect is not needed. [What is being discussed, however, is a new mission:] "Precision firepower called in by TACPs on the ground [is] GAPS and [needs its own doctrine]. The situation in Afghanistan was unique; there was not a large-standing opposing army that was conducting maneuvers to bring firepower to bear against our forces... Airpower was the maneuvering element that was supported by the small fire support teams on the ground. The small ground units have been instrumental in calling in the precise air strikes [especially when Army Special Forces were augmented with Air Force combat controllers]. This emerging mission goes beyond the joint definition of CAS.
General Chuck Horner, the joint air commander during Operation DESERT STORM, likened it to giving infantrymen a "2000 pound hand grenade" (i.e., a 2000 pound JDAM guided bomb) from a long-range bomber loitering overhead.
In the Air Force GAPS doctrine, ground forces were augmented with Air Force combat controllers, equipped with the Viper, which combined off-the-shelf Leica GeoSystems Viper laser range finder binoculars, with integral compass and inclinometer but no GPS, to triangulate targets in Afghanistan. The Viper is capable of a lasing distance from 25 meters to 4,000 meters. The unit runs off of a commercial camera battery. The Special Forces operator radioed their own location, as determined by a separate GPS, and gave Viper-derived coordinates relative to that position, to the bomber. 
Voice communication did not provide full situation awareness for all forces involved. The Viper system, however, allowed communications between one team and only one aircraft. More advanced systems allow network-centric warfare that can send the optimal aircraft to the target, using linkages with the Joint Tactical Information Distribution System (JTIDS), especially the Link-16  variant that can send information to fighters and Army Enhanced Position Location Reporting System (EPLRS) terminals.
An improved version is the the Long-Range Advanced Scout Surveillance System (LRAS3) for general surveillance, as well as detecting and designating targets. This device is routinely provided to Reconnaissance, Surveillance and Target Acquisition Squadrons of Brigade Combat Teams, as well as to special reconnaissance personnel. The LRAS3 gives 24-hour capability with the combination of:
- Forward-looking infrared (FLIR) sensor
- Global Positioning System
- eyesafe laser rangefinder
- Day TV (DTV) camera.
Where tactical situations permit, the ground troops can use a laser designator to provide terminal guidance to the later bombs, such as the GBU-51, which can home on a laser spot after midcourse guidance by GPS/INS. This also allows moving targets to be hit.
JDAM were first significantly used in Operation ALLIED FORCE in Kosovo in 1998; Operation SOUTHERN WATCH enforcing airspace over Iraq, the Afghanistan War (2001-); and in the Iraq War. In contrast to the 1991 Gulf War, where 95% of the bombs dropped were "dumb", 95% of those used in 2003 were guided. This widely used statistic, however, should be constrained by the realization that Gulf War tactics included large-scale "carpet bombing", largely for psychological effect, which was not appropriate to the Iraq War.
One of the errors of the Kosovo operation was accidental bombing of the Chinese Embassy with JDAMs. The JDAMs performed perfectly, hitting a building that intelligence agencies called a Yugoslav facility, but had been taken over by China. Smart bombs, unfortunately, cannot recognize flags or read signs on doors.
The early Afghanistan attempts, without a data link to the bomber, still required voice coordination to give the bomber the coordinates . This led to one "friendly fire" incident killed three United States Army Special Forces soldiers and wounded 19 others. A controller had been using a hand-held GPS receiver, whose battery failed. On replacing the battery, the unit reinitialized to show the controller's own position, not the offset from it he had been targeting. He passed the coordinates to a B-52 crew, who had no way of knowing it was the wrong position. They entered it as given, and the JDAM flew accurately and unfortunately onto its own controller's position..
A basic JDAM has several subsystems:
- Tail control subsystem, which contains the guidance electronics, the movable fins that actually steer the bomb, and the actuators that move the fins
- Strakes, or metal aerodynamic surfaces that give lift and stability, usually attached to the midsection of the bomb casing but sometimes to the nose
- Hardback, containing the shackles that hold the bomb to the carrying aircraft, as well as MIL-STD-1760 and MIL-STD-1553 electrical/electronic interfaces
Most commonly, a JDAM attaches to a hardpoint on the wing of a tactical aircraft, although it also can be placed in the bomb bay of larger aircraft.
