<title>Pointing, tracking, and acquisition through atmospheric turbulence utilizing reciprocal path techniques</title>

Abstract

A naturally-occurring, enhanced backscatter appears whenever an object being obscured by a turbulent medium is actively illuminated and imaged by a monostatic transmitter/receiver. After making a double passage through the same turbulent eddies, reciprocal scattering paths, which encounter an identical phase delay, create a returning conjugate wave resulting in an enhanced illumination along the boresight of the telescope. Utilizing a dual aperture and orthogonal polarization to isolate the reciprocal paths, the backscatter enhancement occurs in the form of Young's interference fringes. For high visibility and stability of the fringes in the presence of time- varying turbulence, the width of an individual aperture is small compared to the atmospheric coherence diameter. With the separation and the width of the two apertures fixed and known, interferometric sensitivity of the displacement of objects was attained even when viewing through a turbulent atmosphere. Laboratory experimental data is compared with computer simulations and to analytical models. The results demonstrate the possibility of using this technique in a closed-loop pointing and tracking system, which would have potential applications in ground-to-space laser communications, laser power beaming to satellites and theater missile defense scenarios.