High Fidelity Synthetic SAR Image Generation of a Canonical Target

A 3-dimensional canonical target known as SLICY (Sandia Laboratory Implementation of Cylinders) and the corresponding measured SAR (Synthetic Aperture Radar) image data of SLICY are used to validate the synthetic target image generation process. Computed SAR images of the SLICY target from an electromagnetic code sampled over 360 degrees in azimuth and at two elevation angles are evaluated by comparing against measured images. The results indicate that computed images of high fidelity can be generated if the scattering primitives on the target are correctly identified in the computer-aided design model of the target and this information is properly utilized for computation in the electromagnetic code. 1.0 INTRODUCTION Achieving accurate computational electromagnetic modeling (CEM) for generating synthetic target signatures in the radar domain has been a challenge. Validation of model simulated results against measurement on benchmark models has always been a goal in the CEM community [1,2,3]. This is especially true in the defence science community where methods for validating simulated radar images of complex targets such as aircraft, ships and ground vehicles with a consistent level of accuracy are highly sought. Simulated SAR (Synthetic Aperture Radar) images can be used in various applications, such as target database compilation in Non-Cooperative Target Recognition (NCTR) and ISR (Intelligence, Surveillance and Reconnaissance) image analysis of targets of interests. Proper methodology to assess an electromagnetic (EM) code can lead to valuable insights in identifying some of the problems encountered in radar target image simulation and prospective solutions to resolve these problems. A reasonable and logical approach to proper CEM validation is to have a well-defined target model as a benchmark reference for measurements. This benchmark target can be represented by a 3-dimensional computer-aided design (CAD) model for numerical simulation. A target composed of scattering primitives such as flat surface, cylinder, dihedral, trihedral, and elementary cavity provides a well-defined reference. Such a canonical target allows researchers to gain a better understanding of the inner workings of the target signature computational processes. The objective of this paper is to establish a better understanding of the process of radar image simulation using an EM code. A commercial EM code, FACETS (Frequency Asymptotic Code for Electromagnetic Target Scattering) is used as the computational tool for assessing the numerical simulation process. Measured X-band SAR images of a canonical target known as SLICY (Sandia Laboratory Implementation of Cylinders) in the MSTAR (Moving and Stationary Targets Acquisition and Recognition) are used as benchmark 1 FACETS is a commercial high-frequency electromagnetic code developed by Thales UK Limited. RTO-MP-SET-160 KN2 1 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED High Fidelity Synthetic SAR Image Generation of a Canonical Target references for comparing with the computed target images. A direct comparison between computed SAR images and measured data provides a sound basis for the validation of the EM code. 2.0 SLICY, A CANONICAL TARGET SLICY is one of many targets with measured SAR images that are compiled in the MSTAR datasets [4]. An illustration of the SLICY target is shown in Figure 1. According to the MSTAR image provider, the purpose of this target is to provide researchers a means to validate their EM code’s algorithm using a well-defined target [5], “The ‘Slicy’ target is a precisely designed and machined engineering test target containing standard radar reflector primitive shapes such as flat plates, dihedrals, trihedrals, and top hats. The purpose of this target is to allow Image Understanding developers the ability to validate the functionality of their algorithm with a simple known target” 2.1 Computer-aided design model of SLICY SLICY is a vehicle size test target that features a number of scattering primitives. Figure 2 shows the CAD drawings of SLICY. The dimensions of SLICY are 2.75 m in length, 2.445 m in width, and the height of the rectangular box is 0.765 m. The tall cylinder with a close-top is 0.915 m in height with a 0.66 m diameter; the short hollow (open-top) cylinder is 0.458 m in height and a 0.66 m diameter. The inner wall of the short hollow cylinder behaves like a cavity, a special type of scattering primitive. There are also a small trihedral corner reflector, a quarter cylinder and two step-like trihedral corners on SLICY. The scattering primitives on SLICY are identified and illustrated in Figure 3. 