In this letter, the low peak sidelobe level (PSL) mismatched filter (MSF) designed for coherent frequency diverse array (FDA) radar using phase-coded waveform is proposed. Considering the space-time coupled characteristic of coherent FDA, the proposed MSF incorporates the equivalent transmit beamforming and low sidelobe range compression simultaneously. The minimization of PSL in MSF design is modeled as a second-order cone programming (SOCP) problem and solved by convex optimization. Furthermore, two modified algorithms are proposed to improve the spatial clutter suppression and Doppler tolerance in original framework. The proposed methods will help to expand the applications of coherent FDA radar in wide-covered and high-resolution observation.

Abstract Bayesian compressive sensing (BCS) is an important sub‐class of sparse signal reconstruction algorithms. In this paper, a modified complex multitask Bayesian compressive sensing (MCMBCS) algorithm using the Laplacian scale mixture (LSM) prior is proposed. The LSM prior is first introduced into the complex BCS framework by exploiting its better sparse characteristic and flexibility than traditional Laplacian prior. Furthermore, by integrating out the noise variance analytically, the MCMBCS algorithm significantly improves the signal recovery performance than the original CMBCS. More importantly, the authors not only present the iterative algorithm but also develop the sub‐optimal fast implementation method based on the marginal likelihood maximisation, which dramatically reduce the computational complexity. Finally, sufficient numerical simulations validate the better performance of the proposed algorithm in reconstruction accuracy and computational effectiveness than existing work. It is revealed that the proposed algorithm has great potential in the complex‐valued signal processing field.

Frequency agility increases the difficulty of jammers to predict and estimate the carrier frequency and, thus, improves the radar electronic counter-countermeasures (ECCM) performance. In this article, frequency agility is introduced into synthetic aperture radar (SAR). The characteristic of the Doppler history of frequency-agile SAR (FASAR) is analyzed, which shows that the azimuth compression could not be achieved by the classic imaging algorithms. In order to reconstruct the images of interested targets, a three-channel sparse imaging method is proposed based on approximated observation model by using the echo data of different carrier frequencies. By deriving the equivalent observation model, it is concluded that the reconstruction performance can be enhanced by improving the coherence of match filter imaging results in different channels. The image characteristic of different channels in FASAR is analyzed based on the chirp scaling imaging operator. A phase compensation method is then proposed to improve the coherence of images of interested targets in different channels. Specifically, the SAR mode of random frequency agility is proposed to improve the reconstruction performance. Finally, simulations are carried out to demonstrate the effectiveness of imaging methods and ECCM performance. Theoretical analysis and simulation results demonstrate that images generated in random FASAR have better performance than those in stepped FASAR. By performing the phase compensation, the reconstruction performance of small targets can be significantly improved.

The interrupted-sampling repeater jamming (ISRJ) is coherent and has the characteristic of suppression and deception to degrade the synthetic aperture radar (SAR) image quality. The anti-ISRJ methods are studied in this work in order to suppress the ISRJ based on the waveform and filter design for SAR. First, the relationship between the ISRJ and waveform is obtained by analyzing the principle of the ISRJ using the ambiguity function. The ISRJ produces multiple false targets based on the high Doppler tolerance of waveform and the characteristic of the matched filter. Next, a method is proposed to counter the ISRJ by transmitting a phase-coded (PC) waveform with low Doppler tolerance and designing the corresponding mismatched filter. The joint design method is then developed to improve the anti-ISRJ and imaging performance. In the proposed methods, the majorization minimization framework is introduced to solve the nonconvex waveform and filter design problem. Finally, several simulations are conducted to demonstrate the effectiveness of the proposed methods. Simulation results show that the joint design method shows better anti-ISRJ and imaging performance in comparison with the separate design method, but it is more sensitive to the ISRJ sampling duty ratio and period.

A joint waveform and filter design method is developed for suppressing radar pulse compression sidelobe level in this article. The problem is formulated as the minimization of the integrated sidelobe level (ISL) under the constraint of waveform constant modular and filter energy. To control the loss-in-processing gain (LPG), an additional function is introduced to constrain the peak level based on penalty function method. The joint design algorithm is then derived based on the alternating minimization and majorization minimization (MM) schemes. The computation complexity is analyzed and the convergence analysis verifies that the algorithm can converge to a critical point. Specifically, the proposed method is extended to the waveform and filter design for suppressing the peak sidelobe level (PSL). Numerical simulations are carried out to analyze the key parameters, which demonstrate the feasibility of the proposed method. Simulation results show that the ISL and PSL can be significantly reduced with small LPG. Moreover, the proposed method exhibits faster running speed than the existing one and it thus can be applied to longer sequence designs.

