A message-passing-interface (MPI)-based parallel finite-difference time-domain (FDTD) algorithm for the electromagnetic scattering from a 1-D randomly rough sea surface is presented. The uniaxial perfectly matched layer (UPML) medium is adopted for truncation of FDTD lattices, in which the finite-difference equations can be used for the total computation domain by properly choosing the uniaxial parameters. This makes the parallel FDTD algorithm easier to implement. The parallel performance with different processors is illustrated for one sea surface realization, and the computation time of the parallel FDTD algorithm is dramatically reduced compared to a single-process implementation. Finally, some numerical results are shown, including the backscattering characteristics of sea surface for different polarization and the bistatic scattering from a sea surface with large incident angle and large wind speed.

An improved shooting and bouncing ray (SBR) method based on the Blend-Tree structure is presented for solving the scattering from multiple complex movement targets. Since the positions and attitudes of targets change with time, the spatial distribution of facets is variable. Hence, quickly finding illuminated areas and tracing high-order rays are two main bottlenecks. A Blend-Tree structure consisting of the ray aggregation transport layer (RATL) and the multiple octree layer (MOL) is proposed for the above problems. RATL assigns a conversion interface (CI) and target-seeking interface (TSI) to each target. MOL stores each octree which is built at the initial moment and connected to CI. In shadow elimination, targets are rotated and translated for virtual display by CIs, and the scanline algorithm is used for acceleration. In high-order raytracing, the ray which enters and exits the target is transformed quickly by quaternions in CIs. The TSI determines the next target for the exiting ray. Therefore, the raytracing area is extremely limited within each bounding box and time-invariant. Furthermore, the ray marching algorithm is improved to accelerate ray tracing in octrees. Numerical results demonstrate the computational accuracy and efficiency of this method. Moreover, echo characteristics of multiple moving targets are analyzed.

This paper proposes a shooting and bouncing ray (SBR) method based on a dynamic octree structure to accelerate the solution of scattering from a moving target. In this algorithm, three coordinate systems are established: the world coordinate system (WCS), the local coordinate system (LCS), and the perspective projection coordinate system (PPCS). Firstly, an octree structure is created to store the target facets in the LCS that moves with the target in the WCS. The octree remains relatively stationary with the target, which avoids rebuilding the structure at every moment. Secondly, to accelerate the shadow elimination, the scan line algorithm is used to convert the three-dimensional processing into a two-dimensional graphic display in the PPCS. Thirdly, high-order ray tracing is completed in the LCS. A forward-backward ray tracing technology is adopted to guarantee the precision of the illuminated facet identification. Finally, the information of illuminated facets is transferred from the LCS to the WCS, and the scattering of target is calculated in the WCS. Meanwhile, some numerical simulations of our method are compared with those of the FEKO-MLFMM and the traditional Octree-SBR method. The results show that the proposed method greatly reduces the computation time while maintaining satisfactory accuracy.

This letter proposes an improved shooting and bouncing ray (SBR) method to efficiently predict the radar cross (RCS) of electrically large targets. First, the scan line algorithm is used to convert three-dimensional (3-D) processing to the 2-D graphic display, which expedites the elimination of shadows. Then, the ray tracing process is accelerated by the ray-marching technique which greatly decreases the intersection test. The validity of the proposed method is verified by comparing the simulation results with those of the multilevel fast multipole method. The runtime performance contrast of the traditional Octree-SBR method and ours indicates that the accelerated algorithm is more computationally efficient. Therefore, the presented algorithm has an excellent performance in the RCS prediction of electrically large targets.

Finite-difference time-domain (FDTD) algorithm with a pulse wave excitation is used to investigate the wide-band composite scattering from a two-dimensional(2-D) infinitely long target with arbitrary cross section located above a one-dimensional(1-D) randomly rough surface. The FDTD calculation is performed with a pulse wave incidence, and the 2-D representative time-domain scattered field in the far zone is obtained directly by extrapolating the currently calculated data on the output boundary. Then the 2-D wide-band scattering result is acquired by transforming the representative time-domain field to the frequency domain with a Fourier transform. Taking the composite scattering of an infinitely long cylinder above rough surface as an example, the wide-band response in the far zone by FDTD with the pulsed excitation is computed and it shows a good agreement with the numerical result by FDTD with the sinusoidal illumination. Finally, the normalized radar cross section (NRCS) from a 2-D target above 1-D rough surface versus the incident frequency, and the representative scattered fields in the far zone versus the time are analyzed in detail.