Fabrice Comblet

Bistatic SAR: theory and simulation

At present, the most used configuration for synthetic aperture radar imaging is the monostatic one (i.e. transmitter and receiver use the same antenna). However, the multiplication of stealth radar targets and the need for more accurate observations imply new radar imaging configurations and processing developments. The bistatic configuration, i.e. operation with separated transmitting and receiving antennas, should overcome the monostatic limitations. The transmitting antenna can also be used in a monostatic configuration, and consequently it is possible to combine monostatic and bistatic data. Bistatic data acquisition provides additional qualitative and quantitative characteristics of scattering targets. The paper presents bistatic SAR images obtained by bistatic RDA (range Doppler algorithm) processing applied to the polarized electromagnetic field detected by the receiver. The proposed model is qualitatively compared with results already published in the literature (Soumekh, M., 1998; 1999), and quantitatively validated from the analytical resolution equations.

A critical overview of bistatic scattering wave from oceans surface

This paper interests in bistatic scattering wave from oceans surface. We present different scattering method for 2D random rough surfaces in bistatic configuration. The purpose is to study their validity for a particular surface: the ocean surface. The ocean surface characteristics are given to be introducing in the scattering method. Then, the validity and robustness of each method are studied for ocean surface applications.

Exploitation of C-Band Sentinel-1 Images for High-Resolution Wind Field Retrieval in Coastal Zones (Iroise Coast, France)

Synthetic aperture radar (SAR) is one of the favorite tools for earth observation applications, i.e., oceanography, land use mapping, climate change since this device can offer the data at a high spatial resolution and in most meteorological conditions. This is more significant when the data acquired by the Sentinel-1, a new C-band satellite, are exploited. For high-resolution wind field extraction, two different approaches are proposed. In the scatterometry-based approach, wind direction is first extracted by the local gradient method at different scales, i.e., 1–5-km wind resolutions. It is then applied to the empirical geophysical model functions, i.e., CMOD (C-band), for surface wind speed estimation. The advantage of this approach is to deliver accurate wind speed estimates in the range of 2–25 m/s from different SAR data. However, it requires wind direction as an input parameter. This can lead to errors in wind speed estimation due to uncertain wind directions. Therefore, for comparison, in the second approach, we propose the use of the model without wind direction input proposed by Komarov et al. In general, the obtained wind fields based on two proposed approaches are quite similar, and they have good agreement with in situ measurements from the meteorological stations along the Iroise coast.

Numerical simulation of bistatic electromagnetic scattering by contaminated sea surface

The aim of this paper is to study the influence of the pollutants (oil spills) on the electromagnetic signature of sea surface observed in bistatic configuration. Therefore, we will start the numerical analyses of the pollutants influence on the sea surface roughness. Then, we will evaluate the electromagnetic scattering coefficients of the contaminated sea surface (sea surface covered by oil layer) in bistatic case by using the numerical Forward-Backward Method (FBM). The model used for the numerical simulation of bistatic scattering coefficients of clean and contaminated sea surface has been analyzed as a function of various parameters (sea state, type of pollutant, incidence and observation angles, radar frequency and polarizations).

Bistatic scattering from a contaminated sea surface observed in C, X, and Ku bands

The aim of the work presented in this paper focuses on the study and analysis of variations of the bistatic electromagnetic signature of the sea surface contaminated by pollutants. Therefore, we will start the numerical analyses of the pollutant effect on the geometrical and physical characteristics of sea surface. Then, we will evaluate the electromagnetic (EM) scattering coefficients of the clean and polluted sea surface observed in bistatic configuration by using the numerical Forward-Backward Method (FBM). The obtained numerical results of the electromagnetic scattering coefficients are studied and given as a function of various parameters: sea state, wind velocity, type of pollutant (sea surface polluted by oil emulsion, and sea surface covered by oil layer), incidence and scattering angles, frequencies bands (C, X and Ku) and radar polarization.

Bistatic radar imaging system for sea surface target detection

This paper presents an original approach to the bistatic SAR processing. Polarized electromagnetic field received is considered and used in a bistatic RDA processing. Then, some bistatic SAR image characteristics are presented. Latter, a model describing bistatic reflectivity of sea surface is introduced. This model is used in the bistatic SAR processing in order to introduce the speckle effect and to simulate targets on sea surface detection.

Study of inversion EM models for wind speed retrieval from Sentinel-1 data

Sea surface wind speed plays a key parameter in the studies of many oceanic applications, i.e. meteorological forecasting, oil slick observation, ship detection, and wind turbine installation recently. It can be obtained from many available wind sources, i.e. measured data, numeric weather models, etc. However, one of the most well-known ways is the retrieval of wind speed from Synthetic Aperture Radar (SAR) data. For this approach, the studies are based on two principal ways: one uses empirical models and the other is based on electromagnetic calculations. In both indicated approaches, the Geophysical Model Functions (GMFs) are used to describe the dependency of radar scattering from sea surface on surface wind speed and the geometry of observations. By knowing radar scattering and geometric parameters from SAR data, it is possible to invert the GMFs to retrieve wind speed. Then, estimated wind speed by two studied models is compared and evaluated with measured data. Based on the comparisons, the advantages and limits of the studied models are analyzed and discussed.

Application of electromagnetic models for sea near-surface wind speed retrieval from C-band SAR images

Synthetic Aperture Radar (SAR) is one of the favorite sources for sea near-surface wind speed retrieval. For this problem, wind speed is principally estimated based on the empirical (EP) models, namely CMOD functions, which are constructed by the observations from spaceborne microwave scatterometers (ERS-1/2). Little studies have mentioned the use of electromagnetic (EM) models for wind speed estimation, probably due to their complicated descriptions. However, it is reasonable to compare wind speed estimates based on the two approaches, since both of them describe the relation of radar scattering and wind field, directly for EP models and via wave surface roughness for EM models. Based on the comparisons, some ideas are proposed to improve the performance of EP and EM models.

Comparison of empirical and electromagnetic geophysical model function for near-surface wind speed retrieval

Despite based on different approaches and objectives, it is reasonable to compare near-surface wind speed estimated by the empirical (EP) and electromagnetic (EP) geophysical model function (GMF), since both of them describe the relation between radar scattering and wind vector (directly for EP GMF and via surface roughness spectrum for EM GMF). In general, the EP and EM models give quite similar normalized radar cross section (NRCS) for radar incidence angle below 40°. Consequently, wind speed estimated by the EP and EM GMF is very close. However, for incidence angles above 40°, the EM models show poor performance of wind speed estimation.

Pirical approach for C-band VV-polarization wind vector retrieval from Sentinel-1 images

Based on an empirical model without wind direction input for the retrieval of C-band HH-polarization wind speed, we propose a modified model for wind speed estimation in VV-polarization. The obtained wind speed is then applied for the CMOD5.N to estimate wind directions. The comparisons with the scatterometry-based approach demonstrate that the estimated wind speed by the proposed model is closer to in situ measurements than that obtained with the CMOD5.N. Likewise, the extracted wind directions from the CMOD5.N are more accurate that those obtained with the local gradient method.