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[1.0] Evolution Of US Smart Bombs

v1.0.0 / chapter 1 of 2 / 01 nov 16 / greg goebel

* The Vietnam War provided the impetus for American development of smart bombs, producing early electro-optic guided bombs and laser-guided bombs. From the 1990s, the US followed up such smart munitions technologies with weapons using the Global Positioning System (GPS) navigation satellite constellation for precision guidance.

2,000-pound JDAMs on USS HARRY TRUMAN


[1.1] WALLEYE EOGB
[1.2] HOBOS EOGB
[1.3] PAVEWAY LGB
[1.4] LASER TARGETING SYSTEMS
[1.5] GAM & JDAM

[1.1] WALLEYE EOGB

* One of the biggest problems with the radio-guided glide bombs of World War II was the crudity of electronic technology of the era. TV and infrared sensor technology were in their infancy, hampered by poor performance and lack of reliability. By the 1960s they had matured, with the new solid-state electronic technology allowing weapon systems to be much more reliable and compact.

The US air war over Southeast Asia started in earnest in 1964, quickly demonstrating that existing weapon systems left something to be desired. Pilots performing strikes on North Vietnam's bridges -- major targets in the attempt to cut the flow of weapons and supplies to Vietcong insurgents in South Vietnam -- found them to be very difficult targets, both hard to hit and hard to damage when they were hit. The AGM-12 Bullpup air-to-surface missile (ASM) turned out to be almost useless. The Bullpup was no great improvement over the radio-guided bombs of World War II; like them, it was also radio-guided, with a pilot directing it with a hand controller, tracking the missile by a flare in the tail. It was not a scheme that lent itself to accuracy, and the Bullpup's warhead was too small to cause much damage, even when a hit was scored.

Better solutions were needed. Development of improved guided bombs during the 1960s was conducted along two paths: weapons guided by television or infrared sensors, or "electro-optic guided bombs (EOGB), and weapons that zeroed in onto reflections from a laser beam reflected by a target, or "laser-guided bombs (LGBs).

US Navy work on an EOGB led to the Martin Marietta "Walleye". The Navy had begun investigation on the weapon in 1963, with a production contract awarded to Martin's division in Orlando, Florida, in early 1966. The weapon was designated the "AGM-62", where "AGM" meant "Air-to-Ground Missile" -- though that was misleading, a glide bomb wasn't really a "missile" since it wasn't powered.

The "Walleye I" had four cropped delta wings arranged in a cruciform pattern, with control fins at the trailing edge; a vidicon-tube TV camera in the nose; a spinner in the tail to drive a hydraulic pump and electrical generator; and a 375-kilogram (825-pound) warhead. The Walleye I was 3.44 meters (11 feet 4 inches) long, with a diameter of 32 centimeters (12.5 inches); a wingspan of 1.16 meters (3 feet 10 inches); and a weight of 500 kilograms (1,100 pounds).

The Walleye I went into combat in early 1967, being carried by Navy Douglas A-4 Skyhawk light attack aircraft in early 1967. Results were impressive: in the first strike, a Skyhawk pilot sent a bomb directly into a window of a barracks building. Of 68 Walleyes dropped over a period of seven months, 65 were precisely on-target.

Walleye

In operation, the image provided by the Walleye's seeker was displayed on a TV screen in the drop aircraft. The operator locked the crosshairs on the display onto a high-contrast scene element, such as a window or a door; set the warhead fuzing option as desired; and dropped the weapon. The Walleye would then guide itself into the target, zeroing in on the target contrast pattern.

The seeker used an analog pattern-matching scheme, with brightness along the horizontal and vertical axes of the target image converted into dual analog electrical waveforms. The seeker control electronics would then detect the transitions in the waveforms corresponding to abrupt changes in scene brightness, and use them as "goalposts" to sent the missile into its target. It was a "fire-&-forget" weapon, able to perform a "terminal attack" on its own.

The seeker needed to have a stable high-contrast pattern to work properly; it could break lock if the day was gray and hazy, or there were shifting clouds or sources of transient sun glare. In such cases, "fire-&-forget" actually turned out to mean that the weapon was fired, and then forgot where it was going. However, when the weapon worked, which was well more often than not, it worked very well, with accuracies of a few meters.

The Navy was enthusiastic enough about the Walleye to also configure A-6 Intruder and A-7 Corsair II strike aircraft to carry the weapon, and develop a 900-kilogram (2,000-pound) "Walleye II", nicknamed "Fat Albert". The Walleye II was built by Hughes under subcontract to Martin Marietta. It looked like a scaled-up Walleye I, being 4.04 meters (13 feet 3 inches) long, with a diameter of 46 centimeters (18 inches), and a wingspan of 1.3 meters (4 feet 3 inches). The Walleye II went into service in 1974.

