In order to improve the performance of terrestrial free-space optical communication systems in adverse visibility conditions, we present a method for estimation of the atmospheric channel impulse response function which governs the optical intensity propagation. This method reduces run-time computational demands and system complexity in comparison to our previously proposed dual-wavelength channel estimation technique. We consider propagation of optical wavelengths in fog, where the droplet diameters are close to the wavelength and thus scattering and absorption effects are significant. A method for rapid calculation of a channel response function based on estimating the effective optical depth of the channel and curve-fitting is described. The channel response estimate can then be used to design a receiver-side equalizer (minimum mean-squared error linear equalizer) to correct the signal distortion due to propagation through the dispersive channel. The channel estimates are based on parametric curve-fitting functions which have been developed using the modified-vector radiative transfer theory to model the channel response. The optimal fit parameters are found using particle-swarm optimization to minimize the simulated bit-error rate of the received signal.

A combined spatial and frequency domain interferometer or angular and frequency correlation (ACF/FCF) between two radar beams in the VHF-band is applied for the direct measurement of sea-ice thickness. This measurement is critical because the thickness of sea ice within the polar region indicates the state of ocean circulation and the associated air-sea heat exchange, which profoundly affects the global heat balance and ocean thermohaline circulation. This new instrument technology - cryospheric advanced sensor (CAS) - can measure sea-ice thickness, tilling a critical gap in measuring the polar region. In this paper, we present the algorithm development and demonstration by simulations of estimating the height of the sea-ice that led to the robust design of CAS interferometric system. Sea-ice thickness is derived from the interferometric phase of the ACF/FCF function of two VHF-band-scattered returns of two radar waves that have different frequencies, incident angles, and observation angles. The inversion calculation to estimate the ice thickness is based on several methods, gradient-descent (GD). least-square (LSQ) method, and genetic algorithm (GA). Compared with a GD method, and LSQ method, GA does not require the knowledge of the derivative of the ACF/FCF function. Good agreement is shown with GD and LSQ results, when a single unknown variable - sea-ice thickness - is to be determined. To support the inversion calculations and analysis, we developed an analytical model. The analytical model used to formulate the ACF/FCF function depends on the age of the ice being measured. The analytical model for first-year ice is based on the small perturbation method (presented here) and, for multiyear ice, the Kirchhoff approximation (presented in accompanying paper by the authors).

An accurate measurement of snow-layer thickness on the ground is a critical process for estimating the equivalent water content of snow. Previous studies were mainly based on the analysis of the backscattering cross-section of two co-polarized signals at different frequencies. The snow thickness was inferred by comparison with the analytical model. Recently, we have applied the angular and frequency correlation functions (ACF/FCF) for the estimation of sea-ice thickness. One of the advantages of the ACF/FCF method is suppression of the interfering volume scattering, which results in better accuracy and reliability of thickness estimation. We apply a simplified 1D three-layer model for the analysis. The layers are air, snow, and ground. The interfaces between layers are modeled as rough surfaces. Within the layers, there are small inclusions which introduce the volume scattering. Rough surfaces are modeled by the Kirchhoff approximation methods. The volume scattering is calculated using the quasi-crystalline approximation with coherent potential approximation (QCA-CP) for small particles. The ACF/FCF works by correlating two signals with different frequencies and/or incident angles. Using this model, we can determine the behavior of ACF/FCF and use it for snow thickness retrieval.

Imaging and detection of objects at optical wavelengths offers better resolution than at microwave or millimeter wavelengths. However, the imaging is severely affected by scattering from fog and clouds. This paper presents a study of optical imaging through clouds by using the point-source vector radiative transfer theory. The point-spread function including complete polarization characteristics is presented with numerical examples at 1 /spl mu/m wavelength showing the effects of aperture size and optical depth on the shower curtain effects.

There has been an increasing interest in the use of spaceborne very high frequency ultra high frequency (VHF-UHF) synthetic aperture radar (SAR) for measuring forest biomass and for detecting underground facilities. The propagation characteristics of the low-frequency electromagnetic wave are severely affected by the ionosphere. Recently, Faraday rotation effects and SAR image degradation have been studied using an analytical model and a homogeneous ionosphere. In this paper, a numerical model is developed to investigate the SAR image degradation caused by an inhomogeneous ionosphere. Both horizontal and vertical structures of the ionosphere are considered in this model. Three different cases are studied. The first is a vertically homogenous ionosphere, where the simulation condition is the same as in the analytical study by Ishimaru and others. The second is a vertical profile, which is introduced using the Chapman formula. The ray-bending effect is added for the ionosphere with a layered structure. Finally, both the vertical profile in electron density and the horizontal gradient in total electron content are considered in the simulation. Simulation results show good agreement with the theoretical analysis under the same conditions of the ionosphere. When both horizontal and vertical structures and the inhomogeneity of the ionosphere are considered in the model, the simulation result shows further image degradation and shift caused by the ray-bending effect. The simulation results also show the strong frequency dependence of the SAR image resolution.

