Anatoli Chaikovsky, Michail Korol, A. Malinka, E. Zege, I. Katsev, A. Prikhach, S. Denisov, V. Dick, P. Goloub, L. Blarel, L. Chaikovskaya, A. Lapyonok, T. Podvin, N. Denishchik-Nelubina, A. Fedarenka, V. Svidinsky
The paper presents lecture materials given at the Nineteenth International Conference and School on Quantum Electronics “Laser Physics and Applications” (19th ICSQE) in 2016, Sozopol, Bulgaria and contains the results of the 10-year research of Belarusian Antarctic expeditions to study the atmospheric aerosol and Earth surface in Antarctica. The works focus on the studying variability and trends of aerosol, cloud and snow characteristics in the Antarctic and the links of these processes with the long range transport of atmospheric pollutants and climate changes.
The conventional method used to detect the range to an underwater object is by sending and receiving some form of
acoustic energy. However, acoustic systems have limitations in the range resolution and accuracy they can provide under
certain conditions. The potential benefits of a laser-based range finder include high-directionality and covertness, speed
of response, and the potential for high-precision, range accuracy. These benefits have been exploited in the above-water
environment where kilometer propagation ranges are achieved with sub-meter range precision. The challenge in using
optical techniques in the underwater environment is overcoming the exponential loss due to scattering and absorption.
While absorption extinguishes photons, scattering redistributes the light and produces a 'clutter' signal component from
the surrounding water environment. Optical modulation techniques using compact laser diode sources are being
investigated to help suppress this 'clutter' and provide accurate target range information in a wide range of underwater
environments. To complement the experimental efforts, a theoretical model has been developed to help optimize the
system parameters and test the performance of various configurations as a function of different water optical properties.
Results from laboratory water tank experiments will be discussed and compared with model predictions.
A new general analytical inversion of the lidar equation with multiple scattering is presented. This inversion of the lidar
equation includes all known solutions as particular cases. It works under single and multiple scattering, for any
geometry, and for any reference point in the retrieval procedure. This inversion serves as a theoretical base for the newly
developed techniques to retrieve the TOP profiles from measured lidar waveforms. The iterative procedures as well as
software INVERTER to perform the lidar waveforms inversion are outlined. INVERTER also serves as a tool for
verification and refinement of various retrieval techniques. Iterative retrieval algorithms for different lidar systems, their
stability and sensitivity to the accuracy of a priory information used are discussed. The examples of the performed
retrieval simulations show a reasonable coincidence of the retrieved IOP profiles with the sea truth.
A theory of imaging of underwater targets with a time gated polarized imaging lidar is briefly presented. This approach
is grounded on the analytical theory of polarized light propagation developed by E. Zege and L. Chaikovskaya earlier.
An algorithm and software for computer simulation of a performance of ocean imaging lidars with polarization devices
are outlined. A vehicle for this model is software simulating the performance of ocean time gated imaging lidars without
polarization devices developed by authors earlier. Results of the simulation of images with time-gated CO-POLAR and
CROSS-POLAR cameras are presented and discussed.
Optical imaging in turbid ocean water is a challenge due to the high probability that light will scatter multiple times as it propagates to and from the object of interest. Techniques have been developed to suppress the contribution from scattered light and increase the image contrast, such as those using a pulsed source with a gated receiver or a modulated source with a coherent RF receiver. While improving the amplitude contrast of underwater images, these two approaches also have the capability of providing target range information. The effectiveness of each approach for both 2D and 3D imagery depends highly on the turbidity of the intervening water medium. This paper describes a system based on the optical modulation approach, the Frequency Agile Modulated Imaging System (FAMIS), and the techniques that have been developed to improve both amplitude and range imaging in turbid water.
This paper overviews a recent advancement in computer simulating of the performance of lidar sounding and imaging systems. A new iterative technique to retrieve the inherent optical characteristics from lidar waveforms is presented. A few approaches needed for simulation of the high spatial resolution 3D imaging in the surf zone and very shallow waters (regard to the forward pulse stretching, the depth correlation of the random realizations of the background images, including elevations in the sea surface model) as well as modeling of optical properties of bubbles and sediments are
briefly discussed.