The Walleye II was followed in 1975 by the "Extended Range / Data Link (ERDL)" Walleye, which featured bigger wings for longer glide range and a more sophisticated guidance and control system. Most of the Walleye I and II weapons were upgraded to "Walleye I ERDL" and "Walleye II ERDL" configuration. The ERDL system allowed an operator to monitor the Walleye's seeker over an "AN/AWW-9" (later "AN/AWW-13") radio datalink pod, and change the target lock after launch if necessary.

The controlling aircraft wasn't necessarily the same as the launch aircraft; it was often better to have one aircraft stand off from the target and guide the bombs, while others released them and got out of the target area. Such "buddy targeting" not only enhanced effectiveness and increased crew safety, but also meant that not all the strike aircraft needed to be fitted with the datalink pod. An updated "digital phase-shift keying (DPSK)" guidance system with a more reliable communications channel was retrofitted in the 1980s, resulting in the "Walleye I ERDL DPSK" and "Walleye II ERDL DPSK".

The Walleye was also obtained in small quantities by the USAF, as well as the Israeli Air Force. The Israelis used Walleye IIs in the Yom Kippur War in 1973 with mixed results; to then come up with a modified version, the "Tadmit", with an improved seeker and a booster rocket, that was used successfully in the 1982 war in southern Lebanon. The US Navy used Walleyes to good effect in the First Gulf War, dropping 124 of them on Iraqi targets.

The weapon was finally withdrawn from service in 1995, at the time the A-7 was retired, no other Navy aircraft being qualified for the weapon. About 5,000 Walleyes of all types were produced, including inert training rounds were built, and even a nuclear-armed Walleye, the "Guided Weapon Mark 6". It doesn't appear the nuclear Walleye ever reached operational status.

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[1.2] HOBOS EOGB

* In 1967, the US Air Force, working with Rockwell International, began development of their own EOGB. The first weapon in this series materialized in combat over Southeast Asia in 1969 as the "GBU-8/B HOBOS (Glide Bomb Unit 8, Homing Bomb System)".

Unlike Walleye, HOBOS was a modular system. In the postwar period, the US Navy conducted research on developing a system of more streamlined bombs for carriage on the faster jet aircraft then emerging, resulting from the 1950s in the "Mark 80" series of "Low Drag General Purpose (LDGP)" bombs. These munitions, known as "slick bombs" (as opposed to the WW2 "fat bombs"), are spindle-shaped munitions, filled with about 50% explosive by weight. Four variants were introduced, going into service with the Air Force, Navy, and Marines:

Boxing categories are used here as informal labels for the weight class. A 900-kilogram (2,000-pound) "penetrator" bomb, the "BLU-109/B", was added to the set in the 1980s -- "BLU" meant "Bomb Live Unit", by the way. It featured a thick, hard casing, allowing it to penetrate reinforced concrete bunkers and such. The fuze was fitted to the tail -- a better location than the nose for a penetrating weapon -- and was set for delayed action, so the bomb wouldn't detonate until after it had broken into a bunker.

In any case, these bombs were modular, in that they could be fitted with different nose and tail kits. This proved handy in development of guided bombs, allowing any of the bombs to be converted into guided munitions by addition of a "smart" guidance kit.

HOBOS consisted of a kit that could be fitted to a Mark 84 heavy bomb -- or it seems, on occasion, to a "Mark 118" 1,350-kilogram (3,000-pound) bomb, an older munition, not part of the LDGP set, this smart munition being designated "GBU-9/B". The kit included a tailfin section that had four square fins, containing contained a battery, plus control and communication electronics; and a nose section that contained an EO seeker system. The two sections were linked by four long slender fins, or "strakes", and an umbilical conduit that ran along the length of the bomb to electronically link the tailfin and nose sections. A HOBOS weapon based on the Mark 84 slick bomb was 3.78 meters (12 feet 5 inches) long, had a span of 1.12 meters (3 feet 8 inches), and weighed 1.016 tonnes (2,240 pounds).

HOBOS

The EO seeker head was based on either TV or imaging infrared technology, though it appears there were a number of experiments with other options. Like the Walleye, the seeker in principle had a fire-&-forget capability, in which the operator could lock it onto a target, with the bomb then directing itself into the target. Also like the Walleye, occasionally "fire-&-forget" meant the weapon would forget where it was going -- but when it worked, it worked well.

