The echo separation issue of different transmit antennas is the most technical challenge in realizing multiple-input and multiple-output synthetic aperture radar (MIMO SAR) with same frequency band, especially for low-computing echo separation, making it extremely difficult towards the practical application for the spaceborne MIMO SAR. Based on the orthogonal frequency-division multiplexing (OFDM) chirp waveforms, this letter proposes an innovative echo separation scheme with digital beamforming (DBF) and bandpass filtering (BPF) on board and bandpass-null steering on the ground for the spaceborne MIMO SAR. This scheme transfers the complex computing process on board to the ground, thus significantly reduce the computational load and relieve the resource occupation on board. Also, the perfect separation of interested echoes from interference can be achieved by this scheme. Finally, performance comparisons and simulation results show the effectiveness of the proposed scheme. The proposed scheme enables a high-efficiency and great-performance echo separation for the spaceborne MIMO SAR and makes the MIMO SAR a more promising technique for future SAR missions.

An advanced sparse multi-channel system is proposed for spaceborne digital beamforming synthetic aperture radar (DBF-SAR), which can suppress pulse extension loss (PEL) and frequency dispersion loss (FDL) without increasing the computational load and system complexity. First, conventional scan-on-receive (SCORE) technique is reviewed and a matching ratio (MR) is proposed to evaluate the mismatch between the formed beam pattern and the pulse signal amplitude. To mitigate the PEL and FDL, the novel sparse SCORE (S-SCORE) based on the optimization of the sparse channel distribution is proposed. The impact of sparse channel distribution is analyzed and the method to optimize the distribution based on the maximized MR is proposed accordingly. Finally, the results of simulations and experiments are provided to demonstrate the superiority of the proposed S-SCORE technique. The work in this paper can be seen as an important candidate for future spaceborne DBF-SAR.

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.

Supervised clustering methods cluster images using graph convolutional networks (GCN) via linkage prediction, and have shown significant improvements over the traditional clustering algorithms (e.g., K-means, DBScan, etc.) in terms of clustering effectiveness. However, existing supervised clustering approaches are always time-consuming, which may limit their usage. The high computation overhead is mainly resulted from generating and processing a large amount of subgraphs, each of which is generated for one image instance in order to infer the linkage between them. To tackle the high computation problem, we propose a new density division clustering approach based on GCN, and our experiments demonstrate that the new approach is both time-efficient and effective. The approach divides the data into high-density and low-density parts, and only performs GCN subgraph link inference on the low-density parts, which highly reduces redundant calculations. Meanwhile, to ensure sufficient contextual information extraction for low-density parts, it generates adaptive subgraphs instead of fixed-size subgraphs. Our experimental evaluations over multiple datasets show that our proposed approach is five-time faster than state-of-the-art algorithms with even higher accuracy.

Insipred by the cocktail party effect, an advanced scheme based on blind source separation is put forward to suppress the range ambiguity of spaceborne SAR in this paper. In the scheme, multiple sub-antennas are used to collect multiple echo data sets, which are given different patterns. In this way, the echo signal of the desired region and that of the ambiguous region are weighted using different coefficients. During post-processing, a detailed flow with the blind source separation algorithm is proposed to separate the desired echoes from those data sets. To verify the scheme, the point targets simulation is carried out. The results indicate that range ambiguity can be reduced by more than 14 dB using the proposed scheme, the processing flow is effective, and the scheme has a good performance without increasing system complexity. The scheme has the potential to be applied to the future spaceborne SAR missions, such as LuTan-l (LT-l) misson.

Space-Time Waveform-Encoding (STWE)-synthetic aperture radar (SAR) enables waveform diversity in the space-time domain to meet the requirements of future high-resolution and wide-swath (HRWS) missions. The STWE-SAR receives echoes from multiple sub-swaths simultaneously with a single receive window. The overlapping echoes are usually separated based on the linear constrained minimum variance (LCMV) beamformer. However, the energy of the echoes from different sub-swaths can have a huge difference in the time domain. The conventional LCMV beamformer cannot effectively separate the overlapped echoes because the echo energy difference is not considered. Based on airborne STWE-SAR data, this paper performs energy equalization pre-processing before echo overlapping. Moreover, this paper confirms that the echo discrepancy is worth considering in the STWE system by comparing the separation results of the LCMV beamformer before and after energy equalization. This paper recommends that the design of future echo separation schemes needs to focus not only on the echo arrival of angle but also on the echo energy based on the actual situation.

