Tilt anisoplanatism in extended turbulence propagation

Abstract

The use of high-energy laser (HEL) weapon systems in tactical air-to-ground target engagements offers great promise for revolutionizing the USAF's war-fighting capabilities. Laser directed-energy systems will enable ultra-precision strike with minimal collateral damage and significant stand-off range for the aerial platform. The tactical directed energy application differs in many crucial ways from the conventional approach used in missile defense. Tactical missions occur at much lower altitudes and involve look-down to low-contrast ground targets instead of a high-contrast boosting missile. At these lower altitudes, the strength of atmospheric turbulence is greatly enhanced. Although the target slant ranges are much shorter, tactical missions may still involve moderate values of the Rytov number (0.1-0.5), and small isoplanatic angles compared to the diffraction angle. With increased density of air in the propagation path, and the potential for slow-moving or stationary ground targets, HEL-induced thermal blooming will certainly be a concern. In order to minimize the errors induced by tracking through thermal blooming, offset aimpoint tracking can be used. However, this will result in significant tilt anisoplanatism, thus degrading beam stabilization on target. In this paper we investigate the effects of extended turbulence on tracking (or tilt) anisoplanatism using theory and wave optics simulations. The simulations show good agreement with geometric optics predictions at angles larger than about 5 micro-radians (asymptotic regime) while at smaller angles the agreement is poor. We present a theoretical basis for this observation.