_JDAM.docx

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JDAM MaturesParts 1 and 2

Australian Aviation, December 2002 - January 2003Updated August, 2008.

by Dr Carlo Kopp

The JDAM will greatly expand the capabilities of theatre deep strike fighters such as theF-15E and F-111C, by providing near precision or precision strike capabilities through an

overcast. Laser guided bombs such as the baseline GBU-10/12 and GBU-22/24 areunusable under conditions where the laser illumination is impaired, conditions which areof no consequence to a JDAM tracking L-band microwave emissions from low orbitingsatellites. The use of platform referenced and wide area differential GPS techniques push

the accuracy of the JDAM into the domain traditionally occupied by laser guided bombs.This Boeing F-15E is pickling off no less than five 2,000 lb GBU-31 JDAMs, each of whichcan autonomously fly to its preprogrammed target (Boeing).

The US Joint Direct Attack Munition (JDAM) family of inertial/GPSguided bombs became a household word with the extensive use ofthese weapons during the Enduring Freedom air campaign in

Afghanistan. This was not the first use of the JDAM, delivered bythe B-2A during the Allied Force campaign in 1999, the JDAM iscredited with providing a critical all weather strike capabilityduring periods of dense cloud cover, when the primary laserguided weapons used by the NATO force proved ineffective.

The JDAM has proven to be a highly effective weapon, offering newcapabilities and very significant long term growth potential, but it is notwithout its critics. This two part feature will explore the current status ofthe JDAM and a number of related growth programs currently under way.


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Inertial/GPS Guided Bombs

The origins of modern GPS guided bombs such as the JDAMs lie not in thedomain of GPS satellite navigation, but in inertially guided bombexperiments performed during the 1980s.

Until that period, the dominant guided bomb technology was the laserguided weapon, first introduced like television guided weapons during theVietnam war period. That conflict saw a long running and sustained war ofattrition conducted by the US Air Force and US Navy against NorthVietnam. While average loss rates of US aircraft to Russian supplied AAAand SAMs were fairly low, the cumulative effect over a decade long warwas telling. This produced significant pressure for precision weapons, and

the early GBU-2 laser guided bombs and GBU-8 HOBOS television guided

bombs evolved primarily to reduce the number of aircraft exposed todefensive fire. The GBU-8 and the GBU-2 had significant limitations butwere nevertheless highly successful compared to dumb bombs.

The guidance packages in these weapons were trivially simple bycontemporary standards, reflecting the low density of period electronics.The cheaper and simpler laser guided weapons rapidly displaced the morecomplex television guided bombs, despite the higher accuracy of thelatter.

The standard low cost GBU-10/12/16 series Paveway II laser guidedbomb kit is a case study in simplicity. The quadrant seeker is fitted undera thick lens, and embedded in an aerodynamically aligned seeker head.Electronics in the guidance package sense the angular error between thebomb's velocity vector and the laser spot, illuminated by an aircraft ofground based laser designator. The angular error is then used to controlsolenoid valves which vent gas from piston / cylinder actuator assemblies,pressurised by a burning gas cartridge. The canard controls are eitherfully deflected or neutral in position, providing the simplest possible bangbang or non-proportional guidance.

The relatively dumb guidance technique in such weapons results inaggregate guidance errors of the order of several metres, generallyirrelevant for a 2,000 lb bomb lethal radius.

Laser guided weapons have some very important limitations. Perhaps themost important of these is their dependency upon continuous laserillumination of the target aimpoint. If the laser is shut down, or the targetis obscured by rain, water vapour (cloud/fog), dust or smoke, the bombseeker is blind and the weapon is apt to follow a ballistic trajectory like avery ordinary dumb bomb.


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This limitation was less important in the latter portion of the Cold Warsince low altitude delivery was considered an acceptable risk in a centralEuropean battle with the Soviets. Therefore fighters and bombersdelivering these weapons would typically attack from short distances, well

below cloud cover in most situations.

With the end of the Cold War tactics shifted. Loss of aircraft and aircrewbecame politically unacceptable, and bombing campaigns were mostlyprosecuted from medium altitudes, well above the reach of AAA andshoulder fired SAMs. The latter accounted for the largest number ofcoalition aircraft losses in the 1991 Desert Storm campaign.

Medium altitude delivery presented serious issues for laser guided bombs.Loss of the sightline to the target would cause the weapon to go ballistic

and frequently impact hundreds of metres from the intended aimpoint. Inurban areas this would result in serious collateral damage, and politicallydamaging loss of civilian lives.

Another issue was the robustness of a simple non-redundant laserguidance system. Whether the guidance signal was lost through hardwarefailure, or loss of illumination, the weapon was almost guaranteed to goastray.

Adverse weather conditions and embarrassing collateral damage incidents

in Desert Storm created the impetus for a production all weatherinertial/GPS guided bomb kit.

Inertially guided bomb technology was the subject of intense US Air Forceinterest during the 1980s. Such a weapon would be initialised over adigital umbilical with target and aircraft coordinates before release, andthen it would autonomously fly to impact using flightpath position andvelocity information produced by its onboard Inertial Measurement Unit.Microprocessor and Kalman filter technology permitted these weapons touse very refined guidance and autopilot algorithms. The weapon'strajectory could be optimised for range, impact velocity or impact angle.Since the inertial system was self contained, the weapon could not bejammed.

An inertially guided bomb presented the prospect of a robust, digitallyprogrammable, highly reproducable weapon which was jam proof andwholly oblivious to ambient weather conditions. The perfect precisionguided bomb?

No inertially guided bomb ever entered production, since the cost ofinertial units with the required accuracy proved to be prohibitive. The

perfect yet unaffordable guided bomb.


