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GPS Part IV - US Direct and Indirect Attack Munition Programs

by Carlo Kopp

published in Australian Aviation, November, 1996

(c) 1996 Aerospace Publications, Pty Ltd, (c) 1996 Carlo Kopp

The MDC GBU-31/32 JDAM Continued ...

USAF mission planning will use the existing AFMSS/TAMPS software tools which would be used to program a data transfer cartridge with weapon delivery parameters for upload into the launch aircraft's mission computer. Once the aircraft approaches the target area, the JDAM is powered up using aircraft electrical power. With power applied, the JDAM will execute an internal self test, warm up and align its IMU, and download target data from the launch aircraft. The aircraft computer, using an appropriate protocol, talks to the bomb and downloads GPS timing, Almanac (ie nav message), Ephemeris (constellation) and PPS crypto key to initialise the GPS receiver. Once the GPS is initialised, the computer downloads target coordinates, fuse settings, impact parameters. These can be reloaded at any time prior to release. Once the targeting data is loaded, the computer downloads IMU position and velocity data from the aircraft. The weapon is then ready for release. The weapon's guidance control laws provide for both steep and shallow dive trajectories to attack both horizontal and vertical targets, and allow an aircraft to release several independently targeted JDAMs from a single point in space, each of which can fly a unique trajectory type.

When the weapon is released, the thermal battery is fired, the JDAM acquires a self-determined, optimum satellite constellation, and flies itself autonomously to impact. The JDAM delivery envelope is identical to that of the dumb Mk.84, Mk.83 and BLU-109 bombs. The weapon can be delivered from up to 50 kft, at speeds of up to 1.3 Mach, using direct (boresight) and off axis trajectories. Accuracy has not been disclosed but will depend critically upon the launch aircraft's capability. A system such as the B-2 GATS would provide JDAM with similar accuracy to the Northrop GAM. Planned JDAM Early Operational Capability (EOC) for B-2 is 1997 and IOC for the weapon is cca 1997. It is expected to be used extensively by all three US services and US allies. The long term intent is to replace the Paveway family of weapons with JDAM.

Carried by fighters such as the F-15E and F-18E, which have APG-70 series radars with synthetic aperture high resolution imaging capability (The B-2 APQ-181 shares much hardware and software with these fighter radars) the JDAM can be expected to achieve similar accuracies to the Northrop GAM if similar delivery techniques to the B-2 GATS scheme are used. Aircraft equipped with older and less capable radars will be limited to the weapon's basic CEP.

In the Australian context, the JDAM is a viable weapon for internal carriage on the F-111. As the weapon has a similar form factor and ballistic properties as the Mk.84, which is smaller and lighter than the USAF SRAM and B-series "special" devices which the F-111 was designed to carry, carriage is feasible. The existing AUP aircraft design however has wing pylon decoder boxes mounted on the sides of the weapon bay, where they protrude into the space which would be occupied by the bombs. Weapon station decoders are conventionally fitted in the wing pylons, the AUP arrangement was done at the time to avoid the costs of a pylon rework and associated flight test program. To carry the JDAM internally it will therefore be necessary for the RAAF to relocate the pylon decoders either into the wing pylons or along the aft roof of the weapon bay (the latter a technically much simpler process, but requiring clearance from the stowed Pave Tack pod and bomb tails), put the original MAU-12 ejectors back into the weapon bay, add a pair of decoders for the weapon bay stations and make some appropriate wiring, connector and software alterations.

It must be argued that now is the appropriate time to do this, as few aircraft have been upgraded as yet and the changes are significantly cheaper to do now than refitting all 22 airframes in 2002. In defence of the RAAF's existing weapon bay usage, it must be stated that at the time of the AUP project definition the JDAM did not exist even on paper and laser guided bombs using Pave Tack were expected to be the only precision direct attack munitions to be used over the life of the aircraft, which is now expected to extend to 2020. Hopefully the RAAF will act on this matter soon as the long term savings outweigh the short term inconvenience. Paveways may not be as affordable or available in 2005 as production winds down and Paveway stocks are expended or timexed.

