Technology Explained — GPS Part 3 — US Direct Attack Munitions

The Northrop Gam

The Northrop GPS Aided Munition (GAM) kit was devised by Northrop engineers as a means of providing the B-2 bomber with a precision conventional attack capability. The B-2 was initially developed to defeat the Soviet integrated air defence system in a nuclear war scenario, using its 18,160kg (40,000lb) payload of SRAM missiles and free fall nuclear devices to hit key strategic targets, as well as carve corridors through defences to allow the conventional B-1B and B-52 to attack other strategic targets. With the demise of Reagan’s ‘Evil Empire’ and the shifting bias toward conventional war, the B-2 needed a precision conventional capability, something not achievable by dumping 20 tonnes of Mk.82s.

The GAM is the first GPS/inertial guided munition to be operationally deployed, with the first batch of weapons achieving operational status earlier this year. The GAM is often described as an ‘expensive JDAM’, and the weapon owes its origins in part to early USAF studies and technology demonstrations under the Inertially Aided Munitions (IAM) program, which determined the feasibility of using inertial guidance on a bomb and eventually led to the JDAM program (described below).

The Mk.84 GAM comprises a 45kg (100lb) tailkit which fits into the standard Mk.84 configuration. The tailcone contains a pair of thermal batteries which power the munition, a servo motor assembly which actuates the four fully movable tailfins, and a guidance system, which comprises a high performance GPS receiver, an inertial package with accelerometers and rate gyros (the same as used in the AIM-120 Amraam), and a computer running the guidance algorithm and autopilot software. Two GPS antennas are used, one dorsal and one at the end of the tailcone, the latter to provide a strong GPS signal during the terminal phase when the bomb is pointing downward. The interface to the launch aircraft is through a Mil-Std-1760 umbilical, which incorporates the Mil-Std-1553B serial databus and prelaunch power feed. The dorsal umbilical connector feeds through a rigid channel into the tailcone assembly.

The GAM has proven to be highly accurate during trials, with better than 6m (20ft) CEPs achieved consistently for launches from 15,000 to 45,000ft. The reason for this high level of accuracy is the B-2’s GPS Aided Targeting System (GATS). The GATS is built around the B-2’s Hughes APQ-181 J band phased array Low Probability of Intercept attack radar (a worthy TE topic within itself), which is capable of producing highly accurate focussed Synthetic Aperture Radar (SAR) imagery of a target area as the bomber approaches. The B-2 will attack its target flying a curved trajectory to enable the SAR to generate images. Nine minutes out and with the target cca 45 degrees off boresight, the B-2 will image the target and the copilot/mission commander/navigator/bombardier (all in one) will use crosshairs on his cockpit scope to designate aimpoints for the weapons on the radar map. Ninety seconds off the target, the radar again generates an image and the aimpoint(s) are if necessary refined. The GAMs assigned to the target are then initialised via the 1760 interface with the target coordinates and the constellation of satellites which the bomber’s GPS receiver is tracking. The bombs are then released and track to impact.

High accuracy is achieved because the bombs see the same satellite constellation the bomber sees, and thus experience almost identical GPS errors to the bomber. The bombs are initialised with an aimpoint relative to the bomber, rather than an absolute set of map coordinates, and the primary errors are then determined by the inaccuracy of the bomb’s guidance algorithms and the range/bearing calibration error of the radar. This scheme is very clever and a tribute to Northrop and Hughes’ engineers.

The standard GAM is built around a Mk.84 blast fragmentation warhead. Experience from the Gulf however indicated that standard 2000lb (910kg) bombs were ineffective against deeply buried bunkers, this leading to the hurried development, deployment and use of the laser guided GBU-28 and the BLU-113 4500lb (2045kg) penetration casing warhead. The warhead was clearly a candidate for GPS guidance and Northrop Grumman engineers adapted the existing GAM design to fit the weapon. The 2130kg (4700lb) 5.13m (202in) long BLU-113 GAM uses a modified GAM tailkit with an adaptor fairing, extended 1760 umbilical and autopilot software changes. As well the weapon has a cruciform wing assembly to improve lift and thus both range and manoeuvre performance, the latter contributing to accuracy.

The B-2 will carry up to 16 2000lb (910kg) Mk.84 GAMs or a much smaller number of BLU-113 GAMs, the latter presumably carried on a modified rotary launcher. While the ability to carry in effect four F-111 loads of precision weapons at intercontinental ranges under any weather conditions is impressive within itself, the ability to engage targets with total surprise and virtual impunity makes the B-2/GATS/GAM the most potent conventional bombing capability in existence today. The BLU-113 provides the further capability to attack hardened and buried targets such as key command posts and nuclear biological and chemical weapons storage sites.