* After the end of the war, the Air Force and Rockwell continued development of the weapon through the 1970s, eventually coming up with the better "GBU-15/B EOGB", originally the "AGM-112" -- the TV-guided "GBU-15(V)1/B" becoming operational in 1983, and the infrared-guided "GBU-15(V)2/B" following two years later. The weapon was fielded with the USAF and the Israeli Air Force, with the Israelis using it effectively in Lebanon in 1982.

The GBU-15/B was actually a family of weapons based on a kit of standard components, known as the "modular guided glide bomb" system. The Air Force also worked with Hughes to build a glide bomb with a pair of "switchblade" pop-open wings for extended glide range, but this "GBU-20/B Planar Wing Weapon (PWW)" never reached production.

The GBU-15/B was conceptually similar to HOBOS, with tail and nose sections strapped to a heavy Mark 84 GP bomb. The kit could be attached to some other munitions, and was also evaluated with a cluster munition canister, though that option never went into service. The standard GBU-15/B had a length of 3.5 meters (12 feet 10 inches), a wingspan of 1.49 meters (4 feet 11 inches), and a weight of 1.125 tonnes (2,500 pounds).

The EO seeker permitted fire-&-forget operation or, unlike the GBU-8/B, command guidance through an AN/AXQ-14 or the later AN/ZSW-1 datalink. Datalink guidance allowed the bomb to be released from above an overcast layer, with the bomb falling under command guidance until it penetrated the cloud layer, where it then locked on to the target.

GBU-15

The GBU-15/B had a distinctively different appearance from HOBOS, however, with triangular nose fins on the seeker section and large truncated-delta fins on the tail section, both sets of fins being arranged in a cruciform pattern. As a result, the GBU-15/B was sometimes referred to as the "cruciform wing weapon (CWW)". The bigger fins gave the GBU-15/B a longer glide distance than HOBOS.

The GBU-15/B was carried by the F-15E Strike Eagle and, while it was still in service, the F-111 Aardvark. The F-111 made very effective use of the GBU-15/B during the First Gulf War, dropping 71 to seal off oil manifolds wrecked by the Iraqis to spew out oil, and other targets.

GBU-15/B kits were developed that featured smaller "short chord" wings; these kits could be used on the BLU-109/B 900-kilogram (2,000-pound) penetrator warhead -- which, for whatever reasons, could not be fitted with the long-chord wings. This gave a "Chinese menu" of possible weapon configurations:

In 1999 and 2000, many GBU-15 munitions were updated with a midcourse guidance system, based on the US military's Global Positioning System (GPS), which involves a constellation of satellites that broadcast signals to allow a GPS receiver on Earth to pinpoint its location to within roughly ten meters. Since GPS broadcast power is low, GPS is easily jammed, and so weapons with GPS also need a backup inertial navigation system (INS) -- with lower accuracy -- to keep them on track if GPS is jammed.

GPS-INS provided a highly-reliable fire-&-forget guidance system, if at some reduction in accuracy compared to LGBs -- though LGBs don't work well in murky or dusty conditions. In any case, GBU-15s with GPS-INS guidance were introduced to combat in the First Gulf War. It is unclear how many GBU-15s remain in US service, but they do apparently persist in inventory for the time being.

* There is of course a fine line between a glide bomb and an air-to-surface missile -- with this line being crossed by the "AGM-130" ASM, a direct derivative of the GBU-15 EOGB.

It consists of a heavyweight Mark 84 GP or BLU-109/B penetrating bomb, body with a short-chord wing kit and a solid-rocket booster to give it considerably extended range, roughly 64 kilometers (40 miles). A terrain-following guidance system based on a radar altimeter is used for midcourse navigation to the target. This would appear to be a simple enough concept, but the weapon's development was apparently troublesome. Although development began in 1984, the AGM-130 didn't become operational until after the First Gulf War.

Production versions of the AGM-130 featured a GPS-INS subsystem backing up the terrain-following navigation. The optical seeker was updated to include a 256-by-256 pixel cadmium-telluride infrared imaging array, along with a CCD-based television imager.

The AGM-130 saw its first use in combat in the air strikes conducted by the US military against Iraqi air-defense sites that began in December 1997. Ten were fired, but in late January 1998 one of the AGM-130s missed its intended military target and hit a residential area, suggesting that not all the bugs had been worked out. They were employed again in the Afghanistan war in 2001, being used to perform precision strikes into the entrances of caves used as hideouts by al-Qaeda terrorists. There was talk of improvements to the AGM-130, but only about 500 were built, and there seems to be little activity in the weapon today. It does appear to still be in inventory.

AGM-130

* Boeing, which eventually became the owner of the GBU weapons and the AGM-130 through the US defense industry consolidation binge of the 1990s, worked with the US Air Force on a turbojet-powered variant of the AGM-130, known informally as the "AGM-130TJ", to provide a longer-range and lighter weapon that could be carried on F-16 fighters as well as F-15Es.

