Deep learning-based methods hold significant potential for synthetic aperture radar (SAR) target detection, but they still face numerous challenges, including difficulty extracting global contextual features for large-scale targets, significant multi-scale issues, and the problem of feature extraction of SAR targets with large aspect ratios, which hinder further performance improvement. To this end, this paper proposes a WaveFormer module, which decomposes the image through wavelet convolution and uses convolution and Transformer to process the frequency domain components they are good at, respectively, to expand the receptive field with low parameter overhead and enhance the target feature extraction ability. To address cross-layer information attenuation during feature fusion, a Gather-and-Distribute(GD) mechanism is introduced to reconstruct the Neck network, enhancing multi-scale feature fusion and detection capabilities. Furthermore, given the large aspect ratio and distinct principal axis orientation of SAR targets, a Weighted Strip-Convolution(WSConv) is proposed to effectively improve detection performance. Experiments on the largest multi-class SAR target detection dataset, SARDet-100K, demonstrate that our method achieves a mean average precision (mAP) of 61.5%, reaching state-of-the-art performance and validating its effectiveness.

The high-resolution wide-swath synthetic aperture radar (HRWS SAR) system enables achieving comprehensive and extensive ground information more accurately and rapidly, enhancing the precision of target detection, identification, confirmation, and description. A common implementation approach of it is azimuth multichannel synthetic aperture radar (SAR), which has become a research hot spot in the field of SAR in recent years. In practice, there is a situation where a channel failure leads to the loss of corresponding data, and in such cases, it is impossible to obtain a high-resolution wide-swath image of quality. Currently, there is no good method to address the data recovery issue for one channel data completely missing. To solve this problem, in this letter, a scheme based on iteration adaptive approach (IAA) and weighted least squares method is proposed for azimuth multichannel SAR missing channel data recovery. Point target simulations and data generated from airborne SAR system demonstrate that the proposed scheme is effective.

High-resolution wide-swath (HRWS) imaging has always been a primary demand for synthetic aperture radar (SAR) remote sensing. Multiple-input multiple-output (MIMO) is a feasible scheme for achieving HRWS imaging and is a hot research topic among scholars. However, in contrast to classic single-input multiple-output (SIMO) SAR, MIMO-SAR schemes require the isolation of transmitted waveforms. The transmitted waveforms and the corresponding echo separation methods are essential techniques in MIMO-SAR. Orthogonal frequency-division multiplexing (OFDM)-chirp waveforms and an OFDM-beamforming (OB) method have been proposed, but have not been validated experimentally. In this letter, an airborne X-band digital beamforming (DBF)-SAR system is used to demonstrate MIMO-SAR with improved OFDM-chirp waveforms for the first time. Experimental results of frequency- and spatial-domain methods demonstrate that the mixed echoes can be effectively separated by OB scheme. The utilization of bandpass filters (BPFs) in OB scheme can conserve computation resources. Signal-to-interference ratio (SIR) is employed to test the performance of BPFs, where the indicator shows that the SIR loss is within 0.2 dB.

The electromagnetic environment is becoming complex as the usable spectrum will be allocated for more services. As a result of this situation synthetic aperture radar (SAR) missions are frequently perturbed by radio frequency interference (RFI) that jeopardizes their scientific observations all over the world. The state-of-the-art multichannel SAR has anti-RFI capability since it’s capable of digitally modulating the antenna pattern (AP) in postprocessing, thereby steering the null toward the angle of arrival (AOA) of the RFI in the spatial domain. However, the AOA of RFI is space-variant, meaning that the mitigation performance of beamformers sensitive to AOA will greatly deteriorate. In addition, the AOA of in-swath RFI and the target echo arrive simultaneously, thus the traditional beamformer will generate a distorted AP, deteriorating the SAR imagery. In light of these considerations, this article studies the in-swath and out-of-swath RFIs in elevation multichannel SAR and develops their countermeasures. Thereinto, a least$\ell _{1}$-norm model is developed to estimate the AOA of the RFI, followed by two schemes developed to separate the RFI. The former develops a beamformer that joint sidelobe control and null expanding to mitigate the space-variant out-of-swaths RFI, whereas the latter develops a blind source separation (BSS)-based technology to mitigate in-swath RFI, avoiding AP distortion and restoring SAR imagery. The effectiveness of the proposed approaches is supported by experiments based on the measured X-band airborne DBF-SAR data as well as the simulated SAR data.