In this letter, an orthogonal frequency-division multiplexing (OFDM) chirp waveform diversity design scheme is proposed to achieve better correlation properties. First, the subchirp duration and bandwidth are optimized simultaneously to increase the degree of freedom in waveform diversity. Second, an optimization procedure is proposed based on the idea of Gram-Schmidt orthogonalization. To reduce the problem complexity, the design problem is separated into several subproblems. Each subproblem corresponds to designing an OFDM chirp composite waveform and is optimized by the Pareto optimization framework. Simulation results show that the proposed method outperforms the conventional methods in terms of both autocorrelation peak sidelobe level (APSL) and cross correlation peak level (CPL) under the circumstance of the same mainlobe width.

As a typical propagation error, tropospheric delay has been fully considered during synthetic aperture radar (SAR) interferometry and differential interferometry, while the influence on the SAR imaging process is just concerned in advanced configurations, e.g., staring mode, high-resolution, geosynchronous/geostationary (GEO) SAR. In this article, a comprehensive model of turbulent tropospheric delay (TTD), involving atmospheric sciences and fluid mechanics, is introduced, and the influence of which on high-resolution SAR is analyzed. In order to compensate for the influence of TTD in advanced SAR missions, the autofocus method used in motion error compensation is extended, and a 2-D phase error estimation and compensation algorithm is proposed. Subsequently, the simulation experiments of both dot-matrix targets and TerraSAR-X data are performed to validate the effectiveness of the proposed algorithm. Conclusions and prospects are reached in the end.

This letter proposes a method for eliminating the paired-echo-like spurious peaks in sliding spotlight synthetic aperture radar (SAR) images caused by the electronic step-by-step azimuth beam steering. The basic idea of the method originates from the extended ideal filtering for suppressing the SAR azimuth ambiguities. The concept of deconvolving filtering is further extended and a new form of deconvolving filter is presented. Using the new filtering, not only the first and odd spurious peaks of point-like targets but also the even spurious peaks can be removed all at once from the detected SAR images. Simulated data from a sliding-spotlight SAR system with quantized azimuth steering are used to verify the improvement in image quality by using the new method.

This article presents an efficient 2-D baseband azimuth scaling (2-D BAS) approach for the focusing of data acquired in the spaceborne squinted sliding spotlight and high-resolution wide-swath (HRWS) terrain observation by progressive scan (TOPS) imaging modes. The existing approaches can become inefficient or invalid due to the coexistence of central “absolute” and marginal “relative” squint in the above-mentioned modes. In this article, the signal properties of spaceborne squint synthetic aperture radar (SAR) with the antenna electronically steering in the azimuth dimension are analyzed first, based on which a new parity-decomposition-based range equation (PDRE) is presented to dedicatedly model the range histories of targets illuminated by the rotating beam. A novel 2-D BAS approach is then developed to remove the odd asymmetric part but preserve the even symmetric part of PDRE, which means a “true derotation” for eliminating the effect of both absolute and relative squints caused by the beam rotation. An even-order-multinomial perturbation function is applied and integrated into a range-Doppler-based SAR processing kernel to efficiently compensate the azimuth variation of high-order range cell migration (RCM) and azimuth phase modulation caused by the 2-D BAS. The proposed processor is efficient, since none of the data extension, the postprocessing for resolving focused-domain folding, or the 2-D frequency-domain interpolation at high squint is needed. Simulations with point targets in the squinted sliding spotlight and HRWS TOPS modes are used to validate the developed algorithm.

Synthetic aperture radar (SAR) is an advanced imaging sensor that can image side-looking terrain. Until now, SAR schemes and imaging theories have been researched for stationary scenes. Unlike a stationary scene, due to unknown motion parameters, a ground moving target (GMT) is shifted and defocused in a background image, which causes difficulties in their use for further applications. This paper proposes a multiple pulse repetition frequency (PRF) airborne SAR scheme for background-free GMT imaging. In the proposed scheme, pulses are transmitted by a nonuniform pulse repetition time, and echoes are divided into groups with different PRFs. For spectra with different PRFs, due to their diverse respective spectra extending periods, periodical extending GMT spectra will not appear at the same location except for the original spectra. A theorem is proposed to prove that the spectra can be extracted intact, undistorted, and unpolluted. In light of the divergence of the GMT spectra location from that of the clutter, filter banks are designed to locate, extract, and reconstruct the GMT spectra out of the mixed spectra that contain the GMT and clutter. For GMT SAR imaging, a moving target is regarded as a steady target under an equivalent geometry with a new equivalent squint angle and a new SAR platform velocity. The SAR image is obtained with the range-Doppler algorithm with equivalent geometry parameters that are estimated from the Doppler parameters. Experimental results obtained from using extensive numerical simulated data validate our proposed approach.