The AGM-130TJ was smaller than the original AGM-130, with weight cut from 1,360 kilograms (3,000 pounds) to 1,072 kilograms (2,365 pounds), and was powered by a French-designed Microturbo small turbojet engine. Range was over 120 kilometers (80 miles), twice that of the original AGM-130. The AGM-130TJ followed studies performed some years earlier on a turbojet-powered derivative of the AGM-130 for the British CASOM (Conventionally Armed Stand-Off Munition) competition, as well as more recent experiments with a lighter version of the AGM-130 powered by a solid-rocket motor. A USAF F-15 performed the first test launch of the AGM-130TJ in September 1998, but the USAF decided not to pursue development of the weapon.

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[1.3] PAVEWAY LGB

* The invention of the laser in the early 1960s led immediately to excited visions of "death rays" that could shoot down enemy missiles, but such "directed energy" weapons proved a major challenge, and are only now beginning to seem practical. The laser did, however, have significant short-term potential for use in combat.

Using a laser as a weapon itself places enormous demands on device physics and energy supply, but the fact that a laser beam can be precisely pointed and remains tightly organized ("coherent" in laser terminology) over long range meant that it could be used as a precise pointing device. A laser could be strapped to a telescopic camera with crosshairs so that the beam could be focused to "illuminate" a particular target to "mark" or "designate" it. The fact that the laser also generates a narrow range of colors ("monochromatic") also meant that the light reflected off such a target could be easily detected by simple sensors through a filter lens. A guided weapon could be fitted with such a sensor, with the sensor linked to feedback-control mechanisms so that it would home in on an illuminated target.

The idea of using a laser to designate targets apparently was devised in 1960 by two civilian engineers, David J. Salonimer and Norman Bell, at the US Army Missile Command in Huntsville, Alabama. The two were interested in building laser-guided artillery shells, and conducted studies on laser designator and seeker systems. Salonimer managed to get a little funding, and worked with Weldon Word of Texas Instruments (TI) to modify a Shrike anti-radar missile -- a variant of the Sparrow air-to-air missile -- as a laser-guided surface-to-surface weapon. The experiment didn't work out, but the idea of laser-guided weapons didn't go away.

Inspired by the Shrike experiment, Martin Marietta performed experiments of their own with laser targeting systems, and in 1964 demonstrated such a device to the Air Force, leading to a modest contract to TI the next year for an experimental demonstration of a "laser-guided bomb (LGB)". TI engineers built a laser seeker, based on an airflow test probe fitted to the nose of a bomb on a universal joint. The unit looked like a badminton "birdie", and so was called a "birdie head". It controlled the movement of four fins, which were originally fitted to the tail of the guided bomb.

The seeker had an optical sensor, shielded by a filter lens that was transparent to laser light, but blocked light of other wavelengths. The sensor was a simple array of photodiodes, arranged in quadrants. The quadrant that picked up the most light energy activated the fins, which were operated in a "bang-bang" control mode: they were either deflected completely to one or the other limit of their range of movement, or remained straight. The fins operated in pairs arranged symmetrically around the guided bomb to shift the weapon up and down, or left and right.

Early tests were conducted with M117 bombs -- like the M118 but smaller, with a nominal weight of 340 kilograms (750 pounds) -- with moveable tailfins, resulting in the TI "BOLT-117", the first LGB, which was tested in April 1965. Results were poor, but much improved accuracy was obtained with the use of the more aerodynamic Mark 84 "slick" bomb, with the control fins attached to the nose of the weapon, instead of the tail.

The TI group, working on a shoestring budget with the USAF Armament Development & Test Center at Eglin, conducted tests through the rest of 1965 and 1966. After some work, the bombs became very accurate -- though due to the bang-bang control scheme, the bombs did have a tendency to bob up and down along the laser beam until they locked on target. Despite the lack of a gyrostabilization system, the bombs generally locked on solid after a few seconds.

TI was finally awarded a contract for 50 "Paveway" guidance kits, where "Paveway" is sometimes said to be derived from the "Precision Avionics Vectoring Equipment (PAVE)" -- though it seems more likely that the name was arbitrary, and the acronym invented after the fact. Prototype LGBs were sent to Vietnam in 1968 for operational testing, with mixed results. F-4 Phantom fighter-bombers were used in the tests, with the "weapons system officer (WSO)" in the back seat marking a target with a hand-held laser system designated the "Airborne Laser Designator (ALD)". It proved very difficult to keep the laser aligned on the target, but half the LGBs hit the target anyway.

