Target vibration introduces unexpected vibration phases in the echo signal of inverse synthetic aperture LADAR (ISAL). The vibration phases cause the imaging results to be defocused. How to estimate the vibration phases has always been one of the difficulties in ISAL imaging, and there has not been an effective solution. In this paper, two methods are proposed to estimate the vibration phases in different situations. The method based on multi-channel along-track interferometry (MCATI method) is for some cases that the target velocity vector is parallel to some baselines of ISAL. The method based on orthogonal interferometry (OI method) is for other cases that the target velocity vector is not parallel to any baseline of ISAL. In addition, a high signal to noise ratio (SNR) is essential for the MCATI and OI methods to get high estimation accuracy. Therefore, a large power-aperture product should be carefully considered in the ISAL system design. Furthermore, on the condition of low SNR, range cell accumulation is also helpful to reduce the influence of noise. Take the ISAL, which is utilized to image the satellite in 500 km orbit, as an example, the estimation accuracy can satisfy the demand of synthetic aperture imaging with 0 dB single-pulse SNR and 300 range cell accumulation. The proposed methods are verified with simulation and experiment.

While imaging geosynchronously orbiting objects with ground-based inverse synthetic aperture LADAR (ISAL), target rotation and vibration will introduce phase errors into the echo and cause the two-dimensional imaging result to be defocused. To solve this problem, one layout of the ISAL receiving channels is designed, combining the orthogonal short baselines in the inner field with the orthogonal long baselines in the external field. On this basis, an imaging method based on orthogonal interferometry processing is proposed. The proposed method estimates and compensates for the phase errors introduced by the rotation and vibration of the target such that the focus of the two-dimensional image is improved, and three-dimensional imaging results with high accuracy are obtained. The effectiveness of the proposed method is verified by simulation data.

A dual-channel inverse synthetic aperture ladar imaging experimental system based on wide-pulse binary phase coded signals and its moving target imaging are introduced. The analysis, simulation, and experimental data processing results of binary phase coded signal Doppler compensation and pulse compression are included. The method of motion phase error estimation based on interferometric processing and the imaging method with small computation in the case of large squint angles are proposed, and the simulation results are presented. The effectiveness of the imaging method is verified by experimental data processing. Doppler frequency curves are estimated based on time-frequency analysis of echo signals, and the coarse compensation of motion phase error is realized. According to the interferometric phase and coherence coefficient of dual-channel echo signals’ time-frequency analysis, the coherence of the dual-channel echo signals is checked, and along-track interferometry can be applied to the precise compensation. The stable interferometric phase and increased coherence coefficient of actual dual-channel data imaging results indicate the effectiveness of the motion phase error compensation method proposed. Considering characteristics of inverse synthetic aperture ladar (ISAL) imaging, after dividing echo signals into multiple sub-apertures, range-Doppler algorithm and sub-aperture stitching are adopted, the stitched image is corrected geometrically through Stolt transformation, and the computation is reduced.

Through the relative motion of ladar and object, inverse synthetic aperture ladar can form the virtual synthetic aperture and improve the imaging resolution. On the basis of coherent array detectors with the laser local oscillator, the problem of laser synthetic aperture coherent imaging for micro-rotating objects is studied. Firstly, structure and parameters of the telescope system based on the diffractive optical system are introduced. Then, the signal model of the imaging method is built, the imaging method is proposed, and influence of noise is analyzed. Afterwards, the signal model of the vibrating object is established and the vibration phase compensation method is proposed. The imaging method proposed forms a virtual aperture through micro-rotation of the object, and realizes high-resolution imaging in the pitch direction and azimuth direction. Without transmitting or processing broadband signals, the imaging system is easy to realize in engineering, and simulation results verify the effectiveness of the imaging method. Meanwhile, Fourier ptychography in the far-field condition is analyzed, and characteristics of the two methods are compared. It is concluded that the imaging method proposed belongs to the concept of traditional matched filter coherent imaging while far-field Fourier ptychography is classified into the category of super-resolution imaging.

To improve the target detection performance of inverse synthetic aperture ladar (ISAL) under the situation that there is Doppler bandwidth ambiguity, we consider energy accumulation of echo signals along a range through matched illumination. However, since the target impulse responses change with the target aspect angle, the phase shifts along slow-time between range cells are different, and the output of matched illumination fluctuates along slow-time. To reduce the slow-time fluctuation of matched illumination under additive white Gaussian noise assumption, we have proposed an interferometric ISAL (InISAL) target detection method based on interferometry processing and principal component analysis (PCA) in this paper. After the interferometry processing, the interferometric phase shifts along slow-time of all range cells are approximately the same. Then PCA is introduced to reduce the impact of noise, interference, and cross-terms of interferometry processing. Thus, the slow-time fluctuation of matched illumination can be decreased, and the target detection performance is improved. The proposed method has been validated with both simulated data of InISAL and real data of millimeter-wave interferometric synthetic aperture radar.

According to the self-heterodyne signal obtained by lidar under different fiber delay times, the model of the local oscillator signal was established, and the maintenance method of signal coherence in lidar based on the digital delay was improved by using multiple sinusoidal frequency modulation components. An imaging detection experiment was carried out at a distance of 5.4 km. The coherence of the lidar signal was maintained by combining the transmitting reference channel correction method and the local oscillator reference channel compensation method, accompanied by the use of a phase spectrum to analyze the improvement effect. The processing results of the echo signal showed that the method could remove the high-order phase errors that cannot be compensated by the phase gradient autofocus algorithm and improve the signal coherence, which could be used for the detection and imaging of long-range targets. (C) 2022 Optica Publishing Group.