SAR Imaging of Surface Target Using High Frequency Electromagnetic Method Wei Yang, Tse-Tong Chia, Chun-Yun Kee and Chao-Fu Wang Temasek Laboratories National University of Singapore, Singapore 5A Engineering Drive 1, #09-02, Singapore 117411 Email: [email protected] Abstract—The physical optics and shooting bouncing ray method are combined to rapidly compute the electromagnetic (EM) scattering from surface targets. In order to avoid modeling the infinite rough sea/ground, image theory and its modification are used to account for the EM interactions with minimal additional computation resource. Therefore, this proposed method is beneficial to generate massive backscatter data for the synthetic aperture radar (SAR) imaging. Index Terms—EM scattering, surface target, synthetic aperture radar, multipath effect, physical optics, shooting and bouncing ray method. I. INTRODUCTION For the last couple of decades, the synthetic aperture radar (SAR) has been primarily utilized for surveillance applications to better understand and interpret terrains and associated geological events [1], as well as surface targets such as ship on sea surface and tank on ground. In order to generate the massive backscatter data required for SAR imaging, an analytical method combining physical optics and the shooting and bouncing ray method with image theory is proposed to effectively solve realistic engineering problems. In our approach, the rough surface is not physically modeled, which significantly reduces memory and computation time. The influence of multipath on the overall scattering and the SAR image are handled by modification of the image theory. II. EM SIMULATION For the imaging scenario shown for the case of a ship on a sea surface in Fig. 1, the overall backscattered field in general consists of five scattering components (due to five different paths). The sum of the first four scattering components of the ship (with its electromagnetic (EM) interactions between it and the sea) is usually called the difference-field scattering . The fifth component, , is the backscatter from the sea. Thus, . As the higher order scattering components are typically much smaller than these five components, they can be ignored. Fig. 1: Five components of scattering for a ship on a sea surface. A. Difference-Field Scattering The aforementioned four scattering components are as follow: 1) the direct backscatter from the ship, 2) the interaction from ship-sea-radar, 3) the interaction from sea- ship-radar, and 4) the interaction from sea-ship-sea-radar. The scattering components of 2 to 4 arise from multipath effects. The four components can be obtained with the help of an imaged induced electric current and an imaged incident plane wave as shown in Fig. 2. In Fig. 2(a), the induced electric current and its imaged current correspond to the direct scattering from the ship and the scattering from ship to sea, respectively. In Fig. 2(b), the imaged incident wave (represented by which is the “image” of the original incident wave vector ), the induced electric current and its imaged current correspond to scattering from sea to ship and from sea to ship then to sea, respectively. For the paths 2 to 4, the Fresnel reflection coefficients and the roughness parameter are used to account for the reflection from the rough (dielectric) sea surface [2]. Therefore, only the rays illuminating the ship are required without the need to model the rough surface. For each incidence angle, the physical optics and shooting and bouncing rays [3,4] are used to rapidly compute the scattering as a function of frequency so that the rays are only traced once. (a) Imaged induced electric current (b) Imaged incident plane wave Fig. 2: Image theory for four scattering components. 161 978-1-4673-7297-8/15/$31.00 c 2015 IEEE