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: tslyw@nus.edu.sg

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.

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B. Sea Surface Scattering The sub-facet model is applied to solve for from the

extra-large sea surface in Fig. 1. The sea surface is firstly divided into sub-facets. The sub-facets are solved via physical optics. The total scattering fields are a summation of the scattering from these sub-facets. More details can be found in [1].

III. SAR SIMULATION An example of a vessel ship model that is 140m long, 18m

wide and 17m high, is shown in Fig. 3. The simulation parameters are as follow: downrange and cross-range resolutions are m, the sea surface is 200 60 m2, frequency range is 2.925–3.075GHz; azimuth range is 88.6°–91.4°, 45°, frequency azimuth samples =

200 60. Three sea states (0, 1, 2) corresponding to the averaged

wind speeds over sea 0.1m/s, 2.5m/s, 4.5m/s are simulated. The SAR images are shown in Fig. 4. It can be observed that multipath effects due to the sea surface and sea states can cause ambiguity in interpretation of the images. This observation is also borne out in the difference field scattering as shown in Fig. 5 for an incidence angle of (45°, 90°). It is interesting that the scattering of the ship in sea state 2 is similar to that of the ship in free-space.

Fig. 3: A frigate model.

(a) sea state 0

(b) sea state 1

(c) sea state 2

Fig.4: SAR image of frigate under different sea states.

As mentioned earlier, only the ray tubes illuminating the ship are required to be traced. There is also no need to model the rough surface. Compared to the free-space problem, the proposed method requires almost the same amount of RAM while consuming less than twice the CPU, as shown in Table 1. Note that the simulations were run on a LINUX platform with Intel® CoreTM E5-4650 2.70GHz processor.

Fig. 5 Comparsions of scattering under different sea states.

Table 1. Computation costs comparison.

RAM (MBytes) CPU time (minutes) Ship in free-space 167 2,097 Ship in sea state 1 168 2,846

IV. CONCLUSION A high frequency method combining physical optics and

the shooting and bouncing ray method with image theory has been developed to calculate the EM scattering from surface targets. The method does not require the physical modeling of the rough sea/ground surface. We plan to further improve the computational efficiency of the method with the use of GPU so that wide-angle wide-bandwidth scattering data of surface targets can be expeditiously generated.

REFERENCES [1] M. Zhang, H. Chen, H. C. Yin, “Facet-based investigation on

EM scattering from electrically large sea surface with two-scale profiles: theoretical model,” IEEE Transaction on Geoscience and Remote Sensing, vol. 49, pp. 1967-1975, 2011.

[2] F. Xu and Y. Q. Jin, “Bidirectional analytic ray tracing for fast computation of composite scattering from electric-large target over a randomly rough surface,” IEEE Transaction on Antennas Propagation, vol. 57, pp. 1495-1505, 2009

[3] L. Lozano, I. González, M. J. Algar, and F. Cátedra, “Efficient RCS analysis of complex targets on infinite ground plane,” Antennas and Propagation Society International Symposium, Chicago, IL, pp.1-2, 2012

[4] S. Kashyap and A. Louie, “RCS of an object inclined to a ground plane,” Microwave and Optical Technology Letters, vol. 18, no. 1, pp. 50-54, 1998

162 2015 IEEE 5th Asia-Pacific Conference on Synthetic Aperture Radar(APSAR)