Regardless of the utility of the JDAM when used with the F-111C AUP, this weapon is the cheapest and most practical means of providing the F-111G with an off the shelf precision strike capability. The F-111G does not have a Pave Tack capability, but it does have a usable internal weapon bay with MAU-12 ejectors. Published USAF tables indicate that the aircraft could carry up to six SRAMs or B-series weapons, all similar in size and weight to the GBU-31 2,000 lb JDAM. Subject to clearance testing and stores control support to initialise the weapon (see AA July 96), the F-111G could carry either four JDAMs externally, two JDAMs internally or up to six JDAMs in total. This would allow the RAAF to truly get its money's worth from the additional airframes, at a minimal cost.

The Rockwell Mk.82 GPS guided Tailkit

The USAF has nearly 100 B-1B aircraft in service, and these are now assigned to perform both conventional and nuclear missions. In the conventional strategic strike role, the aircraft can at this time only deliver either Mk.82 or Mk.84 dumb bombs, the former off bomb racks and the latter off a rotary launcher. USAF ACC were unhappy with this limitation and contracted Rockwell to design a new bomb rack to support ten 1,000 lb cluster bombs rather than 28 Mk.82. A further contract has been let to provide support for the Wind Correct Munitions Dispenser (WCMD), an inertially guided cluster bomb.

Clearly impressed with the B-2 GAM/GATS program, the USAF is seeking a similar capability for the B-1B and USD 15M has been allocated to modify two aircraft and supply 200 weapons by late 1997. The USAF at the time of writing were yet to decide whether to tender for the supply, or award the contract directly to Rockwell. MDC are offering a Mk.82 compatible JDAM derivative, using existing JDAM guidance in a smaller tailcone. The B-1B is expected to carry 24 2,000 lb GBU-31 JDAMs on rotary launchers when the weapon becomes available in 2000, the planned Block D avionic upgrade will include support for JDAM. Three JDAMs were successfully test dropped from a B-1B earlier this year.

Rockwell have offered the USAF a GPS/inertial tailkit for the Mk.82, which is designated the Mk.82 GT (GPS-guided Tailkit). The Mk.82 GT uses an infrared transceiver on each bomb rather than a Mil-Std-1760 bus interface (the test program accordingly designated the BVUD or B-1 Virtual Umbilical Demonstration). The Rockwell offer involves 5,000 Mk.82 GT kits at USD 15k/round, and the modification of six bombers to carry up to 84 rounds.

The Mk.82 GT is like the JDAM, a compact tailkit assembly (see photo) which allows the weapon to be carried in place of the standard dumb Mk.82. Rockwell's press release indicates that the weapon will also be offered for use on other aircraft types, where it will compete with MDC's planned Mk.82 JDAM version. The development program lasted 18 months from concept definition, and a live weapon was tested in a 25,000 ft altitude drop from a B-1B at the China Lake test range. Rockwell have stated that the weapon's achieved test performance exceeded the intended goal for the test drop (we can presume this refers to accuracy). Production rounds are expected to become available for deployment in 1997.

The Texas Instruments AGM-154 Joint Stand Off Weapon

The JSOW glide weapon is the most sophisticated, complex and expensive member of the new family of weapons planned for US deployment. It is intended to equip the USAF F-15E, F-16C, F-111F (again before its early retirement), F-117A, B-1B and B-2A, and the USN/USMC F/A-18C/E and AV-8B aircraft. The JSOW, like the BAeA AGW, is an offspring of the USN Advanced Interdiction Weapon System (AIWS) program. The AIWS was conceived to plug a hole in existing capabilities as well as to replace the obsolescent post-Vietnam era Walleye glidebomb. The primary role of the weapon is to enable indirect attack against vehicular, air defence and other soft or semi-hard targets from outside the range of point air defences, with a lethality similar to that provided by cluster weapons such as the Rockeye and APAM, and with high accuracy. The initial AIWS requirement was for a weapon compatible with existing USN air assets and the now dead A-12 Avenger II, with a stand-off range in excess of 5 NMI, cluster warhead and inertial guidance.

