Lunar wide-swath imaging with Earth-based radar is of great significance for lunar scientific research. Conventional Earth-based radars with high frequencies and large-aperture antennas usually operate in the spotlight mode, resulting in a relatively narrow imaging swath. With the increasing demand for large-scale mapping of the lunar surface, it is necessary to develop a kind of wide-swath imaging mode. Hence, this paper proposes a novel imaging mode, namely terrain observation by beam steering (TOBS) mode. The key feature of TOBS mode is to steer the antenna beam at a non-uniform angular speed along the geographical orientation of the scene swath during the data acquisition. The key techniques are: 1) a dynamic beam control method is proposed to achieve uniform azimuth resolution; 2) a variable PRF design method is proposed for effective data acquisition; 3) an improved ground Cartesian back-projection (GCBP) algorithm based on affine geographic coordinate (AGC) system is proposed for efficient TOBS mode imaging. This paper reports the first demonstration of the TOBS mode lunar imaging with an Earth-based radar prototype system. A scene swath of about 750 km has been imaged utilizing the TOBS mode with an azimuth resolution of about 25 m, and the effectiveness of the proposed method is successfully validated.Unlike the traditional "mosaic mode", where multiple separate observations are required to obtain a long swath image leading to reduction in efficiency and enhanced complexity of data processing. The TOBS mode provides a new technical approach for lunar wide-swath imaging utilizing large-aperture and high-frequency Earth-based radars, translating to high-resolution lunar image datasets.

The back-projection (BP) algorithm has been regarded as a robust high-resolution imaging algorithm, particularly suitable for Earth-based radar lunar imaging. To offset phase errors induced by non-ideal effects, autofocus is essential to obtain well-focused lunar images. However, traditional back-projection autofocus algorithms are often computationally burdensome under long synthetic aperture time. In this paper, an effective back-projection autofocus algorithm is developed for Earth-based radar lunar imaging. To ensure that the defocusing caused by phase error lies in the azimuth direction of the BP image, the delay-Doppler coordinate system is introduced into the BP algorithm, and lunar images are generated on the delay-Doppler grid arranged on the lunar surface. Then, the Fourier transform relationship between the BP image and its wavenumber spectrum is established to facilitate the application of phase gradient autofocus (PGA) algorithm to the BP image. To further improve the estimation accuracy of phase error, the spectral characteristics are analyzed in detail. A multi-subband autofocus method based on local scene is proposed to mitigate frequency-dependence of phase error and space-variance of wavenumber spectrum. The processed results of real lunar data validate the effectiveness and efficiency of the proposed algorithm.

A ground-based radar is a potential technique for lunar surface imaging. However, due to the Earth’s rotation, the maximum azimuth resolution is limited for a single observation. To solve this problem, this letter analyzes the feasibility and performance of obtaining multiangle data from different observations and forms large virtual apertures through aperture synthesis. To ensure the quality of synthesized images, an observation baseline selection method is proposed based on the principle of spectrum continuity, specifying that for two noncontinuous observations, there should be a point on each of them whose target-to-radar vectors share the same direction. Besides, an aperture synthesis algorithm based on spectrum compression is proposed to eliminate spectrum aliasing. The effectiveness of the algorithm is verified via computer simulations and real data experiments. By forming a well-focused 500-m resolution image from two noncontinuous observations, the validation of the proposed algorithm has been proved.

Ground-based distributed radar is a potential technique for space target detection. However, in the case of low signal-to-noise ratio (SNR), it is difficult to achieve long-term integration due to the limited ephemeris guidance accuracy and complex motion model. To solve this problem, a space target detection algorithm based on digital beamforming (DBF) and generalized Radon-Fourier transform (GRFT) is proposed in this paper. To avoid the gain loss caused by ephemeris errors, small-scale beam-searching is conducted through DBF technique, which also enables the measurement of target angle and even angular velocity. Besides, transforming the problem of energy accumulation into parameterized model matching, the GRFT process can achieve long-term integration effectively in the case of complex motion models. The effectiveness of the algorithm is verified via real data experiments based on a ground-based distributed radar. By showing an effective 30-second integration and a computational efficiency improvement of 40%, the validation of the proposed algorithm has been proved.

