Synthetic Aperture Imaging Ladar Research Articles

2-D inverse synthetic aperture Ladar imaging and tracking of maneuvering extended Micro-Doppler UAVs in atmospheric turbulence with skewness

The inverse synthetic aperture Ladar (ISAL) is an important system for observation of space objects. Here, a 2-D ISAL imaging and tracking algorithm is proposed for low observable maneuvering extended objects tracking (M−EOT) such as small space UAVs. When imaging maneuvering objects with ISAL, dispersion and Doppler frequency time-variation exist in the range and cross-range echo signal, respectively. Additionally, the ISAL system can achieve high-resolution images for long-range moving objects, while its performance is affected by atmospheric turbulence. Atmospheric turbulence is a very complicated random state formed by the instability of regular laminar flow at a high maneuvering velocity, which is caused by the random motion of the atmosphere and lead to the random fluctuation of the atmospheric refractive index. Therefore, in real-time M−EOT scenario, the measurements distribution over the whole object is not symmetrical but distributed and skewed in some portions while a target maneuvers due to the unbalanced of reflection that caused by the atmospheric turbulence. In practice, the micro Doppler effect caused by micro moving components such as drone rotors will seriously affect the focused ISAL imaging of the M−EOT’s rigid body. To solve these problems in real-time low observable M−EOTs imaging and tracking from 2-D ISAL system, a novel robustness measurement model and recursion multi-Bernoulli track-before-detect filter based on a multiple model skewed non-symmetrically normal distribution is represented with more than one ellipse and lower complexity. In this paper, the Radon-Laplace method is presented to reduce this micro-Doppler effect in ISAL imaging. Additionally, we will eliminate the effect of atmospheric turbulence in ISAL Imaging by using the network-based variance–covariance estimation (NVCE) weighting method. In particular, we describe the M−EOT’s state through the random matrices model (RMM). In RMM, the extent is applied by one ellipse, which often lose information about size, heading, and shape, when the scatter centers are distributed symmetrically around the M−EOT’s centroid. In this paper, a new observation model used to modify a sub-RMM by considering the characteristics of 2-D ISAL data is proposed. Simulation results demonstrates the proposed robustness algorithm with the influence of the atmospheric turbulence and micro-Doppler.

Synthetic Aperture Ladar Motion Compensation Method Based on Symmetric Triangle Linear Frequency Modulation Continuous Wave Segmented Interference

Synthetic Aperture Ladar (SAL) is a sensor that combines laser detection technology with synthetic aperture technology to achieve ultra-high-resolution imaging. Due to its extremely short wavelength, SAL is more sensitive to motion errors. The micrometer-level motion will affect the target’s azimuth focus. This article proposes an SAL motion compensation method based on Symmetric Triangular Linear Frequency Modulation Continuous Wave (STLFMCW) segmented interference, utilizing the characteristics of a triangular wave, to solve the problem of target azimuth defocusing. This article first establishes an STLFMCW echo signal model based on the SAL system under the influence of motion errors. Secondly, the radial velocity gradient along the azimuth direction is extracted using the triangular-wave-positive and -negative frequency modulation signals segmented interference method. Then, for the initial phase wrapping problem, the frequency spectral cross-correlation method is used to accurately estimate the initial radial velocity error. The radial velocity gradient is integrated along the azimuth to obtain the platform motion trajectory. Finally, the compensation functions are constructed to complete the echo Range Cell Migration (RCM) correction and residual phase compensation, resulting in a focused SAL image. This article verifies the practical effect of this method in eliminating motion errors using only one-period STLFMCW signal through simulation and real experiments. The quantitative results show that compared with the traditional method, the proposed method reduces the azimuth Peak Sidelobe Ratio (PSLR) by 8dB and the Integrated Sidelobe Ratio (ISLR) by 9 dB. This method has significant improvements and is of great significance for high-resolution FMCW SAL imaging.

SAL vibration estimation and compensation based on triangular interferometric signals.