* The tests led to the "Paveway I" munitions. They consisted of a kit that was attached to ordinary Mark 82, Mark 83 and Mark 84 bombs -- at the time, the bantamweight Mark 81 was seen as too light to be worth making a kit for. LGBs based on the Mark 82 were designated as the "GBU-12/B" series, while those based on the Mark 83 were the "GBU-16/B" series, and those based on the Mark 84 were the "GBU-10/B" series. Each series had variants, such as "GBU-10A/B", "GBU-10B/B", "GBU-10C/B", and so on, that distinction being ignored here for the sake of simplicity.

The guidance kit included a laser seeker head with four control fins in a cruciform arrangement, which was attached to the front of the bomb, and a set of four larger fins, also in a cruciform arrangement, which was attached to the rear of the bomb to provide limited glide capability. All the LGBs used the same seeker head, but had different fin assemblies to accommodate different types of bombs.

The LGBs had their own power supply, consisting of a thermal battery for the electronics, along with a hot gas-driven actuator to move the seeker head. They did not require any electrical connection to the aircraft. They could be launched by any aircraft that could carry ordinary bombs of the same size, and could be guided by a laser designator on the launch aircraft; on a spotter aircraft; or operated by ground forces. The Air Force developed a laser designator for the F-4 under the code name "Pave Knife". This consisted of a laser slaved to a TV camera, and allowed the Phantom back-seater to spot a target while the pilot concerned himself with evasive action.

GBU-3/B

Kits were also developed for other unitary bombs and cluster munition canisters, but these munitions were not fielded. The LGB kits only cost a few thousand dollars US, and were produced in great quantity. Tens of thousands of them were used in Vietnam, proving themselves in the North Vietnamese ground offensive into South Vietnam in the spring of 1972, and the LINEBACKER bombing campaign late in that year. Lightweight Paveways scored direct hits on North Vietnamese tanks, while heavyweight Paveways destroyed bridges that had survived repeated conventional bombing raids.

* The initial Paveway design gave way to an improved series, known as "Paveway II", in the early 1970s. These weapons featured an enhanced -- as well as simpler and cheaper -- seeker head, plus distinctive pop-out wings for the rear assembly to improve the weapon's glide characteristics, and make it easier to fit to aircraft. They retained the designations of the Paveway I munitions -- GBU-12/B, GBU-16/B, and GBU-10/B -- with new variant codes.

The British RAF also adopted a Paveway II variant known as the "Mark 13/18", based on a British 450-kilogram (1,000-pound) GP bomb, this munition being used by Harrier strike aircraft during the Falklands War in 1983. GBU-10C/B heavyweight bombs were used by F-111 strike aircraft in the 1986 EL DORADO CANYON punitive raid on Libya.

The US Navy wanted to increase the range of their Paveway II weapons, and so devised a variant fitted with the solid rocket motor from a Shrike missile. This weapon is based on the Mark 83 bomb and is known as the "AGM-123 Skipper II". A Skipper II was used in 1988 during the Persian Gulf convoy operations to sink an Iranian frigate.

* The Paveway II required the launch aircraft to operate from medium altitude so that the bomb would have direct path to the target. That left the aircraft vulnerable to ground defenses, and so in 1976 the USAF issued a requirement for another generation of Paveway weapons, the "Paveway III".

The Paveway III used larger rear fins and a "smart" seeker head that incorporated a microprocessor-based digital autopilot, coupled to a more sensitive seeker with a wider field of view. Unlike earlier Paveway seekers, the seeker head on the Paveway III did not pivot, and the weapon did not use "bang-bang" control -- instead following as efficient a path to the target as possible. Bang-bang control was simple, but wasted much of the bomb's kinetic energy and reduced its range. A Paveway III bomb, in contrast, could be released at low altitude and then sail almost level into the target area, where it could then dive directly into a target.

The USAF only fielded the heavyweight Mark 84 / BLU-109 penetrator versions, with test and evaluation completed in 1986; these munitions were designated as the "GBU-24/B" series. For whatever reasons, the lightweight and middleweight LGBs continued to be designated as Paveway II weapons -- though they have been refined and upgraded, so eventually they have obtained Paveway III technology in all but name.

GBU-24/B

An improved Paveway III kit was used with the F-117 Stealth fighter, with the munition being designated the "GBU-27/B". The USAF worked on an "Advanced Unitary Penetrator (AUP)" munition externally similar to the BLU-1 that could achieve twice the depth of penetration by the interesting method of shedding its outer layers, but it was not adopted for service.

