Alexander G. Luchinin

Theory of underwater LIDAR with a complex modulated illumination beam

The main effects accompanying the propagation of an initially narrow complex modulated light beam in sea water are studied. Sea water, with respect to modulation waves, is shown to be similar to a medium with frequency dispersion with respect to electromagnetic waves; in the medium, the effects of the time focusing of a wave packet are possible. Based on the self-similar small-angle solution to the equation of radiation transfer, we estimate spatial-temporal characteristics of pulse-signal amplitude modulated by a complex signal. We propose a scheme for constructing underwater LIDAR using a complex modulated illumination beam and the processing of a received echo signal. The processing allows the separation of the modulated component and its matched filtration. Values of possible delays and broadening of the separated echo signal caused by dispersion properties of sea water are estimated.

Effect of sea waves on the limiting resolution of aircraft oceanographic lidars

The effect of sea waves on the characteristics of bathymetric lidars is studied. The main effects responsible for the distortion of bathymetric information are discussed. The displacement of the sounding beam and its broadening in a horizontal plane, the statistical mean variation in the signal’s delay time, the variance of signal arrival times, and the increase in the duration of the received signal are evaluated. The calculations are performed allowing for the absorption and multiple scattering of light in water.

Model of an underwater imaging system with a complexly modulated illumination beam

The optical transfer functions of underwater imaging systems that use narrow beams modulated by a complex high-frequency signal to form an image are studied. Images of simple objects observed with the use of such systems are calculated. The image distortions caused by the interference of modulation waves are estimated, and a method for eliminating these distortions is proposed. The potential for improving the image quality through the application of complexly modulated illumination beams is demonstrated using explicit calculations.

Water-scattered signal to compensate for the rough sea surface effect on bottom lidar imaging

We investigate the possibility of using the water-backscattered radiation from a bottom sounding airborne imaging light detection and ranging (lidar) system to determine the surface slope at the point where the laser beam intersects the surface. We show that the refraction angle of the beam can be determined using receivers whose sensitivities vary linearly over their field of view. Equations are derived to estimate the statistical mean and variance values of this refracted angle. We demonstrate that the proposed algorithm improves lidar imaging. Numerical examples with reference to typical marine conditions are given.

Influence of illumination conditions on the sea-bottom visibility

Statistical models of the useful and background signals forming an image of the sea bottom observed through a rough sea surface are constructed. The methods for estimating the signal-to-noise ratio in the image of the bottom with the test distribution of the reflectance are described. The dependence of the signal-to-noise ratio on observational conditions, in particular, on the mutual orientation of the directions of sighting, wind, and solar vertical, is analyzed. Recommendations on the choice of optimal conditions for bottom observations from airborne carriers are given.

Image transfer through rough sea surface: computer simulations

This paper is devoted to computer simulation of random realizations of bottom images. Simulations of random images are considered to be a straightforward way to predict bottom visibility under realistic conditions. A simplified version of a bottom imaging model using a fast simulation algorithm has been developed. Simulated results presented here allow the visual evaluation of image quality for different signal/noise ratio (SNR) values. We show how the simulation of random images can be used to predict bottom visibility over a variety of environmental conditions and also determine an optimal observation strategy.

Time characteristics of lidar signals during sensing through roughed sea surface

We have studied the influence of a roughed sea surface and the dispersion of photons over pathlengths on the time characteristics of a pulsed oceanologic airborne lidar. We have derived equations describing the first two temporal moments of a return signal for two types of lidar (a lidar with an isotropic receiver directional pattern and a lidar with an extremely narrow receiver pattern). It is shown that the delay of the return signal and its effective width depend substantially on both the characteristics of the lidar itself and on the parameters of the sea-surface roughness and inherent optical properties.

Structure of a modulated narrow light beam in seawater: Monte Carlo simulation

Dispersion properties of photon density waves propagating in seawater from a unidirectional point source have been studied using the Monte Carlo technique. It is shown that the spatial distribution of irradiance of the photon density waves at high modulation frequencies is significantly different from that of the stationary light field. Principal dispersion effects predicted earlier in the approximation solutions of the nonstationary radiation transfer equation are confirmed. An increase in the wave decrement and a decrease in the transversal cross section of the initially narrow beam at the modulation frequency with the increase of the modulation frequency are demonstrated. Frequency dependencies of phase and group velocities of photon density waves are calculated. It is shown that seawater possesses anomalous dispersion in a wide frequency range with respect to these waves.

Nonstationary optical transfer functions of underwater imaging systems

Optical transfer functions of underwater imaging systems employing narrow pulses or sinusoidally modulated beams for image formation are studied. A modified Monte Carlo technique allowing for direct statistical modeling of these functions accounting for temporal dispersion is proposed and implemented. The optical transfer functions are calculated for various modulation frequencies of the illumination beam and for the case of pulsed illumination. The employment of high-frequency sinusoidal or pulsed modulation with consistent processing of the received signal is shown to significantly increase the contrast sensitivity of underwater imaging systems as compared with systems with stationary illumination.

Influence of breaking waves on the resolution of oceanologic lidars

The influence of breaking waves on characteristics of a lidar echo signal while sounding a sea bottom from the atmosphere is investigated at strong winds. Relations are derived allowing the average statistical delay and broadening of the signal to be estimated taking into account the sea surface roughness, breaking waves, and multiple scattering of light in water. It is shown that breaking waves weakly influence average statistical characteristics within a range of wind speeds up to 20 m/s. This influence becomes substantial when measurements of the delay and, accordingly, the determination of the depth of the bottom or reflecting object are performed by the time of the first arrival of a signal of backscattering from the water surface.