Due to the large diving trajectory, the design of the imaging algorithm remains an exceptionally difficult task for hypersonic synthetic aperture radar (SAR). In this article, an accurate range model that accounts for the impact of higher-order motion factors on imaging performance is established. Based on the model, the signal characteristics are analyzed, which indicates that the design of the imaging approach faces greatly challenges such as severe cross-couplings and significant spatial variations (SVs). In view of these issues, a time–frequency hybrid domain imaging algorithm (TFHDIA) is proposed for hypersonic vehicle (HSV)-borne SAR systems with a large diving trajectory. The cross-couplings are significantly reduced by azimuth preprocessing. The 2-D SV in the range cell migration (RCM) and the first-order SV in the Doppler parameters (DPs) are eliminated through the fractional Fourier transform (FrFT) and the extended keystone transform (EKT). Additionally, the higher-order SVs of DPs are removed by the phase filter bank, which includes range, azimuth, and cross-coupling spatially variant phase error compensation filters. The proposed algorithm exhibits strong capabilities in eliminating SVs and effectively reducing cross-coupling, making it particularly suitable for HSV SAR systems with large diving trajectories. Simulation and actual acquired data results validate the efficacy of the proposed algorithm.

Radar forward-looking imaging is a leading topical issue in the radar research domain, which plays a significant role in many fields such as target estimation, forward-looking detection, and precision guidance. By generating a stochastic radiation field through the wavefront modulation of the transmitted signals and associating the target echo with the radiation field to achieve the forward-looking imaging of the target in the beam. This method has superior performance on sparse targets, while the performance for complex targets will degrade severely. Therefore, this paper proposes a radar forward-looking imaging method for complex targets based on sparse representation with regularized approximated K-singular value decomposition (RAK-SVD) and damped Gaussian generalized approximate message passing-based variational sparse Bayesian learning (GGAMP-VSBL) algorithm. Firstly, the imaging principle and model are introduced to realize forward-looking imaging. Secondly, a regularization parameter is added to the classical dictionary learning K-SVD algorithm to obtain the regularized AK-SVD algorithm, and it is used for complex targets to improve the accuracy of sparse representation. Thirdly, the variational Bayesian inference (VBI) methodology is applied to SBL to form the VSBL algorithm, and GGAMP is embedded into VSBL to obtain the GGAMP-VSBL algorithm to speed up image reconstruction. Meanwhile, a new variance updating formula is adopted to further reduce the computational complexity and improve the robustness of the algorithm. The experimental results show that the proposed method can effectively image complex targets and has favorable performance in different complex scenes.

The azimuth extent of imaged scene is relatively small under staring spotlight synthetic aperture radar (SAR) mode. The sliding spotlight SAR mode can enlarge the extent of the azimuth scene, but at the cost of azimuth resolution loss. In this paper, an intrapulse azimuth frequency scanning spotlight SAR mode is presented. Through azimuth frequency scanning, the proposed spotlight SAR mode can significantly enlarge the extent of the azimuth scene without causing any loss in the azimuth resolution. Moreover, the proposed mode can separate the azimuth ambiguities in the range frequency domain. Therefore, the extent of the azimuth scene can be enlarged without increasing the pulse repetition frequency (PRF). Then, the range swath width can be maintained. However, to overcome the reduction in the effective range-frequency bandwidth caused by frequency scanning, an ultrawideband transmitting signal is required. The signal model of the proposed mode is derived. Based on the analysis of the characteristics of the echo spectrum, the design criteria for this mode are developed, followed by a design example. Finally, an imaging algorithm for this mode is presented. Numerical simulations verify the effectiveness of the proposed mode.

The frequency dispersion of wideband phased array will lead to the variation of beam point angle with the frequency of transmit signal. In this letter, such phenomenon will be exploited for squint video SAR. Firstly, the frequency dispersion will make the centroid Doppler frequency independent from the range frequency. The spectrum support region will be rectangular in such case. Therefore, the different frequency components of the transmit signal will illuminate the same ground target in different slow time duration. A SAR video can be obtained through splitting the range frequency spectrum instead of the Doppler spectrum. Then, the azimuth resolution can be maintained. Meanwhile, the large bandwidth being employed can not only maintain the range resolution after splitting but also can enlarge the time duration of squint SAR video. Numerical simulations verify the effectiveness of the proposed method.

In this article, a novel 2-D scanning synthetic aperture radar mode for high-resolution and wide-swath imaging is introduced. Instead of the interpulse progressive azimuth beam scanning used in Terrain Observation by Progressive Scans (TOPS) mode, the intrapulse continuous azimuth frequency scanning is employed in this mode. The same azimuth resolution as the multichannel TOPS mode can be obtained through this mode in the case of single channel transmitting and receiving. And the wide-swath imaging in range is still achieved through the cyclically elevation beam scanning from burst to burst. Moreover, the antenna aperture can be fully exploited for transmitting in this mode. Therefore, it will outperform the multichannel TOPS mode especially in the case of active array antenna. The signal model of this mode is derived. Based on the analysis of the characteristics of echo spectrum, the design criteria for this mode are developed, followed by a design example. Finally, an imaging algorithm for this mode is presented. Theoretical analysis and numerical simulations verify the effectiveness of the proposed design criteria and imaging algorithm.