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The great enabler for inertially guided bombs was the US Air ForceNavstar GPS satellite navigation system. By using a GPS receiver tobound the cumulative error produced by the inertial unit, an inertial bombwith GPS could achieve equal or better accuracy at very low cost,

compared to a purely inertially guided bomb.

The first GPS aided inertially guided bomb to be built and deployed wasthe US Air Force's Northrop GBU-36/B GAM84 (GPS Aided Munition) 2,000lb weapon, deployed on the B-2A as a gapfiller prior to production of thethen embryonic JDAM. While the GAM was a relatively expensive weaponat cca USD 40k / round, engineered for early deployment rather thanminimal mass production cost, it did prove the concept convincingly. Moreover, it also proved an important refinement for improving the accuracyof such weapons. This refinement was the use of platform referenced

differential GPS, or GATS (GPS Aided Targeting System). When the B-2programmed its GAMs before release, it included a list of which GPSsatellites it was tracking. The bomb would track only these satellites,ignoring all others, and thus would see identical GPS position errors to thebomber. A GPS aided bomb without differential techniques would have acircular error probable of the order of 7 to 12 metres, using differentialtechniques the B-2A/GAM combo repeatedly demonstrated 6 metres orless, making it directly competitive against the established precision GBU-10 Paveway II.

The Joint Direct Attack Munition

The JDAM was the result of a hotly contested flyoff between McDonnell-Douglas (Boeing) and Martin-Marietta (Lockheed-Martin), bidding theGBU-31/32 and GBU-29/30 respectively. Boeing won what is likely toprove in time to be one of the most lucrative contracts for decades.

The baseline JDAM was to be an accurate rather than precision weapon,with a planned CEP without enhancements of 12 to 13 metres,corresponding to the systemic GPS P-code error and some guidance loop

error. The initial plan was to enhance this basic weapon with futureseeker technology to provide genuine precision capability.

The heart of the JDAM is a Honeywell HG1700 Ring Laser Gyro (RLG)inertial unit, which measures position, velocities and accelerations in allthree axes. The brain of the JDAM is in its Guidance and Control Unit(GCU), which contains an embedded microprocessor running a Kalmanfilter, which accepts position measurements from the GCU's HG1700 anda Rockwell GEM-III low cost military GPS receiver. The Kalman filtercontinuously computes a best estimate of the bomb's position in space.This information, and the preprogrammed target GPS coordinates, arethen used to feed a flight control algorithm. HR Textron actuators are


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used to drive three of the four tail surfaces. Power is provided by athermal battery in the JDAM tailkit. Most JDAM variants employ a set ofstrap on aerodynamic strakes, intended to increase body lift and alsoreduce the weapon's stability to improve its pitch and yaw rates, and thus

manoeuvrability.

The flight control algorithm can be configured before launch for vertical orhorizontal (ie shallow dive) terminal trajectories, selected by the user fora specific type of target. A weapon intended for the basement of a tallbuilding could be programmed to enter at ground floor level, wheres aweapon intended to enter a bunker shaft could be programmed for avertical trajectory.

The use of Kalman filter technology allows for refined midcourse flight

algorithms, which can manage the weapon's kinetic energy and maximiseglide range. Compared to the primitive analogue guidance in a baselinePaveway II, the JDAM achieves close to twice the glide range undersimilar launch conditions.

The JDAM employs the US standard Mil-Std-1760 umbilical interface,incorporating the Mil-Std-1553B digital multiplex bus. Before launch theJDAM's embedded software communicates with the launch aircraft'sstores management processor, no differently than a computer peripheral.Prior to release the JDAM is powered up using an umbilical feed from the

launch aircraft. The JDAM executes an internal self test, warms up andaligns the HG1700 inertial unit. Once the JDAM is ready, it communicatesstatus information to the launch aircraft, which then downloads GPStiming, GPS Almanac (ie nav message), GPS Ephemeris (constellation)and the GPS PPS crypto key. This information is used to initialise theGEM-III receiver.

Once the inertial unit is aligned and the GPS receiver initialised, thelaunch aircraft can download into the bomb the target GPS coordinates,fuse settings and impact parameters, all of which can be reloaded at any

time before release. Prior to release the aircraft's position and velocitiesare downloaded.

After the weapon is released, the thermal battery is initiated, the GPSreceiver acquires a satellite constellation, and the weapon autonomouslyflies itself to impact, using pre-programmed parameters, penetratingcloud with no loss in accuracy. Should the GPS signal be impaired, lost orjammed, the weapon can rely on its inertial unit and will suffer somemodest loss in accuracy, dependent upon how late in the flight the signalwas lost, and also depending on the tolerance errors in the HG1700(some units may be slightly more accurate than others).


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The autonomous capability in the JDAM is without precedent and a keyadvantage of this weapon against laser guided bombs. The latter aredependent upon laser illumination, as a result of which the aircraft canengage only one target at a time. While a good operator can pickle off

bombs several seconds apart for a level medium altitude strike, and movethe laser spot from aimpoint to aimpoint during an attack, in practicalterms this permits strikes only on clusters of targets and dependscritically on operator proficiency. The JDAM has no such limitation.

The JDAM can fly a boresight trajectory similar to a ballistic drop, but canalso fly off axis trajectories, to engage targets to either side of the flightpath, with some loss in range. Therefore, an aircraft can pickle offmultiple JDAMs almost simultaneously, each independently targeted, withthe sole limitation that the targets must be within the kinematic footprint

of the weapon. The weapon can be released from altitudes as high as 50kft, at speeds up to Mach 1.3, with medium altitude drops yieldingstandoff ranges of several nautical miles. A supersonic high altitude drop(F/A-22A) almost doubles range performance due to the much higherinitial energy of the bomb.

