Characterization of underwater scattering layers based on variance components of LiDAR backscattering.

Range-resolved detection of submerged scattering layers was investigated in the Gulf of Mexico based on vertical profiles made with a LiDAR (Light detection and range) system having a green laser (wavelength λ = 532 nm). The backscattering power (Sd) variability was decomposed in principal components (PCs) and related to non-polarized Sd, the Sd ratio between cross- and co-polarized waveforms, the chlorophyll-a fluorescence (Fchl), and the ratio between volume scattering angles of 150° and 100°. The variance of PCs was dominated by non-polarized Sd followed by Fchl. Correlation between PC1 scores and Fchl anomalies suggested that Sd was mainly originated from pigmented particulates.

Weibull approximation of LiDAR waveforms for estimating the beam attenuation coefficient.

Tank experiments were performed at different water turbidities to examine relationships between the beam attenuation coefficient (c) and Weibull shape parameters derived from LiDAR waveforms measured with the Fine Structure Underwater LiDAR (FSUIL). Optical inversions were made at 532 nm, within a c range of 0.045-1.52 m-1, and based on a LiDAR system having two field-of-view (15 and 75.7 mrad) and two linear polarizations. Consistently, the Weibull scale parameter or P2 showed the strongest covariation with c and was a more accurate proxy with respect to the LiDAR attenuation coefficient.

Experimental study of a compressive line sensing imaging system in a turbulent environment.

Turbulence poses challenges in many atmospheric and underwater surveillance applications. The compressive line sensing (CLS) active imaging scheme has been demonstrated in simulations and test tank experiments to be effective in scattering media such as turbid coastal water, fog, and mist. The CLS sensing model adopts the distributed compressive sensing theoretical framework that exploits both intrasignal sparsity and the highly correlated nature of adjacent areas in a natural scene. During sensing operation, the laser illuminates the spatial light modulator digital micromirror device to generate a series of one-dimensional binary sensing patterns from a codebook to encode the current target line segment. A single element detector photomultiplier tube acquires target reflections as the encoder output. The target can then be recovered using the encoder output and a predicted on-target codebook that reflects the environmental interference of original codebook entries. In this work, we investigated the effectiveness of the CLS imaging system in a turbulent environment. The development of a compact CLS prototype will be discussed, as will a series of experiments using various turbulence intensities at the Naval Research Lab's Simulated Turbulence and Turbidity Environment. The experimental results showed that the time-averaged measurements improved both the signal-to-noise radio and the resolution of the reconstructed image in the extreme turbulence environment. The contributing factors for this intriguing and promising result will be discussed.