Cyril Dahon

Capabilities of a forest coherent scattering model applied to radiometry, interferometry, and polarimetry at P- and L-band

The interpretation of radar data would ideally require extensive and numerous observations. However, the number of observations is limited by the difficulty and the cost of acquiring ground truth and radar data. On the other hand, numerical models can provide a wide range of situations, both in inputs and in outputs. More precisely, they have to provide radiometric, polarimetric, and interferometric simulations and be applicable to various forested areas (high density, high/low moisture, inhomogeneous area, etc.) and radar configurations (low/high frequency, bistatic observation, etc.). This paper is dedicated to the presentation of the capabilities of a descriptive coherent scattering model (COSMO) applied to the electromagnetic study of the backscattering by forested areas. Improvements have been implemented in order to produce in output a radar image, which can be treated with the same polarimetric and interferometric tools as those applied to real synthetic aperture radar images. Thus, comparisons are possible. COSMO has been widely tested from P- to L- bands, over temperate and tropical forests and applied to radiometry, polarimetry, and interferometry. It appears finally as an efficient simulating tool to carry out parametric studies and to analyze how the total scattered field is built from canonical mechanisms and individual scatterer contributions.

Analysis of the Main Scattering Mechanisms in Forested Areas: An Integral Representation Approach for Monostatic Radar Configurations

In this paper, a coherent forest scattering model based on the electric-field integral representation is developed. This model gives the scattered field by the forest, resulting from the interaction of an electromagnetic plane wave with its main elements (trunks, branches, and plane ground) in a frequency range of 100 to 400 MHz. The Method of Moments is used to solve the electric-field integral equation. In our treatment, there are three possible scattering mechanisms that contribute to the scattered field defined as follows: 1) single-; 2) double-; and 3) triple-bounce scattering mechanisms. The internal fields inside the tree trunks and branches are calculated by considering all of the multiple-scattering interactions within the forest. Our model is then constructed by coherently summing the three scattering mechanisms previously given. We can also obtain an approximate result by ignoring some or all of the multiple-scattering interactions in the calculation of internal fields inside the trees. The aim of this paper is, first, to determine the relative contribution of each scattering mechanism to the total scattered field, followed by the relative contribution of tree-trunk and branch scattering responses to the total scattering field, and, finally, the multiple-scattering interaction effects on the total backscattered field.

Computing the double-bounce reflection coherent effect in an incoherent electromagnetic scattering model

One of the main limitations of electromagnetic incoherent vegetation models based on the radiative transfer theory concerns their inability to take into account the coherent effect occurring in a double-bounce scattering mechanism. This is particularly important for low-frequency synthetic aperture radar applications over forested areas, where large branch and trunk contributions may be preponderant. In this letter, an easily computable solution based on the reciprocity theorem is proposed. The coherent effect contribution is obtained directly from the radiative transfer incoherent solution through a straightforward correction algorithm. Compared to the classical vector radiative transfer first-order solution, this method provides the exact cross-polarized contribution to the radar cross section or to the polarimetric parameters computation. The theoretical developments are illustrated using electromagnetic full-wave and approximate scattering models.

Full Polarimetric Bistatic Radar Imaging Experiments on Sets of Dielectric Cylinders Above a Conductive Circular Plate

This paper presents fully polarimetric bistatic radar scattering measurements on groups of dielectric vertical and/or tilted square cross-sectional cylinders above a conductive circular plate in the frequency range of 6-18 GHz. The experiments have been conducted in the BABI facility at the French Aerospace Lab (ONERA). The imaging experiments consider an azimuthal bistatic radar configuration with a single incident direction and many scattering directions, in the way that the receiver positions synthesize a 1-D circular aperture in a horizontal plane around the focal point of the anechoic chamber. The bistatic scattering by the different sets of cylinders is also achieved through numerical simulations by using a volume-electric-field-integral-equation model employing a method of moments. First, theoretical and experimental bistatic radar-cross-section values are confronted. The comparisons show a good agreement for all polarization cases (vv, hv, vh, and hh). Then, a near-field reconstruction algorithm has been extended from a monostatic to our bistatic radar configuration in order to generate the reflectivity maps by focusing both theoretical and experimental scattered fields. The comparisons of the 2-D bistatic synthetic aperture radar images show that the two prediction techniques are accurate.

A high performance MPI implementation of numerical modeling of electromagnetic scattering from forest environment

The Message Passing Interface implementation ensures a reduction of the computing time and a distribution of the necessary memory to each processor. The 3D full-wave model for electromagnetic scattering from forest environment, based on the volumetric integral Equation formulation of the electric field, combined with the Characteristic Basis Function Method (CBFM) and implemented in Message Passing Interface (MPI) parallelization is described in this paper. This combination of the CBFM and the Method of Moments (MoM) allows us to reduce significantly the size of the initial electromagnetic scattering problem.

A new reciprocal 3D model of scattering by a finite dielectric cylinder: Application to forest remote sensing

Herein, we suggest a fast approximate 3D numerical model for the interaction of an electromagnetic wave with a forest. Contrarily to other such approximate models found in the literature, this model accounts for the reciprocity of the double bounce scattering mechanism observed in particular configurations. This reciprocity is enforced at the scattering matrix level. The model is tested and compared to a rigorous model based upon an integral representation of the fields.

Full wave analysis of VHF-UHF forest bistatic scattering mechanisms an investigation on the influence of electromagnetic coupling

A 3D coherent scattering model simulating the interaction of electromagnetic waves with forests has been developed. It is obtained by means of a full wave approach, based on an integral representation of the electric field. A method of moments is used to solve the integral equation and compute the scattered fields related to the various scattering mechanisms as well as the contribution of tree-trunks and branches. This model is used here to evaluate the impact of electromagnetic coupling effects between a group of scatterers (which can be the branches and the trunk of a single tree, multiple tree-trunks, or multiple trees) for monostatic and/or bistatic radar configurations. To validate our model, we compare our simulation results with anechoic chamber measurements.

Integral Representation of the Electromagnetic Field for the Description of the Main Mechanisms Appearing in Forested Area for Monostatic Radar Configurations

In this paper, an electromagnetic scattering model based on the electric field integral representation is used to separate the scattering mechanisms involved in the interaction of an electromagnetic field with forested areas. This model is used to examine the different coupling effects between the main scatterers (trunks and branches) and those between the scatterers and the ground.