Synthetic aperture Ladar (SAL) is an extension of synthetic aperture technology in the optical frequency band. Owing to the short wavelength of lasers, the system has high-resolution, high-data-rate, and refined imaging capabilities, which has potential in high-resolution observation fields such as ground observation and space target observation. However, the short wavelength of lasers also makes SAL severely sensitive to vibrations even on the micron order which cause azimuth defocusing and range cell migration. To address this problem, we establish a de-chirp signal model under vibration environment, and propose a vibration error estimation and compensation method using triangular interferometric signals. According to the symmetrical characteristics of triangular frequency modulated continuous wave (T-FMCW) and the time-frequency information introduced by the azimuthal vibration phase, we use a two-stage interferometry method to estimate instantaneous frequency introduced by the vibration errors that cause range cell migration. For the scenarios without obvious range cell migration, we use a one-stage interferometry method to estimate the instantaneous frequency. Subsequently, we establish a vibration compensation filter using the estimated instantaneous frequency to compensate for the vibration errors. We use two experiments to verify the effectiveness and superiority of the proposed method. The results show that the proposed method effectively eliminates range cell migration and azimuthal phase errors introduced by vibration errors, producing SAL imaging results with higher resolution than the conventional spectral correlation method.

Effects of Atmospheric Coherent Time on Inverse Synthetic Aperture Ladar Imaging through Atmospheric Turbulence

Inverse synthetic aperture ladar (ISAL) can achieve high-resolution images for long-range moving targets, while its performance is affected by atmospheric turbulence. In this paper, the dynamic evolution of atmospheric turbulence is studied by using an infinitely long phase screen (ILPS), and the atmospheric coherent time is defined to describe the variation speed of the phase fluctuation induced by atmospheric turbulence. The simulation results show that the temporal decoherence of the echo induced by turbulence causes phase fluctuation and introduces an extra random phase, which deteriorates the phase stability and makes coherent synthesis impossible. Thus, we evaluated its effects on ISAL imaging and found a method to mitigate the impact of turbulence on ISAL images. The phase compensation algorithm could correct the phase variation in different pulses instead of that within the same pulse. Therefore, the relationship between the atmospheric coherent time and pulse duration time (rather than that between the atmospheric coherent time and ISAL imaging time) ultimately determines the ISAL imaging quality. Furthermore, these adverse effects could be mitigated by increasing the atmospheric coherent time or decreasing the pulse duration time, which results in an improvement in the ISAL imaging quality.

Moving target imaging of a dual-channel ISAL with binary phase shift keying signals and large squint angles.

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.

Inverse synthetic aperture ladar imaging based on modified cubic phase function.

Inverse synthetic aperture imaging ladar (ISAL) can achieve high-resolution images, and yet it faces pulse-to-pulse high-order phase errors that the microwave radar can ignore. The high-order phase errors are almost caused by mechanical vibrations in general, which blur the azimuth focusing effect. This paper presents an ISAL imaging model to obtain high-resolution images. A novel modified cubic phase function (CPF) algorithm is proposed to compensate the additional high-order phase errors. Some high-resolution well-focused ISAL simulation images and real target images are shown to validate the methods. It is shown that the third-order phase errors are compensated by the distinctive digital signal process and the image entropy of real target images is reduced significantly.

Multi-Channel Inverse Synthetic Aperture Ladar Imaging Detection Technology and Experimental Research

Multi-Channel Inverse Synthetic Aperture Ladar Imaging Detection Technology and Experimental Research

Sub-aperture phase error stitching for full aperture airborne SAL data processing method based on azimuth deramp

Synthetic Aperture Ladar (SAL) is a combination of synthetic aperture and lidar. At present, sub-aperture stitching with PGA algorithm is used to realize SAL imaging. However, this algorithm has a low rate of data utilization, resulting in a worse image SNR and resolution. For solving this problem, a method of sub-aperture phase error stitching for full aperture airborne SAL data processing based on azimuth deramp is proposed in this paper. The data is divided into a sequence of sub-apertures and the standard Standard Minimum Entropy Autofocus (MEA) is used to estimate the phase error of each sub-aperture. Then all the phase errors of the sub-apertures are stitched to obtain the full-aperture phase error. After the compensation of the full-aperture phase error, the full-aperture image is obtained by deramp. Compared with the SAL image obtained by sub-aperture algorithm, the method we proposed can offer better resolution and SNR. Finally, the imaging results of airborne SAL real data verify the effectiveness of the proposed algorithm.

Neural network based synthetic aperture ladar imaging through marine atmosphere

The atmospheric turbulence can severely affect the azimuth imaging of the Synthetic Aperture Ladar (SAL). Based on a neural network, this study generates atmospheric phase screens complied with marine atmosphere spectrum. The SAL imaging results with different turbulence intensities and laser wavelengths are calculated, and the changing law of the ratio of real aperture length to atmospheric coherence diameter with turbulence intensity under different slant ranges and laser wavelengths are analyzed in this paper. The results suggest that the SAL imaging becomes worse as the turbulence intensity is stronger, and even the target is not able to be distinguished. A longer laser wavelength can be used to alleviate the defocus effect, and the phase error compensation algorithm based on marine phase screens can effectively improve the SAL imaging quality to guarantee the real–time process of echo signals of SAL in the marine atmosphere.