Elevation multichannel synthetic aperture radar (SAR) utilizes digital beamforming (DBF) technology to achieve high-resolution and wide-swath (HRWS) imaging. However, electromagnetic jamming sometimes occurs in SAR images, resulting in the loss of scene information. A DBF-SAR system is weak in resisting jamming signals, and the barrage jamming signals within an imaging swath are not easy to remove using traditional suppression methods for DBF-SAR. Thus, an advanced barrage jamming suppression scheme based on elevation multichannel cancellation is proposed. First, the jamming signals in the echoes are removed using a well-designed channel cancellation filter to obtain the jamming-free signals. In accordance with the characteristics of jamming-free signals, the weighting coefficients are rededuced, and a modified DBF method is presented to achieve HRWS imaging and improve the DBF-SAR signal-to-noise ratio (SNR). In this way, the proposed scheme can effectively eliminate barrage jamming signals. In addition, some simulations and a 16-channel airborne DBF-SAR experiment were executed to demonstrate the proposed scheme.

Space-time waveform-encoding (STWE) SAR has great potential to improve the performance of the future spaceborne SAR system. With the aim of solving the problem of overlapped echoes, one effective approach involves the use of digital beamforming (DBF) in elevation combined with the null-steering techniques. However, the traditional real-time echo separation scheme with single-null steering techniques occupies huge digital resources. In this letter, a novel real-time echo separation processing scheme in intermediate frequency domain is proposed to overcome the disadvantage of the conventional scheme. Based on the two-dimensional simulation results, the proposed scheme has the same accuracy of separating the overlapped echoes as the traditional scheme. Detailed analysis of the system complexity indicates that the proposed scheme can greatly reduce the resource occupation. Moreover, the effectiveness of the proposed scheme is further verified and demonstrated by the experimental results on the X-band 16-channel DBF-SAR system.

Frequency diverse array synthetic aperture radar (FDA-SAR) has been recognized as a potential technique for high-resolution wide-swath imaging, and employs a slight frequency increment to the transmitting sub-apertures/channels to form a range-dependent beam pattern for swath widening. The key challenge of FDA-SAR is echo separation at the receiver. Former studies have employed quasi-orthogonal waveforms with low cross-correlation energies as the transmitted signals, which can not ensure reliable echo separation, especially for distributed scatterers. In this letter an FDA-SAR processing framework is proposed based on the segmented phase code waveform. An encoding scheme and a notch-expanding receiving beamformer based on convex optimization are used for precise echo separation. Then, using the degrees of freedom on transmitting, a beamformer is applied to resolve range ambiguities. The proposed scheme avoids the performance deterioration caused by the echo separation issue, even for distributed scatterers, while allowing other multiple-channel techniques to be integrated. Simulation experiments have validated the effectiveness of the proposed scheme.

Synthetic aperture radar (SAR), an active remote sensing equipment, shares the spectrum with devices in the same frequency band and is therefore easy to affect by pulse radio frequency interference (PRFI). The wideband version of PRFI, i.e., linear-frequency-modulation PRFI (LFM-PRFI), derived from the ground- and space-based radar sensors is a challenging issue for SAR because it usually has a large bandwidth and pulse width compared with the traditional PRFI. The well-known notch filtering methods, including time- and frequency-domain versions, are robust approaches against PRFI, which has been integrated into some SAR ground processing systems. However, the down-range frequency domain notch filtering will increase the sidelobes of the targets when mitigating wideband LFM-PRFI, whereas the time-domain version will introduce ghosts when notching large pulse-width ones. This paper proposed a filtering method that consists of three steps to tackle the above problems. The first step is focusing the energy of LFM-PRFI down-range and cross-range simultaneously, which can be done by match filtering and Fourier transform. The second step is mitigating the LFM-PRFI, which can be done by a designed filter. The last step is data restoration, which can be done by multiplying the conjugate of the phase function used before. Numerical experiments based on simulated SAR data and measured spaceborne SAR data acquired by European Sentinel-1 are performed to test the mitigation performance, which verified the effectiveness and superiority of the proposed approach.

