PAUL LEWIS / WASHINGTON DC
As precision-guided weapons improve with each conflict, so do defences. But there may not be sanctuary against a new US initiative
Precision-guided weapons (PGM) have made a quantum leap in capability since the first Paveway laser-guided bombs (LGBs) were used in Vietnam 30 years ago. But while technology advances with each successive conflict, so do enemy counter tactics. Iraq in 1991 took full advantage of cover of darkness to move and fire its Scud missiles with virtual impunity, while eight years later adverse weather sheltered Serbian forces in Kosovo from all but the latest generation of GPS satellite navigation-guided weaponry. More recently, Taliban vehicles relied on speed and manoeuvrability to try and dodge allied air attacks. Now, thanks to a US Defense Advanced Research Projects Agency (DARPA) demonstration, agility and mother nature may no longer provide sanctuary in the next war.
The Affordable Moving Surface Target Engagement (AMSTE) programme was launched following allied combat experience in the Balkans, where mobility and short dwell times made targets difficult to destroy. Among the PGMs available today, LGBs are capable of hitting a moving target, but require clear skies and close-in weapon delivery. While GPS-guided weapons such as the Boeing GBU-31/32 Joint Direct Attack Munition (JDAM) are immune to bad weather, they are confined to static targets. The only other possible solution, the acoustic/Infrared-guided Northrop Grumman Brilliant Anti-Armour submunition, was effectively killed by Congress in the latest defence bill.
AMSTE is not a weapon, but a network. It harnesses current intelligence, surveillance and reconnaissance (ISR) platforms, fuses their ground moving-target indicator (GMTI) data to form a single integrated picture and datalinks a continual feed of targeting updates to GPS/intertial navigation (INS)-guided weapons already in flight.
Target tracking
"AMSTE can track a target nominated for engagement and maintain positive control until a weapon is in place to engage it. This is not as easy a task as one might believe on a battlefield where you have random positions, confusion over high-density traffic and ISR platforms that don't always have line of sight," says Chuck Taylor, DARPA AMSTE programme manager.
At least two GMTI radars are required to generate a cross reference or bi-lateration radar track. During the initial competitive phase in 2001, Raytheon employed an ASARS-2A-equipped Lockheed Martin U-2 and Northrop Grumman RQ-4A Global Hawk unmanned air vehicle (UAV) sensors. The rival Northrop Grumman team, which was later to emerge as the winner, has used a combination of E-8C Joint Stand-off Targeting Attack Radar Systems (JSTARS) with modified software and additional communications, together with the company's BAC One-Eleven Joint Strike Fighter (JSF) radar testbed.
In addition to serving as a JSF surrogate in the GMTI forward-looking mode, the One-Eleven radar was modified this year to look sideways to simulate the planned future Global Hawk equipped with an electronically scanned array. Under the US Air Force's Multi Platform-Radar Technology Insertion Programme (MP-RTIP), scalable phased- array antennas will be fitted to the high-altitude RQ-4A and the planned Spiral One variant of the Boeing 767-400-based Multi-sensor Command and Control Aircraft (MC2A) that will eventually succeed JSTARS from around 2012.
While future strike aircraft such as the Lockheed Martin F-35 JSF and Lockheed Martin/Boeing F/A-22 Raptor will have GMTI radar modes that promise to work well in the final stages of an engagement, neither is ideal for fully exploiting AMSTE's stand-off tracking capability. "MP-RTIP will right away be able to implement AMSTE, and when you're talking about bi-lateration using two GMTI radars to create a precise location for weapon quality tracking, Global Hawk teamed with JSTARS is probably ideal," says Bill McCall, Northrop Grumman AMSTE programme manager.
AMSTE not only lends itself to a range of current and future ISR platforms and sensors, the system's other major selling point is its ability to use adapted GPS/INS weaponry already in the USAF and US Navy arsenal or under development. The demonstration to date has logged six separate weapon drops since the summer of 2001, all of them successful engagements with a 10m (33ft) circular error of probability, according to DARPA. During the competitive phase, the Northrop Grumman and Raytheon teams employed the Lockheed Martin Precision Direct Attack Munition and a modified Raytheon AGM-65 Maverick missile, respectively. The former is a seekerless, winged bomb that until recently was a classified experimental development (Flight International, 15-21 January).
