Increasing application demands are driving the need for future spaceborne synthetic aperture radar (SAR) systems with high resolution and continuous ultrawide swath capabilities. Azimuth multichannel staggered SAR, which integrates variable pulse repetition interval (PRI) and multichannel techniques, presents a promising solution. However, the resulting nonuniform sampling invalidates conventional frequency-domain reconstruction algorithms and increases signal processing complexity. To address this challenge, this paper proposes a uniform reconstruction scheme based on phase center adjustment (PCA). By introducing a phase center variation, the scheme compensates for nonuniform components to achieve equivalent uniform sampling during data acquisition. The PRI design criterion is established to minimize the maximum PCA value and provide the allowable range of the initial PRI. Furthermore, activation strategies for both transmit and receive antenna elements are defined to jointly achieve the required PCA. Simulation results validate the effectiveness of the proposed scheme.

Typically, synthetic aperture radar (SAR) utilizes linear frequency modulation (LFM) signal to acquire high-resolution images, requiring spectral windowing to suppress sidelobes while sacrificing signal-to-noise ratio (SNR). In contrast to LFM signal, nonlinear frequency modulation (NLFM) signal can reconstruct the signal power spectral density (PSD) without sacrificing SNR, providing autocorrelation outputs with lower sidelobes. Despite the excellent application potential of NLFM signal, the real-time generation of NLFM faces numerous challenges due to the high complexity of the systems involved and constraints imposed by waveform generator devices. In this letter, a low-complexity, high-precision and high-resolution intermediate-frequency NLFM signal generation device is developed, requiring only eleven parameters to generate real-time NLFM signal of arbitrary time width and bandwidth, with a maximum bandwidth reaching 1.2 GHz. This NLFM signal generator will be employed in the unmanned aerial vehicle (UAV) SAR system. Finally, the performance of the NLFM signal generator has been validated through ground experimental results.

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.

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.

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) 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.

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) 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 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.

The software of artificial organ is more and more complex, but it lacks real-time operating system to manage and schedule its resources. In this paper, we propose a lightweight real-time operating system (RTOS) Cyborgan OS based on the SmartOSEK OS. Cyborgan OS optimizes and improves it from the code size, context switch, low power consumption, and partial dynamic update, making it suitable for the artificial organ control system. Finally, we use the heart blood pump model to analyze the task allocation and execution sequence as well as the code size of the whole program. In this application, the maximum space occupied by the code is only 15 kB, which is suitable for most microcontrollers.