This paper describes a novel approach for three-dimensional (3-D) imaging in a heavy-clutter environment. The proposed approach consists of two techniques. The first calls for the use of a synthetic aperture radar (SAR) moving in a circular orbit, whereas the second involves using angular correlation function (ACF) measurement for achieving clutter suppression. To demonstrate the imaging and clutter suppression capabilities of this combined approach (circular-correlation SAR), microwave imaging experiments are conducted at X-band frequencies (7-13 GHz). It is found that in heavy-clutter environments, this combined SAR approach outperforms conventional SAR, resulting in clearer images with enhanced target-to-clutter ratio.

This paper presents a theory and its experimental demonstration of an imaging technique based on three-dimensional (3D) space-time confocal imaging and circular synthetic aperture radar (SAR). The theory is an extension of the conventional straight-path SAR-to-SAR on an arbitrary curved path. Next, a general formulation for the curved SAR is applied to circular SAR geometry, which has two important features. First, it allows the maximum attainable resolution to be an the order of a wavelength. Second, it makes 3D confocal imaging possible, X-band (7-13 GHz) imaging experiments are conducted to demonstrate this technique.

The polarimetric angular memory effect is applied to obtain the average topographic height of a rough surface. This novel effect improves the height sensitivity while maintaining a good degree of correlation between the sensors. By using a reference flat surface, the interferometric phase is linearly related to the mean topographic height. The combination of angular memory and wideband frequency interferometry (AMFI) is realized and offers a means to design a robust interferometric system. Extension of the technique to the pulse scattering problem is studied through the two-frequency mutual coherence function, and its time-domain transform provides an equivalent way to obtain the mean topographic height by combining both spatial and temporal diversity. Millimeter-wave (MMW) experiments are conducted with rough surfaces of different statistics and scattering media of different types (gravel, sand, and rough surfaces) to prove the effectiveness of the technique.

Despite the recent development of analytical and numerical techniques for problems of scattering from two-dimensional rough surfaces, very few experimental studies were available for verification. The authors present the results of millimeter-wave experiments on scattering from two-dimensional conducting random rough surfaces with Gaussian surface roughness statistics. Machine-fabricated rough surfaces with controlled roughness statistics were examined. Special attention was paid to surfaces with large rms slopes (ranging from 0.35 to 1.00) for which enhanced backscattering is expected to take place. Experimentally, such enhancement was indeed observed in both the copolarized and cross-polarized returns. In addition, it was noticed that at moderate angles of incidence, the scattering profile as a function of observation angle is fairly independent of the incident polarization and operating frequency. This independence justifies the use of the geometric optics approximation embodied in the Kirchhoff formulation for surfaces with large surface radius of curvature. When compared with the experimental data, this analytical technique demonstrates good agreement with the experimental data.

It is known that a change in the direction of an incident wave on a random medium is "remembered" by the angular correlation characteristics of the scattered waves. This "memory effect" is studied for rough-surface scattering by means of theoretical [second-order Kirchhoff approximation (KA)], numerical (Monte Carlo simulations), and experimental (millimeter-wave range) approaches. The second-order KA has been found to be effective for wave scattering from very rough surfaces with large radii of curvature and high slopes (0.5-1.5). Although the second-order KA is based on a number of approximations including the geometrical optics approximation and the approximate forms of the shadowing functions, excellent agreement with Monte Carlo simulations and millimeter-wave experiments was achieved. The results are presented in a form of memory signatures which clearly exhibit the important features of this effect.

The author presents a comprehensive review highlighting historical as well as new developments in the area of random media. Both discrete and continuous media are considered as well as rough surfaces. The author discusses wave propagation in turbulence and in a random continuum where the refractive index is a random function of space and time. Examples are optical propagation in the atmosphere, microwaves in the troposphere, ionosphere, planetary atmosphere, and solar wind, and acoustic scattering in ocean turbulence. The author describes multiple scattering by random distributions of discrete scatterers. Examples are optical and microwave scattering by rain, fog, smog, snow, ice particles, and vegetation, optical and ultrasound scattering by tissues and blood, optical and acoustic scattering in the ocean, and scattering in composite materials. Scattering by rough surfaces and interfaces is discussed. Examples are acoustic scattering by ocean surfaces, microwave and optical scattering by vegetation, terrain, and snow cover, and ultrasound scattering by rough interfaces in biological media.>