Nicole de Beaucoudrey

Resonance Behavior of Radar Targets With Aperture: Example of an Open Rectangular Cavity

In the resonance region, the radar scattering response of any object can be modelled by natural poles with the formalism of the singularity expansion method. These natural poles are resonance parameters which provide useful information for the discrimination of radar targets as their general shape, characteristic dimensions and constitution. In the case of an open radar target, high-Q internal resonances and low-Q external resonances occur respectively inside the target and on its surface. Because internal resonances have a higher Q, they may have a higher total energy and can thus be used for target identification. In this paper, we choose to study the resonance behavior of a perfectly conducting rectangular cavity with a rectangular aperture. With this simple example, we intend to show how to distinguish between the two origins of these resonances: external resonances corresponding to traveling waves on the surface of the target and internal resonances corresponding to cavity waves. Indeed, this can be applied to characterize aircrafts, whose apertures (such as inlets, open ducts, air-intakes, cavities etc.) contribute significantly to the overall radar cross section.

A New Algorithm of 3D Image Reconstruction of Radar Targets from Ramp Responses in Low Frequency

Low frequency imaging in radar domain can have applications for stealthy or buried targets. The transient scattering response from a ramp waveform is related to the proflle function of the target, namely its transverse cross-sectional area along the line-of- sight, and thus provides information about the target size, orientation and geometrical shape. Such ramp responses can be used to generate a 3-dimensional image of the global shape of the target. Former imaging algorithm uses approximate limiting surfaces and is therefore limited to single convex objects. Here is proposed a new algorithm able to reconstruct non-convex as well as separated targets, from their ramp response signatures.

Statistical Analysis of Real Aperture Radar Field Backscattered From Sea Surfaces Under Moderate Winds by Monte Carlo Simulations

The statistical properties of the electromagnetic field backscattered from sea surfaces are studied by using asymptotic numerical methods. Sea surfaces are modeled by using the Elfouhaily et al. spectrum model. The influence of the radar spatial resolution on the field statistics is studied for various wind conditions and radar configurations, by considering equal range and azimuth resolutions. It is observed that the backscattered field phase can be assimilated to a uniform distribution and that the amplitude resembles a Rayleigh distribution. Moreover, reducing the radar spatial resolution d induces a stronger variability of the backscattered field amplitude and a departure from the Rayleigh distribution. This departure is enhanced particularly when the radar look approaches the cross-wind direction and also when increasing the wind speed. Also, a fitting of the amplitude distribution with various theoretical statistical laws (gamma, Weibull, K, and so on) highlights the general good fitting of the K distribution, which is in agreement with previous work led from measurements.

Modeling and simulation of sea surface radar observations

This paper describes a methodology to model and simulate the reflectivity measured by radar observing a maritime environment. The simulation principle consists of reproducing the acquisition of a Real Aperture Radar (RAR) moving along its axis. Pulse after pulse, high range resolution profiles are successively computed by summing the contribution of backscatters comprised in the radar footprint. The scatterer contributions are calculated from typical statistical distribution estimation. The simulation output, called raw data, is the concatenation of time profiles (short time) successively obtained for each radar pulse (long time). A SAR algorithm is then applied in order to form a high resolution image. The generation of the sea surface is achieved by a multi-scale model. This approach takes into account phenomena at different scales (from long wave to small objects), all in interaction in a hydrodynamic environment. It is thus possible to focus locally on contributions considered as more significant. To improve the processing time some contributions can also be retrieved from Look-Up Tables. Hence, our method performs a realistic simulation of electromagnetic interactions in a maritime environment.

Resonances of complex shape scattering objects: Modelling by resonant circuits

In resonance domain, the radar scattering response of any object can be modelled by natural poles of resonance with the formalism of the Singularity Expansion Method (SEM). The mapping of poles gives useful information for the discrimination of radar targets. In this paper, we propose to use resonant circuits modelling to characterize the resonance behavior of complex shape targets from the quality factor Q and the natural pulsation of resonance ¿0. For perfectly conducting (PC) canonical and complex shape targets, we present results exhibiting advantages of these two parameters {¿0 ; Q}.

Algorithm for profile function calculation of 3D objects: Application for radar target identification in low frequency

Ramp response technique in low frequency can be used for generating 3-dimensional images of radar targets (even stealthy or buried targets) so as to identify them. This technique uses the target proflle function, which is deflned as its transverse cross- sectional area versus distance along the observing direction. For mutually orthogonal observing views, reconstructed 3D images are quite accurate. However, in practice, due to the bias introduced from the response in shadow region and from limited non-orthogonal observing directions, reconstructions become distorted. To evaluate the quality of the reconstruction and to further identify objects from their reconstruction, we need to calculate proflle functions of 3D reconstructed objects in arbitrary directions. Therefore, in this paper, we propose an algorithm meeting this needs.

Radar sea surface modeling and simulation

This work is a significant part of the MODENA project, aiming at modeling and simulating the maritime environment remotely sensed by a radar [1].The main steps of the project go through a modeling of the ocean surface, the man-made objects on the surface as well as of the interaction between the electromagnetic waves with this surface and the objects. One of the main interests of the radar simulation is SAR imaging. Usually SAR imaging is directly simulated from a sea spectrum, through an appropriate transfer function. The drawback of this method is the impossibility to simulate a phenomenon whose size is inferior to the SAR resolution. The methodology developed in this paper is different since the simulation is done before SAR processing. By choosing to simulate the backscattered field toward the radar antenna, it is then possible to define the scene mesh independently of the final SAR image resolution. Furthermore the use of irregular mesh provides opportunities to focus more finely on specific phenomena locally defined. The simulation principle was explained in [6]. It consists of reproducing the acquisition of a Real Aperture Radar (RAR) moving along an axis. An important part of the simulation is the generation of the sea surface. It is achieved by a multi-scale model whose description is given in [2] and [3]. This model gives the possibility to manage and represent dynamically the maritime environment at different scales: large scale for the long waves of the sea surface (swell-like); short scale for small waves (wind-driven ones). To improve the processing time some contributions can also be retrieved from Look-Up Tables. Hence, our method performs a realistic simulation of electromagnetic interactions in a maritime environment. This paper will focus on the results obtained from the theoretical and practical developments achieved since the description given in [4] at last year conference.

Radar imaging using the ramp response technique in arbitrary directions

Radar imaging in low frequency can reconstruct the global shape of targets (buried or stealthy) using the ramp response technique, which only needs no more than 3 observing angles. With the ramp response technique, existing three-dimensional (3D) image reconstruction algorithms work well for orthogonal observing directions, but produce distorted images for the non-orthogonal case. Therefore, we apply the level set method, which has been widely used in capturing dynamic interfaces and shapes, to optimize the image reconstruction from arbitrary observing directions. Satisfactory results are obtained.