Inverse Synthetic Aperture 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.

Doppler shift compensation algorithm based on inverse synthetic aperture ladar in binary phase-coded signal

The binary phase-coded signal is used in inverse synthetic aperture ladar (ISAL) as a commonly used pulse compression signal. However, due to its sensitivity to doppler shift, it will lead to poor or even uncompressible pulse compression results. Combined with the principle of inverse synthetic aperture ladar, the doppler shift introduced by target motion will have a very significant impact, further degrading the imaging results. By using the digital moving target detection (DMTD) method, the doppler information of each distance unit is extracted by performing Fourier transform on the same distance unit, and then a compensation factor is constructed in the frequency domain to compensate for this doppler, correcting the adaptation problem brought by it and improving the imaging result.

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.

Low sampling rate digital dechirp for Inverse Synthetic Aperture Ladar imaging processing

The application of digital dechirp technology to Inverse Synthetic Aperture Ladar (ISAL) can greatly reduce the sampling rate burden on analog-to-digital converters. We address the disadvantage of signal sampling and imaging processing in linear frequency-modulated continuous waveform (LFMCW) ISAL systems and suggest a delay estimation method based on a reference channel to reduce the impact of echo delay error. Our method uses the strongest point in the scene as the scene center to estimate the delay and target distance via dechirped pulse compression. The reference channel data are also used to align the dechirped pulse signal and, in turn, reduce clock errors and ensure azimuth signal quality. To our knowledge, our experiment represents the first successful application of digital dechirp to ISAL systems. Experimental results confirm the applicability of our digital dechirp method to ISAL system in order to reduce sampling requirements.

Vibration phases estimation based on orthogonal interferometry of inner view field for ISAL imaging and detection.

Inverse synthetic aperture ladar (ISAL) has the capability to achieve high-resolution imaging of long-distance targets in a short time because of the laser's short wavelength. However, the unexpected phases introduced by target vibration in the echo can cause defocused imaging results of the ISAL. How to estimate the vibration phases has always been one of the difficulties in ISAL imaging. In this paper, in view of the echo's low signal-to-noise ratio, the orthogonal interferometry method based on time-frequency analysis is proposed to estimate and compensate the vibration phases of ISAL. The method can effectively suppress the influence of noise on the interferometric phases and accurately estimate vibration phases using multichannel interferometry in the inner view field. The effectiveness of the proposed method is validated through simulations and experiments, including a 1200m distance cooperative vehicle experiment and a 250m distance noncooperative unmanned aerial vehicle experiment.

Design of LEO target imaging mode with space-based Inverse Synthetic Aperture LADAR

Design of LEO target imaging mode with space-based Inverse Synthetic Aperture LADAR

Laser Synthetic Aperture Coherent Imaging for Micro-Rotating Objects Based on Array Detectors

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.

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.

Noise-Robust Vibration Phase Compensation for Satellite ISAL Imaging by Frequency Descent Minimum Entropy Optimization

Inverse synthetic aperture ladar (ISAL) can perform high-resolution imaging for satellites. However, due to the short wavelength of the laser, satellite micro-vibration will introduce space-variant vibration phase error (SVVPE) and space-invariant vibration phase error (SIVVPE) in the echoes, which seriously blur the ISAL image. In this paper, we propose a noise-robust vibration phase compensation algorithm to accurately estimate and correct these two types of vibration phase errors by frequency descent minimum entropy optimization. Firstly, considering the characteristics of the micro-vibration of satellites, we establish a novel phase error model based on the Fourier series theory, which only contains low-frequency vibration components. The estimation of the phase errors is then translated into the estimation of the model’s Fourier coefficients, which can be achieved by a multi-dimensional minimum entropy optimization. After that, a frequency descent method (FD) is proposed to transform the multi-dimensional optimization into a group of two-dimensional optimizations so that the proposed algorithm can achieve monotonic iterative convergence. In addition, we introduce a solution space adaptive reduction operation to reduce the computational burden when solving the two-dimensional minimum entropy optimizations by the genetic algorithm (GA) to obtain the global optimal solution. Finally, experiments based on the simulated data and the real measured data confirm the effectiveness of the proposed algorithm. Compared with the traditional methods, the proposed algorithm achieves higher phase error estimation accuracy and better image quality.

Amplitude compensation using homodyne detection for inverse synthetic aperture LADAR.

Inverse synthetic aperture LADAR (ISAL), based on an electro-optic in-phase and quadrature (I/Q) modulator, and homodyne detection, has high pulse repetition frequency, simple structure, and minimum intra-pulse phase errors. Homodyne detection can implement the de-chirp operation in the optical domain, while the output amplitude is susceptible to light intensity. However, the modulated optical signal amplitude fading, induced by radio frequency devices and the I/Q modulator, is quite tricky to measure and be compensated for. In this work, the effect of amplitude distortion on the ISAL system is analyzed. A simple and effective compensation method (based on homodyne detection) is proposed, followed by some experiments. The improvement of homodyne signal power to the compensated signal compared with non-compensated signal was about 1 dB (26%), and the image quality of our ISAL system along range direction was also improved by using the proposed method. Moreover, the image of an extended target was acquired via our ISAL system with 500 mW transmitting power at a distance of 1.1 km.

