DARPA eyes missile-defense system for vehicles

WASHINGTON-While anti-missile defenses of cities and nations have captured public attention since the Persian Gulf War, engineers at the U.S. Defense Advanced Research Projects Agency (DARPA) also have been working toward a system capable of small-site and platform protection from missile threats at ranges from 50 to 400 meters.

Oct 1st, 1997
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DARPA eyes missile-defense system for vehicles

By J.R. Wilson

WASHINGTON-While anti-missile defenses of cities and nations have captured public attention since the Persian Gulf War, engineers at the U.S. Defense Advanced Research Projects Agency (DARPA) also have been working toward a system capable of small-site and platform protection from missile threats at ranges from 50 to 400 meters.

DARPA`s Small Low-cost Interceptor Device (SLID) program is to provide self-defense capability to combat vehicles and high-value fixed sites such as command centers, parked aircraft, and radars. SLID`s capability may be extended to naval ships and low-speed aircraft.

The system will use a small, maneuverable hit-to-kill interceptor, high-speed launcher, passive threat warning sensor, and precision fire-control system that can react with split-second speed, given the range, speed, and nature of the incoming threats. SLID can defend against anti-tank guided missiles, high-explosive anti-tank (HEAT) rounds, mortar rounds, and artillery shells. Designers are placing an emphasis on maximum component commonality between the vehicle self-defense system and the area defense system.

Engineers at Raytheon Co. of Lexington, Mass., and the Boeing Infor-mation, Space & Defense Systems Group of Duluth, Ga., are developing competing prototypes of SLID and have taken decidedly different technological approaches to the question of battlefield guidance. Raytheon`s version is computer-controlled from the launch platform, which sends information to the projectile through a trailing fiber-optic cable. Boeing designers, on the other hand, have elected to use a platform-based laser designator to "paint" the incoming target for a laser-guided projectile.

Raytheon`s director of Army programs, Rick Macey, says he believes this would be the first fielded fiber-optic-guided system. In addition, he says Raytheon`s entry would be far less susceptible to countermeasures than Boeing`s laser-guided system.

"When you start working with countermeasures and counter-counter, the cleaner or more closed-loop - such as with a fiber optic line - the better off you`ll be," Macey says. "If you are using RF or laser or anything that is spoofable to direct your round, you will put things on your own outgoing missiles to defeat those, so it just escalates and you just keep adding another counter in front of that as the technologies escalate."

Boeing SLID program manager Ben Fraser takes a different view: "There may conceivably be countermeasures out there in the future we would need to work around, but we currently are not in a countermeasures-rich environment. Fiber optic is a clean connection, but if you tie yourself to a fiber-optic cable technology, you limit your range on down the line when you may want to extend your capability. And against our current threat sets, our accuracy is just as good at 250 meters as at 100 meters because of how we do terminal homing. We`re also running on coded laser signals - codes of the day the enemy probably wouldn`t know about - and a narrow beam, so we`re not exactly naked to counter-measures."

The technology that makes it feasible to place another piece of equipment on vehicles such as Abrams main battle tanks or Bradley fighting vehicles is largely an amalgamation of existing elements rather than any new breakthrough. Nonetheless, Macey says it cannot be categorized as a commercial-off-the-shelf effort because "this program does have development elements to it."

A primary concern is creating a sensor that can, with equal high-speed capability, not only look outward for missiles, but also look up for mortar and artillery shells. "I`m not aware of anything that has been found to be insurmountable in accomplishing this, but it has not been accomplished yet in the field," Macey says.

Raytheon`s SLID design incorporates existing sensors such as millimeter wave radar, with the fiber-optic guided round. "It does not need to be a standalone sensor. It could use an existing sensor already designed to identify incoming," Macey says. "All you have to do is find out where [the threat] is and then guide to it."

Thus an existing sensor that detects an incoming round to slew a tank`s main gun around for return fire also could also provide preliminary area guidance to SLID. But to accomplish hit-to-kill capability, the system needs much more precise knowledge of the target than existing sensors can provide.

"Conceptually, you would want the thing to be automatic to the point where it would be quick enough to respond, but with some override control in the event you might be jeopardizing somebody else," Macey says. "But since what you`re trying to do is destroy an incoming round, there doesn`t have to be too much there."

Boeing designers take a rather different approach for their three-part prototype, which combines threat warning and fire control, a four- or six-barrel launcher, and the interceptor. Early interceptor designs are 21 inches long, 4 inches in diameter, and weigh about 16 pounds. Boeing engineers expect eventually to shrink that down to 10 pounds.