Synthetic aperture ladar motion compensation method based on symmetrical triangular linear frequency modulation continuous wave

Synthetic Aperture Ladar (SAL) has the ability to achieve fast high-resolution imaging due to the characteristics of ultra-high resolution and ultra-short synthetic aperture time. However, because synthetic aperture ladar operates in the near-infrared band and the operating wavelength is extremely short, small variations in flightpath can produce artifacts in the phase history that corrupt SAL image generation. This work proposes a motion compensation method based on symmetrical triangular linear frequency modulation continuous wave (T-FMCW) combining with Phase Gradient Autofocus (PGA). Firstly, the ambiguity function of T-FMCW is deduced and its characteristics are analyzed. Then the principle of velocity measurement and Cramer–Rao lower bound of speed estimation error based on this waveform are given. T-FMCW is used to estimate the radial velocity error of the platform in SAL system, and then the radial motion error is estimated by integration along azimuth to realize coarsely focused SAL image. For the coarsely focused SAL image, the residual motion error is extracted to improve the image quality by using PGA. The effectiveness and accuracy of the method is verified by simulation experiments.

Extended Target Three-Dimensional Reconstruction From Inverse Synthetic Aperture Ladar Image Sequence

Inverse Synthetic Aperture Ladar (ISAL) can continuously capture images of the target in complex motion, which provides an opportunity for the 3-D reconstruction. However, the existing 3-D imaging methods need to meet some specific conditions, including multiple isolated prominent scattering points in the image, known or parametric motion of the target, and 360° observation of the target. To relax these restrictions, a novel 3-D reconstruction method is proposed. In this method, the corner of the target is first detected and used as a reference point. Then, the positioning accuracy of these fiducial points is greatly improved by the relationship between the radial range and velocity. Finally, the 3-D reconstruction is realized by the singular value decomposition of the measurement matrix. The data obtained from the outdoor experiment are used to test the proposed method. By this process, a series of images of a satellite model which is 2 km away are acquired. The size and shape errors of the reconstruction are less than 10%. The result demonstrates the effectiveness of the 3-D reconstruction method.

Estimation and correction of vibration-induced range cell migration for FMCW synthetic aperture ladar.

Synthetic aperture ladar (SAL) is a newly developed imaging device for remote sensing application. Owing to its short wavelength (3-5 orders of magnitude shorter than radar), SAL is very sensitive to platform vibration. For frequency-modulated continuous-wave SAL (FMCW-SAL), the platform vibration induces an additional range cell migration (RCM) to the SAL image. The vibration-induced RCM (VI-RCM) deteriorates the image quality. The VI-RCM is a unique problem for the FMCW-SAL imaging. To address this problem, a raw-data-driven method is proposed to correct the VI-RCM in this paper. First, the signal model was developed to show the VI-RCM in FMCW-SAL echo. Then, based on the model, the differential phase function (DPF) is constructed for the adjacent range profiles. The DPF is a single-frequency signal with its frequency being proportional to the relative range shift between the adjacent range profiles. Based on the DPF, the relative range shift is estimated. After the estimation of all the relative range shifts, the VI-RCM is calculated and corrected. Experiments are performed. The simulated experiment demonstrated the feasibility, accuracy, and efficiency of the proposed method, and the real data processing result verified the effectiveness of the proposed method for FMCW-SAL in practical applications.

An airborne inverse synthetic aperture ladar imaging method based on sparse sampling

The inverse synthetic aperture ladar combines coherent laser technology with synthetic aperture imaging to achieve centimeter-level resolution over a distance of thousands of kilometers. Due to the short wavelength, the required pulse repetition frequency of ladar is very high. Furthermore, because of the uncertain trajectory of the non-cooperative target, the problem of sparse sampling always exists in azimuthal direction. This paper applies inverse synthetic aperture ladar technology into airborne platform, and studies the reconstruction of ladar image under the condition of azimuthal sparse sampling. A frequency spectrum unwrapping method is proposed based on linear observation matrix and Compressed Sensing theory, to restore the image from sparsely sampled echo. Simulation results demonstrate the effectiveness of proposed method.

