Albert Guissard

Mueller and Kennaugh matrices in radar polarimetry

This paper is concerned with the notions of the Mueller matrix and the Kennaugh (or Stokes scattering) matrix, that are commonly used in the field of radar polarimetry. Although these two matrices are quite similar, there exists a basic difference between them that seems to have been overlooked by a number of researchers, resulting in a great deal of confusion. An attempt is made to clarify this point and to present the basic relationships of radar polarimetry in a compact form. >

An Approximate Model for the Microwave Brightness Temperature of the Sea

Abstract A modified two-scale model is proposed for scattering and emissivity calculations for certain classes of random rough surfaces. It is based on an approach by Burrows and by Brown, but it has been extended to bistatic scattering by lossy dielectric surfaces, and it incorporates modified Fresnel reflection coefficients and a simple correction for multiple-scattering effects. The method is shown to be applicable to the ocean surface for light and moderate winds. A contracted form of the radiative-transfer equation is proposed and the included Wentz correction for surface scattering is discussed. This could lead to a method that could be both simple and accurate enough for real-time inversion algorithms in microwave remote sensing.

Full-range sea surface spectrum in nonfully developed state for scattering calculations

A new form for the spectrum of the ocean surface vertical displacements is derived for the case of nonfully developed states. The gravity range is expressed as a function of the fetch x and the significant slope /spl conint/ as well. The capillary-gravity range is assumed dependent on the wind friction velocity only. Recent wavenumber spectrum measurements in this spectral domain and ocean conditions are analyzed. Tobas spectral shape is shown to represent correctly these experimental data when updated with an equivalent amplitude factor. An expression for this factor is proposed. It is weakly wind friction velocity dependent, as observed by Mitsuyasu in the late 1970s. The proposed spectrum is then combined with a boundary perturbation model for electromagnetic scattering computations. Empirical scattering models and radar data collocated with assumed ground-truth data are used for comparison. This is shown to give consistent results for both C- and Ku-bands as well as large ranges of wind speeds and incidence angles. Comparisons of backscattering coefficients computed using other sea spectra from the literature are presented. The significant slope is found to be an important factor for scattering at low incidence angles. The proposed spectrum thus constitutes a useful basis for physically based inversion algorithms.

Synergic inversion technique for active and passive microwave remote sensing of the ocean

A unified approach for combining active and passive microwave measurements for remote sensing applications is described. A synergic inversion technique has been developed and applied to the retrieval of geophysical parameters of the ocean surface and of the atmosphere. It is based on the combination of radiometric and radar measurements at the electromagnetic and cell level and not only on the correction of radar measurements by radiometric measurements, or conversely. Such a combination is performed through a common quantity: the bistatic scattering coefficient of the observed surface. This is used in a direct model to simulate combined measurements from active and passive sensors. It requires a rather complete and accurate calculation of the scattering of microwaves by the rough sea surface. >

Ring-waves generated by water drops impacting on water surfaces at rest

Radar observations of the ocean surface can be affected by impacting raindrops. Ring-wave measurements are presented for drops of 2.2 and 2.8 mm in diameter impacting on fresh and salt water surfaces initially at rest. They are based on the observation of the mirror image of a sharp edge on the perturbed surface. The retrieved wave profiles show a rather stable characteristic wavenumber (0.2 mm/sup -1/) and very small wave amplitudes: the fraction of the incident kinetic energy converted into ring-waves is of the order of 1%.

Fully and nonfully developed sea models for microwave remote sensing applications

A new statistical description of the ocean surface is proposed and described. Classically it is assumed that the spectrum of the surface vertical displacements depends only on one external, or forcing parameter-the wind speed (or the wind friction velocity); this is, true for fully developed sea states only. In the more general case of nonfully developed sea states, the spectrum should depend as well on internal parameters describing the characteristic features of the surface. Such a representation is discussed in this article for applications to active and passive microwave remote sensing of the oceans.

Phase calibration of polarimetric radars from slightly rough surfaces

The phase calibration problem for polarimetric radars is discussed. More specifically, the assumption that the copolar vertical and horizontal returns from a slightly rough surface are in phase is critically reviewed. It is concluded by means of a theoretical analysis that the phase difference for moist soil targets can reach up to 10/spl deg/ at 15 GHz and for incidence angles of 50/spl deg/. However the assumption of zero phase difference is a good approximation for dry soils, lower frequencies, and low incidence angles. >

Directional Spectrum of the Sea-surface and Wind Scatterometry

This article is concerned with the spectral description of the ocean surface, as required for microwave remote sensing applications. Users are interested in the availability of efficient inversion algorithms for the determination of geophysical parameters such as the wind speed or the wave heights. An accurate statistical description of the surface is required as an input for electromagnetic scattering and emissivity calculations. The basic question of the azimuthal symmetry of the surface spectrum is discussed in detail, with some emphasis on the interpretation of scatterometric measurements. Two definitions of the spectrum can be found in the literature: the classical statistical spectrum, here called the roughness spectrum, and the wave spectrum as used by oceanographers. The first one is symmetric in the (k, omega) space, the second one is not. Conditions of validity of the random plane wave representation, on which the wave spectrum is based, are discussed and the links with the roughness spectrum are stated explicitly. Finally measuring methods of the wave spectrum are briefly summarized.

Sea surface scattering calculations in maritime satellite communications

A method extending the Kirchhoff classical approach for mobile maritime satellite communications is presented. Carrier to specular and carrier to multipath ratios are evaluated from scattering by the rough sea surface calculations using the boundary perturbation method. They are compared with experimental results and with other experimental models. The results illustrate the limited effects of the wind speed and significant wave height, the important dependence on the elevation angle and the very large effects of the ship motions. >

Two‐scale models for rough surface scattering: Comparison between the boundary perturbation method and the integral equation method

This paper presents a comparison between two models for rough surface scattering computations: the boundary perturbation method (BPM) and the integral equation method (IEM). They differ in two fundamental aspects: the method used to compute the electric and magnetic fields on the surface and the surface information required for the computation. The two approaches lead analytically to the same solution in the two asymptotic cases of very large and very small vertical displacements, with no intermediate scales. For a composite surface the solution of the BPM is expressed as the sum of two terms, while a series development up to higher orders can be formulated with the IEM. In this paper, the comparison is restricted to composite surfaces and, particularly, to the ocean surface. After presenting a method for the two-scale decomposition of a rough surface, which satisfies the constraints of the electromagnetic model for each scale, we compute the scattering by the ocean surface using both models for various instrumental configurations and surface conditions. We show that considering the accuracy of usual radar measurements and under the assumptions made for the development of the models, both methods give very similar results. Since the BPM is based on a simple physical argument and appears to be more efficient than the two-scale IEM with regard to the computation time, the BPM should be preferred for ocean-like rough surfaces.