Since there is no atmospheric influence, space-based terahertz inverse synthetic aperture radar (THz-ISAR) can obtain high-resolution images of space targets at a certain distance, so it has important application prospects. In addition, the relative motion of space-based platforms and space targets is complicated. For some satellites that are tumbling out of control or are undergoing attitude change, the imaging angle formed by the target relative to the radar can be considered to be non-uniform, which in turn leads to imaging defocus.In this paper, a minimum entropy-based THz-ISAR rotation parameter estimation and motion compensation method for nonuniformly rotating space targets is proposed. Firstly, the radar echo is compensated for the translational motion and corrected for the range migration to obtain an envelope-aligned one-dimensional range profile. Then, the minimum entropy (ME) algorithm based on Newton’s method (NM) is used to estimate rotational velocity and centre of the targets, and to compensate the range second-order spatial-variant (SV) phase. After that, the golden section method is applied to estimate the target rotational acceleration and compensate the azimuth second-order spatial-variant phase. Finally, the cyclic variable method is used to iterate these two compensation steps alternately to avoid the coupling effect of range and azimuth second-order spatial-variant phase, while obtaining the target rotation parameters and the well-focused imaging results.
In the study of spatial target component recognition based on terahertz radar imaging using the traditional YOLOv5network, the recognition performance of the model decreases due to large overlapping areas of components in some samples and unclear imaging of small components. To address this issue, this paper proposes a typical component target recognition model, BoT-YOLO+, based on an improved YOLOv5 network architecture. On one hand, the proposed model enhances performance by introducing the BoTNet backbone architecture, which incorporates the attention mechanism from Transformers and improves the feature extraction capability for small components and thereby increasing the recognition rate of small components, without significantly increasing computational costs.
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.
Traditional waveform design methods mostly assume that the noise in the environment is Gaussian white noise. However, as the electronic warfare environment for the jammer becomes increasingly complicated, there is more likely to be a noise source that can emit fuzzy colored noise and a situation where the radar and jammer game, and the existing waveform design methods cannot meet jammer’s performance demand. Therefore, to reduce the radar’s estimation performance in complex environments, a fuzzy colored noise environment and a robust interference waveform design method under the hierarchical game model between the radar and jammer are proposed successively, which can minimize the mutual information of the radar echo signal and the target impulse response. Extensive simulation experiments show that the designed robust interference waveform can finally optimize the strategy of the jammer and provide a meaningful reference for the interference waveform energy allocation strategy in the future.
Imaging, classification, and recognition techniques of ballistic targets in midcourse have always been the focus of research in the radar field for military applications. However, the high velocity translation of ballistic targets will subject range profile and Doppler to translation, slope, and fold, which are especially severe in the terahertz region. Therefore, a two-step translation compensation method based on envelope alignment is presented. The rough compensation is based on the traditional envelope alignment algorithm in inverse synthetic aperture radar imaging, and the fine compensation is supported by distance fitting. Then, a wideband imaging radar system with a carrier frequency of 0.32 THz is introduced, and an experiment on a precession missile model is carried out. After translation compensation with the method proposed in this paper, the range profile and the micro-Doppler distributions unaffected by translation are obtained, providing an important foundation for the high-resolution imaging and micro-Doppler extraction of the terahertz radar.
Research on the scattering behavior of terahertz (THz) waves plays a prominent part in the development of remote sensing and imaging systems. Due to the relatively short wavelengths of THz waves, materials that could be considered smooth at microwave band may begin to display rough surface in the THz region. To study the scattering characteristics and implement parameter estimation and imaging of rough surface rotating targets in the THz band, THz radar systems with carrier frequencies of 220 and 440 GHz were built, and experiments based on a series of rough surface cylinders were carried out. From the time–frequency distributions and imaging results, the transition of scattering characteristics from smooth surface to rough surface is clearly visible, which is consistent with the fact that as the roughness increases, there is an overall increase in diffuse scattering. In addition, methods of rotation period estimation and target size estimation were proposed and applied to experimental data of the THz radar systems, and their validities were verified by the high-precision estimation results.
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