Anisoplanatic effects in moderate aperture laser communication system uplinks

Atmospheric turbulence plays an important role in the performance of ground-to-space laser communication systems. Many of the effects of turbulence in the downlink path can be compensated using high-bandwidth precision tracking and adaptive optics techniques. The point-ahead angle required by the relative satellite velocity results in an uplink data beacon which traverses a different atmospheric path than the downlink beam, causing anisoplanatic differences between the downlink and uplink wavefront phase. In his foundational paper on anisoplanitism in adaptive optics, David Fried noted that the magnitude of the anisoplanatic effect can be characterized by an isoplanatic patch size, and is less severe in systems with smaller transmit diameters. In contrast to traditional space-imaging systems, laser communication systems use moderate aperture sizes, a few r₀ in diameter. The isoplanatic patch size also scales with wavelength, resulting in a larger isoplanatic patch and lower losses at typical communication system wavelengths. The anisoplanatic loss data as a function of aperture diameter and offset angle presented by Fried is fit to provide a direct calculation of the residual phase error of an uplink data beacon in ground receiver systems that provide high bandwidth tilt correction or full adaptive optics system correction of the downlink beam path. The effects of anisoplanitism on both full aperture and sub-aperture uplink beams is considered. Significant correction of the uplink beacon is shown for moderate aperture laser communication systems. The analysis results are shown to be in good agreement with Monte-Carlo analyses of wave optics propagations.