Pengjiang Hu

IAA-Based High-Resolution ISAR Imaging With Small Rotational Angle

The Fourier transform-based range Doppler method is commonly used in an inverse synthetic aperture radar. Although it has achieved good success in most scenarios, its performance is determined by the rotational angle, and the cross-range resolution is extremely low in the case of a small rotational angle. In this letter, to improve the cross-range resolution, a novel cross-range compression scheme based on the iterative adaptive approach (IAA) is proposed. In addition to the standard IAA to achieve high resolution, the efficient IAA is introduced to suppress the sidelobes due to noise. Both the simulation and experimental results demonstrate that the proposed method has the advantages of parameter-free, high accuracy, and high efficiency.

Adaptive ISAR Imaging of Maneuvering Targets Based on a Modified Fourier Transform

Focusing on the inverse synthetic aperture radar (ISAR) imaging of maneuvering targets, this paper presents a new imaging method which works well when the target’s maneuvering is not too severe. After translational motion compensation, we describe the equivalent rotation of maneuvering targets by two variables—the relative chirp rate of the linear frequency modulated (LFM) signal and the Doppler focus shift. The first variable indicates the target’s motion status, and the second one represents the possible residual error of the translational motion compensation. With them, a modified Fourier transform matrix is constructed and then used for cross-range compression. Consequently, the imaging of maneuvering is converted into a two-dimensional parameter optimization problem in which a stable and clear ISAR image is guaranteed. A gradient descent optimization scheme is employed to obtain the accurate relative chirp rate and Doppler focus shift. Moreover, we designed an efficient and robust initialization process for the gradient descent method, thus, the well-focused ISAR images of maneuvering targets can be achieved adaptively. Human intervention is not needed, and it is quite convenient for practical ISAR imaging systems. Compared to precedent imaging methods, the new method achieves better imaging quality under reasonable computational cost. Simulation results are provided to validate the effectiveness and advantages of the proposed method.

Inverse Synthetic Aperture Radar Imaging of Targets with Complex Motion based on Optimized Non-Uniform Rotation Transform

Focusing on the inverse synthetic aperture radar (ISAR) imaging of targets with complex motion, this paper proposes a modified version of the Fourier transform, called non-uniform rotation transform, to achieve cross-range compression. After translational motion compensation, the target’s complex motion is converted into non-uniform rotation. We define two variables—relative angular acceleration (RAA) and relative angular jerk (RAJ)—to describe the rotational nonuniformity. With the estimated RAA and RAJ, rotational nonuniformity compensation is carried out in the non-uniform rotation transform matrix, after which, a focused ISAR image can be obtained. Moreover, considering the possible deviation of RAA and RAJ, we design an optimization scheme to obtain the optimal RAA and RAJ according to the optimal quality of the ISAR image. Consequently, the ISAR imaging of targets with complex motion is converted into a parameter optimization problem in which a stable and clear ISAR image is guaranteed. Compared to precedent imaging methods, the new method achieves better imaging results with a reasonable computational cost. Experimental results verify the effectiveness and advantages of the proposed algorithm.

Time division in a pulse polarimetric radar based ISAR fusion imaging technique

Fully polarimetric radar shows advantage in measuring scattering mechanisms and target scattering characteristics over single polarimetric radar. Aiming at dealing with the distortion of the target polarimetric properties caused by traditional time division between pulses (TDBP) polarimetric radar, time division in a pulse (TDIP) polarimetric radar is studied in this paper. The model of this novel polarimetric radar is established firstly. By analyzing the envelope and phase property of the intermediate frequency (IF) direct sampling echoes, pre-processing is carried out to different polarimetric channels based on the delay and the velocity measured by the narrow band. In an effort to achieving a fusion of different polarimetric channels, the translation motion is compensated jointly under the constraint of reciprocity. Pauli decomposition is employed to construct an RGB image which can describe the scattering mechanisms better. The results of simulated data prove the effectiveness of the proposed approach.

A ROBUST SUB-INTEGER RANGE ALIGNMENT ALGORITHM AGAINST MTRC FOR ISAR IMAGING

Range alignment plays an important role in the inverse synthetic aperture radar (ISAR) imaging. The performance of the traditional range alignment algorithms decreases when the migration through resolution cells (MTRC) is much severe. In this paper, a measure of MTRC is defined, and the effect of MTRC on range alignment is analyzed. Taking MTRC into account, a robust sub-integer range alignment algorithm is proposed. Firstly, each range profile is interpolated to remove the precision limitation of integer range resolution cell. Subsequently, the matrix formed by all the range profiles is partitioned into several matrix blocks on the slow-time domain. For each matrix block, the range profiles are aligned by minimizing the entropy of the average range profile (ARP). Finally, the matrix blocks are coarsely aligned using the maximum correlation method, followed by a fine alignment based on the minimization of the ISAR image entropy. The effectiveness of the proposed algorithm is validated by simulations and real-world data. Results demonstrate that the proposed method is robust against MTRC and can reduce the alignment error. The resultant ISAR image is much better focused.

Synthetic bandwidth method for ISAR in DIFS mode

Theoretical analysis of the time domain synthetic bandwidth method applied in synthetic aperture radar (SAR) is formulated in this paper firstly. Some problem arising in practical implementation of the method in inverse synthetic aperture radar (ISAR) direct intermediate frequency sampling (DIFS) mode is discussed. The solution to the problem is proposed in this paper. First of all, the transmitted sub-pulses should overlap each other. Based on the time-frequency property of linear frequency modulated signal, the sub-pulse echoes are truncated by equal length before superposition. In addition, the parameter choice of sampling window is discussed. Simulated data results confirm the validity of the proposed solution.

Analysis of novel approaches for SAR-GMTI based on modulated stepped-frequency signals

SAR-GMTI is of great significance and attraction in target surveillance. Modulated stepped frequency (MSF) signal is studied in this paper as a competent choice to satisfy the cooperation of SAR and GMTI. The characteristics of SAR-GMTI and relevant signal forms are presented firstly. Several MSF based approaches are analyzed for SAR-GMTI application, including MIMO configuration, PRF transformation and compound velocity measurement.

A modified time-frequency approach for radar signal analysis based on spectrogram and entropy evaluation

A modified spectrogram approach is presented for the analysis of frequency modulated radar signals, taking the entropy as the evaluation criterion. The characteristics of spectrogram are firstly introduced. After the analysis of performance evaluation for time-frequency distribution, entropy is studied as a quantitative parameter for concentration and cross-term suppression. An improved spectrogram method based on multi-window is proposed and verified by simulated data through entropy criterion.