Charles L. Rino

Numerical simulation of backscatter from linear and nonlinear ocean surface realizations

In this paper, numerical simulations of the scattering from time-dependent realizations of one-dimensional ocean surface waves are described. A new technique is used that allows efficient generation of ocean surface realizations that preserve the dominant nonlinear hydrodynamic characteristics. Thus unique scattering effects of real ocean surface waves can be explored. Until very recently, numerical simulations of rough-surface scattering were used mainly to test and/or improve theoretical models that predict the average bistatic scatter cross section. We carry the simulations further by generating Doppler spectra from dynamically evolving surface realizations. Doppler spectra of signals scattered from the ocean surface are affected by both hydrodynamic nonlinearities and higher-order scatter terms. The simulated Doppler spectra from nonlinear surface realizations reproduce the measured characteristics of ocean and wave-tank data for low and high wind conditions. We also show that the results are essentially reproduced by the second-order Kirchhoff approximation.

Application of beam simulation to scattering at low grazing angles: 1. Methodology and validation

Numerical simulations of rough surface scattering at near-grazing incidence require very large surfaces (≳500λ). Conventional methods of exact solutions require the inversion of a very large matrix, which can exceed the memory and speed capabilities of even modern supercomputers. The beam simulation method proposed by Saillard and Maystre circumvents this problem by decomposing the large incident beam into narrower subbeams and then synthesizing the large beam by coherent superposition. The radius of these narrower subbeams is determined by the local interaction distance on the surface, which is found to increase with incidence angle, ultimately forcing a single beam in the limit of strict grazing incidence. This paper demonstrates that this technique gives essentially the same results as can be obtained by the method of moments and can handle surfaces as large as 1000λ for grazing incidence angle as low as 10°.

Forward propagation in a half-space with an irregular boundary

The parabolic wave equation (PWE) has been used extensively for modeling the propagation of narrow beams in weakly inhomogeneous random media. Corrections have been developed to accommodate wider scattering angles and boundaries have been introduced. Nonetheless, the formalism remains approximate and irregular surfaces with general boundary conditions present difficulties that have yet to be overcome. This paper presents an alternative approach to the entire class of propagation problems that strictly involve forward propagation. Forward-backward iteration has been shown to be a powerful procedure for computing the source functions that support propagation over irregular boundaries at low grazing angles. We show that the source functions for any unidirectional sweep can be computed by using a marching solution. This is not only more efficient than the single-sweep computation, but it facilitates accommodation of inhomogeneities in the propagation media. An exact equation for forward propagation in unbounded inhomogeneous media is used to derive a correction term that is applied at each forward-marching step. Results that combine ducting atmospheres and rough-surface scattering effects are presented for both the Dirichlet and Neumann boundary conditions.

A spectral-domain method for multiple scattering in continuous randomly irregular media

A spectral-domain method for computing the first and second-order moments of a scalar wavefield propagating in a continuous randomly irregular medium is described. The scattering is characterized by the moments of incremental forward and backward scattering functions. Approximate forms for these moments are given in terms of the spectral density function of the relative permittivity fluctuations. Solutions are developed for incident plane and spherical waves. Well-known results that are usually derived from the parabolic wave equation using the Markov approximation are easily recovered from the general solutions. The results show that backscatter enhancements depend on the correlation between the forward and backward scattered waves. Computations are performed to illustrate the effects of backscatter on VHF transionospheric radiowaves. >

On propagation in continuous random media

Abstract The analysis of wave propagation in continuous random media typically proceeds from the parabolic wave equation with back scatter neglected. A closed hierarchy of moment equations can be obtained by using the Novikov–Furutsu theorem. When the same procedure is applied in the spatial Fourier domain, one obtains a closed hierarchy of coupled moment equations for the forward- and back-scattered wavefields that is not restricted to narrow scattering angles nor to small local perturbations. The general equations are difficult to solve, but a Markov-like approximation is suggested by the form of the scattering terms. Simple algebraic solutions can be obtained if a narrow-angle-scatter approximation is then invoked. Thus, three distinct approximations are explicit in this analysis, namely closure, Markov and narrow-angle scatter. The results show that the extinction of the coherent wavefield has a distinctly different form from the corresponding result for propagation in a sparse distribution of discret...

