KEYWORDS: Detection and tracking algorithms, Reconstruction algorithms, Radar, Stereoscopy, 3D image processing, 3D acquisition, Image restoration, Radar imaging, 3D modeling, Tin
High-resolution near-field three-dimensional (3D) radar imaging techniques with millimeter and terahertz wave have played an increasingly significant role in applications including hidden object detection and nondestructive testing. Currently, imaging methods suffer from low efficiency when imaging defects in stratified media. This is caused by the inadaptability of the fast Fourier transform in the frequency domain due to the alteration of wave numbers in different media. We propose an imaging method based on the nonuniform fast Fourier transform to overcome the problem. Numerical simulations and experiments were carried out to verify our idea. Results show that our proposed method can greatly improve the imaging efficiency while maintaining the imaging quality. This work is of great significance for future development of nondestructive testing technology based on millimeter wave/terahertz imaging.
In recent years, cylindrical millimeter-wave (MMW) holographic imaging technique has attracted a lot of attention for its special advantages, including high resolution, large aperture, and strong penetrability. A modified wavenumber-domain three-dimensional imaging algorithm applied for near-field cylindrical MMW holography is proposed. The core of the proposed algorithm is to generate an accurate and efficient phase compensation factor in wavenumber-domain. Compared with the existing algorithms, the proposed algorithm holds advantages in both imaging speed and imaging quality and is more suitable for security inspection application. Both the simulation results of point targets and the experimental results of the resolution test panel validate the effectiveness of the proposed algorithm.
Attribute scattering centers model (ASCM) can provide important geometric information regarding the illuminated target. However, sliding scattering center (SSC) cannot be well extracted based on the current ASCM. This paper proposes an efficient method to extract SSCs based on density-distance (DD) matching. First, the scattering characteristic of SSC is derived theoretically from the perspective of physical optical (PO). Then, the frequency dependence and estimated position are analyzed by the multi-peak model. The distance and density of each scattering center are constructed and cluster by the proposed DD -based matching algorithm. Finally, the geometrical structure corresponding to each scattering centers can be retrieved. Simulation results validate the feasibility and accuracy of the proposed method.
In the terahertz (THz) band, the scaling relation between scaled models and prototypes with coatings needs to be reconsidered due to the dispersive nature of the coating materials. Besides, it is difficult to obtain the material parameters, such as the permittivity. Based on the comparison of RCS between a metal plate and a coated plate, a key parameter extraction method is proposed. Combining with the scaled method which is based on the dimensional analysis theory, an improved scaled method is proposed for predicting the RCS of a coated prototype. The parameter extraction method can provide the key parameter corresponding to the coated material at certain frequency when the intrinsic parameters are unknown. The validity of the proposed method is proved by simulation. The scope of RCS scaled measurements on coated targets will be expanded in the THz band.
The plasma sheaths generated by vehicles flying at hypersonic speed in the near space will interact with electromagnetic waves. This creates series of problems to communication and detection. Terahertz (THz) technology holds large potential in solving the problems caused by the plasma sheath for its special characteristics. However, due to the limitation in computation capability and experimental equipment, the research on the radar cross section (RCS) of the target covered with the inhomogeneous plasma sheath in THz band is still insufficient. The echo attenuation is the basis of considering the RCS attenuation, so studying echo attenuation characteristics can provide support for further study of radar scattering characteristics. In this paper, a simplified calculation method is proposed to analyze the echo attenuation characteristics of a vehicle covered with the inhomogeneous plasma sheath in THz band. Firstly, the echo attenuation model of the target covered with the inhomogeneous plasma sheath is established. Then, considering that the total echoes include the echoes reflected by the surface of the plasma sheath and reflected by the target surface through the plasma sheath, the calculation process of the total echo attenuation is derived. The variations of echo attenuation with THz frequency, plasma electron density, plasma collision frequency and incident angle are further studied. The results validate that the shielding effect of the plasma sheath is weakened in the THz spectrum. This indicates the great prospect of THz technology in the communication and detection of hypersonic vehicles in the future.
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