Precision Guidance Reaches Small Munitions

March 2004
By Robert K. Ackerman
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Simpler seekers and simpler steering can lead to more on-target warheads.

A new approach to guided munitions may empower small warheads with the same targeting precision employed by larger glide bombs and missiles. The technology takes a low-cost approach to guidance that could improve precision for artillery rounds, mortar shells and grenades for as little as $100 per warhead. Mass-production ultimately could open up the technology for bullets at an even lower cost.

Known as low-cost course correction, or LCCC, the technique eschews existing autonomous satellite-guided technologies in favor of a synergistic approach that combines human targeting with in-flight adjustment. Personnel operating weapon systems would use traditional best aiming and fire control techniques to fire the munition to a point as close as possible to the target. The LCCC system then would use impulse guidance to correct for residual error, adjusting the warhead’s course and bringing it down right on the target.

This type of steering can be achieved without resorting to expensive electronics. A simple electronic system that can be ruggedized easily would provide the guidance based on signals from a directional optical sensor. The target would be identified by a laser designator.

Small explosive charges along the sides of the munition detonate to steer the warhead, even while the projectile is spinning rapidly. Its accuracy can be improved by more than a factor of 10 or even as much as a factor of 50. Potentially, the targeting of a projectile with an accuracy of 1 milliradian can be improved to 50 to 100 microradians by this system.

Dr. Tibor Horvath, head of TGC Associates Incorporated, Falmouth, Virginia, explains that the system attains its low cost by eliminating the expensive components of traditional precision-guided munitions. Horvath’s company is working with General Dynamics Ordnance and Tactical Systems on an experimental LCCC system. Instead of elements such as mechanical aerodynamic actuators, electric motors and high-power batteries—which are difficult to harden—LCCC uses direct energy transfer to steer. This also produces savings in weight and volume by as much as a factor of 100.

The system’s seeker also takes a new approach. Instead of using focal plane arrays, the system uses an uncooled quadrant detector comprising four silicon photodiodes. The detector is positioned behind a lens that conforms to the aerodynamic shape of the projectile. This large-diameter lens provides increased light-gathering capability, which allows de-focusing the image on the seeker. This out-of-focus image of a targeting laser spot is projected on the quadrant seeker. The strength of the fuzzy area of light on each quadrant determines the linear guidance that the system provides to the projectile. If the light area falls equally on all four quadrants, then the warhead already is on target to impact precisely on the target and no impulse guidance is necessary.

Horvath explains that this blurred light spot provides effective guidance even as the warhead closes in on the target. Because the focal length of the lens is only 8 millimeters, the soft-edged area does not begin to expand appreciably on the detector array until the warhead is within about 2 or 3 meters of the target. By this time, any impulse guidance is neither effective nor necessary.

This short-focal-length lens offers other advantages. Where conventional lenses tend to have focal ranges (the lens focal length divided by its diameter) of 1.5 or 2, the LCCC’s high-speed lenses have focal ranges of 0.2 or even 0.15. Horvath describes one lens already built with a diameter of 2 inches, a focal length of 9 millimeters and a focal range of 0.15. The field of view is defined by size of the quadrant detector and the focal lengths of the lens.

The laser designator can take several forms. The standard targeting laser, which is not eye-visible, has a range of about 10 kilometers. It has been tested with LCCC munitions at a range of 5 kilometers. One option is to combine it with a laser rangefinder. If LCCC technology is adapted to small munitions such as rifle ammunition, a sniper rifle could be equipped with this laser designator to supplement the marksman’s expert eye.

These current technologies can be mass-produced for a cost of about $100 per warhead, Horvath claims. He continues that the best way to realize the economic benefits would be to integrate all of the electronics on a single chip. The result would be a single silicon chip containing the quadrant detector and all of the necessary signal processing circuitry. This chip would be far less complex than an Intel Pentium chip, Horvath notes. Additionally, by making this device part of an injection-molded lens, the system would be a single integrated component.

This chip-integrated system also would be much less expensive than its progenitor. A mass-produced bullet-mounted version could provide an infantryman with precision targeting for as little as $20 per bullet, Horvath suggests.