3.0 COMPUTATIONAL ELECTROMAGNETIC CODE, FACETS The FACETS code is capable of generating synthetic SAR images of complex targets such as aircraft, ships and ground vehicles. But it is also a convenient tool to use for investigating the basic image computation process of a simple target. It has a modular algorithmic structure that allows it to facilitate the computation using a combination of different scattering processes, for example, single bounce (flat-plate), double bounce (dihedral), triple bounce (trihedral), edge diffraction (cylinder and top-hat), cavity (hollow cylinder) and shadowing (obstructions between parts on the target). This modular function provides a flexible combination of computing parameters and allows the computed images to be characterized as a function of a number of radar scattering processes available in the code. Thus a well-defined methodology to evaluate simulation-to-measurement validation is available through the use of the FACETS code as a computational and investigative tool. FACETS computes the SAR image of a target using a number of scattering methods. For example, shooting-and-bouncing ray method is used to illuminate the target and define the target’s boundary for scattering computation. Single-bounce and multiple-bounce scatterings on the target due to High Fidelity Synthetic SAR Image Generation of a Canonical Target KN2 2 RTO-MP-SET-160 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED surface-to-surface interaction and reflection (e.g., dihedral and trihedral scattering sites) are computed by a combination of geometrical and physical optics. Edge scattering is handled using various diffraction methods, such as curved surface diffraction, reflection diffraction, edge travelling wave and surface creeping wave. FACETS can also be used to compute scattering from hollow cylinder cavity and more complex structure such as aircraft intake duct with engine components inside the cavity. FACETS is built with a modular structure for computing various scattering processes. It has a singlebounce scattering algorithm that computes the basic scattering from surface types such as flat plates, cylinders and spheres. FACETS has recognized that it would be difficult to accurately account for, in an automatic manner by the algorithm, the more complex scattering processes such as double-bounce (dihedral), triple-bounce (trihedral), cavity (hollow structure) and diffraction from edges. Thus, FACETS requires all of these scattering processes to be specified for computation; that is, the locations of these types of scattering primitives on the target must be identified and fed into the EM code manually. Graphical illustrations of these scattering primitives are shown in Figure 2. For realworld complex targets such as ships and aircraft, the nominations of all the scattering primitives could be quite labor intensive. However, for a relatively simple canonical target such as SLICY (Figure 1), this is quite manageable. It will be shown that being able to identify and account for all the appropriate scattering types correctly on the target is crucial to the simulation of high fidelity SAR images. It will be demonstrated in the following analysis that this human-assisted algorithmic structure in FACETS can offer some useful and insightful glimpses of what some of the requirements and challenges are for extending the generation of high fidelity simulated SAR images to more complex targets. 4.0 SAR IMAGE SIMULATION AND MEASURED DATA To analyze the FACETS computations, SAR images of SLICY are computed at two elevation look angles, 15 degrees and 30 degrees, and a full 360-degree coverage in the azimuth direction at X-band radar frequency (9.6 GHz). The computed images are then compared with the measured images of SLICY for validation. The measured SAR images were collected using the Spotlight SAR mode as part of the MSTAR datasets of ground targets [4, 5]. The SLICY target was stationary; data were captured at a number of depression angles. The Spotlight SAR radar was flown several times around the target, providing multiple 360-degree azimuth coverage at several elevation angles in the measurements. The SAR images of SLICY were cropped from larger scenes of the captured SAR data. The size of the SLICY target image is 54 by 54 pixels. The measured images have down-range and cross-range resolutions of 0.254 m. These correspond to a radar bandwidth of 591 MHz and an angular aperture of 3.5 degrees. The image data are, however, over-sampled to give effective down-range and cross-range pixel spacing of 0.2 m, as described in the header file of each SAR image data file. For comparative purposes, provision is made so that the 0.2 m pixel spacing is also incorporated in the computed image. This is achieved by using a radar bandwidth of 750 MHz and an angular High Fidelity Synthetic SAR Image Generation of a Canonical Target RTO-MP-SET-160 KN2 3 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED aperture of 4.5 degrees. Thus the computed SAR images have a slightly better spatial resolution. This slightly sharper resolution helps to make it easier to identify various scattering centres on the computed images of SLICY. The SLICY target is computed as an isolated object in the simulation; that is, there is no groundplane interaction included in the computation.