A heavy bomber carrying dozens of JDAMs can obliterate dozens oftargets within a given footprint, in a single large drop, as each bomb canbe independently preprogrammed before release. The catchcry for thelaser guided bomb was one aircraft, one bomb, one target - in the JDAM

era this becomes one aircraft, many JDAMs, many targets.

Integration of the JDAM is relatively simple, the principal prerequisitebeing that the launch aircraft is equipped with a Mil-Std-1760 digitalweapon station interface. With this capability, software changes are theonly modification to the launch vehicle. Clearance testing is required sincethe JDAM is aerodynamically different to the Mk-84/83/82 series slickbombs.

The JDAM GCU module was sized from the outset to fit the internal

volume of a Mk.84, Mk.83, Mk.82, BLU-109/B and BLU-110/B tailcone. Atthis time production of the JDAM encompasses the GBU-31 (Mk.84/BLU-109), GBU-32 (Mk.83), GBU-35 (BLU-110) models, with the GBU-38(Mk.82) in development with a planned 2004 IOC.

The GBU-31 has been most widely used, primarily as a replacement forthe GBU-10 in strategic strike (Serbia/Afghanistan), battlefield interdictionand close air support roles (Afghanistan). The US Navy has used the GBU-32 and GBU-35 widely during the Afghan campaign. It is expected thatthe GBU-38 will become a preferred weapon for battlefield interdiction,close air support and especially urban combat - in these roles low

collateral damage is more important than lethal blast effect. Directly


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interchangable with the Mk.82 slick, the GBU-38 will provide aircraft likethe B-52H, B-1B, B-2A, F-111C and F-15E with formidable firepower.

To date the JDAM has been used only in its basic configuration, without

additional seekers installed. Even with this limitation, the weapon hasproven to be a robust replacement for the Paveway II.

The capability of the JDAM to punch through a solid cloudbase hasrevolutionised close air support and battlefield work, since historicallysuch combat required either very low level strikes using dumb bombs, ormedium to low altitude strikes using laser guided bombs. Inclementweather offered cover to a clever opponent. The JDAM has closed thisstrategic loophole forever.

JDAM Accuracy and Jam Resistance

The accuracy of the JDAM is frequently criticised, the bomb being oftendescribed as much less accurate than the widely used GBU-10/12Paveway II weapons. This argument is lame and not representative ofmore recent developments in technique and technology.

The baseline accuracy of the weapon cited in mid 1990s glossy brochuresis a very pessimistic number, based on worst case GPS accuracy for theperiod. Since the 1999 Allied Force campaign, the US Air Force has

generated predictions of GPS accuracy variations over a 24 hour cycle fortargets of interest, or areas of interest. These computer models analysean effect termed Geometrical Dilution Of Precision (GDOP), which arisesas a result of the relative positions of satellites in the constellation areciever can see at a given point in time and space. As the orbitalpositions of the satellites in time, the GDOP error increases or decreases.Where and when an unusually favourable constellation is seen, the GDOPerror can be very low, and GPS errors resulting can be a fraction of thetextbook figure. The practice followed by the US Air Force since 1999 is toplan non-time critical strikes to fall into time periods of minimal GDOP for

the target of interest, to achieve defacto precision accuracy.

The US Air Force planned in the late 1990s a series of ProductImprovement Program (PIP) incremental block upgrades to the JDAMguidance package, but no details have been disclosed more recently as towhich have been implemented to date.

One candidate is the use of platform referenced differential GPS, which isrelatively undemanding to implement since it involves only softwarechanges to the aircraft and bomb embedded code (OFP), and GPSreceiver operating code. These force the bomb to acquire only aprogrammed constellation of satellites. The principal errors in bomb


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delivery are then dominated by the accuracy of the synthetic apertureradar or thermal imager/laser rangefinder used to produce targetcoordinates, and the guidance loop error in the bomb. Experience with theB-2A suggest this technique results in 6 metre or better CEPs, with the

GDOP error dominating the GPS error under most circumstances.

Another more potent candidate is the use of Wide Area Differential GPS(WADGPS) techniques, pioneered in the US Air Force EDGE and WAGEtrials. This family of techniques involves the deployment of a network ofprecisely calibrated GPS receiver ground stations surrounding the theatreof operations, which continuously measure the error in the recieved GPSsignal against the precisely surveyed location. Data from these groundstations is fed over low data rate landlines or satellite links to a centralground station, which runs a complex computer model incorporating

parameters such as solid earth tide (bulge) and wet / dry troposphericdelay. The system continously computes a set of correction parametersfor use in an enhanced Kalman filter, these are encrypted and broadcastvia a radio link (EDGE) or unused encrypted GPS Almanac page (WAGE).The compensated GPS errors achieved using this technique are as low asseveral inches in all three axes.

An aircraft and JDAM configured to use WADGPS techniques can achievetrue precision accuracy, 100% of the time, without the cost penalty of aseeker package.

Experience from Afghanistan suggests that the most frequent cause ofJDAMs going astray were either bent fins resulting from mishandling, ormore frequently the fat finger factor to use the colourful americanism.Human errors in entering aimpoint coordinates on keypads, entry of otherthan the intended coordinates, and in one instance possibly a groundforward air controller mistakenly transmitting over the radio his owncoordinates rather than those of the enemy!

Like the alleged inaccuracy of the JDAM, its vulnerability to jamming is

very frequently overstated by its critics. To date there is no publishedevidence of successful use of jamming to defeat a JDAM, or indeed anyGPS aided weapon.