Imaging of a ground moving target (GMT) with synthetic aperture radar (SAR) is a challenging task, particularly when the Doppler ambiguity happens. This paper proposes a novel GMT range model and a novel Doppler ambiguity-tolerated GMT imaging algorithm based on modified keystone transform. In the proposed GMT range model, the range from radar antenna to the scattering center on the target is divided into two parts: the range from radar antenna to the target centroid and the projection length on the light of sight from the target centroid to the scattering center. Based on the new range model, a Doppler ambiguity-tolerated range cell migration correction (RCMC) method is proposed, in which keystone transform is modified to realize the differential RCMC by interpolation with nonzero phase sinc kernel. Finally, the GMT image is obtained in the range-Doppler domain by matched filtering in the slow-time domain and is restored into the 2-D space domain. The effectiveness of our proposed model and imaging method is demonstrated by both simulated and real airborne SAR data.

Phase synchronization is a key problem which has to be carefully considered in the distributed radar systems. In this letter, the use of a continuous duplex phase synchronization method for the distributed frequency modulated continuous wave synthetic aperture radar system is investigated. The bistatic raw signal model containing the phase difference due to the spatially separated oscillators is introduced. The continuous duplex phase synchronization procedure is proposed later and the synchronization method performance is predicted.

HRWS-SAR steers the antenna to increase the synthetic aperture which causes the radar changes beam squint angle gradually. The squint angle increases with the demanding of the resolution and swath width. With the squint increasing, the relative velocity between radar and lighting swath may reach several km/s. It causes the intra-pulse modulation on the echo. The traditional pulse compressing under the stop-go-stop assumption has bad performance including resolution decrease and PSLR increase. The paper established the echo model. The chirp rate is slightly changed because of the relative velocity. And velocity compensation method is put forward. The experiment results show that the pulse is well compressed. The PSLR and the resolution are restored to normal level.

The coherence and blind velocity are two important questions in multiple-input multiple-output (MIMO) radar ground moving target indication (GMTI). Multi-frequency signal is an effective way to solve the blind velocity problem, and multi-frequency signals are usually transmitted by MIMO radar. However, by this way the coherence is lost. This paper proposed an efficient way by circular-shift transmission of multi-frequency signals between pulses. Comparing to the traditional MIMO radar, the coherence of the same frequency is kept, the baselines become longer, and the blind velocity can be removed. The GMTI performance improvement is verified by a simulation.

For sparse signals (direct or indirect), sparse imaging methods can break through limitations of the conventional SAR methods. In this paper, we introduce the basic theory of sparse representation and reconstruction, and then implements several imaging algorithms including FFT and sparse methods. Through comparison, we conclude a good sparse reconstruction algorithm in SAR imaging. Besides, a new strategy of finding a better regularization parameter in sparse reconstruction is implemented. The imaging result of us has a higher resolution and much lower side lobe than the conventional algorithm based on matched filter theory.

This letter presents a novel extended space-time coding waveform scheme for multiple-input-multiple-output synthetic aperture radar implementation. The extensions include the fulfilling of the two Alamouti periods in one transmit duration minimizing the time-variant channel effect and the new form of the basic orthogonal waveforms. The main advantages of the proposed waveform scheme include ghost image prevention, cross-channel interference mutual cancelation, simultaneous transmission via orthogonal Tx channels, free selection of PRF, etc. The proposed techniques are verified by a simulation for the case of point-like targets.

This letter investigates the performance of a waveform-separation approach employing a posteriori digital beamforming (DBF) in elevation for multidimensional waveform encoding SAR sensing. The signal-to-noise ratio scaling factor in elevation and full-Doppler-band range ambiguity-to-signal ratio are defined to give global assessment of the DBF approach. Constraints for the selection of elevation subaperture number and their relevant influence on image quality are investigated. By theoretical analysis and simulation, the subaperture number can be appropriately selected by a tradeoff between the data volume and imaging performance. Further strategies for data reduction and performance optimization are briefly discussed.

Terrain observation by progressive scans (TOPSAR) solves the scalloping effect of ScanSAR by steering the radar beam in the azimuth direction. The steering enables TOPSAR to achieve more qualified images than ScanSAR with the same coverage and resolution. Nevertheless, when TOPSAR is used for district observation, the steering enables it to get higher resolution and/or wider coverage. This letter compares the resolution limits of TOPSAR and ScanSAR and then proposes four possible modes of using TOPSAR for district observation. For each mode, the steering angle is discussed, and a design result is provided.