There are a large number of JDAM designations, not all in service. The main model number (e.g., GBU-31 for a 2000 pound weapon) designates the bomb weight supported by the JDAM kit. Additional suffixes identify using service (Air Force or Navy), and versions of the guidance kit. The JDAM guidance kits, designated in the KMU-55x series, attach to bombs including 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's GBU-39 Small Diameter Bomb or the 500-pound MK 82/BLU-111 bombs.
While the Navy and Air Force versions are ballistically identical, the Navy versions usually have a wrinkled gray heat-resistant paint and internally use insensitive high explosive, while the Air Force versions have been painted green. For simplicity in logistics, the Navy variant is becoming the standard.
Operational use specifications also evolve. The higher and faster the point of launch, the longer is the range. Early ceilings were 45,000-plus feet (13,677 meters). Supersonic release at altitudes of 50,000 feet and above have been emphasized for the F-22 Raptor aircraft.
A variety of fuzes are used with JDAM, depending on precisely where the targeting specialist desires the bomb to explode. The most common fuze is of the FMU-139 series, some models of which are no longer in production, and some of which may be supplemented with optional sensors.  Fuze systems often are updated so consult current documentation to find the exact version in use. These fuzes are not necessarily used with laser-guided or advanced JDAMs.
- Instantaneous explosion on the ground, or slight delay (e.g., explode after then nose penetrates but the tail fins are still above ground): FMU-139
- Air burst above ground: FMU-139 with DSU-33 sensor
- Deep penetration into ground, concrete, or rock before penetration: FMU-157/B
In the drawing to the right, a U.S. FMU-139 fuze is shown, symbolically, within a common bomb case. The FMU-139 itself is roughly can-shaped, and contains the actual initiating explosive (i.e., detonator). The components screw into pre-machined wells in Mark 8x series conventional bomb cases. The FMU-139 proper can be set to detonate the bomb charge on impact, or with a slight delay that allows some surface penetration. If more complex penetration is needed, the FMU-157/B would replace it. If it is desired to have the bomb burst in the air, before it hits the ground, the optional DSU-33 proximity sensor is screwed into the nose. This sensor, as well as the main fuze, go to the safety switch, which may be a Mark 122 or FZU-38. Internal cabling connects these two or three subcomponents.
500 pound class
GBU-38 is the general designation for 500 lb class JDAM bombs with guidance kits from Boeing. At least two different types of warhead can be used with the 500 lb JDAM tailkits:
|Designation||Basic bomb||Bomb filler||Notes|
|GBU-38(V)*1/B||Mark 82 or BLU-111/B||TNT or PBXN-109||Air Force|
|GBU-38(V)*2/B||BLU-111/B||PBXN-109||Navy heat-resistant paint|
|GBU-38(V)*3/B||BLU-126/B||Reduced explosive PBXN-109 Low Collateral Damage Bomb; also has carbon fiber nonfragmenting bombcase||Air Force|
|GBU-38(V)*4/B||BLU-126/B||Reduced explosive PBXN-109 Low Collateral Damage Bomb; also has carbon fiber nonfragmenting bombcase||Navy heat-resistant paint|
The original, unguided Low Collateral Damage Bomb (LCDB) was developed by the U.S. Navy for use in situations where friendly forces and/or civilians are close to the target.  Even further reducing collateral damage are pure kinetic versions filled with concrete. 
1000 pound class
Current production JDAMs with 1000 lb class warheads are designated in the GBU-32(V)/B (and formerly also GBU-35(V)/B) series.
|Designation||Basic bomb||Bomb filler||Notes|
|GBU-32*(V)1/B||Mark 83||TNT replacement||Air Force|
|GBU-32*(V)1/B||BLU-110/B||PBXN-109||Navy heat-resistant paint|
2000 pound class
GBU-31 series designations use 2000 pound bombs. The 2000 pound bomb form factor is most commonly used for "cargo JDAM" derivatives with other than high explosive filler.
|Designation||Basic bomb||Bomb filler||Notes|
|GBU-31*(V)1/B||Mark 84||TNT replacement||Air Force|
|GBU-31*(V)2/B||BLU-117/B||PBXN-109||Navy heat-resistant paint|
|GBU-31*(V)3/B||BLU-109/B||PBXN-109||Air Force hard target penetrator|
|GBU-31*(V)4/B||BLU-109/B||PBXN-109||Navy hard target penetrator with heat-resistant paint|
There is a trend to make the bombs both more accurate, but, sometimes counterintuitively, to have smaller areas of effect. 