The baseline GEM-III receiver has built in provisions to resist GPSjamming. Regardless of these, successful jamming of a GPS guided bombis not as simple as JDAM critics like to suggest. For a jamming effort towork properly, the jamming signal must be coupled into the mainlobe ofthe bomb's antenna, preferably from the very instant the bomb isreleased, or even earlier. This is easier said than done, since the GPSantenna on the JDAM is mounted on the tail, and therefore if the jammer

is colocated with the target, the antenna mainlobe is always pointing


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away from the jammer. The only jamming signal which can couple in iswhat little attaches to the skin of the bomb and tailkit as a creeping wave.Creeping waves tend to be weak in magnitude, and are easily suppressedwith coatings.

Even should GPS jamming increase in popularity (US reports suggestmore recent AGM-88 HARM versions will have provisions for homing onGPS jammers), the installation of improved GPS antennas and receiverswould defeat most techniques. Neither represent unusual integrationchallenges for a modular design such as the JDAM.

One issue JDAM critics seem to universally overlook is the reality that ittakes very little effort in any inertial/GPS system to incorporate codewhich monitors the difference between the GPS and inertially predicted

bomb positions. Should the GPS position read from the receiver suddenlychange by a large amount, the software can simply reject the GPSmeasurement and continue to fly the bomb using inertial data untilimpact, or until the GPS signal behaves as it ought to. Unless the jammeris unusually effective, odds are that gaps in jamming will occur and thebomb guidance can use these to grab valid GPS measurements. With aflight time of mere minutes or tens of seconds, the cumulative inertialsystem error seen since the last valid GPS measurement could be verysmall indeed.

It is worth noting that a JDAM is potentially more robust than an analoguelaser guided bomb in the event of a guidance component failure. Forinstance a hardware failure in a GPS receiver or inertial unit could behandled by rejecting its output and flying to impact on the remainingsource of position and velocity data. Boeing have not disclosed whetherthis technique is used.

In summary, most of the criticisms directed at the JDAM (and verypopular in some Canberra circles) are very lame and assume a very clevertechnological peer competitor opponent. Whatever limitations the JDAM

might have, these are generally of less significance than the enormousgains in capability and firepower offered by this weapon. At unit costsunder USD 20M, the JDAM is one of the best bang for buck choices in themarket today.


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JDAM Cutaway. The JDAM is a GPS aided inertially guided bomb. The Guidance and

Control Unit containing a HG1700 RLG, GEM-III GPS receiver and computer package isinstalled inside the bomb tailkit. The GCU was designed from the outset for tailkitvolumes compatible with the Mk.84, Mk.83 and Mk.82 low drag bombs, and has beenadapted to the tungsten tipped bunker busting BLU-109/B and BLU-110/B warheads(Boeing).

Without doubt the most important near term application of the JDAM has been its use asa near precision conventional weapon for US Air Force heavy bombers, previously limited

to dumb bombs. The 2,000 lb GBU-31 fitted to the Mk.84 or BLU-109/B warheads wasthe first to see widescale combat use. The JDAM was blooded in 1999 when the B-2Abombed Belgrade with the weapon. In 2001, the decisive blows to the combinedTaliban/Al Qaeda ground forces in Afghanistan were inflicted by B-52H and B-1B

bombers delivering GBU-31s against a wide range of battlefield targets (Boeing/USAF).


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The US Navy's primary JDAM variants are the GBU-32 and GBU-35, designed for the

1,000 lb Mk.83 and BLU-110/B warheads standard for this service. The Boeing F/A-18C/D/E/F will be the primary near term delivery platform for naval JDAMs. Loadouts arelikely to be identical to the existing Mk.83, but using smart Mil-Std-1760 racks with Mil-

Std-1553B bussing to the bomb umbilical connectors. The baseline JDAM can be

retargeted up to the point where it is released (Boeing).


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The smallest member of the JDAM family is the new GBU-38 500 lb weapon, designedfor the Mk.82 warhead. This weapon is easily identified by the absence of the largecruciform strakes used on the 2,000 lb and 1,000 lb variants, with small nose mountedvanes substituted. The 500 lb JDAM will become a mainstay of close air support,battlefield interdiction, airfield attack and urban bombardment roles, as it offers goodlethality against soft targets yet a much smaller collateral damage footprint than itslarger siblings. A B-52H carrying 48 rounds, or an F-111C carrying 24 rounds, eachindependently targeted, offers a dramatic increase in deliverable precision firepower on a

single pass. It is not unreasonable to argue that this weapon will revolutionise bombingtechnique (Boeing).


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Imaging seekers are one technique which will provide the JDAM with genuine precision

capability. A typical design for such a seeker will see the JDAM seeker take a snapshot of the

target surroundings, which is then compared with a preprogrammed image to fix the bomb's

position. Once the error is found, the target aimpoint is corrected and the bomb dives into the

target. MilliMetric Wave Imaging techniques were demonstrated in the Orca program, while

DAMASK demonstrated an IIR seeker. Both techniques have growth potential for attacks on

moving targets such as vehicles or shipping (Author/USAF).

From the very outset of the JDAM program, the intention of the US Air Force

was to equip the basic weapon with a range of precision terminal homing

seekers. The basic idea was to provide an accurate basic weapon, with the

terminal seeker providing the remaining precision capability.


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To that effect, the JDAM Guidance Control Unit (GCU) was designed with

additional growth capacity in empty slots for more cards, but also with unused

spare interfaces to permit additional hardware to be integrated with minimal

effort. In this fashion, specific software could be written for seeker equipped

variants and loaded into the standard low cost mass production GCU. A unique

seeker would then be plugged into the unused GCU interfaces via an umbilical

routed from the nose of the bomb.

This highly flexible model was devised to accommodate as many different

options in seeker technology as the user might ever want. By dividing the

system into discrete modules, where the mass produced baseline hardware is

kept unchanged, it is possible to achieve the large economies of scale which

are characteristic of very large, uniform and mature mass production builds.