Confronted with deeply-buried Iraqi command bunkers in the First Gulf War, the USAF found that even Paveway IIIs built around the hardened BLU-109 bomb weren't good enough, and a crash program was put in motion to build something more fearsome. The new "bunker-busting" bomb, the "GBU-28/B", was prototyped in 17 days, using scrapped 203-millimeter (8-inch) artillery barrels with Paveway III kits attached.

A single test drop was performed at the Tonopah Test Range in Utah, with the bomb burying itself so deep that it could not be found. More GBU-28/Bs were quickly built, airlifted to the Gulf, and immediately used to attack hardened Iraqi installations. Each GBU-28/B weighed 2,250 kilograms (5,000 pounds). It consisted of a guidance kit attached to a BLU-113/B 2.1 tonne (4,700 pound) penetrator bomb.

The GBU-28/B was also used in the American war in Afghanistan in the winter of 2001:2002, carried by B-52s and F-15Es. Only a limited batch was built in the first place, and stocks ran low during the campaign. The Air Force has since obtained a batch of improved GBU-28/B bombs, based on the new BLU-122/B penetrating bomb -- at 2.02 tonnes (4,450 pounds), slightly lighter than the BLU-113/B, but more effective.

GBU-28/B

Some 9,000 Paveway II and III munitions were dropped during the First Gulf War. Video recordings taken by targeting systems showed Paveways and other smart munitions dropping into the front doors of aircraft shelters and falling into the ventilation shafts of buildings, blowing them apart with satisfying effect. These proved a striking and impressive propaganda tool for US forces. Videos that showed the gruesome effects of high explosive blasts on humans were, understandably but a little dishonestly, not released to the public. One Paveway was even used to blast an Iraqi helicopter out of the sky, an action-movie stunt that probably amused one side of the transaction immensely.

During the Anglo-American invasion of Iraq in the spring of 2003, US forces used lightweight LGBs filled with concrete as "minimum collateral damage" munitions to perform precision attacks on targets in built-up urban areas. These weapons were designated "GBU-45/B". The British also used practice LGBs filled with concrete during the campaign. The idea of turning an LGB into a "smart rock" was dreamt up by the Israelis during their squabbles with the Palestinians. The Americans later fielded a "Low Collateral Damage Bomb", which was a core munition with a composite case and modified explosive filler to reduce damage outside the immediate impact area, allowing the weapon to be used in built-up areas.

Hundreds of thousands of Paveway LGBs have been produced, and tens of thousands used in combat. Upgrade kits were introduced in the late 1990s to add GPS-INS guidance to LGBs, these being referred to as "dual-mode" munitions. They were originally designated "EGBU", for "Enhanced Glide Bomb Unit" -- this usage was dropped, with the GPS-INS guidance being designated by an "E" variant code, for example "GBU-24E/B".

The end result of such upgrades has been a bit of designation confusion, with Lockheed Martin selling GPS-INS-enhanced LGBs as "Paveway II Plus", leading to a refined "Dual Mode Plus" munition. Raytheon similar sells a dual-mode munition as the "Paveway IV", based on Paveway III technology and featuring a "smart" fuzing system that allows the pilot to select airburst, burst on impact, or post-impact time-delay burst before weapon drop.

* As a footnote, Lockheed Martin produces an LGB "training round", the "laser-guided training round (LGTR)", which the company introduced into production in 1992, with an "enhanced LGTR (ELGTR)" introduced in 2006. It weighs about 40 kilograms (88 pounds), but handles like a full-size LGB. It is only armed with a small "spotting charge" -- but Lockheed Martin has been pushing the ELGTR with a more potent charge as a low-collateral-damage munition for piloted aircraft and drones, under the name of "Scalpel", with an improved “Scalpel Plus” with both laser and GPS-INS guidance now on offer. It doesn't appear anyone's adopted Scalpel munitions just yet.

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[1.4] LASER TARGETING SYSTEMS

* As a footnote before proceeding, development of LGBs was shadowed by the development of handheld laser designators for ground forces, and improved targeting pods. A typical targeting pod features a motion-stabilized turret in the nose containing a camera boresighted with a laser system that illuminates a target, with the laser system also providing rangefinder capabilities. The camera may be a daylight or low-light-level TV (LLLTV) camera; or a forward-looking infrared (FLIR) imager; or both may be included. The camera or cameras will have multiple levels of zoom to provide either a wide field of view or a close-up on a target.

The pod is typically carried by a two-seat strike aircraft, with the weapons system officer (WSO) in the back seat viewing the target image provided by the pod, and using a joystick or similar hand controller to keep the target lined up in the display crosshairs. The pod may have "smarts" that allow it to keep the crosshairs on the target automatically. Modern US laser designators use a NATO-standardized pulse-code scheme to allow multiple LGBs to be dropped simultaneously during an engagement, without confusion.