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

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

Feature extraction algorithm of precession target based on image length and Doppler broadening.

In space defense, utilizing the micromotion features to distinguish real targets from interfering targets and decoys is effective. Due to the imaging of the high-speed precession target by microwave radar consisting of isolated scattering centers, there are many difficulties in using inverse synthetic aperture radar (ISAR) images for feature extraction. On the other hand, the inverse synthetic aperture ladar (ISAL) image is relatively continuous because of the short wavelength of laser, and the image sequence contains information about the variation in image length and Doppler width caused by target precession, which can be used for inverse motion parameters. By establishing an observation model of the precession target and performing image processing on the obtained ISAL image at different times, the image length sequence and Doppler width sequence can be obtained. Using the ellipse fitting method to process the obtained sequence, the precession parameters of the target can be obtained. The algorithm does not require prior information such as the radius and speed of the target motion, effectively improving the practicability of the algorithm. Finally, the effectiveness of the algorithm is verified by experimental results, and the error is controlled within 2%.

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.

Phase Gradient Matrix Autofocus for ISAL Space-Time-Varied Phase Error Correction

A new autofocus method of the space-time-varied phase error (PE) correction is presented for inverse synthetic aperture ladar (ISAL). The PE is corrected by a two-dimension matrix that is estimated in the form of phase gradient both in frequency and time domains. The autofocus method we named as PGMA is capable of compensating non-parametrical motion errors varying from pulse to pulse and is robust for widespread scene contents where any isolated strong scatterer is not necessary. The effectiveness is verified by comparing the imaging results of extended targets between PGMA and the conventional phase gradient autofocus (PGA) both in simulations and experiments. When the motion of targets contains both translational and angular motion errors, PGMA produces well-focused images while PGA merely corrects translational motion error, leaving the formatted images smear by angular motion error, especially to the edges.

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.

Inverse synthetic aperture ladar imaging algorithm for space spinning targets

Due to the limitation of laser modulation technology, the azimuth Doppler ambiguity problem exists in the process of inverse synthetic aperture ladar (ISAL) imaging for spinning targets. The traditional azimuth imaging method will not be used to obtain a good two-dimensional image. Therefore, we consider using the target's spinning information for imaging. The spatial geometric model of the spinning target ISAL imaging is established, and the characteristics of the echo signal are analysed. An ISAL imaging algorithm based on the backward projection transform is proposed. First, the spinning angular velocity of the target is obtained by the generalised autocorrelation method, and then the envelope and phase of the distance and the slow time domain are transformed into a backward projection to achieve coherent accumulation, and the two-dimensional high-resolution image of the spinning target is finally obtained. Due to the use of echo phase information, the sidelobe effect is low and the resolution is high. The simulation results show that the algorithm can still get well-focused images under low SNR and Doppler ambiguity.

Inverse synthetic aperture ladar autofocus imaging algorithm for micro-vibrating satellites based on two prominent points.

As an important imaging method for long-range satellite targets, inverse synthetic aperture ladar (ISAL) has the characteristics of high-resolution imaging and competitive detectability. Since the working wavelength of the ISAL is comparable to the micro-vibrations generated by mechanical moving components of satellites, which will cause image defocusing, motion compensation is of great significance. In this paper, an autofocus algorithm is proposed for estimating and compensating the phase error relating to both translational and rotational micro-vibrations. Comparing with non-parametric algorithms like phase gradient autofocus and parametric algorithms like contrast-based autofocus and entropy-based autofocus, the proposed one, which is based on two prominent points, is especially effective for the rotational phase error oscillating numbers of cycles. Simulations and experiments are conducted to validate the feasibility of the proposed algorithm.

Image processing for GEO object with 3D rotation based on ground-based InISAL with orthogonal baselines.

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.

Novel Scaling Approach for ISAL via the Fractional Fourier Transform

Inverse Synthetic Aperture LADAR (ISAL) can obtain high resolution images of remote targets. In order to measure the dimension of the target, the ISAL image must be scaled. Many existing scaling methods require isolated strong scattering centers which are not easily found in ISAL images. To solve this problem, a novel ISAL scaling method is proposed. The method estimates the Doppler frequency modulation rate of each range row by the fractional Fourier transform (FrFT) and image entropy. The linear relationship between the frequency modulation rate and the range is used to obtain an estimate of the target equivalent rotational angular velocity which is the key parameter for scaling. The outfield experimental results confirm the effectiveness of the 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.