Cost has traditionally been the greatest impediment to the large scale use of

precision munitions. While a well guided GBU-10/12 Paveway II laser guided

bomb can be very accurate, and is cheap due to its primitive seeker design, the

weapon is also in many respects fragile since the seeker's simplicity denies

redundancy to protect against hardware failures, and the guidance technique is

vulnerable to the loss of laser illumination. Opting for more sophisticated

proportional navigation style laser semiactive homing, with an inertial capability,

as used in the later GBU-22/24 Paveway III bombs drives up the cost.

Television guided bombs have also proven expensive. The GBU-8 HOBOS,

which evolved into the cruciform wing GBU-15 family of weapons, proved to be

amongst the most expensive guided bomb kits ever mass produced. The

requirement to provide a stabilised platform for the bomb's seeker, and robust

radio datalinks, resulted in a cost structure which effectively compromised

these capable weapons in large scale use. The key difficulty with the GBU-15

series was its uniqueness - the airframe components were unusable for other

purposes and this drove up the unit cost.

The advent of the JDAM as a platform for range of precision seekers or

guidance packages changes the basic economic equation. The unique portion

of the precision weapons kit is the seeker hardware/software alone, with the

remainder of the weapon being essentially standard low cost mass production

hardware. Therefore nearly all of the investment in developing and producing

the precision weapon is concentrated into the seeker alone.

To date no precision seekers have been deployed operationally, or at least not

announced in the public domain. In part this is because the basic JDAM has

proven generally more accurate than originally expected. Operational use of


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techniques such as strike planning in optimal GDOP windows, deployment of

improved later generation GPS satellite vehicles have clearly driven accuracy

close to the GBU-10 class, and with the eventual use of wide area differential

GPS (eg WAGE) and B-2 derived platform referenced differential GPS, there will

be little pressure for precision seekers. Why add US$10k to 20k to the cost of

each bomb if you can get 80% of its accuracy via cheaper techniques?

However, this does not by any measure mean that seekers are dead. On the

contrary, many situtations will demand seekers. Moving targets in a jamming

environment will almost certainly require seeker technology to retain precision

accuracy if the GPS channel is lost.

The US Air Force ran two technology demonstrations during the late 1990s. The

classified Raytheon/Sandia Hammerhead program demonstrated the use of

Synthetic Aperture Radar (SAR) active seeker for the JDAM, with a 3 m CEP.

While details have not been released as yet, it is reasonable to speculate that

the design uses a scene matching area correlation technique to fit a SAR map

against a preprogrammed target area map.

At that time the US Air Force also sponsored the classified Orca program, to

demonstrate a millimetric wave (MMW) radar seeker with a 3 metre or better

CEP. MMW seekers have been used for instance on radar guided anti-tank

mortar rounds, and the technology is central to the latest variants of the Hellfire

missile carried by the AH-64D Longbow Apache. No details have been released

on Orca to date. Given the potential of the technology, an MMW seeker could

be used for attacking moving targets like shipping or armour, and using scene

matching area correlation techniques in the manner of the Pershing II IRBM, it

could also be used for precision strikes on fixed targets.

Clearly there is considerable potential in radar seeker technology for the JDAM,

and many possibilities exist.

At this time there are very few electro-optically (EO) guided bombs in

operational service. The US Air Force retains residual stocks of the GBU-15,

which have been since upgraded to EGBU-15 configuration by the additional of

a GPS receiver and IMU to provide JDAM-like midcourse guidance. The Israelis

have a range of weapons, but stocks and configurations remain largely

undisclosed.


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A key obstacle to the use of autonomous and datalink supported EO guidance

techniques has been cost. To achieve a respectable acquisition range of

several miles, the seeker optics must be stabilised down to tens of

microradians or better jitter performance. Typically multiple fields of view are

required. The result was an expensive to produce gimballed optical package

with the additional encumbrance of cryogenic cooling if infrared day/night

capability was needed. If the weapon was to be remotely guided from a

cockpit, then the weapon would also require an expensive jam resistant

wideband video datalink to carry the seeker image to the launch aircraft. While

autonomous target recognition techniques have matured in recent years, one to

two decades ago they were both expensive and unreliable.

Much has changed since in basic technology. In daylight imaging, high

resolution CCDs and CMOS imagers are now much cheaper and immeasurablybetter than the vidicon tubes of the 1970s. In infrared imaging, bolometric

uncooled and cryogenically cooled Indium Antimonide, Mercury Cadmium

Telluride, Platinum Silicide and Aluminium Gallium Arsenide Quantum Well

Imaging Photodetector (QWIP) focal plane or staring arrays are now available.

Of particular interest is the QWIP technology since it permits high resolution

imaging chips operating in the MWIR (midwave or 4-5 micron band) and LWIR

(longwave or 8-12 micron band), but also allows a single imaging chip of the

proper architecture to concurrently image in both the MWIR and LWIR bands -

effectively two band specific thermal imagers in one slab of Aluminium Gallium

Arsenide semiconductor producing two video signals at the same time. Not

surprisingly, the leading wave of QWIP imagers is in the high volume

commercial medical/industrial markets rather than low volume military market.

No less important is the uncooled bolometric thermal imaging technology,

which is much less sensitive than cooled semiconductor imaging chips, but

also much cheaper, and not requiring the dollar hit of a refrigeration package.

It's principal market lies in automotive thermal imagers, popular in top tier US

limousines.

Electro-Optical guidance, be it autonomous or datalink aided, is potentially

valuable to the JDAM family of weapons. While it cannot penetrate cloud, it is

compact and extremely precise. With the weather immune GPS/IMU guidance,

an EO seeker equipped JDAM can fly under the cloudbase to acquire its target.