The baseline system uses a wide-field-of-view midwave infrared (IR) focal plane array sensor of 640-by-480-pixel resolution. Four sensors on the vehicle will do the cueing against missiles. Fraser says the microwave sensor has proven adept at picking up such indicators as launch flares against a clutter background.

For colder threats from higher elevations, "we will use longwave IR cueing, which gives a better signal to noise against a sky background," Fraser says. "And we`re looking at both staring wide-field-of-view and SADA technology, which is a 4-by-480 focal plane.

"The time line is a bit longer for a top attack," Fraser continues. "A HEAT round fired from a tank at a 1-kilometer range requires an instantaneous cue in order to shoot it down before it hits us. Mortar and artillery don`t come at us nearly as fast. So a step-stare looks at a portion of the sky for a time, then steps around to look at another section and so on rather than using an array of wide-field-of-view systems."

While midwave and longwave focal plane array technologies have been around for a while, the "cutting edge part comes when you try to build up systems with extremely large focal plane arrays," Fraser says. "Standard now is 256 by 256, but we have developed them up to 640 by 480. That technology will be incorporated when we make the midwave tactical system. The step-stare technology for the longwave would be considered cutting edge."

Software is a separate challenge. "The software that actually picks out the targets and rejects clutter is based on legacy IR tracker technology we`re keying off of, so the fundamental pieces of the algorithms are things we`ve worked on at Boeing for the past several years," Fraser says. "But we have to fine-tune those algorithms so the system functions most efficiently against the targets we`ll face."

Based on the system to be defended and the anticipated threats against it, engineers may choose either an outlooking direct-attack warning or upward-looking top-attack warning.

"Or you could build up a system that incorporates both. The system is modular enough to do that," Fraser adds. "Currently our concept calls for the threat warning system to be an integral piece of the fire-control system, but that`s not a mandatory requirement. The bottom line is, from any cueing system you might choose to use, you have available assets. You could even use an existing radar from a radar site being defended as the cueing system."

If that system only narrows the field down to a half-quadrant or so, however, it would not be accurate enough to be of much help. The system needs to pinpoint the threat to within at least a few degrees if it is to focus in and acquire it in time to destroy it. Minimum range must be roughly 50 to 150 meters for a Bradley or Abrams, and about 250 meters for a soft target, although Boeing officials claim their SLID can engage a threat as far away as 400 meters.

Tracking software picks out the threat and the midwave IR locks on, sharing the optical path with a co-boresighted 1.06-micron laser designator/tracker/rangefinder. Thus the IR fine track sensor tracks the target and holds the laser beam on the candidate threat. The red laser return is then read by a laser receiver in fire-control system, pulsing the threat at a 120 Hz rate in the tactical configuration. The computer uses this data to quickly define the speed, direction, range, and time of arrival of the potential threat. It also can estimate the impact point, including the possibility it will hit somewhere else.

"If you are defending a helicopter pad or radar site rather than just the platform vehicle, you are interested in knowing if the threat is going to strike somewhere in the defended area," Fraser says. "All of which we can tell by evaluating that laser return. We can evaluate three threats in rapid order, for example, determine which threats to shoot at and in what order. The real threats are then stacked into a threat table and [the computer] determines the optimum time to shoot."

Once a commitment to engage has been made, the laser pulse rate increases to 720 Hz and the tracking function passes from the midwave IR to the laser receiver. The system then goes into autotrack mode, sending a command to the launcher telling it where to point, and launches the interceptor.

The interceptor has a short-burn rocket motor that burns out about 15 meters out of the launch tube, at which it is moving at 250 meters per second. It carries a strapdown laser seeker, similar to Hellfire technology, but not gimbaled, which homes in on the reflected laser energy coming in off the threat.

The SLID program is entering its final phase of testing in 1998 as an advanced technology demonstrator.

"DARPA is working with the Army to determine where to go with the program next, having pretty much demonstrated what they set out to do," Macey says. "What is left for the next year`s work is indirect fire - taking artillery and mortar rounds. They`ve proved they can sense incoming, and fire the canister in the general direction of the target, but this year the actual hit-to-kill and indirect fire will be done."

DARPA`s program plans for to extend the SLID protection range in 1999, and demonstrate its potential to protect high-value fixed sites and switch the program to the U.S. Army.

In a parallel effort, Boeing experts are evaluating using SLID to protect against anti-radiation missiles, cruise missiles, and unmanned air vehicles. Also under evaluation is an extended-range SLID system concept for defending a wide spectrum of military assets from farther away than 2 kilometers.

Click here to enlarge image

Artist`s rendering of a developmental system intended to detect, track, and destroy incoming missiles, mortars, and artillery rounds before they strike vehicles and fixed sites.

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