ABSTRACT
This work explores the conditions resulting in the saturation of angular anisoplanatic error. When turbulence is modeled with a von Kármán outer scale or when the piston and aperture tilt are compensated the anisoplanatic error can saturate to less than one squared radian. In Kolmogorov turbulence anisoplanatic error is limited to values smaller than one when the ratio of the Fried parameter to the outer scale is 0.349. To understand the effect of compensation on saturation both a first-order asymptotic approach and numerical integration are considered for both plane and spherical wave sources and in non-Kolmogorov turbulence. Asymptotic expressions are found to agree with the numerical results as long as the ratio of the outer scale to aperture size is less than five. For a plane wave propagating in Kolmogorov turbulence, the compensated anisoplanatic error is found to saturate when D/r 0=3.9, and the outer scale is equal to the aperture size. When a spherical wave source is considered D/r 0 increases to 5.8; as expected these values are related by a factor of 1.8. This work also formulates the anisoplanatic error in terms of an integrated strength parameter and the mean turbulence height allowing extension to arbitrary path geometries and power law exponents. Using this approach I find smaller power law exponents increase the mean turbulence height, thereby decreasing the isoplanatic angle; the opposite applies as the power law exponent is increased relative to Kolmogorov turbulence.
ABSTRACT
An ad hoc autonomous mobile microgrid system requires electrical connections to be formed between physically separated resources. This work proposes the use of unmanned ground vehicles (UGVs) as the means to deploy the electrical cable that creates these connections. This operation requires careful control of the cabling at variable speeds to avoid entanglement with the deploying UGV or obstacles in complex outdoor environments. Searching for a product that could supply the needed control and flexibility revealed a lack of compact and low-cost options. Existing options are very heavy ( > 100 lbs) and do not supply precision in their deployment. There is no commercial off-the-shelf option available for small-scale cable deployment operations with size and weight constraints. To fulfill the application requirements and to combat this deficiency, a custom design and build of an "Adjustable Cable Management Mechanism" (ACMM) was required. This ACMM provides a low cost, compact platform for powered and controlled deployment and retraction of different-sized cable under moderate loads, utilizing Commercial Off-The-Shelf components (COTS). Employing this design has enabled a variety of tasks that require distribution of electrical or data cables to be accomplished for small-scale projects. The goal of this paper is to give detailed design specifications of the ACMM and instructions on how to recreate it and calibrate it to be useful for tethering robots in various applications such as steep terrain, internet connection through tight spaces, or electrical connection between nodes for complex microgrids.
ABSTRACT
We use Mellin-transform techniques to derive generalized expressions for the piston-removed and piston-and-tilt-removed anisoplanatic error in non-Kolmogorov turbulence with a finite outer scale. We use these expressions to investigate the behavior of the anisoplanatic error when imaging over long horizontal paths where the angular extent of the scene is often many times the isoplanatic angle. By evaluating these expressions, we first find that in many cases, the anisoplanatic error saturates to a value less than 1rad2. Next, as power law increases, the contributions due to piston and tilt dominate the anisoplanatic error expression. Last, the size of the outer scale contributes primarily to the piston and tilt terms. Together, these behaviors imply that when piston and tilt are removed, anisoplanatism is reduced by as much as 60%.
ABSTRACT
We derive a generalized expression for the differential piston phase variance in non-Kolmogorov turbulence. Specifically, our result applies in the case where index of refraction is described by a power-law medium with an exponent between 0 and 1. Kolmogorov assumptions of homogeneity and isotropy are maintained. After some development, our expression is derived using the Mellin-transform techniques and may be generalized to other forms for the three-dimensional index of refraction turbulence power spectrum. This analytical result has two regions of convergence. The separation between these regions is defined by a characteristic time given as the ratio of the mean wind speed and aperture size. By evaluating this expression, we find the differential piston phase variance exhibits a power-law behavior roughly proportional to that of the medium. In addition, we find that piston phase variance decreases with increase in aperture size. We also find that the differential piston phase variance is independent of aperture size as the power law approaches unity.
ABSTRACT
This paper presents a massively parallel method for the phase reconstruction of an object from its bispectrum phase. Our aim is to recover an enhanced version of a turbulence-corrupted image by developing an efficient and fast parallel image-restoration algorithm. The proposed massively parallel bispectrum algorithm relies on multiple block parallelization. Further, in each block, we employ wavefront processing through strength reduction to parallelize an iterative algorithm. Results are presented and compared with the existing iterative bispectrum method. We report a speed-up factor of 85.94 with respect to sequential implementation of the same algorithm for an image size of 1024×1024.
ABSTRACT
We describe a modification to fast Fourier transform (FFT)-based, subharmonic, phase screen generation techniques that accounts for non-Kolmogorov and anisotropic turbulence. Our model also allows for the angle of anisotropy to vary in the plane orthogonal to the direction of propagation. In addition, turbulence strength in our model is specified via a characteristic length equivalent to the Fried parameter in isotropic, Kolmogorov turbulence. Incorporating this feature enables comparison between propagating scenarios with differing anisotropies and power-law exponents to the standard Kolmogorov, isotropic model. We show that the accuracy of this technique is comparable to other FFT-based subharmonic methods up to three-dimensional spectral power-law exponents around 3.9.
