Daiyin Zhu

A Keystone Transform Without Interpolation for SAR Ground Moving-Target Imaging

Synthetic aperture radar (SAR) image formation for a ground moving target necessitates the compensation of the unknown target trajectory. The keystone transform has been employed to remove the linear component of the range migration for the moving target, where interpolation is required. In this letter, a realization of the keystone transform avoiding interpolation is presented. The kernel of this transform, i.e., the range-frequency-dependent azimuth time rescaling, is implemented using only complex multiplications and fast Fourier transforms based on the scaling principle, which has been successfully applied in the equalization of the space-variant range cell migration in SAR processing. In addition, the moving target is coarsely focused according to the SAR geometry and the platform velocity while exploiting the scaling principle. This preliminary focusing is helpful in the isolation of the moving target from ground clutter, so as to facilitate a more refined processing with respect to each mover. SAR raw data combined with simulated echoes of moving targets are utilized to validate the presented approach

Robust ISAR Range Alignment via Minimizing the Entropy of the Average Range Profile

In this letter, a novel global approach to range alignment for inverse synthetic aperture radar (ISAR) image formation is presented. The algorithm is based on the minimization of the entropy of the average range profile (ARP), and the processing chain is capable of exploiting the efficiency of the fast Fourier transform. With respect to the existing global methods, the new one requires no exhaustive search operation and eliminates the necessity of the parametric model for the relative offset among the range profiles. The derivation of the algorithm indicates that the presented methodology is essentially an iterative solution to a set of simultaneous equations, and its robustness is also ensured by the iterative structure. Some alternative criteria, such as the maximum contrast of the ARP, can be introduced into the algorithm with a minor change in the entropy-based method. The convergence and robustness of the presented algorithm have been validated by experimental ISAR data.

New Antivelocity Deception Jamming Technique using Pulses with Adaptive Initial Phases

An effective electronic counter-countermeasures (ECCM) technique for a moving target indication/pulsed Doppler (MTI/PD) radar system against velocity deception jamming is proposed. A novel radar signaling strategy is outlined based on variations of the initial phases of the transmitted pulses in pulse repetition interval (PRI) domain. It makes the Doppler spectrum of the false targets, created by a digital radio frequency memory (DRFM) repeat-back jammer, form a notch around the Doppler frequency of the true one. The penalty function and corresponding algorithm for designing adaptive initial phases are given. We also present an approach of the multi-channel matched filter processing to estimate the manner, by which a DRFM repeat jammer operates, and the parameters of the false targets. The working flow of an MTI/PD radar for countering velocity deception jamming is investigated. Simulation results show the effectiveness of the above methods.

SIGNATURES OF MOVING TARGET IN POLAR FORMAT SPOTLIGHT SAR IMAGE

The synthetic aperture radar (SAR) signatures of moving target are the basis of ground moving target detection and imaging (GMTI&Im). However, previous studies are mainly based on the 2-D separable SAR processing, and little work has been done to investigate the signatures of moving target after the application of a particular fine resolution SAR image formation algorithm. In this paper, the spectrum of moving target after polar format algorithm (PFA) processing is derived. Based on this spectrum, detailed analysis on the SAR signatures of moving target, including the geometric displacement, residual range migration, and the defocusing effect in both range and azimuth dimensions are performed. Simulation results validate the theoretical analysis.

Multi-Subaperture PGA for SAR Autofocusing

For spotlight mode synthetic aperture radar (SAR) autofocusing, the traditional full-aperture phase gradient autofocus (PGA) algorithm might suffer from performance degradation in the presence of significant high-order phase error and residual range cell migration (RCM), which tend to occur when the coherent processing interval (CPI) is long. Meanwhile, PGA does not perform satisfactorily when applied directly on the stripmap data. To address these shortcomings, we present a multi-subaperture PGA algorithm, which takes advantage of the map drift (MD) technique. It smoothly incorporates the estimation of residual RCM and combines the subaperture phase error (SPE) estimated by PGA in a very precise manner. The methodology and accuracy of PGA-MD are investigated in detail. Experimental results indicate the effectiveness of PGA-MD in both the spotlight and the stripmap modes.