Pave Spike

As mentioned earlier, the USAF used the ALD targeting system early on, later fielding the Pave Knife pod. It left something to be desired, and was quickly followed by better technology:

The early targeting pods were followed by still better systems, most significantly the Martin-Marietta (now Lockheed Martin) "Low Altitude Navigation and Targeting, Infrared, for Night (LANTIRN)". LANTIRN actually includes a pair of pods:

LANTIRN was carried by USAF F-15s and F-16s, and has been manufactured in improved variants. At last notice, the LANTIRN navigation pod was still in service, but the targeting pod had been replaced by the Lockheed Martin's "AN/AAQ-33 Sniper" targeting pod, itself available in a series of refined variants. The US Navy has fielded their own series of targeting pods, the Lockheed "AN/AAS-38 Nite Hawk" pod having been followed by the improved Raytheon "AN/ASQ-228 Advanced Targeting FLIR (ATFLIR)" pod.

Some aircraft built specifically for the strike role may actually have a built-in targeting system, eliminating the need to carry a pod. As a prominent example, from the late 1970s the US Navy's Grumman A-6E Intruder strike aircraft were fitted with an undernose turret with a FLIR and laser designator / rangefinder, designated the "AN/AAS-33 Target Recognition and Attack Multisensor (TRAM)". The new Lockheed Martin F-35 strike fighter also has a built-in "Electro-Optical Targeting System".

AN/PED-1

Of course, laser target designators can be carried by ground forces. US Army troops use the Northrop Grumman "AN/PED-1 Lightweight Laser Designator Rangefinder (LLDR)", which is an infantry-portable unit, mounted on a tripod. It features a day camera and an infrared imager; a laser designator and rangefinder; a laser "spot tracker" to spot another laser beam; a GPS and digital magnetic compass locator system; and a digital output port.

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[1.5] GAM & JDAM

* With the completion of the GPS navsat system in the early 1990s, the US military quickly went on to develop guided weapons that could leverage off of it. After the First Gulf War, the USAF decided to develop a GPS-guided bomb known as the "Joint Direct Attack Munition (JDAM)". In particular, the USAF wanted to have a guided weapon for their new B-2 stealth bomber to make use of the aircraft's impressive attack capabilities. The B-2 featured a precision, all-weather "GPS-Aided Targeting System (GATS)", and a GPS-guided bomb coupled to GATS was exactly what was needed.

However, JDAM wasn't scheduled to become available until almost the end of the century, which left the B-2 without a precision attack capability for several years. As a result, Northrop Grumman proposed a fast-track program to provide a "GPS-Aided Munition (GAM)" until JDAM came along. The result was a kit for the heavyweight Mark 84 bomb. The kit consisted of a tailpiece that contained control electronics, and a "jacket" that was wrapped around the nose of the bomb. The tailkit had moveable fins, along with a guidance system and thermal battery, and was linked to the launch aircraft over an umbilical connection that allowed downloading GPS coordinates. The jacket had strakes mounted on it at an angle to keep the bomb properly oriented as it fell.

The weapon was originally designated "GAM-84", but later redesignated "GBU-36/B". Initial drop tests were conducted in June 1995, leading up to a demonstration in October 1996: three B-2s dropped 16 live GAMs on 16 targets, with the first bomber dropping eight GAMs, and the following two bombers dropping four each. The last bomber performed damage assessment, to showed that all 16 targets had been hit and destroyed in a single pass of the three aircraft. GAM could hit a target up to 24 kilometers (15 miles) away when dropped from altitude, and its "circular error probability (CEP)", which is defined as the radius into which a munition can be placed at least half the time, was less than six meters (20 feet).

The Air Force also wanted a GPS-aided munition based on a heavy penetrator bomb for bunker busting, and so developed a GAM kit for the BLU-113/B 2,100-kilogram (4,700-pound) penetrator bomb. The weapon that resulted was originally designated "GAM-113" but later redesignated "GBU-37/B"; the first inert drop of this weapon from a B-2 was in April 1997. It was dropped in combat by B-2s beginning in the fall of 2001, during strikes on terrorist camps in Afghanistan following terrorist attacks on the US.

* As noted, GAM was strictly an interim development, produced in small quantities at relatively high unit cost, and soon expended. JDAM was the way of the future, designed to provide the same capabilities as GAM at low cost in high volume, for use on several different aircraft.

The JDAM GPS guidance kit provides a tail with a GPS-INS guidance system, along with a set of strakes that are strapped around the bomb midbody to control the bomb's fall. The guidance system is programmed by an umbilical connection; there has been talk of an infrared link instead, though status of that exercise is unclear.