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.

Due to the minimum antenna area constrain of synthetic aperture radar (SAR), high-resolution and wide-swath (HRWS) imaging is difficult to be achieved using classical modes, such as scan, spotlight, and so on. Range ambiguity is one of the main technical challenges limiting HRWS imaging in spaceborne SAR. Insipred by the cocktail party effect, an advanced scheme based on blind source separation is put forward to suppress the range ambiguity of spaceborne SAR in this paper. In the scheme, multiple subantennas are used to collect multiple echo data sets, which are given different patterns. In this way, the echo signal of the desired region and that of the ambiguous region are weighted using different coefficients. During postprocessing, a detailed flow with the blind source separation algorithm is proposed to separate the desired echoes from those data sets. To verify the scheme, point target and scene simulations based on GF3 are carried out. The results indicate that the range ambiguity can be reduced by more than 15 dB using the proposed scheme, the processing flow is effective, and the scheme has a good performance without increasing the complexity of the system design under the current SAR system conditions. The scheme has the potential to be adopted in future spaceborne SAR missions.

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) is a fundamental technique for synthetic aperture radar (SAR) to get high-resolution wide-swath (HRWS) images, which significantly increases the signal-to-noise ratio (SNR) of the system and improves range ambiguity performance. Moreover, the performance of the DBF-SAR system can be improved by using the nonlinear frequency modulation (NLFM) waveform, which can provide a matched filtering output with lower sidelobes without the loss of SNR compared to the linear frequency modulation (LFM) waveform. Combining the DBF technique and the NLFM waveform will enhance the system performance of DBF-SAR from an additional degree of freedom, which has essential engineering significance for reducing the transmit power of the system. However, the previous system architecture and processing method of DBF-SAR are generally based on the LFM waveform and are not practicable in DBF-SAR adopting NLFM waveform. This manuscript demonstrates the potential of adopting the NLFM waveform in DBF-SAR and analyzes the problems of compensating pulse extension loss (PEL) and frequency dispersion loss (FDL) in the new system. Then, an optimized DBF framework that combines sub-digital beamforming and a bank of unequal-width bandpass filters to suppress PEL and FDL in DBF-SAR adopting NLFM waveform is proposed. Simulations demonstrate that the proposed framework shows greater efficiency and stability in suppressing the severe PEL and FDL in the NLFM and LFM systems than previous methods. This manuscript brings an additional degree of freedom to the next generation spaceborne DBF-SAR and provides sufficient technical support for high-performance DBF-SAR when the LFM waveform is not adopted.

Space-time waveform-encoding (STWE) synthetic-aperture radars (SARs) can realize waveform diversity in the space-time domain and effectively improve the system performance. However, the benefits of the STWE SAR are based on the accurate separation of the overlapped echoes. Digital beamforming (DBF) in elevation using the null-steering techniques has the potential for echo separation in the STWE SAR. However, the performance of the conventional null-steering beamformer deteriorates when it meets with the extended pulse. Constant time delay of the signal in each channel is usually used to relieve the influence of the extended pulse in the DBF SAR. However, for the STWE SAR in elevation with multiple subswaths, the used delay values are only compatible for one single imaging swath. This letter proposes an innovative echo separation beamformer with digital scalloped beamforming (DSBF) and adaptive multiple null-steering for the STWE SAR. The proposed beamformer can relieve the influence of the pulse-extension loss (PEL) for the echo to be extracted and much better suppress the interfering echoes compared with the conventional methods. Simulation results show the effectiveness of the proposed beamformer. The proposed beamformer enables perfect echo separation for the STWE SAR and makes the STWE SAR a more promising technique for future SAR missions.

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.

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.