GPS supplement to intertial guidance
GPS was an enhancement to the original JDAM. Inertial navigation is always available. Once dropped, the JDAM navigates to the target without human input. When basic GPS supplementary data is available, the JDAM system has 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.
There are ways, however, to refine the GPS information and hence the precision, such as differential GPS. In the most common form of differential GPS, a precisely surveyed ground station broadcasts a correction to satellite information received in that part of the Earth. The correction signal may come from an earth station, or be rebroadcast by a satellite different from the GPS satellites.
One challenge is that while the bomb should be continuously updating, the GPS signals may not penetrate a metal internal bomb bay, or even the wing under which JDAMs are externally carried — the satellite signals, of course, come from above the metal wing. GPS retransmission, local to the aircraft, has been used to maintain synchronization. 
Both for increased accuracy and to deal with moving targets, a prototype terminal guidance seeker was evaluated in 1997-1998. Called DAMASK (Direct Attack Munitions Affordable Seeker), it used IIR (Imaging Infrared) seeker and associated control algorithms to improve the circular error probability to 3m (10 ft).
Terminal guidance, however, was abandoned until 2006, with a competition, by the Air Force and Navy, for a Dual Mode Guided Bomb (DMGB) program, which was won by Boeing's LJDAM that Precision Laser Guidance Set (PLGS) to JDAM guided bombs. The other two contenders were Lockheed Martin's DMGB and Raytheon's Enhanced Paveway II (EGBU). Inertial and GPS guidance still gets the JDAM to the target area (i.e., midcourse guidance), but it then follows a laser designator spot onto a potentially moving target.
Research and development deals both with launch aircraft capabilities, the use of "cargo JDAM" to deliver non-explosive payloads, and continuing improvements in terminal guidance.
Delivering large numbers of bombs
A recent demonstration showed the power of preprogrammed targeting for the JDAM: a B-2 Spirit stealth bomber, with intercontinental range, 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.
An initiative to clear land mines and other obstacles from beaches, and potentially other landing zones, called the Countermine System, uses JDAM technology. While the weapon looks like other JDAMs, it actually is one of the class of "Cargo JDAM" applications. A bombcase, whose ballistics match those defined in a JDAM guidance kit, is filled with approximately 4,000 darts and a dispenser. The darts are made of "chemical materials, high-explosives or reactive materials that neutralize the mine by deflagrating mine's explosive charge without detonation."
Continuing JDAM assault breaching into the water, research has been done on using a slightly modified GBU-31 to clear naval mines, with an explosive JDAM guided into the water to create a shock wave that will cause sympathetic detonation of the mines. The main change was replacing the pointed nose cone of the bomb with a flattened ogive MXU-735 nose cone.
A challenge in penetrating water, however, is both the physical survival of the guiding fins, and, as well, their hydrodynamic rather than aerodynamic behavior, stabilizing the bomb as it goes down.
Longshot JDAM torpedo
Another application, called Longshot, combines JDAM guidance with torpedoes. As with sea mine clearing, the hydrodynamics of the fins may be important. The combination, called Longshot, would allow high-altitude drops both from the P-8 Poseidon and the older P-3 Orion.
In development is the second-generation GBU-53 Small Diameter Bomb, which adds the Common Tri-Mode Seeker (CMTS) to the standard inertial and GPS, providing laser, imaging infrared, and radar guidance. It also has a unique explosive filler; the GBU-53 is not an add-on kit to an existing bomb, but still reuses JDAM technology.
We can drop bombs right onto 10 digit grid locations, but miss with logistics drops by half a mile. We need the equivalent of JDAM for logistics drops.
