Margarita M. Krekova

NUMERICAL EVALUATION OF THE POSSIBILITIES OF REMOTE LASER SENSING OF FISH SCHOOLS

The operation of an airborne lidar intended for the detection of fish schools is numerically simulated by the Monte Carlo method. The calculations are performed for schools located at small depths in order to study the regularities in the shaping of the lidar return accurately. Three models of the phase function of scattering of laser radiation in sea water are used. The signals reflected from surface waters that contain a school of fish are determined as a function of the lidar parameters, light scattering and absorption coefficients in the water, stratification of light scattering layers, and fish-school depth. The results obtained can be used for interpreting the signals of the fish-detection lidar.

Laser Sensing of a Subsurface Oceanic Layer. II. Polarization Characteristics of Signals.

In Part I of this paper we calculated depth profiles and polarization characteristics of airborne lidar return signals by the Monte Carlo method. Here we calculate the polarization characteristics of lidar return signals for different types of water. We demonstrate the feasibility of polarization lidar application to the detection of underwater inhomogeneities of different origins. It is shown that simultaneous analysis of depth profiles of the lidar return signal power and signal depolarization ratio substantially increases the information content of airborne lidar sensing of seawater. We compare calculated results with the data of airborne lidar measurements for lambda = 0.53 mum.

Laser sensing of a subsurface oceanic layer. I. Effect of the atmosphere and wind-driven sea waves.

Depth profiles and polarization characteristics of airborne lidar return signals have been calculated by the Monte Carlo method. We analyze some peculiarities of depth profiles of lidar return signals for a rough air-water interface. The distorting effect of the atmosphere on the lidar return signal structure is evaluated as a function of the geometry of the observations. Calculated results are compared with the data of airborne lidar measurements for lambda = 0.53 mum.

Reabsorption of laser-induced fluorescence in a plant canopy: Stochastic model

This paper continues the series of publications on the statistical simulation of transspectral processes. A system of interrelated radiative transfer equations is proposed. This system gives a formal basis for a numerical analysis of a wider range of spectroscopic effects that accompanies the propagation of laser radiation in the environment, such as the reabsorption of fluorescence in dense disperse media containing two and more fluorophores. As applied to the problem of lidar monitoring of the state of a plant canopy, an optical model is developed in which a leaf is not treated as an individual scattering element, but rather as a local volume of a multiphase medium with a complex polydisperse structure. The Monte Carlo algorithms have been modified so that they have achieved the simulation of fluorescence and reabsorption processes. Test calculations have demonstrated the adequacy of the proposed approach.

Radiative characteristics of plant leaf

Existing leaf radiation models are reviewed. A new concept of the optical model of the leaf as a multiphase system containing three aggregate ensembles of particles significantly different in microphysical and optical characteristics is proposed. The proposed model is based on the reconstruction of the particle size distribution function from the experimental leaf absorption spectrum. Based on the obtained microphysical model of the plant leaf, the spectra of optical radiation reflection and transmission in the range of 400–800 nm are calculated for various relative concentrations of light-absorbing pigments (chlorophyll a, b and carotenes) and various leaf thicknesses. Optical radiation propagation was simulated using the stochastic Monte Carlo method. The simulation results are in good agreement with relevant experimental spectra.

Method for reconstructing atmospheric optical parameters from the data of polarization lidar sensing.

Inversion of polarization lidar sensing data based on the form of the lidar sensing equation with allowance for contributions from multiple-scattering calls for a priori information on the scattering phase matrix. In the present study the parameters of the Stokes vectors for various propagation media, including those with the scattering phase matrices that vary along the measuring range, are investigated. It is demonstrated that, in spaceborne lidar sensing, a simple parameterization of the multiple-scattering contribution is applicable and the polarization signals characteristics depend mainly on the lidar and depolarization ratios, whereas differences in the angular dependences of the matrix components are no longer determining factors. An algorithm for simultaneous reconstruction of the profiles of the backscattering coefficient and depolarization and lidar ratios in an inhomogeneous medium is suggested. Specific features of the methods are analyzed for the examples of interpretation of lidar signal profiles calculated by the Monte Carlo method and are measured experimentally.

Influence of the air–water interface on hydrosol lidar operation

The results of seawater sensing by use of an airborne lidar with a changeable field of view (FOV) are presented, together with the results of numerical simulation of lidar operation by the Monte Carlo method. It is demonstrated that multiple scattering and wind-driven sea waves have opposite effects on the measured attenuation coefficient. At small FOVs the wind-driven sea waves cause the lidar signal decay rate to increase compared with the size of the plane surface and hence result in an overestimation of the retrieved attenuation coefficient. Inefficient operation of lidars with small FOVs is caused by strong fluctuations of lidar signal power that cannot be described by a normal distribution. Specific features of the fluctuations can be interpreted as manifestations of the well-known effect of backscattered signal amplification caused by the double passage of radiation through the same inhomogeneities. As for the plane air-water interface, multiple scattering is significant for large FOVs and compensates for the effect of wind-driven sea waves. The applicability of simple sea-surface models to a description of lidar signal power fluctuations is discussed.

Potential of pulsed excilamps for remote sounding of polluted atmosphere

The results are discussed of a closed numerical experiment on lidar sounding of the concentration of small gas impurities in the tropospheric layer of the atmosphere based on a new LIDAR-DOAS hybrid technology that uses a XeCl* excilamp as a source of pulsed broadband radiation. Quantitative estimates con-firm the promise of the approach, which expands the potential of the classical scheme of differential optical absorption spectroscopy (DOAS) with respect to the remote monitoring and localization of hazardous anthropogenic emissions of toxic gases. Combining the Monte Carlo method with the genetic algorithm for solving the inverse problem of reconstructing the profiles of sought gas constituents of the troposphere makes it possible to strictly quantitatively predict the efficiency of new promising lidar systems for monitoring the environment.

Shadow field stop for a hydrooptical lidar

Experiments on sensing of the sea bottom in shallow waters were carried out with an airborne lidar. To reduce glint reflections from the sea water surface in lidar returns, the axial region of the receiving lidar telescope was screened by a shadow field stop. In this case, the entire lidar return from water depth is formed only by multiply scattered radiation. This lidar configuration improves the contrast of signals reflected from the sea bottom.

Influence of multiscattering on the results of airborne hydro-optical sensing

Experimental data on seawater laser sensing with an airborne lidar having a changeable field of view are presented. Wind-driven sea waves and microwaves lead to Fresnels splitting of a sounding beam into smaller sized beams, fluctuations, and escape of a number of signal photons from the detector field-of-view (FOV) cone, that is, to an apparent increase in the attenuation coefficient. Multiple scattering leads to the arrival of additional signal photons in the detector FOV, that is, to an apparent decrease in the attenuation coefficient. For the described lidar, two opposite processes provide its reliable operation at the FOV no less than 6 mrad.