Widely available EO targeting pods, especially on US aircraft, provide a source

of good quality infrared imagery which can be downloaded to a seeker

equipped JDAM before release. With satellite and UAV generated high

resolution imagery, and datalinks to combat aircraft, there are few obstacles to

target imagery being tranmsitted in seconds from a source to a bomber, and

through the Mil-Std-1760 umbilical, to a seeker equipped JDAM before release.


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The first EO seeker demonstrated on a JDAM was the DAMASK (Direct Attack

Munitions Affordable Seeker), sponsored by the Office of Naval Research

(ONR) under a USD 15M contract. The aim of the DAMASK project was to

demonstrate a very cheap yet highly accurate low cost EO seeker, with no

moving parts.

The DAMASK program demonstrated the viability of an uncooled autonomous thermal imaging

seeker on the baseline GBU-31 JDAM. The DAMASK would take a snapshot of the target

scene, and pattern match the image against a stored image of the target area to refine its

position estimate. The result is accuracy of the order of several feet, and trials drops as good

as 2 ft from the intended aimpoint. The HART program will see this technology incorporated

into a production weapon (US Navy).


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The DAMASK design was innovative in many respects. The low cost seeker was

designed around an uncooled imaging-infrared focal plane array (UIIFPA)

device, using low cost optics and a molded composite casing. The imaging

array is based on the same technology used in the Cadillac Seville 2000 head

up FLIR, to achieve exceptionally low unit costs. A commercial signal

processing module was adapted to support the seeker, and installed in theunused tailkit volume. The US Navy estimated the unit cost of a DAMASK kit at

US$12.7k in mass production.

The DAMASK employs scene matching techniques well proven in systems such

as the Tomahawk. Before the bomb is released, the launch aircraft downloads

an image of the target, produced by satellite, the aircraft's SAR or FLIR. When

the bomb is released is flies over the target and then noses over to point down

at a very steep angle. In this terminal flight phase it images the area

surrounding the target, and then performs the correlation operation to

determine the bomb's actual position against its intended position. The system

was to calculate weapon alignment to 100 microradians accuracy, for a 2.6

metre error at impact.

Once the JDAM's position is updated from the target scene, the weapon will

correct its donwward trajectory, pulling multiple Gs if required as it is travelling

down very quickly at several thousand feet of altitude at this point. Once the

trajectory adjustment is completed, the weapon continues on inertial/GPS

guidance to impact.

The DAMASK demonstration presented some interesting problems. The issue of


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seeker alignment was demanding, especially since the minute flexure in the

bomb body was enough to introduce potentially problematic errors. Image roll

alignment proved to be an issue, as did motion induced image blurring and

image distortion resulting from lens behaviour. Image processing speed also

presented challenges, since the time window for processing the acquired image

was very short.

DAMASK proved to be a resounding success, with trial weapon drops including

simulations of GPS jamming by disabling the bomb's GPS receiver. The first

drop saw the weapon impact within 2 ft of the intended aimpoint.

The DAMASK program was essentially a technology demonstration to prove that

the concept of a simple EO seeker worked effectively.

The current US Navy HART (Hornet Autonomous Real-Time Targeting for F/A-

18C/D/E/F) program builds on the DAMASK effort. HART is aimed at providing

a production EO seeker for the JDAM, which incorporates the capability to

download the image from the aircraft's FLIR/EO targeting pod (AAS-38 or

ASQ-228 ATFLIR/Terminator) providing the ability to precisely target pop-up

and relocatable targets. The formal FBO statement for the program specifies

Boeing as the sole source. Whether the HART seeker package will incorporate

the Autonomous Target Recognition (ATR) algorithms devised by Boeing for the

AGM-84E SLAM family of missiles is unclear from published materials. HART

will run until 2007.

Whether the US Air Force adopt the HART seeker, or indeed it becomes

available to export clients, remains to be seen. The nature of the design lends

itself to integration on any FLIR/EO pod equipped Mil-Std-1760 capable

aircraft, which both the RAAF's F-111C Block C-4/5 and F/A-18A HUG will

become in the timelines of interest.


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The DARPA AMSTE program recently demonstrated a successful strike against a moving targetusing a JTIDS datalink aided JDAM. The target was tracked by two separate airborne GMTI

radars, providing a continuous stream of target coordinates which were fused and then

tranmitted over a JTIDS channel to the JDAM in flight. The weapon is reported to have impacted

within the lethal radius of the target (Author).

The limitation of the baseline JDAM guidance package is that it was designed

to engage fixed targets, the original intent being to fit precision seekers for

attacking moving targets. More recent developments in the US suggest that a

radical change may be afoot in this area.

The Affordable Moving Surface Target Engagement (AMSTE) technology

demonstration program is a complex effort which is intended to develop and

prove techniques for the engagement of moving ground targets, using cheap

munitions and standoff radar targeting techniques. In particular, AMSTE is

exploring Ground Moving Target Indicator (GMTI) radar techniques, target

position refinement using information from multiple radars on multiple aircraft,

and the use of datalinks to guided weapons.

Perhaps the most dramatic outcome of the AMSTE effort was the August 22,


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2002 demonstration, in which a JDAM modified with a JTIDS datalink receiver

successfully engaged a moving vehicle in a column, using target coordinates

produced by a distant E-8 JSTARS and a second radar on an airborne testbed.

The inert JDAM was dropped by an F-16C at 20,000 ft, the target was part of a

vehicular column travelling at 30 km/h. Once released, the JDAM acquired the

JTIDS signal and continuously updated its aimpoint position as it flew toward

the target. DARPA have not disclosed the frequency of updates, but it is likely

that a whole JTIDS net was reserved for this purpose.