The warhead guidance system currently is powered by two standard 9-volt batteries. However, an LCCC based on a chip-integrated system could be powered by little more than watch batteries.

The microexplosives used for the impulse course-correction also are inexpensive. The number of these explosive squibs would vary according to warhead size. Smaller projectiles would require only a few thrusters, with the number restricted largely by wiring limitations. Bullets could be course-corrected by only a single charge that would be activated when the trajectory builds up an error rate that reaches a predetermined value.

A 25- or 40-millimeter projectile would require about four of these squibs to obtain a tenfold accuracy improvement over a range of 4 kilometers. Larger projectiles would require more charges as they increase in size. A greater force is required to deflect the course of a larger munition, but the charges cannot be increased in size lest they blow up the projectile. So, the system must fire more charges. A 2.75-inch rocket would require 16 or 24 squibs, while a 120-millimeter mortar shell could require as many as 64 charges.

Horvath offers that larger projectiles with greater potential for error can benefit best from LCCC technology. Systems as large as 155-millimeter artillery can achieve 20-fold accuracy improvements. However, with these projectiles requiring large numbers of deflectors, the cost savings might not be as great as for smaller weapons.

Another determinant is the projectile’s trajectory itself. A flatter trajectory can benefit much more from LCCC technology than one with a greater arc. While a mortar, for example, can realize accuracy improvement from LCCC application, the system runs into a field-of-view issue. Half of the projectile’s path is ascendant with the target below the optics for a significant portion. In a mortar application, the LCCC system activates its guidance only after the shell has passed apogee and has begun its descent. Without this guidance from launch, guidance efficiency suffers because the system must compensate for a larger angle of error. Experiments with mortar simulations have shown that this can be addressed sufficiently, Horvath states.

Horvath relates that recent simulations have indicated that only one or two course corrections are necessary for precision targeting. Illuminating a target for about one second—the flight time of a small-caliber munition such as a bullet—will result in a linear accuracy improvement of about 10. In terms of area, this represents an accuracy improvement of about 100.

This kind of improvement can help address logistics concerns. If fewer shots are necessary to hit the target, then resupply can occur less frequently.

Several programs are underway to test the technology on existing munitions. General Dynamics has been experimenting with an upgrade of the 2.75-inch Hydra rocket. Another effort has concentrated on developing an LCCC 120-millimeter mortar round. Work has focused on building a retrofit guidance package and fuse. Both systems recently have been undergoing testing, Horvath relates.

An independent effort has focused on a 40-millimeter warhead, and this has produced some interesting results, Horvath offers. An experimental seeker based on LCCC technology has been integrated with the 40-millimeter projectile that can be fired from a small experimental unmanned helicopter. However, this system has proved to be an excellent retrofit for personnel-launched munitions such as rifle grenades. These weapons may have trajectories with ranges as long as 2 kilometers, but the inaccuracies inherent in these ranges limit effectiveness to only a little more than 100 meters. By providing a way to enable precision targeting, the improved accuracy increases the effective range of the weapons.

Being able to provide this capability to the foot soldier can be of great interest to special operations forces, Horvath continues. Preliminary discussions are aiming at hammering out the possibilities for a program geared to their needs, he says.

Most of the components needed to produce a working LCCC system are available now, Horvath claims. He declares that, within 18 months of being given the go-ahead and adequate funding from the government, an LCCC 40-millimeter rifle grenade could be fielded for use by special forces. Roughly 10,000 warheads could be produced for less than $1,000 apiece without chip-level integration. Given more time, chip-level integration could enable LCCC systems on 40-millimeter rifle grenades for less than $100 each.

Horvath allows that foreign governments have expressed some interest in the technology. Turkish officials have made inquiries, and General Dynamics has obtained an export license to discuss this technology with them. Turkey has its own rocket company that could incorporate LCCC as a less-expensive means of precision guidance.

French officials also have approached Horvath’s company and General Dynamics. Their application involves 120-millimeter autoload rifled mortar rounds.

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