Air Particles Weaken Antimissile Laser Beams

Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva, Israel

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MAXIMIZING OPERATION OF AIRBORNE ANTIMISSILE LASER WEAPONS WILL REQUIRE BETTER KNOWLEDGE OF ATMOSPHERIC PARTICLES

Beer-Sheva, April 17, 2001 - Improved targeting of theater ballistic missiles by airborne laser weapons - a top-priority research and development project of the US Air Force - may be possible thanks to largely neglected atmospheric features under study at Ben-Gurion University of the Negev.

Researchers, headed by Prof. Natan S. Kopeika at BGU's Department of Electrical and Computer Engineering, have found that under many atmospheric conditions, suspended aerosol particles in the path of a laser beam will significantly broaden its strike area, thereby decreasing weapon effectiveness. Optimal positioning of high-power laser-bearing aircraft must therefore take into effect the presence of atmospheric aerosols - solid particles, often enveloped by water, which are presently neglected in estimating beam spreading. The BGU studies indicate that air-turbulence conditions, taken today as the sole determinant of beam spreading, must be augmented by additional factors.

These findings will be presented today at the SPIE 15th Annual International Symposium on Aerospace/Defense Sensing, Simulation, and Controls in Orlando, Florida. Collaborating with Prof. Kopeika are BGU graduate students Arkadi Zilberman and Yakov Sorani.

During the Gulf War of 1991, it became obvious that the United States required better antimissile defenses. To obviate the problems associated with targeting incoming missiles, the U.S. Air Force has chosen to develop a weapon capable of downing missiles soon after launch. It would involve a high-power laser installed on a specially equipped jumbo jet that would detect and track a missile seconds after launch, using the heat of a laser strike to drop the climbing missile on enemy territory.

"Because airborne lasers (ABL's) are expected to be positioned hundreds of, if not a thousand miles from a missile launch site," notes Prof. Kopeika, "the width of the focused high-power laser beam will inevitably be broadened by the intervening atmosphere, lessening its effectiveness. We have, therefore, studied the widening of laser-radar beam pulses sent vertically into the atmosphere and reflected back to earth. Our measurements of the return of high-intensity pulses at three different wavelengths - infrared, visible and ultraviolet - enable us to determine the distribution of aerosol particles, which reflect the three wavelengths to different extents, depending on particle size. We can also measure turbulence directly from the 'dancing' of the position of return of our laser-radar beam."

The BGU researchers found that turbulence strength is greater than what had been assumed previously. Investigators generally determine turbulence indirectly via temperature measurements. Since the Kopeika group uses light, it can measure total turbulence, which includes the effects of humidity and aerosols, as well as temperature. The investigators also measured the direct effect of aerosols on laser-beam widening and found that these effects are often more significant than those of turbulence. "This means," stresses Kopeika, "that researchers must take the aerosol effects into consideration when estimating laser-beam broadening."

For positioning the flying platform carrying ABL weapons, operators should know the most effective altitude for laser activation, namely the height above cloud cover that would result in minimum beam widening. This determination would require a mathematical model able to predict beam widening. Designing such a model, which is now under development at BGU, would involve knowledge of surface weather and the properties of the aerosol layers at various elevations, as well as how these change with seasons, times of day, geographical locale, high-altitude winds, and cloud layers.

Developing a reliable model will take years, particularly since measurements must be carried out 24 hours a day 365 days a year and at strategic points over large geographic regions.

"While the U.S. Airforce is gathering atmospheric data over Korea and the Persian Gulf," says Kopeika, "we are interested in parallel information closer to home. By the time the initial ABL aircraft will be operational, in about 2005, we expect to be a lot more knowledgeable about laser-weapon broadening in our region."

This work has been supported by the Airborne Laser Division of the U.S. Air Force, through the European Office of Aerospace Research and Development. Prof. Kopeika occupies the Reuben and Frances Feinberg Chair in Electro-optics at Ben-Gurion University.