Theory of polarized lidar sounding of multiply scattering media with highly anisotropic phase functions which describes angular patterns with representative azimuth structure of the backscattered signal observed through the polarizer and analyzer is proposed. Such patterns were visualized by other investigators in the plane perpendicular to the receiver optical axis in experiments performed on the base of mono-static polarized lidar systems. Previously obtained approximate equations of the vector theory along with generalized approximations earlier developed in the scalar theory form the basis for our theory. Solution expressed through multidimensional integrals which determine return polarization parameters is presented and explained. It accounts for formation of a signal as a process consisting of single scattering events into the backward region and propagation and small-angle scattering into near-forward directions before and after the back-scattering events. The developed solution gives the return angular pattern for any initial and received states of polarization and includes the multiple scattering. Half-analytical fast techniques that have been developed on the base of the obtained solution are used to simulate the near-backscattering patterns. Computed examples of the patterns for the case of scattering in seawaters are shown and discussed.
A semi-analytical approach to compute the power and polarization of pulsed return signal in the bi-static sounding of a multiply scattering medium with forward elongated phase function, such as the sea water, tissue or a cloud, which is based on the signal consideration as resulting from the single scattering into backward directions and small-angle multiple scattering into near-forward directions, is briefly described. Possibility of radical simplification of the approach for small angles between the receiver and source axes is shown with using examples of light scattering in the sea water and tissue. Evaluated values of the return polarization as a function of the angle between source and receiver axes in the case of sea water sounding are presented.
An analytical approach for modeling Raman lidar return with multiple scattering is presented. The approach is based on a small-angle quasi-single scatteirng approximation developed earlier for elastic lidar sounding. Spatial-angular structure of Raman lidar return is investigated. For particular case of warm clouds it is shown that multiple-field-of-view lidar technique allows one to retrieve the effective size of scattering particles.
This work overviews recent advances that have been made in an analytical theory of elastic and Raman lidar returns with multiple scattering and polarization from clouds and seawaters and outlines newly developed software for computer simulation of airborne oceanic lidar performance.
This study is devoted to the development of a semi-analytical algorithm for the determination of the otpical thickness, the liquid water path and the effective size of droplets from spectral measurements of the intensity of solar light reflected from water clouds with large optical thickness. The algorithm is planned to be aplied to the data fromteh Scanning Imaging Absorption Spectrometer for Atmospheric Chartography, launched on March 1st, 2002 on board of the ENVIronmental SATellite. The probability of photon absorption by droplets in the visible and near-IR spectral regions is low. This allows us to simplify and modify well known asymptotic equations of the radiative transfer theory, taking into account the fact that the single scattering albedo is close to one. Modified asymptotic equations are used to develop the inverse algorithm. We also avoid the use of the Mie theory, applying parameterization and geometrical optics results with account for wave corrections. The main advantage of the method proposed lays in the fact that the equations derived not only provide a valuable alternative to the numerical radiative transfer solution. They are also much more simple than equations of a conventional asymptotic theory. This simplicity allows both the simplication of the cloud retrieval algorithm and, even more important, insight into various factors involved in cloud retrieval schemes.
The real-time simulation of broken and distorted images of a submerged target observed through a random realization of wind roughened sea surface produced by airborne gating imaging lidar is presented. The user is allowed to watch the sequence of images at the computer screen, which the lidar operator vies sat the TC screens of an operating airborne imaging lidar. Th inclusion of all types of noise as well as spatial correlation of receiver noise and noise due to a windy roughened sea surface with detailed treatment of seawater scattering provides a very realistic model. Two image processing techniques to deal with actual images - Matched Filter and Optimal Integration - are compared as to signal-to-noise ratio and probability of detection. Optimal Integration is appropriate for use by an experienced lidar operator.
A modulated light detecting and ranging system has been developed to improve underwater imaging. This system uses the modulation information encoded on an optical signal to distinguish between the backscatter signal and the signal reflected from an underwater target. Through choice of the appropriate modulation frequency, this technique has the ability to improve underwater target contrasts by reducing backscatter noise. Both laboratory tank experiments and in- situ pier measurements have been completed with a modulated lidar prototype. The results show that the target contrast improved as the modulation frequency. Concurrent with the experimental measurements, a theoretical model is being developed for the modulated lidar system. This analytical model incorporates both the Small Angle Diffusion Approximation and the Multi-Component Method developed by Zege et al to solve the radiative transfer equation. The various experimental characteristics are included in the model and the results are compared with relevant experimental data. Preliminary results show good agreement with experimental data, including the reduction of backscatter with increasing modulation frequency.