The design was constrained in size by the AV-8B which needed a 1,000 lb class weapon, as well the diversity of target types quickly led TI's designers to look at variants with different warhead types. The current design has provisions to support USN, USMC and NATO launchers, and is built for a gross weight of up to 1,500 lb.

In 1992 the USAF bought into the AIWS program, which was then redesignated the JSOW and became a tri-service program with the USN the lead service.

The JSOW is a modular weapon and four variants are currently in the pipeline. All variants share the basic airframe and navigation guidance system, but differ in payloads and in some instances, a seeker will be added. The intent was to produce a reconfigurable "bomb truck". The nucleus of the JSOW navigation/guidance is a mission computer package with a pair of Milspec Intel 486 33 MHz CPUs, a Singer Kearfott inertial package and a GPS receiver. Power to the guidance systems and Lucas Aerospace control actuators is provided by Eagle Pitcher thermal batteries. The weapon is programmed through a Mil-Std-1760 or 1553B interface much like a JDAM.

The JSOW design will provide a standoff range of 15 NMI for a low level release, and 40 NMI for a high altitude release. The weapon can turn through 180 degrees to engage off boresight targets. Moreover, the smarts provided by two fast CPUs allow the weapon to perform some very clever tricks. On release the weapon will separate laterally from the launch aircraft, before it deploys its wings and commences its glide, to ensure safe clearance. Once programmed, as long as it is released at such an altitude and range to be able to aerodynamically reach its target, it will autonomously calculate the flightpath and profile to correctly engage its target. If released at high speeds, it will delay wing deployment to avoid penalising its range by drag, the wing is deployed when most appropriate. The weapon can also be programmed to attack a target from a specific heading, and to fly between multiple programmed waypoints. A typical profile will see the weapon glide in at several hundred feet, pop up close to the target and dive in to dispense its payload from several hundred feet.

The first variant of the weapon is the AGM-154A, termed the baseline JSOW. This weapon is intended for use against soft targets such as parked aircraft, vehicles, SAM sites and mobile command posts, and for close support of troops on the deck. It carries 145 BLU-97A/B Combined Effects Bomblets (CEB), a munition which was used in cluster bombs during the Gulf campaign and was very popular with its USAF users. Each CEB has a conical shaped charge which can punch through 5 to 7" or armour, a main charge which produces about 300 high velocity fragments, and a Zirconium sponge incendiary element. The CEBs are deployed in a dive, the JSOW uses pyrotechnic charges to blow off the payload bay doors, after which a gas generator inflates an internal aluminium bladder which breaks a set of retaining straps and ejects the CEB payload.

The second variant of the JSOW is the AGM-154B. This variant is a specialised anti-armour weapon, which carries six sticks of Sensor Fused Weapon (SFW) submunitions. The SFW is the production derivative of the Skeet weapon developed for the original DARPA Assault Breaker program (Tacit Blue, Pave Mover, JSTARS see AA Sept 1984). Each SFW stick is retarded by drogue chute on release, upon which it fires a solid rocket which via canted nozzles spins the device to a high RPM whilst converting its descent into a climb. At this point the four skeet submunitions are released in a clover leaf pattern. Shaped like ice hockey pucks, and unbalanced so they wobble as they fly, the Skeets each have a simple two colour infrared sensor which searches for a tank signature using the wobble to produce a classical conical scan pattern. Once the Skeet detects a tank, it fires its shaped charge warhead which propels a metal pellet formed from the Skeet body (termed a self forging penetrator) through the soft top armour of the victim tank. The JSOW's intelligent navigation system allows it to be programmed to follow a road and dispense the SFW sticks at programmed aimpoints, accounting for wind and weapon velocity. Planned USAF improvements to the SFW submunition will include a better infrared sensor and a warhead which will produce a slug and a shrapnel pattern.

The third variant of the JSOW is the AGM-154C, intended specifically to replace the USN Walleye glidebomb. This variant carries a 500 lb BLU-111 Mk.82 blast fragmentation warhead, and a thermal imaging terminal seeker and datalink. The datalink is compatible with the Walleye AWW-13 pod, and allows the operator to select an aimpoint for weapon impact. TI are proposing an autonomous version.

