Space–time waveform-encoding (STWE) SAR can receive echoes from multiple sub-swaths simultaneously with a single receive window. The echoes overlap each other in the time domain. To separate the echoes from different directions, traditional schemes adapt single-null steering techniques for digital receive beam patterns. However, the problems of spaceborne DBF-SAR, in practice, such as null extension loss, terrain undulation, elevation angle of arrival extension, and spaceborne antenna beam control, make the conventional scheme unable to effectively separate the echoes from different sub-swaths, which overlap each other in the time domain.A novel multi-null constrained echo separation scheme is proposed to overcome the shortcomings of the conventional scheme. The proposed algorithm can flexibly adjust the width of the notch to track the time-varying pulse extension angle with less resource consumption. Moreover, the hardware implementation details of the corresponding real-time processing architecture are discussed. The two-dimensional simulation results indicate that the proposed scheme can effectively improve the performance of echo separation. The effectiveness of the proposed method is verified by raw data processing instance of an X-band 16-channel DBF-SAR airborne system.

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) 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.

The frequency diverse array (FDA) technique is a novel scheme for high resolution wide swath synthetic aperture radar (SAR) imaging, which employs a frequency increment across the array elements. This introduces a range-angle-dependence to the transmission steering vector, which is exploited for range ambiguity resolution in strip-map SAR. Generally in spotlight mode, scatterers dispersively distributed in azimuth have different Doppler histories, in which the range ambiguity resolution for strip-map SAR fails. To extend the flexibility of FDA, in this paper a novel FDA imaging scheme for spotlight mode SAR is proposed. Exploiting the property of the same illuminated period of all scatterers in spotlight mode, the proposed scheme is carried out entirely in the azimuth time domain, which allows for higher processing efficiency and real-time implementations. Still, excessive Doppler history differences among scatterers deteriorate the scheme performance for azimuth-edge scatterers. Aiming at this situation, a residual angle phase compensation in time domain is considered for the cases of a large azimuth beam width, improving the applicability of the proposed scheme. Compared with existing methods, the proposed spotlight FDA-SAR offers the possibility of achieving simultaneously high azimuth resolution and wide swath performance with high efficiency. Simulations and analyses are performed to demonstrate the effectiveness of the proposed spotlight FDA-SAR scheme.

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.

High-resolution, wide-range imaging is crucial for contemporary and future remote sensing surveys. Multi-channel synthetic aperture radar (SAR) is an effective tool to fulfill this requirement. Among various multi-channel technologies, null steering digital beamforming is commonly used for ambiguity suppression, SAR echo separation, and interference and clutter suppression. However, existing null steering beamforming algorithms lack robustness and are sensitive to channel errors. This paper proposes an improved SAR echo separation scheme based on complex independent component analysis (ICA) to address the channel error issue. Initially, the problem caused by channel error leading to the failure of the linear constraint minimum variance (LCMV) beamformer is analyzed. Then, non-Gaussian and non-circular measurements of SAR echoes from different scatterers are conducted, concluding that multi-channel SAR echoes satisfy the conditions for ICA application. For an elevated multi-channel wide-swath SAR, an ambiguity signal separation scheme based on ICA is proposed. Block processing or pure-decimal-constraint LCMV (PDC-LCMV) beamforming is first employed to compensate for the spatial dependence of the mixing matrix. A complex entropy bound maximum algorithm, based on source signal independence, is then used for further signal separation. Our proposed scheme effectively addresses the channel error issue in traditional null steering beamforming schemes and efficiently separates ambiguity echoes. It leverages the statistical characteristics of the signal itself and expands the processing dimension. Capable of handling multiple errors, it is suitable for systems with limited degrees of freedom and does not require additional hardware resources. Simulation experiments verify the effectiveness of this scheme.