Spaceborne multi-channel synthetic aperture radar (SAR) is an effective means to realize high-resolution and wide-swath imaging. However, for spaceborne multi-channel SAR imaging of maritime moving targets, the target motion will cause undesired channel imbalance, i.e., phase error, and further introduce the spurious targets in the image. To solve this problem, this paper proposes a novel spaceborne multi-channel SAR imaging algorithm for maritime moving targets, which consists of sequential coarse imaging and accurate imaging. The key strategies are to separate different moving targets by coarse imaging and to estimate the phase error based on the relationship between phase errors and amplitudes of spurious targets. First, the quantitative relationship between phase errors and amplitudes of spurious targets is established. Second, based on coarse imaging results, different maritime targets are effectively separated. Then, based on the measured amplitudes of spurious targets, a cost function, which represents the difference between the real target velocity and estimated target velocity, is constructed and minimized to separately estimate the velocities and phase errors of targets. Moreover, to further improve the accuracy of estimation and to suppress the undesired effects caused by target defocusing, clutter, and noise, an iterative strategy is adopted. Last, by auto-focusing, a well-focused SAR image is obtained. The GF-3 dual-channel real data experiment is conducted. The results indicate that the spurious targets are well suppressed, which validates the effectiveness of the proposed algorithm.

High-squint imaging is an effective means to enhance the flexibility and coverage ability of spaceborne synthetic aperture radar (SAR). Although existing imaging algorithms based on linear range cell migration correction (LRCMC) and nonlinear chirp scaling (NCS) can reduce the range-azimuth coupling of the spectrum and the spatial-variant of the Doppler parameter to some extent, they become invalid as the resolution increases. On one hand, the beam rotation of sliding spotlight SAR results in nonlinear azimuth-variant of the Doppler centre, and the traditional deramping operation, which removes the linear variation, will cause spectrum aliasing. On the other hand, these algorithms assume the azimuth-variants of range cell migration (RCM) are consistent in the total swath. However, the azimuth-variants of RCM are different in different range cells, which cannot be neglected in high-resolution imaging. To solve these problems, a novel imaging algorithm based on two-dimensional spatial-variant range cell migration correction is proposed in this paper. First, LRCMC is utilized, and the nonlinear azimuth deramping operation is conducted to obtain aliasing-free spectrum. Then, the azimuth-variant of RCM is corrected by azimuth interpolation and polynomial compensation. Noting that the interpolation coefficient varies linearly with slant range, this can weaken the azimuth-variant differences of RCM in different range cells. Meanwhile, azimuth polynomial compensation can correct the consistent azimuth-variant of RCM, and hence the azimuth-variant of RCM can be totally corrected. Finally, the compression is performed via the range chirp scaling and azimuth NCS. The effectiveness of the proposed algorithm is verified by computer simulations.

Yaw steering is an important technique for bistatic spaceborne synthetic aperture radar (SAR), traditionally implemented separately at the master and slave satellites, such as the X-band TerraSAR-X/TanDEM-X system. This separate yaw steering method, however, will degrade the illumination synchronization between the master and slave satellites. With the increment in frequency band and master–slave distance, the degradation will get worse, leading to intolerable azimuth resolution degradation or even failure of imaging. To solve this problem, a new joint yaw steering (JYS) method is proposed for bistatic spaceborne SAR with an arbitrary configuration in this letter. The word “joint” means that the master and slave satellitescooperativelywork to always make their beams point to an identical point on the Earth’s surface. In this way, the best illumination synchronization can be achieved, contributing to the best available azimuth resolution. The presented approach has been evaluated through computer simulations.

Dual-channel synthetic aperture radar (SAR) system is promising in high-resolution wide-swath (HRWS) imaging. However, gain and phase unbalance in the dual-channel SAR system will severely degrade the performance. To make clear the channel error effect on the image of the reconstruction algorithm, a precise analysis of the channel error effect is proposed in this letter. Based on the channel error effect analysis, the amplitude and position of the ambiguity caused by the channel error can be calculated, which is significant for understanding the image quality degradation caused by channel unbalance. Finally, based on the Chinese first dual-channel spaceborne SAR system-GF-3, the validity of this letter is verified by the simulated and real data, respectively.