Northrop Grumman has made the interesting observation that a single B-2 with 16 GAMs has the equivalent capability to a pair of B-52s delivering 32 cruise missiles, with a total munition cost of $US640,000 ($US40,000/round) vs$US32m for the cruise missiles. While the exact figures in this scenario can be debated, it does provide an excellent order of magnitude indication of the force multiplication provided by combining stealth and GPS guided weapons. It is clearly the way of the future.

The Mcdonnell Douglas Gbu-31/32 Joint Direct Attack Munition

The JDAM program is the direct offspring of the 1980s IAM program. While IAM initially sought to improve the accuracy of tossed unguided weapons, the incorporation of GPS into the concept improves accuracy to a point where it compares very well with less accurate types of laser guided munitions. This concept was validated by the USAF’s Operational Concept Demonstration (OCD), an end to end demonstration of INS/GPS guidance including targeting, weapon development, and flight test. OCD proved conclusively that the JDAM concept was a low technical risk and ready for accelerated development, leading to early deployment.

The JDAM development was then initiated in the early 90s, when Gulf War experience indicated the need for an all weather highly accurate or precision munition. Poor weather conditions on many occasions compromised sorties armed with laser guided weapons and an alternative was sought to arm USAF, USN and USMC strike aircraft.

The baseline JDAM program provides a design for the GBU-31/Mk.84 and BLU-109 2000lb (910kg) weapons and the GBU-32/Mk.83 and BLU-110 1000lb (455kg) weapons for use on the USAF F-15E, F-16C, F-117A, F-22, B-1B, B-2A and B-52H, the USN F/A-18C/D F-14A/B/D and the USMC AV-8B and F/A-18C/D. The baseline accuracy for the weapon is a CEP of 13m (42ft) with a target volume production cost between $US14,000 and$US25,000, which is highly competitive with laser guided weapons. Martin-Marietta (prior to the merger with Lockheed) and MDC competed for the lucrative contract, with MDC winning the eventual prize.

The USAF’s JDAM Product Improvement Program (PIP) is currently in requirements definition and is evaluating concepts for increased accuracy, improved anti jam capability, increased range, and compatibility with various warheads, including several small, highly lethal warheads under development by the USAF’s Wright Laboratory in Florida. The small bombs are intended to provide aircraft such as the F-22 and JSF (formerly JAST) with a credible strike capability using internally carried weapons.

US industry sources suggest that JDAM accuracy improvement may involve the use of a millimetric wave radar seeker, which would employ SAR techniques and terrain contour matching to achieve precision delivery accuracy. Whether this is required, given the availability of Scene Matching Area Correlation algorithms and existing millimetric wave seekers such as that used on the BAe Merlin mortar round, is clearly open to debate. In any event, a number of techniques exist for using miniature radar seekers to refine the bomb’s aimpoint.

The MDC JDAM kit comprises a tail kit and a set of cruciform body strakes, pairs of which are shipped to a deployment site in a hermetically sealed, stratified polythene bags inside foam lined fibreglass shipping container. On site the kits are attached to warheads and loaded on aircraft. The shelf life of the sealed package is 20 years and maintenance is not required. The weapon is compatible with the standard US AERO-51, MHU-141, MHU-191 and MJ-40 bomb trailers/hoists.

The strakes on the JDAM increase the body lift of the weapon (cf Standard SAM) and thus contribute to better manoeuvrability and accuracy, as well as a slight gain in delivery range against a standard bomb.

The tailkit structure is low cost sheetmetal, and comprises the tailcone, the cruciform fins, three of which are moveable and one fixed, and the guidance package. The latter consists of a tail actuator package with three servoes, a GPS antenna, a thermal battery and Guidance and Control Unit (GCU). The GCU contains an inertial package (IMU), GPS receiver, mission computer, and electrical power conditioner. The weapon uses a Mil-Std-1760 interface, although other alternatives could be supported with appropriate hardware and software interfaces.

Part 4 will complete our series on GPS aided weapons and address the Joint Stand Off Weapon (JSOW) and other emerging US GPS aided weapon programs.

Acknowledgments: Special thanks to Dr Don Kelly then of the USAF EDGE Program and Lt Col Greg Teman of the USAF JDAM Program Office for their reviews and comment on the draft of this article.