The AMSTE demonstration is important since it proves the feasibility of

continuosly datalinking a moving target's position to a JDAM in flight. The

position information could be produced a GMTI radar on a distant aircraft, be it

a fighter with a larger radar, an ISR platform or a UAV, or it could be producedby a FLIR/EO/laser targeting system on a fighter or an endurance UAV such as

a Predator or a Global Hawk. Once the targeting sensor is measuring the

location of the target vehicle, it takes little effort to pump this information out

on a datalink radio channel to a bomb in flight.

Handling the target coordinates at the bomb end is perhaps the most

challenging aspect of such systems. The guidance software will have to

incorporate a Kalman filter which estimates the position of the target vehicle

based upon a track history of continuously transmitted coordinates. A

prediction of the target's position based on this data is then used to adjust the

bomb's aimpoint. Since the JDAM is flying blind toward its target, the quality of

the prediction algorithms is critical to success.

Another important aspect of seekerless JDAM engagement of moving targets is

the accuracy of the transmitted coordinates, since these are added to the

JDAM's guidance error. While many radars support GMTI techniques, very few

support the more accurate multi segment Differential Phase Centre Antenna

(DPCA) techniques, as these require specific adaptations to the radar antenna

design, and feed designs. As a result, the range and bearing accuracy of GMTIradars usually does not match that achieved in SARs. The AMSTE program

works around this limitation by fusing GMTI tracks from multiple airborne radars,

to yield a best estimate of target position. The target bearing error can be

modest, and triangulation of the target using bearings from two or more radars

separated by several miles evidently makes the difference.

When the AMSTE derived technique does eventually become operational, it will

permit the concurrent engagement of multiple ground vehicles in all weather

day/night conditions. Whilst it may not match the accuracy of seeker equipped

JDAMs, it makes up for that limitation in much lower weapon costs.


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Combining a datalink midcourse system with a cheap autonomous short range

seeker, such as a device derived from an anti-armour submunition, of course

yields the best of both worlds.

What is clearly evident is that the sanctuary of motion will not last long for

evaders of the JDAM.

The HdH JDAM-ER is being designed for very low mass production unit cost, which is reflected

in a number of design features. The most evident is the revival of the DSTO GTV untapered

wing planform, which sacrifices a little range performance but is significantly easier to

manufacture. The baseline GBU-31/32/35/38 tailkit is used, with software alterations to support

the changed aerodynamics and wing deployment functions (HdH).

Additional HdH JDAM-ER line drawings here[1],[2],[3](HdH).
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The notion of a GPS aided inertially guided glide bomb is nothing new, but

fielding one has proven to be a time consuming task. Australia is in a unique

position insofar as the DSTO GTV/Kerkanya demonstration put it in the forefront

of glide bomb kit research - until recently this innovative DSTO effort sat in

limbo.

The first attempts to convert the GTV/Kerkanya concept into viable production

weapons never got off the ground, in both senses of the phrase. During the1990s Hawker de Havilland pursued the Icarus I and II concepts, the former

using a BAe ALARM anti-radiation seeker, the latter using a JDAM-like

GPS/inertially guided tailkit. A lack of funding saw both efforts confined largely

to paper studies. AWADI also pursued the idea of a production GTV/Kerkanya

derivative, but aimed from the outset at a GPS/inertially guided tailkit solution

under the Agile Gliding Weapon (AGW) designation. With the entry of the JDAM

into full scale production, the idea of fusing the AGW wing kit with the JDAM

tailkit was explored as a joint effort between AWADI and Boeing. The AWADI

effort collapsed after the company was acquired by BAeA. Thus, it appeared,the effort to revive the GTV/Kerkanya as a production effort was doomed to

failure.

Last year Hawker de Havilland (now Boeing owned) at Fisherman's Bend were

awarded RAAF funding to pursue a Concept Technology Demonstration of a

GTV/Kerkanya derived wing kit for the GBU-38 500 lb JDAM. HdH licenced the

DSTO intellectual property in the GTV/Kerkanya and acquired all archived DSTO

design data, reports, and remaining demonstrator hardware components to

support this effort. HdH have received great support from DSTO, RAAF

Capability Development, the DoD CTD program office and DMO.


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Over the last 2 years, the HdH development team at Fisherman's Bend have

been working in earnest to convert the GTV/Kerkanya research findings into a

viable design for mass production. This effort has involved analysing the basic

design issues for the wing from the ground up, and re-evaluating nearly all

basic design assumptions.

The current intent is to perform a critical design review at the end of 2002,

resulting in a qualified design by mid 2003 and flight trials in late 2003. Should

no unforseen difficulties arise, the HdH Range Extension Kit for the GBU-38

JDAM (JDAM-ER for Extended Range) could enter Low Rate Initial Production

(LRIP) some time in 2004.

The basic JDAM tailkit is well suited to such an adaptation since the Guidanceand Control Unit (GCU) has available internal growth capacity, and spare

unused interfaces to permit the control of additional hardware. The wing kit

would thus be connected to the GCU via an umbilical, and additional code

added to the baseline JDAM to provide for release of the wing, and provide a

unique autopilot for the winged variant. In the simplest of terms, the JDAM

tailkit hardware would remain unchanged, but software would be added to

adapt the tailkit to the glide wing.

The HdH design uses an untapered wing planform like the GTV demonstrator,

but differing from the later tapered wing planform on the Kerkanya. This

reversion loses a few percent in aerodynamic efficiency, but improves the radar

scattering behaviour of the wing, and is much easier to mass produce at low

cost. Unlike the DSTO demonstrators which used differential pressure sensing

ports and a pitot tube to achieve optimal gliding performance, the baseline HdH

design will derive its velocity from GPS/inertial outputs. While this does not

extract the full glide range potential from the design, it does reduce cost and

complexity considerably, and improves the reliability of the wing kit.