3-D Inverse Synthetic Aperture Ladar Imaging and Scaling of Space Debris Based on the Fractional Fourier Transform

The inverse synthetic aperture ladar (ISAL) is an important method for observation and imaging of space targets. Here, a 3-D ISAL imaging algorithm is proposed for spinning targets such as space debris. Since laser wavelength is 4–5 orders of magnitude smaller than that of microwave, the Doppler frequency caused by target motion is more pronounced in ISAL. Doppler frequency modulation rates can be estimated by the fractional Fourier transform with respect to azimuth slow time even when the rotation angle is small such that scattering centers do not migrate through a range cell. Then, slant range, Doppler frequency, and Doppler frequency modulation rates form a 3-D space. The angular velocity and the incident angle can be estimated by the position relationship between the scattering centers in two observations. After image scaling, the 3-D shape and size of the target can be obtained. The 3-D structure of the target in the simulation experiment is accurately reconstructed. Monte Carlo experiments are conducted to discuss the effect of the signal-to-noise ratio and observation time on the algorithm. Finally, the effectiveness and robustness of the algorithm are verified.

大随机相位误差下条带模式合成孔径激光雷达成像实验

大随机相位误差下条带模式合成孔径激光雷达成像实验

Large-aperture synthetic aperture imaging ladar

A strip-mode side-looking large-aperture synthetic aperture imaging ladar (SAIL) based on free-space optics is designed to experimentally verify the key performances for further airborne application. The optical difficulties in the spatial domain, including optical antenna and imaging resolution for large-aperture SAIL, are considered systematically. The optical antenna consists of a circulated duplex including the tunable transmission and reception channels for the flexible wavefront control. An irradiating beam with a spatial quadratic phase-bias for the suitable and controlled phase history can be transmitted from the transmission channel, and the wavefront aberration in the reception channel resulted from the diffraction can be eliminated for the optimized heterodyne detection. The construction and 2-D imaging algorithm of the SAIL are also proposed. The performances, including wavefront, heterodyne efficiency, field of view, imaging resolution, and 2-D imaging algorithm, have been systemically verified in the short-distance of laboratory space. The well-focused 2-D SAIL imaging of a diffusely scattering target with submillimeter imaging resolution is given. The experimental results are in good agreement with the theoretical design. The results have the great significance for the future development of airborne large-aperture SAIL.

A novel sidelobe-suppression algorithm for airborne synthetic aperture imaging ladar

A novel sidelobe-suppression algorithm is proposed according to the problem of sidelobe in the real data of airborne Synthetic Aperture Ladar (SAL) imaging. Compression Sensing (CS) theory is proposed to recover a sparse signal as well as to suppress the sidelobe of the signal. However, the SAL images are not sparse in a general way. To solve such a problem, a novel sidelobe-suppression algorithm based on modified Spatial Variant Apodization (SVA) technique connected with CS theory is proposed. Firstly, the sparsification of SAL images can be achieved by using the modified SVA algorithm, which deals with not only the linear but also the non-linear sampling situations. And then, the sidelobe-suppression can be achieved by recovery processing using the CS algorithm. Finally, the simulation shows that the sidelobes of the signal can be suppressed as low as −34 dB without losing resolution. Meanwhile, the real data result proves the validity of the proposed algorithm.

Wide-scene airborne ladar imaging exploiting the synthetic aperture technique with terrain observation by progressive scans

The image scene is limited by the small size of the spot, which is linked with the short wavelength of the laser in synthetic aperture ladar (SAL). The common wide-scene imaging mode in synthetic aperture radar, i.e., the terrain observation by progressive scans imaging mode, is extended and applied in SAL. A suitable imaging algorithm is also proposed according to the characteristics of the signal. First, the azimuth prefiltering based on the spectrum analysis is adopted to eliminate the azimuth spectrum aliasing caused by the steering of the antenna beam. Then, the improved frequency scaling algorithm is applied to complete the range compression and range cell migration correction. Meanwhile, the Doppler frequency shift induced by the continuous moving of the platform is compensated. Finally, the azimuth compression is completed in the Doppler domain, and the SAL image without ambiguity is obtained with the application of the dechirp technique. Simulation results provide the evidence on the effectiveness of the proposed method.

本振增强直视合成孔径激光成像雷达的二维成像实验

本振增强直视合成孔径激光成像雷达的二维成像实验

多通道宽幅度合成孔径激光成像雷达收发装置优化研究

多通道宽幅度合成孔径激光成像雷达收发装置优化研究