Improving airborne accuracy

Improved targeting of theatre ballistic missiles by airborne laser weapons may be possible thanks to neglected atmospheric features being studied at Ben-Gurion University in Israel.

Improved targeting of theatre 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 being studied at Ben-Gurion University.

Researchers, headed by Professor 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 and decrease 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 how far a laser beam will spread. The BGU studies are said to indicate that air-turbulence conditions, taken as the sole determinant of beam spreading, must be augmented by additional factors.

‘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.’

The BGU researchers found that turbulence strength is greater than what had been assumed previously.

Investigators generally determine turbulence indirectly via temperature measurements. Kopeika’s team uses light, which can measure total turbulence, including 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.

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, according to the BGU team, 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 US Airforce is gathering atmospheric data over Korea and the Persian Gulf,’ said 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.’