Ocean microwave backscatter from the LOGAN experiment

High resolution, X-band (9.75 GHz) sea-clutter data were collected at grazing angles of 1/spl deg/ and 2/spl deg/ under a variety of sea states as part of the LOGAN experiment. Backscatter from sea spike events in the vicinity of wave crests allowed the dispersive properties of the ocean surface waves to be estimated from measurements of the radar cross section (RCS) as a function of range and time. Modulations of the RCS are shown to be consistent with the presence of two types of sea-spike scattering phenomena. Variations in Doppler shift and RCS in response to the passage of wave crests through single range cells reveals an asymmetry between upwave and downwave sea spikes.<<ETX>>

A comparison of forward-boundary-integral and parabolic-wave-equation propagation models

The parabolic-wave equation and its variants have provided the theoretical framework for most practical forward-propagation models. Split-step integration generates an easily obtained, robust solution for most applications. Irregular boundaries can be incorporated by using a conformal mapping technique introduced by Beilis and Tappert (1979) and refined by Donohue and Kuttler (see ibid., vol.48, p.260-77, 2000). In an earlier paper, we demonstrated an alternative method that incorporates a numerical solution to the forward-boundary-integral equation within each split-step cycle. This paper compares predictions of forward propagation obtained by these two distinctly different methods. The results confirm that the PWE-based method is very accurate for smoothly varying surfaces and that it captures the primary forward structure even in the presence of unresolved surface detail. The moderate loss of fidelity is often an acceptable trade for increased computational efficiency. There are situations, however, where the details of the surface structure are important. Furthermore, the induced surface currents are unique to the forward-boundary-integral method. We illustrate their use by calculating the bistatic scatter that would he measured from an isolated surface segment. We show that the scattered field measured in this way can be normalized to form a bistatic scatter function only when the illuminating beam is tilted slightly toward the surface. We interpret this disparity as a breakdown in concept that underlies a local scattering function.

Wave scattering functions and their application to multiple scattering problems

Abstract Scattering functions arise naturally in standard treatments of the effects of a material object or surface embedded in a uniform field. The most commonly used scattering function describes the far-field modulation imparted at large distances to a spherical wavefront eminating from the scatterer. The purpose of this is to develop the properties of the spectrum of scattered plane waves as an exact generalized scattering function. The linearity of the wave equations guarantees that such a representation exists; moreover, it is possible to derive the generalized scattering function from the far-field scattering function by analytic continuation. Although these properties are known, recent theoretical developments have motivated us to reexplore the interrelations among the far-field scattering function, the Greens function and various forms of the generalized scattering function as well as the symmetry properties of the generalized scattering function imposed by reciprocity. For multiple-scattering o...

Low‐frequency acoustic scatter from subsurface bubble clouds

At high sea states, acoustic reverberation from the ocean surface achieves anomalously high levels that cannot be explained by bottomside roughness induced by wind‐driven surface waves. It has been hypothesized that bubble clouds generated by breaking or spilling waves are the source of the increased reverberation. Evidently, when the bubble‐cloud size exceeds the acoustic wavelength, backscatter from the bubble cloud dominates the surface‐scatter contribution. By using a new scatter formalism, we are able to quantitatively evaluate the bubble‐cloud hypothesis. Computations have been performed for a rudimentary bubble‐cloud model in the presence of a highly rough surface. For a flat surface, the bubble cloud and its negative image tend to cancel one another when they are in near contact, but they can interfere constructively as the bubble cloud moves away from the surface. Surface roughness tends to diminish the cancellation effects whereby the average scattered signal strength lies at or above its level ...

Results from the 1993 LOGAN (Low Grazing ANgle) radar experiment

During July and August of 1993, a low grazing angle (LOGAN) radar experiment was conducted from the Chesapeake Light Tower which is located on the continental shelf approximately 24 km east of Cape Henry, Virginia. High resolution (0.15 m) and calibrated radar measurements were made at low grazing angles of 0.5/spl deg/ to 20/spl deg/ over a spectrum of frequencies (0.35 to 18.0 GHz) and for polarizations of VV, VH, HH and HV. The radar measurements were supported with environmental measurements from the NOAA C-MAN weather station, a meteorological buoy, a wave buoy, a thermistor chain, a current meter, and video and photographic records of the sea surface. Measurements were made over a variety of sea conditions, including a sea surface with both natural slicks and a man made slick. Various approaches were used to investigate multi-path and glistening zone processes. The LOGAN measurements are to be used to evaluate hypotheses pertaining to sea surface backscatter and the doppler associated with backscatter observed at low grazing angles.<<ETX>>