DARPA and Northrop Grumman have since expanded testing against remotely controlled targets to include single and multiple 900kg (2,000lb) JDAM drops from Grumman F-14D Tomcat and Lockheed Martin F-16 fighters, respectively. The F-14D demonstration on 30 September was conducted from a height of 20,000ft (6,100m) and a range of 11km (6nm), simultaneously targeting the second and third vehicles in a five-vehicle convoy moving at 18km/h (11mph) across the China Lake, California, range. Later that same day a Boeing F/A-18D launched a Raytheon AGM-154 Joint Stand-Off Weapon (JSOW) from 30,000ft and a range of 65km against an M60 main battle tank manoeuvring at 13km/h.
Target location
In both cases, an E-8C and the One-Eleven testbed provided precision real-time target location and velocity to the weapons via a datalink - Link 16 in the case of JSOW and a UHF anti-jam link for JDAM. The former has the advantage of being in production and in the NATO inventory, but is not small or inexpensive enough to fit to weapons. UHF, while relatively inexpensive and compact, is not widely used and would have to be retrofitted to current ISR platforms. "Northrop Grumman is working several offerings for the US military, looking at a particular datalink so that they could implement this capability quickly," says McCall.
The demonstration programme has been designed to get progressively more complex and demanding as it moves into the third and final year of testing in 2003. Set for December is an exercise at the USAF's Eglin range in Florida involving around 30 different vehicles and different scenarios, including overtaking and intersecting traffic. During the Kosovo conflict it was not uncommon for Serbian military vehicles to mix with civilian traffic. AMSTE in high-density traffic will rely on other radar modes such as Doppler or high range-resolution to detect more discriminatory signatures coming off individual vehicles, such as the number of axles, to help maintain an identifiable target. AMSTE will include a maritime mode for tracking small fast boats.
DARPA and Northrop Grumman will conclude the demonstration towards the end of next year with a graduation exercise at Eglin that will require targets tracks to be maintained not only through complex traffic patterns, but also where vehicles stop and start and mask themselves behind terrain. According to pilot action reports from Afghanistan, it was not uncommon for Taliban and al-Qaeda vehicles to deliberately stop and start along a road to avoid being targeted by a laser designator. This presents a challenge for GMTIs such as on JSTARS, which have a minimum detectable velocity (MDV).
"Velocity on the ground is not really an issue, what's more important is that a target doesn't go too slow. These are moving target radars and, in comparison to either of the radars, if a vehicle starts to drop below MDV, then you have a problem," says Taylor. In an effort to maintain a track when a vehicle slows, the GMTI automatically switches to a move-stop-move model, which projects a track to where the vehicle will stop based on the rate of deceleration. At this point other sensors take over, such as synthetic-aperture radar that can be used for targeting or maintaining a track until the vehicle starts moving again.
Similarly, if a vehicle disappears behind a mountain and is moving at a constant speed, a "coaster track" will be declared until a similar profile target emerges on the other side of the terrain. The target will remain ambiguous, however, until confirming evidence can be found to ensure that it is the same vehicle. As more sensors are added to the net, the possibility of a target being able to disappear totally from view behind terrain becomes more remote. The ultimate solution might be space- based radar and, although it is not on the planning scope for AMSTE, Taylor says it could be "a possibility" in the future.
While $50 million has already been sunk into the demonstration and, from a technology standpoint, it is viewed as relatively mature, additional work is required before AMSTE weapon-modification kits could be put into series production. Further investment is also needed to develop the system's supportability and sustainability before it could be fielded.
McCall nonetheless estimates that "within nine to 12 months we could have a capability in the field". With war clouds once again gathering over Iraq and mobile surface-to-surface missile launchers a clear and present threat, AMSTE could find itself on a fast track to deployment.
Source: Flight International
