Title

Signal to noise ratio effects on aperture synthesis for digital holographic ladar

Date of Award

2012

Degree Name

M.S. in Electro-Optics

Department

Department of Electrical and Computer Engineering. Electro-optics Graduate Program

Advisor/Chair

Advisor: Edward A. Watson

Abstract

The cross-range resolution of a laser radar (ladar) system can be improved by synthesizing a large aperture from multiple smaller sub-apertures. This aperture synthesis requires a coherent combination of the sub-apertures; that is, the sub-apertures must be properly phased and placed with respect to each other. One method that has been demonstrated in the literature to coherently combine the sub-apertures is to cross-correlate the speckle patterns imaged in overlapping regions. This work investigates the effect of low signal to noise ratio (SNR) on an efficient speckle cross-correlation registration algorithm with sub-pixel accuracy. Specifically, the algorithms ability to estimate relative piston and tilt errors between sub-apertures at low signal levels is modeled and measured. The effects of these errors on image quality are examined using the modulation transfer function (MTF) as a metric. The results demonstrate that in the shot noise limit, with signal levels as low as about 0.02 signal photoelectrons per pixel in a typical CCD, the registration algorithm estimates relative piston and tilt accurately to within 0.1 radians of true piston and 0.1 waves of true tilt. If the sub-apertures are not accurately aligned in the synthetic aperture, then the image quality degrades as the number of sub-apertures increases. The effect on the MTF is similar to the effects due to defocus aberrations.

Keywords

Synthetic apertures, Signal processing Evaluation, Imaging systems Image quality Evaluation, Optical radar, Holography Data processing, Image processing Digital techniques, Ladar; Synthetic Aperture Registration; SNR; Speckle Cross-Correlation; MTF; Spatial Heterodyne; engineering; optics; remote sensing; scientific imaging

Rights Statement

Copyright © 2012, author

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