Laetitia Thirion-Lefevre

Investigating Attenuation, Scattering Phase Center, and Total Height Using Simulated Interferometric SAR Images of Forested Areas

The objective of this paper is to examine the link between the attenuation coefficients and the interferometric phase center heights, for several frequencies from P- to L-band, and to study the extent to which it depends on the canopy architecture and description. This paper relies on the use of a coherent and full polarimetric scattering model, which simulates the fields backscattered by a forested area. In the first part, we study the behavior with a frequency of the interferometric phase center heights, and in the second part, we focus on the attenuation coefficients. Then, we compare the behaviors of these two quantities, and we propose to empirically derive a relation between these two quantities and the mean forest height. Finally, we investigate if a change in the initial forest or radar configuration has an impact on the determination of this relation.

Bistatic scattering from forest components. Part II: first validation of a bistatic polarimetric forest model in the VHF-UHF band [225–475 MHz] using indoor measurements

This paper introduces the validation of the extension of a scattering model of forests to the bistatic configuration (COBISMO). The measurement in an anechoic chamber is first described. The various stages of the validation process are presented. One dielectric cylinder on a metallic plate is chosen as the canonical element to be tested. Indoor measurements are confronted with the results predicted by the model, first in the horizontal/azimuthal plane, then in the vertical/elevation plane. Then mutual coupling is also investigated using a group of three cylinders. The agreement between simulation and measurement is surprisingly good in light of the precision of such indoor measurements. Several other aspects are discussed: the influence of the frequency, of the shape of the section of the cylinder, and polarimetric effects.

SAR Processors Based on Signal or Interference Subspace Detector

In this paper, we present a new synthetic aperture radar (SAR) processor relying on subspace detectors. The scattering pattern of the constituent element of the man-made target (MMT) to detect is used to improve the detection capabilities. In the particular case of foliage penetration (FOPEN), we propose to reduce the probability of false alarms by considering the scattering pattern of almost vertical trunks (modeled here as dielectric cylinders). New subspace detectors are derived to look for either an interference or a target element in a noisy environment. A new processor based on these subspace detectors is successively applied to simulated data. In addition, results obtained with real data reveal a significant improvement in the detection capabilities both in comparison with a classical SAR processor and with the subspace signal detector SAR (SSDSAR) processor (without interference model).

New SAR target imaging algorithm based on oblique projection for clutter reduction

We have developed a new synthetic aperture radar (SAR) algorithm based on physical models for the detection of a man-made target (MMT) embedded in strong clutter (trunks in a forest). The physical models for the MMT and the clutter are represented by low-rank subspaces and are based on scattering and polarimetric properties. Our SAR algorithm applies the oblique projection of the received signal along the clutter subspace onto the target subspace. We compute its statistical performance in terms of probabilities of detection and false alarms. The performances of the proposed SAR algorithm are improved compared to those obtained with existing SAR algorithms: the MMT detection is greatly improved, and the clutter is rejected. We also studied the robustness of our SAR algorithm to interference modeling errors. Results on real foliage penetration data showed the usefulness of this approach.

New SAR Algorithm Based on Orthogonal Projections for MMT Detection and Interference Reduction

We develop a new synthetic aperture radar (SAR) algorithm based on physical models for the detection of a man-made target (MMT) embedded in strong interferences (trunks of a forest). These physical models for the MMT and the interferences are integrated in low-rank subspaces and are based on scattering and polarimetric properties. Several images, called subspace SAR images, can be generated and combined considering these subspace models. We then propose a new approach for target detection and interference reduction based on the combination of SAR subspace images. We show that our SAR algorithm outperforms the classical SAR imagery algorithm on both simulated data and real data in the context of foliage penetration detection.

Back Projection Version of Subspace Detector SAR Processors

A new version of the signal subspace detector synthetic aperture radar (SSDSAR) processor developed in [1] is proposed. Actually, the SSDSAR processor can be divided in two different parts: a subspace basis computation part and an imaging part. The new version proposed here consists in performing the imaging part on fast-time compressed data and not raw data in order to reduce the computational cost from O(N4) to O(N3). We note that this new processor is the subspace detector version of the classical back projection SAR processor.

Bistatic scattering from forest components. Part I: coherent polarimetric modelling and analysis of simulated results

This article deals with the study of bistatic scattering by forested areas using a coherent scattering model. The contribution of the scatterers is considered (trunks, branches), as well as their interaction with the ground. This code has been extended from monostatic to bistatic configurations and we present all the tests we have performed with this tool in order to validate it as far as possible. The transmitter is at a fixed location and the receiver moves either in the incidence plane (only θ s varies) or in all positions (θ s and φ s both vary). Our objective is to determine the main trends characterizing bistatic scattering by a forest. We study the variation of the scattering coefficient, when evolving from the backscattering to the specular configuration (only θ s varies). Both radiometric and polarimetric aspects are analysed, first when considering a simple forest of trunks and then a forest with trunks and branches. Finally, we analyse the impact of both θ s and φ s for two cases: a vertical dielectric cylinder in free space and a set of vertical dielectric cylinders, arranged in different ways. Here again, the radiometric and polarimetric aspects are investigated to check the electromagnetic behaviour.

Determination of mechanisms that can occur in NLOS urban canyon

In this paper, we are interested in identifying all areas inside a urban canyon that can be illuminated by a radar in NLOS (non line of sight) configuration. We developed a simple model to identify, according to each canyon configuration, the ground canyon areas illuminated by the radar and also the non illuminated areas. We test the results of our algorithm for a specific canyon. To validate our theoretical results we finally present some measurements that will be performed in the anechoic chamber of ONERA on a scaled urban canyon in the case of far field.

A New Light on SAR Backscattering Coefficient and Interferometric Coherence in Layover Areas

This letter focuses on the analysis of layover effects in interferometric synthetic aperture radar (SAR) data of urban areas. In particular, we derive two formulations to express the backscattering coefficient and the interferometric coherence in this case. These equations show that the backscattering coefficient and the interferometric coherence in layover areas can be seen as a combination of the backscattering coefficients and interferometric coherences of the individual scattering mechanisms. These formulations are then tested on interferometric SAR (InSAR) data and analyzed statistically.

Analysis of a NLOS canyon in an InSAR image of a urban area at Ka-band

In this paper, our concern was to help understanding the different scatterers responses in an interferometric image of a urban area by studying the different mechanisms that can occur inside the urban canyon. This study relies on a geometric code we specifically developed for this kind of scene. In this example, because of the reflections on the walls, we highlight the impact of the Brewster angle on the interferometric signature. For this reason, we recommend the use of different polarization and incidence angles for urban areas analysis of InSAR image.