A novel and promising type of Synthetic Aperture Radar (SAR) concept for High Resolution and Wide Swath (HRWS) observation is presented. Different from conventional SAR systems, the proposed WYPSAR transmits a long pulse of Linear Frequency Modulation (LFM), but receives only partial signals of echoes, however, the focused image of the short received signals can be nearly the same length as the transmit pulse. And the rest time of the pulse interval can be used to receive echoes from other swaths. By WYPSAR, the HRWS observation is carried out much simpler than using the technology of multi-channel systems. After an introduction to the new concept, a simple example is presented. At last, the signal processing technique is discussed and a simulation result follows.

When bistatic synthetic aperture radar (SAR) works in alternating bistatic/ping-pong mode, it can perform phase referencing without a synchronization link between different satellites. The key to this is to extract a compensation phase from the bistatic echoes, including oscillator phase differences. An echo-domain phase synchronization algorithm using the correlation of bistatic echoes is proposed. Because of its larger equivalent synchronization frequency, the echo-domain algorithm has higher phase synchronization accuracy than the algorithm in the image domain. Experiments using simulated bistatic echoes with real single-stripmap SAR echo validate the proposed echo-domain phase synchronization algorithm.

To address the new requirements for InSAR raw-signal simulation for hardware-in-the-loop simulation application, a real-time raw-signal simulation algorithm with high accuracy is proposed. The raw signal is expressed as the convolution of the transmitting signal and the scene modulation signal (SMS), and the convolution is then implemented using the fast Fourier transform. The SMS is calculated by the position approximation in the time domain, and an interpolation technique is used to maintain the accuracy of the fast approximation algorithm. The SMS is filtered to the radar sampling frequency for real-time implementation. The approximation error of the fast algorithm is analyzed quantitatively to obtain the minimum interpolation factor. Ground-based hardware-in-the-loop simulation results are provided to validate the algorithm.

Due to the separation of the radar transmitter and receiver, the channel mismatch, the time and phase synchronization error between synthetic aperture radar (SAR) central electronic equipments (CEE) is the key problems of spaceborne distributed SAR system compared with the traditional single-satellite double pass interferometric SAR (InSAR) system, which has to be considered in the SAR system design, index allocation, integration and testing, verification and validation. A hardware-in-loop simulation and evaluation approach is presented for the influence analysis of CEE payload on the InSAR performance. The hardware-in-loop simulation system includes the master and slave CEE payloads, the time and phase synchronization prototypes, the on-board computer simulator, the SAR target echo signal simulator (TESS) and other auxiliary equipments. For obtaining the influence of the error source on InSAR performance, a hardware-in-loop simulation and evaluation approach based on different hardware configuration is adopted. The experiment results validate the theoretical analysis and computer-based simulation results, and show the feasibility of the time and phase synchronization scheme, which is meaningful for the implementation of the spacebome distributed SAR system.

This letter derives a 2-D point target spectrum for general bistatic synthetic aperture radar (SAR). For the bistatic configuration, the contributions of the transmitter and the receiver to the overall instantaneous Doppler are unequal due to the different slant range histories. In this letter, an instantaneous Doppler contribution ratio is proposed to represent the difference between the instantaneous Doppler contributions of the transmitter and the receiver, which varies with instantaneous Doppler and range frequency. Then, the 2-D spectrum is obtained by using the stationary phase principle and Taylor series expansion for general bistatic SAR. The accuracy of the spectrum is verified with a point target simulation of different general bistatic configurations.

An algorithm for achieving High-resolution wide-swath (HRWS) images in bistatic space-borne SAR with multiple phase centers is presented. Based on the analysis of the correlation of echos from all phase centers, a signal model is built that all echoes combined form a periodically nonuniform undersampled signal. Then the echoes with Doppler ambiguities are reconstructed to unambiguous uniform samplings which can be processed with conventional imaging algorithms. Simulation trials prove that the approach is valid and robust.

The range variant bistatic deformation term of LBF is analyzed. By linearizing the difference between the nearest ranges of transmitter and receiver, the range variant bistatic deformation term is approximated as the linear function of nearest ranges summation. Based on this, the new interpolation function is presented. Then the range variant bistatic deformation is compensated by interpolation, this processing is more efficient without blockwise in range blocks. The simulation results prove its effectiveness.

In this paper, we consider bandwidth fusion to improve the range resolution. We propose a method based on spectral estimation and amplitude-phase error model to compensate for the lack of mutual coherent between radar subbands. Then we consider two-dimensional fusion to improve the range and cross-range resolution. A new algorithm, called EMEMP, is developed to resolve the poles mismatch occurred in traditional fusion method based on modified Root-Music. By simulations, good performance of these methods will be show.