This study presents an InSAR flight experiment of the latest mission using helicopter dual-band Synthetic Aperture Radar system (HD-SAR), conducted in southern China by the Xi'an Institute of Space Radio Technology. The HD-SAR system is a dual-band polarization radar based on a helicopter platform, designed for rapid installation and high integration. In every frequency band, the dual receiving channel architecture activates the InSAR mode. High-frequency and low-frequency digital elevation models (DEMs) provide differing benefits depending on the scenario because of the various impacts of ground object scattering qualities. Both high-coherence and low-coherence sectors can achieve optimal performance through the use of dual-band fusion technology. This paper presents the system design and key parameters first. The flying experiments and antenna installation plans for InSAR mode on the Sikorsky S-76 helicopter are described in the section that follows. The results showed that in hilly areas, the relative elevation accuracies of the dual-band fusion DEMs improved by 32% and 25% respectively, whereas the relative elevation accuracies of the C-band and Ku-band DEMs were less than 3 meters. SAR and InSAR mode flight experiments have demonstrated that the HD-SAR system is suitable for most high-resolution detection and interferometry mapping scenarios.

It is important to explore the application of electromagnetic vortex (EMV) wave radar carrying orbital angular momentum in the field of SAR imaging as a new type of radar. Due to the unique Bessel antenna pattern modulation and the vortex azimuth angle phase influence in EMV wave radar, the traditional SAR imaging methods result in a decrease in sidelobe quality, which affects the image quality. To address the aforementioned issues, this paper propose an EMV SAR sidelobe quality improvement imaging method. Accurate geometric and signal model for EMV SAR are established. The Bessel antenna pattern and vortex azimuth angle phase compensation functions are designed. And range block division and pattern modification methods are proposed to address the spatial variation and near zero value issues of Bessel antenna pattern issues respectively. Simulation and image evaluation results verified the effectiveness of the proposed method.

High-resolution wide-swath (HRWS) multichannel synthetic aperture radar (SAR) system possesses a number of receiving channels along the azimuth direction, so it has the capacity of moving target indication and imaging. However, due to the radial velocity of the moving target, false targets occur in the focused image. By estimating the radial velocity and combining it with moving target imaging, false targets can be effectively suppressed. In this letter, a method of radial velocity estimation of a moving target is proposed based on the theory of subspace projection. This method does not need to estimate the real azimuth position of the moving target and can predict the processing time. Simulation and airborne measured data show the effectiveness of the proposed method.

This letter proposes a new method for focusing ground maneuvering targets and estimating the motion parameters with a synthetic aperture radar (SAR) system. In this method, the Hough transform is applied to estimate the cross-track velocity from the slope of the range walk (RW) trajectory, and the RW and Doppler centroid shift are compensated. The second-order Keystone transform is performed to correct the additional range curve caused by the along-track velocity and cross-track acceleration. Then, we adopt the adaptive polynomial Fourier transform to estimate the second-and third-order Doppler parameters from a 1-D parameter interval, and the corresponding motion parameters are calculated. Finally, the moving target is well focused after the motion parameters compensation because the second- and third-order Doppler parameters are efficiently eliminated. Both the simulated and real data processing results are presented to demonstrate the validity of the proposed algorithm.

In the existing time-domain autofocus algorithms, the azimuth deramping operation will change the azimuth-independent phase into the azimuth-dependent phase, which may greatly reduce the accuracy of autofocus processing in squint spotlight synthetic aperture radar (SAR). In contrast, the frequency-domain autofocus algorithms can avoid this problem because it does not involve the azimuth deramping operation. However, the existing frequency-domain autofocus algorithms are proposed based on the assumption of broadside mode, which cannot be directly applied to the squint mode. Therefore, this article extends the existing frequency-domain autofocus algorithm to the squint mode combined with the extended Omega-K (EOK) algorithm. Furthermore, a space division (SD) algorithm is embedded into the proposed algorithm as preprocessing, which can effectively compensate for the azimuth-dependent motion error. The simulation and real data are processed to verify the effectiveness of the algorithm.

ISAR 2D imaging is obtained by projecting the 3D structure target onto a 2D imaging plane. The angle between the imaging plane and the target spinning axis has a great influence on the projection result. Generally, this angle is neglected, which results in the target imaging has smaller size than the real target. This is not conducive to the further application of target detection and target recognition. In a short observation time, this angle cannot be estimated by monostatic radar. In order to solve such a problem, this letter proposes a method using bistatic radar to estimate the angle and accomplish accurate calibration. First, bistatic ISAR model and bistatic echo signal of spinning target are modeled. Then combining monostatic and bistatic 2D imaging, the angle can be calculated based on several prominent scatterers. Recalibration is performed based on this angle. Finally, the effectiveness of the proposed method is verified by different simulation experiments.

Spaceborne synthetic aperture radar (SAR) has a high application value in the observation of ship targets. After the ship is detected, the actual observation position of the moving ship and its motion parameters are worthy of concern, especially for some medium and large size valuable ships. In this paper, we propose a method of extracting the energy center of the ship signal trajectory to locate the ship first. Then according to the difference between the imaging position and the positioning position of the ship, the radial velocity estimation can be calculated. The proposed method does not need to construct a reference data box, and can directly locate the moving ship. The processing of the Gaofen-3 (GF-3) complex data verifies the effectiveness of the proposed method.

A squinted observation geometry along with long integration time significantly aggravates the range walk and spatial variation of a medium-earth-orbit (MEO) synthetic aperture radar (SAR) signal. Variable pulse repeating frequency (PRF) is recommended to avoid the blockage in echo recording and save storage space. The existing wavenumber algorithms cannot handle the nonlinear and range-azimuth-coupled spatial variation (RACSP) over a large scene. In this paper, we propose a modified Stolt mapping method along with a modified joint time and Doppler resampling (JTDR) for highly squinted MEO SAR data processing. An azimuth timescale transformation is used to deal with the nonlinear spatial variation of the azimuth frequency-modulation (FM) rate. An extended Omega-K is used to linearize the range frequency and achieve range cell migration correction (RCMC). To address the RACSP, the Doppler is linearized in the range-Doppler domain using a range-dependent Doppler scale transformation. The computational complexity and geometry distortion correction (GDC) are also discussed. Simulation results are shown to verify the effectiveness of the developed focusing approaches.