Elevation frequency scanning synthetic aperture radar (EF-SCAN SAR) can isolate range ambiguities in the range-frequency domain. Therefore, range ambiguity suppression can be accomplished through range-frequency bandpass filtering, which can outperform spatial filtering-based ambiguity suppression. In this article, the analytic space-time signal model of an intrapulse frequency scanning array is derived. Based on the analysis of the characteristics of this signal model, waveform design criteria for EF-SCAN SAR are developed to effectively separate the range ambiguities in the range frequency domain. The ground echo model of EF-SCAN SAR is also developed based on this space-time signal model. The impact of intrapulse frequency scanning on the performance of high-resolution and wide-swath (HRWS) imaging is analyzed based on the derivation and analysis of the echo spectrum. Finally, an imaging algorithm for EF-SCAN SAR is developed based on this analysis. Numerical simulations verify the effectiveness of the proposed waveform design criteria and imaging algorithm.

With the development of science and requirement, Integration of Communication and Radar with properties of more functions, lower buck and safety is becoming a trend and draws more attention of researchers. Current researches are focusing on communication or radar respectively or just focusing on signal generation. This paper proposes a solution of integrated system for SAR radar communication based on OFDM techniques in high speed scenario. The receiver can reconstruct the reference signal which is adopted for radar pulse compression procedure by communication system directly. In high speed scenario, channel response changes fast, comb type pilots and channel estimation are added to equalize the channel response to reduce bit-error-rate in communication system. Finally, the simulation are used to validate the performance of the proposed algorithm.

Circulating code provides a simple way to achieve transmit diversity with stable gain along wide angular coverage, whereas this property is at costs of poor sidelobe signal suppression ability and degraded range resolution. In this paper, an angle dependent match filter combined with slow-time coding is proposed and investigated to solve the above problem. The angle dependent match filter is designed via convex optimization technique to suppress interfering signals from sidelobes with a controllable penalty in the target signal-to-noise ratio (SNR). Then the slow-time coding technique is used to recover range resolution. Numerical experiments are carried out using simulated data. The performance assessments show that the proposed technique is superior in interference suppression and range resolution improvement.

In geosynchronous (GEO) synthetic aperture radar (SAR), the high orbit height and the long integration time would lead to the significant round-trip delay time and the curved flight path, thus the traditional range model with `stop-go' assumption will be ineffective. In this Paper, based on the analysis of instantaneous Doppler frequency of GEO SAR, an equivalent midpoint mathematical model is proposed to accurately describe the range history, which takes the effect of `stop-go' assumption into account and is more applicable to SAR processing. Based on the equivalent model, the two dimensional (2-D) spectrum of the GEO SAR can be obtained easily by the method of series reversion (MSR). The simulations show the validity of the presented method.

In this paper, a target detector with frequency diverse array (FDA) multiple-input and multiple-output (MIMO) radar in a homogenous environment is proposed. Unlike the conventional MIMO radar, the system uses FDA transmitting array. The essence of the proposed technique is to formulate a range-angle dependent detector with FDA MIMO radar. The detector assumes the signals belonging to conic regions, and relies on secondary data. Then, the detector performance is evaluated via Monte Carlo simulations. The results show that our system has interesting detection capabilities with range-angle dependent property, and it could be easily adapted to real scenarios.

A huge limitation of the correlation analysis in existing scatterer model is that the former model, only consisting of uniformly distributed scatterers, ignores the variability of complex scattering intensity in real situation. In fact, each part of the target has a different scattering architecture. Moreover, the scattering intensity of pinnacle or boundary scattering structure is an order of magnitude larger than the normal scatterers. To solve this problem, this paper studies the correlation coefficient for scattering model which has several dominant scatterers in spatial-frequency diversity MIMO radar. And the positions of the dominant scatterers assumed to be variables to make an equivalent of different real targets. The proposed analysis enhances the authenticity of scattering model and can simulate the real model accurately. Simulation results show the effectiveness of the proposed approach.

A new wavenumber domain mapping method for the highly squinted beam steering synthetic aperture radar (BS-SAR) is analysed. After the mapping operation, the two dimensional (2-D) spectrums have a broadside form which can be regarded as an equivalent broadside SAR. Furthermore, a novel 2-D two-step approach is introduced to avoid spectral aliasing and skewing caused by the beam steering and the high squint angle. With the mapping operation being incorporated into the novel two-step approach, the baseband azimuth scaling (BAS) algorithm which is only used for broadside cases can be applied for the highly squinted BS-SAR without subaperture.

The most serious problem in processing highly squinted data is the strong range–Doppler coupling, which can be reduced by the linear range cell migration correction (LRCMC). After the LRCMC, however, targets with different azimuth frequency modulation (FM) rates are moved into the same range cell, blocking the efficient azimuth compression. In this letter, a new algorithm to equalize the FM rates is presented, which is referred to as azimuth extended nonlinear chirp scaling. The key is to derive a new perturbation function to handle the problem while introducing a negligible residual phase. The proposed method outperforms the existing algorithms, particularly in large bandwidth cases. Numerical simulations illustrate the performance of the proposed algorithm.