Digital beamforming (DBF) with Scan-On-Receive (SCORE) in elevation is a powerful technique for spaceborne synthetic aperture radar (SAR) to achieve high-resolution wide-swath (HRWS) images. DBF in spaceborne SAR should be implemented in real-time to reduce the volume of the downloaded data. Current digital beamformers in SAR systems usually use phase shift instead of a time delay network to ensure real-time implementation. However, conventional phase shift DBF SAR with narrow array bandwidth is not applicable for broadband signals. The beam steering deviation as a function of frequency is obvious for signals with large fractional bandwidth, which appears as a phenomenon of frequency dispersion. If the signal bandwidth is wider than the array bandwidth, frequency dispersion can significantly deteriorate the signal-to-noise ratio (SNR) and must be well relieved. This article analyzes the frequency dispersion in DBF SAR and proposes an innovative DBF-SCORE scheme with specially designed scalloped beam. The proposed digital scalloped beamforming (DSBF) scheme is effective in relieving the influence of frequency dispersion in DBF SAR. Theoretical analysis and simulation results validate the effectiveness of the proposed scheme. The proposed DSBF scheme gives practical solution to the problem of frequency dispersion in broadband DBF SAR, which makes DBF-SCORE a more practical technique for future spaceborne HRWS SAR.

High-resolution wide-swath (HRWS) synthetic aperture radar (SAR) images are valuable for disaster and environment monitoring. Digital beamforming (DBF) in elevation is a powerful technique for future HRWS SAR. However, real-time DBF processing requires massive digital resources, which are precious for spaceborne SAR. The intermediate frequency (IF) process scheme of DBF SAR is advantageous in reducing the required digital resources. Since DBF processing is performed before digital demodulation in IF DBF SAR, the channel imbalance of the system must be compensated with IF real signal. That will be quite different from the conventional scheme and has never been researched. This paper gives solution to this problem based on the IF process scheme without adding to the system complexity. China's next generation airborne DBF SAR (IDBSAR), operated by the Institute of Electronics, Chinese Academy of Sciences (IECAS), will serve as a test bed.

This paper presents a 3.6 GHz X-band wideband airborne SAR system, featured by full-bandwidth transmitting and receiving. After the general introduction, the architecture, inter-connect design and system composition are discussed in detailed. To avoid the main-lobe distortion and asymmetrical side-lobe, the pre-distortion signal is constructed in time domain to compensate the system errors. The results of the experiment were accomplished successfully and validate the effectiveness and applicability of this airborne SAR system.

Bistatic and multistatic synthetic aperture radar (SAR) can greatly improve system performance in various aspects, especially in achieving high-resolution wide-swath (HRWS) images. However, the specific system structure may lead to a higher degree of range ambiguities. This letter studies the structure of multistatic multiple elevation beams (MMEB) system which achieves HRWS images with the drawback of more serious range ambiguities compared with the conventional SAR which must be suppressed to get an acceptable system performance. The character that the receivers form, a prospective distributed multichannel system in azimuth, can give a guide in estimating the strong range ambiguities and remove them. An innovative method in estimating and removing (not just smearing) the strong range ambiguities based on this character is proposed. Theoretical analysis and experimental results show the effectiveness of the method. The proposed method considerably improves the range ambiguous performance of the MMEB system without using digital beamforming or pulse coding and makes full use of the system character, and thus improves the applicability of the system.

Multi-channel synthetic aperture radar (SAR) in azimuth can overcome the minimum-antenna-area constraint of the conventional SAR in high-resolution and wide-swath (HRWS) imaging. However, the SAR system suffers from amplitude and phase mismatch among channels and nonideal antenna pattern, which will result in azimuth ambiguity and ghost targets in the final image. Therefore, taking the nonbandlimited signal and channel errors into account, a practical azimuth ambiguity-to-signal ratio (AASR) model of multi-channel SAR system is established. Meanwhile, the baseband Doppler centroid (DC) frequency related to channel errors also has an influence on image quality. Then, an effective method is proposed to calculate the baseband DC frequency according to the jumping points of the channel phase errors estimate. Subsequently, considering the effect of azimuth antenna pattern (AAP), a corresponding relationship between the ideal steering vectors and the signal subspace from the decomposing covariance matrix is established. After that, based on the uniqueness of the signal subspace and the correct corresponding relationship, an accurate method is proposed to estimate the channel phase errors by minimizing the minimum mean square error (MMSE) of the signal subspace. Finally, an accurate multi-channel SAR imaging diagram is shown to effectively mitigate the azimuth ambiguous energy caused by channel errors. Simulation and real data experiments, including four channel airborne SAR data with a bandwidth of 210 MHz and the Chinese Gaofen-3 dual receiving channel (DRC) spaceborne SAR data, validate the effectiveness of the proposed calibration method, particularly in low signal-to-noise ratio (SNR).