Digital beamforming (DBF) can provide high-gain narrow-beam scanning reception while transmitting wide-beam signals, which greatly improves the signal-to-noise ratio (SNR) of the corresponding systems. It is an effective technique for synthetic aperture radar (SAR) to obtain high-resolution wide-swath (HRWS) imaging capability. However, elevation changes in mountain area will lead to beam-pointing mismatch problems when using the ideal sphere model to calculate the beamforming weighting vector. As a result, the loss of receive gain and the deterioration of the SNR will occur. To solve this problem, adaptive DBF (ADBF) methods based on spectral estimation are typically used, such as Capon and MUSIC. However, the computational complexity of spectral estimation method is high, which is not conducive to on-satellite real-time processing. Therefore, a low complexity ADBF method based on beam-space phase-center cross correlation is proposed. In this method, the whole array is divided into several subarrays, and multiple phase centers are formed by beamforming so that the angle of arrival (AOA) of the signal source can be accurately estimated. Then, the weighted vector of the received beam is updated to mitigate the loss of receiver gain. The simulation results and airborne measured data validate the effectiveness of the proposed method. Compared with methods based on Capon and MUSIC, the proposed method can decrease the computational complexity without reducing the processing accuracy, thus providing a basis for the real-time processing of spaceborne DBF-SAR signals in the future.

The multiple elevation beam (MEB) mode is an effective technique for enhancing imaging width in spaceborne synthetic aperture radar (SAR) systems. This mode combines intra-pulse beam-steering during transmitting and digital beamforming (DBF) during receiving. By sequentially illuminating the far sub-swath followed by the near sub-swath, echoes from different sub-swaths can reach the antenna at the same time and overlap each other in the receiving window. To separate the overlapping echoes, the linear constrained minimum variance (LCMV) beamformer has been used, which is a single-null constrained beamformer. Additionally, a multi-null constraint beamformer has also been proposed on this basis. However, these two methods are insufficient for effectively separating the overlapping echoes when there is a significant energy difference between different sub-beams signals. To solve this problem, an advanced multi-null constrained beamformer with deepened nulls is proposed. Compared with other methods, the proposed method can flexibly adjust the width and depth of the nulls. The simulation results demonstrate that the proposed method can enhance echo separation quality. And the experimental results verify the effectiveness of the proposed method. All the results indicate that the proposed method is helpful to improve high-resolution and wide-swath imaging performance of future spaceborne SAR systems.

Digital beamforming (DBF) is an effective method for improving the signal-to-noise ratio (SNR) in synthetic aperture radar (SAR) images. However, further enhancing the SNR in DBF-SAR is limited by the antenna size and the expensive system cost. To address this problem, in this letter we implement a hybrid strip-map/spotlight mode for DBF. By utilizing the proposed processing method, DBF is adapted for hybrid strip-map/spotlight mode data. Furthermore, the additional synthetic aperture acquired from the hybrid strip-map/spotlight mode is employed to implement multilook processing for further enhancing the SNR of DBF-SAR images. The proposed processing method was verified by an airborne X-band 16-channel DBF-SAR system. The results confirmed that a remarkable additional SNR improvement in DBF-SAR images can be obtained using the proposed method without increasing system complexity.

Digital beamforming (DBF) with scan-on-receive (SCORE) is a state-of-the-art technique for high-resolution and wide-swath observation in synthetic aperture radar (SAR) imaging. However, this technique has a problem with frequency dispersion. Most existing DBF methods treat the frequency as a constant, and little work has been devoted to the analysis of frequency dispersion. To address this shortcoming, this letter analyzes the dispersion effect using the raw data of two digital beamforming synthetic aperture radars (DBF-SARs). Moreover, this letter demonstrates in practice for the first time the effectiveness of the digital scalloped beamforming (DSBF), one of the candidates for solving the frequency dispersion problem in SCORE DBF, using the X-band sixteen-channel DBF-SAR and the C-band four-channel DBF-SAR. The analyses of the impulse response width extension and the signal-to-noise ratio (SNR) improvement are presented in detail to show the performance of the frequency dispersion compensation. The results show that the DSBF is effective in compensating for the frequency dispersion in the DBF technique. With DSBF, a higher SNR improvement can be obtained in DBF-SARs.