Kits were developed for the heavyweight Mark 84 general purpose or BLU-109/B penetrator bombs, resulting in a weapon with the designation "GBU-31/B", and for the middleweight Mark 83 general purpose bomb, resulting in the "GBU-32/B". Stated CEP for the JDAM is 13 meters (43 feet), though is likely better than that in practice.

First tests of JDAM were performed in late 1996, with the weapon being qualified on most US attack aircraft. Production began in 1998, with JDAM introduced to combat during the 1999 NATO Kosovo campaign, the B-2 stealth bomber using the weapon to good effect. Kosovo was the first combat use of the B-2. The USAF also employed JDAMs in 2001, during the campaign against Afghanistan, dropping them from B-52s and B-1s. Post-strike imagery demonstrated the remarkable accuracy of the JDAM, with targeted runways neatly cratered out at all the intersections.

JDAM

In addition, JDAMs allowed the heavy bombers to fly all-weather close support missions, a role which their original designers could not have imagined. The bombers took off with a load of JDAMs without having specific targets and orbited over the battle area, above cloud cover. If ground forces or spotter aircraft found a target, they reported the target coordinates through an intelligence network, which passed attack instructions and coordinates on to a bomber. The bomber crew loaded the coordinates into the required number of JDAMs, set them for air, surface, or penetrating detonation, proceeded to the target area, and released the bombs.

The goal in this process was a ten-minute "kill cycle", but the Air Force admitted that was difficult to achieve. This was no fault of the JDAM itself, whose accuracy was all that was expected. In addition, Boeing claimed that field use to that time showed the reliability of the JDAM kits to be well above the 98% operability rate required by specification. JDAM was also extensively used during the US invasion of Iraq in the spring of 2003, making up a substantial portion of the almost 20,000 guided munitions used in the campaign.

The US Air Force, Navy, and Marine Corps originally planned to acquire a total of over 87,000 JDAM kits from Boeing, with total program cost of over $2 billion USD. By the end of 2013, over 250,000 had been produced, at a cost of a few tens of thousands of dollars per item -- cheap for a weapons system. The success of JDAM in combat led to considerable international interest in the weapon, with several dozen US allies adopting it.

Boeing funded development of a JDAM kit for the Mark 82 lightweight bomb, and the services adopted the weapon, designated "GBU-38/B", finding it useful for attacks in built-up areas where collateral damage had to be minimized. Initial use of the GBU-38/B was in the fall of 2004, during fighting in Iraq. The GBU-38/B is carried by B-1 and B-2 bombers, with the large numbers of munitions in a load allowing the bombers to hit a long list of targets in a single sortie. A smart stores rack was developed for the B-2, allowing the aircraft to carry 80 such munitions.

The US is also developing a guided version of the venerable B61 nuclear bomb -- called a "dial-a-yield" munition, since its explosive yield can be preset from a range before a mission -- with GPS-INS guidance. While specifics of the new "B61 Mark 12" are not all that clear, illustrations do show that it is fitted with a JDAM tailkit and strakes, or derivatives of such.

* Boeing has worked with Australia's Defence Science & Technology Organisation to develop the "JDAM-ER" -- an "extended range" variant with pop-out switchblade wings that triple the reach of the weapon. It is in production in Australia, being fitted to lightweight bombs for Royal Australian Air Force (RAAF) work. The switchblade wings do not increase the weight of the weapon significantly, imposing little effective restriction on carriage.

It is believed that Boeing will push JDAM-ER for export sales; the company has also worked on a switchblade wing kit for heavyweight bombs, but for whatever reason, has not made of the matter in public yet.

JDAM-ER

Boeing has also developed an auxiliary laser seeker for the JDAM, with the first drops of the "Laser JDAM (LJDAM)" in 2005. Boeing decided to perform development of the laser seeker with company funds in hopes that the US armed services might be willing to buy the finished product, and in fact the Navy and Air Force quickly snapped up LJDAM. The new seekers were refitted to existing lightweight JDAMs, with the upgraded munitions being designated "GBU-54", and seeing combat in Iraq in the summer of 2008. Although Paveway and JDAM munitions were once seen as complementary, they are now effectively competitive.

In late 2002, the Air Force also began experiments on attacking moving targets, using JDAMs modified with a datalink to permit course corrections up to weapon impact. There has been further work on this technology, as it provides a relatively cheap option for precision strike. Such a datalink would allow a JDAM-ER to be dropped from a standoff distance and then guided to target by a battlefield surveillance aircraft, or in potential even a space-based surveillance platform.

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