This Joint Precision Airdrop System is an initiative to adapt the guidance technology of JDAMs for droppping other than bombs. Even if it is not an exact match to JDAM technology, the fact that the comparison is made, the fact that the JDAM is the gold standard for cheap precision delivery, is significant. Some of the conditions in which precision supply dropping is required are very similar to the reasons why precision-guided munitions, including those with reduced collateral damage, are necessary:
- Enemy ground fire forces the aircraft to medium or high altitude, or the terrain is extreme and prevents low level flight
- High accuracy is needed, variously because the intended recipients are in close proximity to the enemy, or the drop has to take place in an urban area
- The drop needs to be clandestine, preferably from long range
- During a single sortie, the aircraft needs to deliver multiple payloads to multiple points
Specifics of implementation vary significantly from lethal JDAMs, but there are many guidance commonalities. Rather than fins on a bombcase, the delivery system is a steerable parafoil, guided by GPS and compass bearings, and with weather information provided. Laser guidance and more precise GPS, however, is being examined.
Non-US use and alternatives
Most U.S. allies use the JDAM rather than nationally developed bombs, although there are a good many laser-guided bomb products.
Russia builds its own bombs with satellite-assisted inertial guidance, very similar to the GBU-32: the KAB-500S/S-E series. It was first tested in 2000, and displayed in public in 2003. According to Carlo Kopp, "the cited CEP is 7 to 12 metres which is typical for this guidance type without differential GPS enhancement. The Kompas PSN-2001 (Pribor Sputnikovoy Navigatsii) satellite receiver is claimed to use 24 channels and be capable of using secure Glonass and GPS C/A signals. The weapon uses an impact fuse with three programmable modes."
- Steven L. Basham (April 2001), B-2A and 500 pound JDAM: A New Concept of Mass, Air Command and Staff College, Air University, pp. 11-12
- "Precision Engagement Redefining Warfare: Admiral Jim Ellis", Defender: Spotlight on National Defense Technologies: 2-3, 8, 2004
- Andreas Parsch, Boeing (McDonnell Douglas) JDAM (GBU-29/B, GBU-30/B, GBU-31/B, GBU-32/B, GBU-34/B, GBU-35/B, GBU-38/B, GBU-54/B, GBU-55/B, GBU-56/B)
- "JDAM: A GPS-INS Add-on Adds Accuracy to Airstrikes", Defense Industry Daily, 10 February 2011
- Theisen, Eric E. (2003). Ground-Aided Precision Strike Heavy Bomber Activity in Operation Enduring Freedom. Air University Press. Retrieved on 2007-11-12.
- Clancy, Tom (2000). Every Man a Tiger. Berkley Trade.
- Erwin, Sandra I. (April 2002), "Air Warfare Tactics Refined in Afghanistan: Planners, air crews fine-tuning targeting techniques and rules of engagement", National Defense Magazine. Retrieved on 2007-11-11
- JTIDS - Link 16. The Warfighter's Encyclopedia. Naval Air Warfare Center. Retrieved on 2007-11-11.
- Raytheon, Long-Range Advanced Scout Surveillance System
- David Liberatore (11 May 2006 accessdate = 2011-02-16). FMU-139C/B Electronic Bomb Fuze Design Update.
- FMU-157/B Hard Target Smart Fuze (HTSF), Globalsecurity
- James Dunnigan (7 October 2005), Why Concrete Filled Bombs are So Damn Useful, StrategyPage
- Haendschke, Ernie, Adding Less-Lethal Arrows to the Quiver for Counterinsurgency Air Operations
- Brian Paul (October 2010), "White Paper – GPS Retransmission for GPS Guided Munitions Delivery", GPS Source
- Air Force Link (August 27, 2008), Airmen employ laser joint direct attack munition in Iraq
- "Boeing Develops JDAM Based Countermine Weapon for the US Navy", Defense Update, 10 August 2008
- Jillene Marie Bushnell (December 2009), Master's Thesis: Tail Separation and Density Effects on the Underwater Trajectory of the JDAM, U.S. Naval Postgraduate School
- "Longshot: A Swooping HAAWC for Torpedos", Defense Industry Daily, 31 May 2007
- Erin Staine-Pyne (1 July 2009), Precision Airdrop Improvements, Air Materiel Command Industry Days
- Carlo Kopp (2009), "Soviet/Russian Guided Bombs", Air Power Australia, Technical Report APA-TR-2009-0806