The fourth variant of the JSOW is proposed but at the time of writing not funded for production. This subtype is the powered JSOW, which is equipped with a Williams International turbojet powerplant common to the BQM-74 drone and offers in excess of 120 NMI of standoff range. The weapon aerodynamics were successfully proven in 1995 flight trials and the design has been offered for the RAF CASOM and US JASSM requirements. The basic navigation system can be programmed with 14 waypoints, and provides for throttle control. The weapon would carry a 500 or 800 lb warhead, and use an autonomous or datalink imaging seeker derived from the design used for the cancelled Northrop AGM-137 TSSAM weapon.

US reports indicate that despite its complexity, the JSOW program has been very successful to date and an IOC well before 2000 is expected for the baseline variant. From the Australian perspective, the weapon is potentially useful but due existing policy which discourages the use of cluster munitions, is unlikely to be a candidate for deployment. Should policy on cluster warheads change, then the weapon could be a useful tool for defence suppression and interdiction tasks.

Summary

Australia should not fall behind in the GPS weapons game, the payoff in exploiting GPS is simply too lucrative to ignore. By the same token, Australia should look very hard at how it exploits the technology, and ensure that any GPS based weapons acquired are sufficiently resilient to Electronic CounterMeasures to prevent jamming from compromising what has been gained. As the JDAM is expected to wholly displace the Paveway in US service during the first decade of the next century, the RAAF will have to look very carefully at what direct attack bread and butter munitions it plans to use on the F-111 and F/A-18 in this period. The JDAM may well find itself in service by default.

A follow on TE will address the USAF EDGE wide area differential GPS demonstration program, which has yielded some very impressive results, including positioning errors over wide areas of well below 1 metre.


Acknowledgments:

Special thanks to Dr Don Kelly then of the USAF EDGE Program, the USAF JDAM Program Office and Texas Instruments for their reviews and comment on the draft of this article.


Pic.1/2 (F-18, F-111 / JSOW)

The 1,065 lb TI AGM-154 is an autonomous gliding "bomb truck", built in a number of variants for use by tactical fighters and strategic bombers. The munition's intelligence is provided by a pair of Intel 486 CPUs, which enable it to be programmed with aimpoints, waypoints, delivery profiles and energy management algorithms. The baseline AGM-154A dispenses 145 BLU-97 Combined Effects Munitions, the anti-armour AGM-154B deploys 24 Skeet Sensor Fused Weapon submunitions, while the AGM-154C carries a 500 lb BLU-111 warhead and datalink in the USN subtype. A lightweight weapon built to defeat point defences, the JSOW can be carried one per hardpoint, as illustrated by the depicted F-111 and F/A-18, or in pairs on multiple ejectors for a total of eight rounds on an F-111 (Texas Instruments).

Pic.3 (JSOW submunition release)

This AGM-154A JSOW is releasing its payload of 145 Combined Effects Bomblets, each of which has an armour penetrating, fragmentation and incendiary element. The AGM-154A "baseline" JSOW is intended for use against soft skinned targets such as vehicles, parked aircraft, SAM sites and radars, command posts and personnel. The weapon will be used by the USAF, USN and USMC on a wide range of aircraft, and allows targets to be engaged from outside the range of point defence and many lower performance area defence missiles (Texas Instruments).

Pic.4 (Rockwell Mk.82 GT)

The Rockwell Mk.82 GT is a low cost 500 lb class GPS aided munition which has been proposed for the USAF's B-1B force. Unlike its larger cousins, which employ Mil-Std-1760 bus umbilical interfaces for prelaunch initialisation, the Mk.82 GT employs an infrared transceiver using similar technology to cordless keyboards. This significantly reduces the costs of fitting a bomb bay for guided weapons. The B-1B has a total capacity, within its three voluminous bomb bays, or no less that 84 500 lb Mk.82 bombs. Providing these with accurate guidance means that the B-1B is likely to supplant the B-52 as the USAF's "sledgehammer" heavy bombing capability.

 


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