Key design objectives for the HdH product are lowest possible mass productioncost, zero hardware changes to the existing GBU-31/32/35/38 tailkits, best

possible performance, modularity, ease of maintenance and especially shortest

possible assembly time in the field. The latter will be critical to user acceptance

of the kit, the less time expended and the fewer errors in assembly when

deployed in the middle of nowhere, the more popular the kit will be with its

users. The design philosophy is centred on producing a flexible product which

can further grow as customers request additions. Should a customer pursue a

high wing configuration, improved glide range, or a different wing sweep angle,

the basic design is aimed at accommodating such changes at the lowest

incremental cost.


25/29

HdH intend to offer scaled variants of the kit for the Mk.82, Mk.83, Mk.84, BLU-

109/B, BLU-110/B, BLU-118/B warheads, and any future warheads in this

weight class.

At the time of writing the external design was frozen with detail design currently

progressing to design review. Available illustrations reflect the current

configuration, but are likely to change in detail areas to reflect future customer

requirements.

The importance of the HdH effort cannot be understated. In strategic terms, a

JDAM-ER with 30 to 50 NMI of standoff range for a high altitude release

provides a very cheap mass production standoff weapon which defeats all but

the largest and most capable area defence SAMs in service. As the range of theweapon is well matched to typical combat aircraft radar SAR modes, it provides

a genuine standoff all weather capability. Should the JDAM in the future acquire

a standard datalink, this capability would be expanded to encompass moving

targets.

The JDAM-ER is not a substitute for the AGM-142 SOW, as the latter is a

supersonic weapon with a pinpoint precision imaging seeker and remote

datalink control. When dealing with well defended very high value targets, such

as radar installations, mobile command posts, command bunkers or

communications nodes, or targets of opportunity, the AGM-142 permits

positive operator control of the weapon to impact with a fairly short flight time.

This contrasts with the less precise, much slower but also much cheaper

JDAM-ER. The low cost of the JDAM-ER permits its use against much lower

value targets, even if these are well defended. In practice the RAAF would use

the AGM-142 to engage air defence and command-control-communications

targets, while concurrently using the JDAM-ER to engage the fixed targets

being defended by those same assets.


26/29

The HdH JDAM-ER effort builds on the DTSO GTV/Kerkanya glidebomb effort, using the

standard JDAM tailkit with suitable software alterations. With a standoff range likely to be well in

excess of 50 NMI, the JDAM-ER will revolutionise much of the bombing game. The weapon will

be suitable for medium/high altitude drops, and low level toss deliveries, placing the bomber

outside the range of most air defence weapons (Author).


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Like all other variants of the JDAM, the JDAM-ER will permit massed attacks

against prebriefed targets. A fighter could pickle off an arbitary number of these

weapons, and turn tail while the bombs each autonomously fly to their targets.

Even with a 50 NMI glide range, the footprint the fighter can hold at risk

encompasses roughly a 100 NMI circle. A key issue for the RAAF will be

achieving a mature Mil-Std-1760 capability on its F-111C/G and F/A-18A

fleets before the weapon becomes available.

Exploiting the full potential of the JDAM-ER, especially the 500 lb GBU-38

varianant, will require smart bomb rack technology, with a Mil-Std-1760

capability on each ejector. For the F/A-18A this would require a dual or triple

rack, for the F-111C/G a modified BRU-3/A six hardpoint rack. The GBU-

38/JDAM-ER would be especially well suited to the F-111C/G as with four 6

hardpoint smart racks it has to potential to engage 20-24 aimpoints on a singlepass, subject to clearances. Autonomous targeting of the JDAM-ER will require


28/29

either a good Synthetic Aperture Radar or a high resolution thermal imager with

exceptional jitter performance. The latter makes a good case for some

technology insertion into the Pave Tack, since no existing thermal imaging pods

come near the required performance (doubters might consider looking up the

jitter specifications of such if they choose not to believe this author).

Most observers consider the introduction of the JDAM into the RAAF inventory

as a forgone conclusion, under the AIR 5409 Bomb Improvement Program,

although the JDAM has had its fair share of doubters and critics in Russell over

recent years. One hopes that repeated 6 o'clock news observation of BBC and

CNN TV footage from Afghanistan will have dispelled their fears or indeed

dislike of the weapon! Whether one likes the JDAM or not, it has proven its

effectiveness very convincingly.

Boeing GBU-38 JDAM-ER prototype in 2011. It is based on the Kerkanya wing kit design (

2011 Carlo Kopp).

In conclusion the JDAM is the vanguard of a new generation of low cost,

digital, autonomous weapons, designed for genuine all weather use. It is

revolutionising air warfare in a manner analogous to the laser guided bomb

three decades ago, and promises to develop into a diverse family of derivativeweapons adapted to a range of demanding niche roles. Air forces without


29/29

JDAM capability today will be as handicapped as air forces without laser guided

bomb capability were two decades ago.

Since this article was produced there have been numerous developments in

Australia and the United States. The JDAM-ER ACTD progressed and trial drops

of the weapon were performed in August, 2006. Also initial JDAM integration

work was performed on the F-111C, funded from internal Boeing Australia

budgets[Click for more ...]. The baseline JDAM is being integrated on the

F/A-18A/B HUG Hornet. In late 2006 then Defence Minister Nelson sold toFederal Cabinet the idea of replacing front line F-111s with F/A-18F Super

Hornets, the latter more suited as advanced trainers given the regionalenvironment[Click for more ...]. The JDAM HART/DAMASK achieved IOCin 2007, with claims that the US Navy would acquire up to 6,000 seekerkits. The AMSTE system was trialled in 2004 as an alternative maritimestrike capability CONOPS, during the Resultant Fury Sinkex [Click formore ...]. The GBU-39/B Small Diameter Bomb achieved IOC and is beingflight tested on the F-22A Raptor[Click for more ...].
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