A research project headed by Natan Kopeika at Ben-Gurion University of the Negev (BGU; Israel) has 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 account the presence of atmospheric aerosols--solid particles, often enveloped by water, which are presently neglected in estimating beam spreading. The BGU studies, which were presented at this week's AeroSense conference in Orlando, FL, indicate that air-turbulence conditions, taken today as the sole determinant of beam spreading, must be augmented by additional factors.
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 US 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 (ABLs) are expected to be positioned hundreds of miles, if not thousands of miles, from a missile launch site," says 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 research team, which included graduate students Arkadi Zilberman and Yakov Sorani, found that turbulence strength is greater than what had been assumed previously. Investigators generally determine turbulence indirectly via temperature measurements. Since the Kopeika group used light, they were able to 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, a current project 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 US Air Force, through the European Office of Aerospace Research and Development.