On the base of a newly developed theory of image formation, which includes the effect of shadowing of the space behind an illuminated and observed object, the receiver time gating of an airborne ocean lidar system is simulated. Depending on the timing of the start of the gate, a reflected target or its shadow may be seen. The contrast and signal-to-noise ratio (SNR), for an image of the directly observed object (reflection) and the image of its shadow (obscuration), are compared. It is shown that in many cases the SNR of the object in the `shadow' (obscuration) mode can be greater than the SNR of the object observed in reflection compared at the same depth. In addition, the obscuration mode is advantageous for improving system performance in cases involving a priori choices of the start and duration of the gate. These advantages are most pronounced while observing an object in turbid shallow water through a windy roughened sea surface.
The method and computation algorithm have been developed to assess the effect of scattered light on a signal received by a sun photometer while a cloud optical thickness is measured. The approach provides estimations of impact of scattered light for any receiver field of view with allowance for angular dispersion of sunlight. The accuracy of measurements has been investigated as a function of zenith angle of the Sun, receiver field of view, wavelength, cloud optical thickness. The effect of aerosol scattering is an undercloud layer on the accuracy of cloud optical thickness measurement has also been considered.
A lot of different approaches to the problem of radiation propagation in scattering media is known, but a choice of an appropriate one to solve a specific problem is more art than science. There is no exact analytical solution to the stationary radiation transfer problem up to now. So, numerous approximate approaches should be used, each of them is reliable within its own range of characteristic parameters of the range. When the nonstationary problem is taken into consideration, the mathematical difficulties increase and, hence, a number of reliable approaches decreases.
A new analytical solution for the local optical characteristics (extinction, light pressure, and absorption coefficients, asymmetry parameter of a phase function) of spherical polydispersions with comparatively large particles are derived. The geometrical optics (GO) approximation is used to solve the problem. To improve the accuracy of the GO approximation the edge effects were taken into account. A comparison with the Mie theory data shows a fairly satisfactory accuracy of our analytic formulas.
The features of forming a signal-to-noise ratio (SNR) as applied to the pulse-coded information transfer technique through an optical channel are considered. To maximize a SNR and a range of communication, an optimum receiver resolution time (RRT) should be found and used. An analytical solution as well as an algorithm to calculate the distortion of a pulse, propagating through a cloud layer, and the position and amplitude of a maximum of a recorded signal have been developed. They were realized as an interactive PC program, which calculates the optimum value of RRT and range limit of communication for any parameters of systems and optical channels (atmosphere + cloud).
A simple and convenient tool to investigate the radiative characteristics of the atmosphere- underlying surface system is described here. It is the ATMOTOOLS package implemented for IBM PC and synthesizing up-to-date optical atmosphere models, new procedures for refining these models, and special radiation calculation procedures. Considerable attention is paid to aerosols models and arrangement of data banks on optical aerosol properties. Calculation procedures for radiative transfer are developed with regard to atmosphere stratification, light polarization, multiple scattering, and so on. Some examples of investigating different problems connected with remote sensing of ocean, ozone, and clouds using ATMOTOOLS are given.
A multicomponent approach to calculate a light field structure with allowance for multiple scattering in the media such as clouds, mists and ocean water is given. All characteristic properties of the real phase function are regarded and propagation in a scattering medium of any optical thickness with an arbitrary single scattering albedo can be considered. The phase function is represented as a sum of more simple functions. The radiance is given as a sum of appropriate components. The equations like the radiative transfer equation are obtained for each component. They can be solved using the known methods within domains where they work best of all. Comparison of various solutions (lidar returns, temporal structure of light pulse transmitted by cloud, an angular structure of the light reflected from and transmitted by cloud) with different numerical calculations shows a fairly satisfactory accuracy of our analytical formulas.
A new concept is presented to determine the range of vision of objects in scattering media relied upon the idea of optimum image processing. A program package which realized this concept to calculate the limited range of detection and discrimination of underwater objects by any passive and active laser TV systems, including observation through a rough sea surface, is described. Some examples of vision range calculation when viewing in atmosphere and ocean using different systems from the human eye to pulsed range gating laser TV are given.
We propose simple analytical solutions , which directly relates the optical transfer function (OTF) and the mutual coherence function (MCF) of coarse-dispersed aerosols to aerosol microstructure parameters.This approach relies upon a small-angle approximation of radiative transfer theory and on a special geometrical optics solution for medium local optical parameters.
The known analytical solutions to this problem were obtained as the approximation of low scattering orders and, therefore, can be used for a very narrow range of characteristic problem parameters. Relatively well-developed numerical methods relies upon the Monte-Carlo algorithms, which are inconvenient for operative estimations and. qualitative studies because of time-consuming computation procedure.
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