Abstract
Accurate characterization of atmospheric turbulence is useful for performance assessment of optical systems operating in real environments and for designing systems to mitigate turbulence effects. Irradiance-based techniques such as scintillometry, suffer from saturation, and hence commercial scintillometers have limited operational ranges. A method to estimate turbulence parameters, such as path weighted Cn2 and Fried’s coherence diameter r0 from turbulence-induced random, differential motion of extended features in the time-lapse imagery of a distant target is presented. Since the method is phase-based, it can be applied to longer paths. It has an added advantage of remotely sensing turbulence without the need for deployment of sensors at the target location. The approach uses a derived set of path weighting functions that drop to zero at both ends of the imaging path, the peak location depending on the size of the imaging aperture, and the relative sizes and separations of the features whose motions are being tracked. Using different sized features separated by different amounts, a rich set of weighting functions can be obtained. These weighting functions can be linearly combined to approximate a desired weighting function such as that of a scintillometer or that of r0 in inhomogeneous turbulence. The time-lapse measurements can thus mimic the measurements of a scintillometer or any other instrument. The method is applied to images captured along two different paths, and the estimates are compared to co-located scintillometer measurements.
Highlights
Accurate characterization of atmospheric turbulence and its effects is extremely important for performance evaluation of optical systems operating in real environment and for designing of systems to mitigate turbulence effects
The tilt variances for the two different patch separations were multiplied by their corresponding weights and added together to obtain a path-weighted estimate of C2n
Estimation errors are possibly due to moving shadows, blurring of features, and changes in contrast between neighboring images due to changing cloud cover around noon
Summary
Accurate characterization of atmospheric turbulence and its effects is extremely important for performance evaluation of optical systems operating in real environment and for designing of systems to mitigate turbulence effects. Irradiancebased methods such as the scintillation, detection, and ranging technique have been used in the past by astronomers to obtain a low-resolution vertical profile of turbulence.[1,2] The path-weighted refractive index structure constant C2n is traditionally measured using scintillometers.[3] irradiance-based techniques are of limited use in the saturation regime and are not suitable for measurements over long paths through turbulence. A phase-based technique built on a process known as the
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