High Azimuth Resolution Research Articles

Multiple-input multiple-output synthetic aperture ladar system for wide-range swath with high azimuth resolution.

For the trade-off between the high azimuth resolution and the wide-range swath in the single-input single-output synthetic aperture ladar (SAL) system, the range swath of the SAL system is restricted to a narrow range, this paper proposes a multiple-input multiple-output (MIMO) synthetic aperture ladar system. The MIMO system adopts a low pulse repetition frequency (PRF) to avoid a range ambiguity for the wide-range swath and in azimuth adopts the multi-channel method to achieve azimuth high resolution from the unambiguous azimuth wide-spectrum signal, processed through adaptive digital beam-forming technology. Simulations and analytical results are presented.

COMPREHENSIVE ANALYSIS FOR SHIFTING MIMO SAR SYSTEM

Synthetic aperture radar (SAR) system has inherent constraints between high azimuth resolution and wide swath width. Achieving more phase center samples is one of the key solutions to resolve this limitation. By multiple N transmitting and N receiving channel concept, an increased resolution or a widened swath width could be obtained. In this paper, comprehensive analysis for shifting multiple-input multiple-output (MIMO) SAR system is presented. System resolution enhancement has been demonstrated based on the distributed target simulation by a factor of N compared to conventional displaced phase center antenna (DPCA) system.

High-resolution adaptive beamforming for borehole acoustic reflection imaging

Acoustic reflection imaging in boreholes is believed to be a vast improvement in the evaluation of heterogeneous reservoirs. However, only 2D azimuth information of reflectors can be achieved by conventional methods. To obtain 3D information, a new high-resolution adaptive beamforming method has been developed to apply to the borehole environment, using an array acoustic logging tool with azimuthally distributed receivers. Moreover, a migration method based on the Fourier finite difference method is introduced into the beamforming to effectively reduce computational complexity from 3D to 2D and to reduce the effects of noise caused by velocity changes in the vertical and horizontal planes. Resolution analysis of the imaging results indicates that the tool diameter and source frequency are the key factors in increasing the azimuth resolution. Furthermore, increasing the resolution with the new method can help improve the fidelity of reconstructed objects in the reflection image. A comparison of results obtained by the new method and the conventional method with simulation data demonstrates that the new method can achieve a reflection image of high azimuth resolution with extremely noisy signals. Application to field data also found a good agreement between the reflection image and the borehole resistivity image. Therefore, detailed information regarding the azimuth and the radial distance of reflectors to the borehole can be determined in 3D space.

A novel SAR interferometry processing method in high resolution spotlight SAR

Synthetic Aperture Radar Interferometry (InSAR) can produce accurate Digital Elevation Model (DEM), and a novel spotlight SAR can get higher azimuth resolution than conventional strip-map SAR. Hence, the combination of InSAR and spotlight SAR techniques implies the potential to obtain higher resolution DEMs which will show more tiny terrain variations. However, in spotlight InSAR processing, additional azimuth interferometric fringes and misregistration decorrelation will be induced by conventional co-registration methods which ignore azimuth varied Doppler centroids in interpolations. Meanwhile, because interferometric fringes in regions with high resolution terrain variations can easily be damaged by random phase noise, there will generate more unreliable unwrapped phase areas with conventional phase unwrapping methods. To eliminate residual azimuth fringes, misregistration decorrelation and unreliable unwrapped phase areas, this paper firstly proposes a novel spotlight SAR interferometry processing method based on co-registration related to two-dimensional Doppler centroid estimation and joint phase unwrapping, and it is validated by using TerraSAR-X spotlight data. The final generated 4 m × 4 m grid space DEM not only preserves high resolution, but also has low root mean square error and absolute mean error which are less than 3 m with respect to the Google earth DEM.

SAR signal reconstruction from multi-channel non-uniform sampling near singular points

The technique of multiple apertures in azimuth enables a wide swath synthetic aperture radar (SAR) with a high azimuth resolution. In a traditional multi-channel SAR system, if the pulse repetition frequency (PRF) is not an optimum value it will result in a non-uniform sampling. To overcome this limitation, a reconstruction algorithm has been designed to recover the non-uniform signal. In this letter, we claim that while the system PRF is around some specific values, singular points exist in the reconstruction algorithm and therefore the reconstruction cannot get correct result. To avoid the influence of these singular points, a novel method is presented in this letter which can also improve the system signal-to-noise ratio. Furthermore, simulation results are presented to evaluate the performance of this method. At the end of this letter, the real multi-channel SAR data are processed to demonstrate the effectiveness of the proposed method experimentally.

Wideband underwater sonar imaging via compressed sensing with scaling effect compensation

By exploiting the sparsity of the imaging scene, compressed sensing (CS) imaging may provide an effective solution to cope with the tradeoff of high azimuth resolution and wide swath in synthetic aperture sonar imaging. However, existing CS imaging methods are based on narrowband signal model and the scaling effect on the echoes is normally omitted, which may seriously affect the ultimate image reconstruction performance. This paper establishes the wideband CS underwater sonar imaging model at first, where the scaling effect and Doppler frequency shift are jointly considered. Furthermore, two wideband CS imaging approaches are discussed in uniform scale-range space and velocity-range space, respectively. Besides, modified l 1-norm minimization algorithms are proposed for the scene reconstruction in velocity-range space. Finally, numerical experiments are provided to demonstrate the effectiveness of the proposed model and the reconstruction algorithms.