Some Aspects of Improving the Frequency Scaling Algorithm for Dechirped SAR Data Processing

The frequency scaling algorithm (FSA) was proposed to process the synthetic aperture radar (SAR) data acquired via the dechirp-on-receive approach. Some aspects of improving the FSA are investigated in this paper, based on which an extended FSA (EFSA) is presented. The general purpose of the EFSA is to reduce the effect of range spectrum shift of the intermediate processing results, which occurs during the scaling operation in the FSA, so as to achieve a more effective utilization of the processed bandwidth. The EFSA is implemented through time shifting the scaling and the inverse scaling functions used in the FSA and also the adjustment of the scaling factor. The derivation of the EFSA is detailed in this paper. Point target simulation in squinted imaging geometry indicates that the presented algorithm is more suitable for large-squint applications.

Polar Format Algorithm Wavefront Curvature Compensation Under Arbitrary Radar Flight Path

An improved space-variant postfiltering approach to compensate for the wavefront curvature effect in polar format imagery is presented in this letter. The main contribution of this new method is the construction of a space-variant filter, which is based on exploiting the endomorphism property of the polar format transformation. The new approach provides an accurate and general solution to wavefront curvature compensation under arbitrary radar flight path. Finally, point target simulation has validated the effectiveness of the new approach.

Comparative Study of Rma and Pfa on Their Responses to Moving Target

The synthetic aperture radar (SAR) signatures of moving target are the basis of ground moving target indication and imaging (GMTI&Im) in the framework of SAR systems. However, previous studies are mainly based on the 2-D separable SAR processing, and little work has been done to investigate the signatures of moving target after the application of a particular flne resolution SAR image formation algorithm. In this paper, we derive the image spectrum of moving target after two representative flne resolution SAR image formation algorithms, i.e., the range migration algorithm (RMA) and polar format algorithm (PFA), respectively. Based on the spectrum derived, detailed analysis on the SAR signatures of moving target, including the geometric displacement, residual range migration, and defocusing efiect in both the range and azimuth dimensions are performed. The presented work might be helpful when considering a SAR system with the capability of ground moving target indication and imaging (GMTI&Im).

The Geometric-Distortion Correction Algorithm for Circular-Scanning SAR Imaging

The image formation scheme for circular-scanning synthetic aperture radar includes generating a set of focused subimages and the followed mosaiking processing. However, due to the particular acquisition geometry and irregular motion of the radar platform, the inevitable geometric distortions in the subimages are necessary to be corrected. In this letter, a 2-D geometric-distortion correction algorithm based on projection transformation between the scatterers and the images is presented, in condition of focusing the subimages using a linear range-Doppler algorithm. The point target simulation and real circular-scanning data implementation results are provided to demonstrate the validity of the proposed method.

Autofocus Correction of APE and Residual RCM in Spotlight SAR Polar Format Imagery

Synthetic aperture radar (SAR) images are often blurred by phase perturbations induced by uncompensated sensor motion and/or unknown propagation effects caused by turbulent media. To get refocused images, autofocus proves to be a useful postprocessing technique applied to estimate and compensate the unknown phase errors. However, a severe drawback of the conventional autofocus algorithms is that they are only capable of removing one-dimensional azimuth phase errors (APEs). As the resolution becomes finer, residual range cell migration (RCM), which makes the defocus inherently two dimensional (2-D), becomes a new challenge. In this paper, correction of APE and residual RCM are presented in the framework of polar format algorithm (PFA). First, an insight into the underlying mathematical mechanism of polar reformatting is presented. Then, based on this new formulation, the effect of polar reformatting on the uncompensated APE and residual RCM is investigated in detail. By using the derived analytical relationship between APE and residual RCM, an efficient 2-D autofocus method is proposed. Experimental results indicate the effectiveness of the proposed method.