In this letter, we examine a new measure for high resolution and wide swath (HRWS) synthetic aperture radar (SAR) imaging based on an elevated frequency diversity array (EFDA). By highly integrating digital beamforming (DBF) and frequency diversity array (FDA) techniques, EFDA–SAR achieves range ambiguity resolution in the spatial frequency domain and range ambiguity suppression outside the observed swath in the range space domain. Moreover, the EFDA–SAR system improves the signal-to-noise ratio (SNR) due to its elevated antenna array design. A model is developed for the time-varying filtering of this novel EFDA-SAR system design. Simulation results are provided to demonstrate the efficiency of the proposed design. Using EFDA–SAR, we can obtain an HRWS SAR image without range ambiguity from the observed swath or outside it. Moreover, by combining DBF on reception, the SNR of the EFDA-SAR image is significantly improved.

High-resolution wide swath (HRWS) imaging has become a research hotspot in the field of synthetic aperture radars (SARs) in recent years. Digital beamforming (DBF) denotes a powerful technique for obtaining the HRWS images. The DBF can be implemented in a spaceborne SAR in real time but at the cost of massive digital resources. To solve this problem, an intermediate frequency (IF) DBF real-time processing architecture has been proposed, but it has never been validated in practice. In this letter, the X-band 16-channel DBF-SAR and the C-band four-channel DBF-SAR are used to demonstrate the effectiveness of the IFDBF scheme in practice for the first time. The performance of the IFDBF scheme is verified by the detailed analysis of the signal-to-noise ratio (SNR) improvement and system complexity. The result shows that the IFDBF scheme can achieve the same SNR improvement as the DBF scheme. Moreover, by applying the IFDBF scheme, a spaceborne SAR can save more digital resources in real-time processing.

Digital beamforming (DBF) synthetic aperture radar (DBSAR) is a promising candidate to overcome the constraint of minimum antenna area in the traditional single-channel synthetic aperture radar (SAR) system for achieving high-resolution and wide-swath (HRWS) image. On reception, DBF technology in elevation can significantly improve the system performance such as its sensitivity, ambiguity level, and the output signal-to-noise ratio (SNR) by Scan-On-Receive (SCORE). However, the inevitable channel mismatch will result in a beam-pointing error and further deteriorate the output SNR in the final SAR images. In this letter, the configuration of the 16-channel DBF-SAR system is described. A scheme of semi-physical simulation testbed for 1-D point target imaging is designed. Based on this, a detailed internal calibration technique combined with DBSAR mode is proposed to calibrate the channel mismatch and thus improve the SAR image quality. The real 1-D raw data experiment is performed to validate the proposed scheme. Finally, the imaging results from the practical flight experiment are shown and analyzed.

Digital beamforming (DBF) is a state-of-the-art technique for high-resolution and wide-swath (HRWS) observation in synthetic aperture radar (SAR) imaging. Since azimuth high-resolution imaging will be a popular tendency in future SARs, DBF-SAR needs to become practical. A new problem arising in airborne DBF-SAR application is the aperture extension loss (AEL). Similar to the pulse extension loss, it is confirmed in our study that AEL will widen the point target responses and degrade the signal-to-noise ratio (SNR) improvement in airborne DBF-SAR. However, little work has been devoted to the compensation of the AEL effect. In this article, the effects of AEL are demonstrated and two compensation methods are proposed, discussed, and implemented for the processing of raw data. The above are demonstrated through simulations and an airborne experimental C-band azimuth high-resolution DBF-SAR. The results show that the AEL compensation removes the AEL effects and increases the SNR with the cost of higher impulse response sidelobes. Moreover, with this novel design scheme, it is demonstrated that azimuth ambiguity suppression can be implemented by elevation DBF.

Digital beamforming (DBF) with scan-on-receive (SCORE) technique in elevation is a powerful technique that enables a spaceborne synthetic aperture radar (SAR) to achieve high-resolution wide swath (HRWS) imaging. In the spaceborne DBF-SAR system, sampling signals from each channel are weighted by weights generated by a digital signal processing system in real-time. However, the contradiction between the shortage of spaceborne hardware resources and resource demand of the multichannel real-time signal processing increases the difficulty of system design. In order to solve this problem, a novel weight generator and an improved intermediate frequency (IF) DBF real-time processing architecture are proposed in this article. By taking advantage of the special properties of the SCORE algorithm, the proposed weight generator calculates weights using a linear polynomial algorithm. The simulation result shows that a low-order approximation can achieve high performance. The proposed generator can correct multichannel amplitude and phase error at a low cost on hardware resources. The effectiveness of the proposed method is verified by experiments with a raw data processing instance of an X-band 16 channels DBF-SAR.

The challenging problem to realize high-resolution and wide-swath (HRWS) synthetic aperture radar (SAR) imaging is the ambiguity suppression in the azimuth and range directions. According to the spatial angle difference of each ambiguity component, the current technical approach is to design the spatial filter for achieving the ambiguity suppression based on the 2-D multichannel system. Along with the increasing of HRWS imaging requirements, the number of system channels also gradually increase and further result in the complex structure design of the phased array antenna system. Meanwhile, the traditional phased array antenna cannot effectively control the direction of the transmit beampattern in range. Unlike the traditional phased array, frequency diverse array (FDA) employs a small-frequency increment across the whole array elements and forms the range-angle-dependent S-shaped transmit beampattern, which can be utilized to separate the different range ambiguous region. Considering the above-mentioned characteristics and the range periodicity problem of transmit beampattern, this letter devises a scheme for spaceborne SAR HRWS imaging mode in the view of transmit beampattern utilizing 2-D planar array, i.e., the FDA in azimuth for removing the range nonperiodicity ambiguity and the conventional phased array in elevation for removing the range periodicity ambiguity. Simulation results have been presented to validate the effectiveness of the proposed scheme.

The displaced phase center antennas in azimuth and digital beamforming (DBF) in elevation are two state-of-the-art techniques for achieving high-resolution wide swath (HRWS) imaging in the multichannel synthetic aperture radar (SAR) systems. However, due to the atmospheric turbulence, airborne HRWS-SARs inevitably suffer trajectory disturbances, which will consequently defocus the SAR image. Although various motion compensation (MoCo) methods have been proposed, they are mostly designed for the traditional single-channel SAR and, therefore, ignore the channel-dependent posture error. The posture motion error will introduce residual motion and time-variant channel errors, which not only defocuses the image but also causes azimuth ambiguity and degrades the SNR improvement of the final images of the azimuth multichannel and DBF-SAR, respectively. To solve these problems, a novel MoCo scheme for 2-D multichannel SAR systems is proposed. The posture error is described and calculated in matrix form through the use of quaternions. Then, the posture error is transformed into the translational motion error for each receiving channel and is subsequently compensated precisely. To address the residual aperture-variant motion error, a modified aperture-dependent MoCo is integrated into the proposed scheme. Simulations and airborne experiments, including processing the data of a C-band SAR system with four azimuth channels and an X-band DBF-SAR system with 16 elevation channels, have been implemented to validate the effectiveness of the proposed MoCo scheme.

In the Earth observation mission of the synthetic aperture radar (SAR), wide swath can be used to complete global monitoring in a short time and high resolution can provide rich detailed information about the feature space and prominent structure and texture. However, the traditional single-channel classical SAR system cannot meet high-resolution and wide-swath (HRWS) imaging demand due to the constraint of minimum antenna area. Fortunately, this fundamental limitation can be overcome by using multiple receive subapertures in combination with advanced digital beamforming (DBF) technique. DBF in elevation can provide high gain and better system performance and has recently gained much attention in the field of SAR imaging. This article presents a 16-channel in elevation airborne X-band DBF-SAR system with 500-MHz bandwidth, characterized by high speed data acquisition and storage, as a test bed to provide the technical reserves and support for a future spaceborne DBF-SAR system in China. The hardware configuration of this system is designed according to a realistic flight mission. To verify the feasibility and operability of this advanced 16-channel DBF-SAR system, an outfield airborne flight experiment was successfully conducted in eastern Guangdong Province in November 2019. Meanwhile, considering the inevitable channel mismatch from airborne system, a precise strategy as well as the underlying signal processing is proposed to process the experiment data. In addition to the channel mismatch due to the topographic height, the Scan-On-Receive (SCORE) pattern loss (SPL) is also an inherent factor, which will deteriorate the output SNR in final SAR images. Therefore, this article also implements a quantitative assessment of SPL combined with the practical flight parameters and the real airborne data. Finally, the corresponding processing results are presented and analyzed in detail. The practical SNR improvement of 11.23 dB emphasize that DBF technology can significantly improve the quality of SAR images and will make an essential contribution to next generation of HRWS technology for environment monitoring.