Feasibility of low-frequency directive sound source with high range resolution using pulse compression technique

The lateral resolution of a parametric acoustic array with narrow directivity is expected to be superior to that of a conventional linear sound source. The range resolution of the parametric array, however, is as low as the resolution of the linear source because of its long wavelength. To improve the range resolution in maintaining a high azimuth resolution for parametric sounds, a pulse compression technique using linear frequency-modulated signals or chirp signals is explored in the present study. The generation of chirp-modulated parametric sounds was experimentally confirmed in water, and then the auto-correlation of the parametric sound demonstrates short pulse widths up to 1/10 of the primary pulse duration. The results reveal that the pulse compression is feasible for a low-frequency parametric sound source with narrow directivity in the same manner as the compression of a linear sound wave.

A Novel High-Order Range Model and Imaging Approach for High-Resolution LEO SAR

High-resolution spaceborne synthetic aperture radar (SAR) mainly poses two challenges to signal processing. The first challenge involves the signal model, where a precise range equation of spaceborne SAR should be considered as the conventional hyperbolic range equation fails to precisely describe the range history in the high-resolution case. The second challenge is an efficient focusing algorithm since the existing SAR processors are inaccurate or inefficient for high-resolution spaceborne SAR. Therefore, in this paper, a novel fourth-order polynomial range equation based on Doppler parameters is proposed, and the method for parameter determination is also addressed. Compared with conventional range equations, the presented one is more accurate and concise for low-earth-orbit SAR so that a higher azimuth resolution can be achieved. Based on the range model, a 2-D spectrum is derived, and an extended range Doppler domain algorithm for SAR image formation in the sliding spotlight mode is also developed. Additionally, we carried out several simulations to validate the presented approach. Results demonstrate high performances of the focusing algorithm as well as the range equation.

Frequency-sum passive cavitation imaging

Passive cavitation imaging (PCI) is a method for spatially mapping acoustic emissions caused by microbubble activity, including subharmonic and ultraharmonic emissions that denote stable cavitation. The point spread function (PSF) of passive cavitation images is diffraction limited. When typical clinical diagnostic linear arrays are used for PCI, the diffraction limit results in high azimuthal resolution but low axial resolution. Abadi et al. (2013)recently demonstrated a method called frequency-sum beamforming, which employs second-order or higher products of the acoustic emissions to manufacture higher frequencies, thereby reducing the size of the PSF. We applied this approach to cavitation emissions recorded from albumin-shelled bubbles insonified by 2 MHz ultrasound. Cavitation emissions were recorded on a 5 MHz, 128 element linear array using a Vantage scanner (Verasonics Inc.). Quadratic and fourth-order frequency-sum beamforming was applied to both harmonic and ultraharmonic cavitation emissions. Corresponding simulations were also performed to illustrate frequency-sum passive cavitation imaging of multiple bubbles. In comparison to delay-and-sum PCI, apparent areas of cavitation activity decreased when products of the emissions were used to perform frequency-sum beamforming. However, frequency-sum beamforming also produced artifacts, including the appearance of spurious emission sources.

Overview of Ultrasound Imaging Algorithms Based on Synthetic Aperture Focusing Technique

Synthetic aperture focusing ultrasound imaging technique used signal processing method to analysis ultrasonic echo data. It let the performance of transducer with small apertures be as well as the transducer with large aperture. It can still obtain high azimuth resolution image and form three-dimensional image, when it was operated at lower operating frequency. This paper describes the history and the theory of synthetic aperture focusing ultrasound imaging technology. It also contains the development of the synthetic aperture focusing ultrasound imaging technology in China and at abroad. And lots of excellent synthetic aperture focusing ultrasound imaging algorithms were listed in this paper. We analyzed the deficiencies and problems of the existing ultrasound focusing algorithms. And we made forecast about the development of synthetic aperture technology in nondestructive testing.

Deramping-based Imaging Method for Bistatic Forward-looking SAR

Abstract With the ability to resolve the ambiguity and higher azimuth resolution, bistatic forward-looking synthetic aperture radar (BFLSAR) imaging is strongly desirable in both navigation and safety landing for small platforms. However, due to the Doppler centroid variation along the range direction, azimuth spectrum will cross into the neighboring pulse repetition frequency (PRF) band causing severe azimuth ambiguity. Conventional frequency focusing method cannot directly used in this case. In this paper, deramping technique is employed to realize the coarse focusing and reduce the azimuth bandwidth, thus spectrum folding is mitigated. Incorporated secondary range compression, fine focusing is obtained. Simulation results validate the effectiveness of the proposed method.

Forward-looking bistatic SAR imaging based on high-order range equation and high-order phase compensation

Synthetic aperture radar (SAR) techniques are typically used to achieve a high azimuth resolution, but conventional monostatic SAR is not applicable in forward direction because of azimuth ambiguities and poor Doppler resolution. This paper focuses on SAR imaging processing in a novel forward-looking bistatic configuration that a pilotless plane transmits signals and a guided missile receives echo which can avoid image ambiguities and to improve the azimuth resolution. A fourth-order range equation and a fourth-order phase term are adopted to improve the accuracy in the approximation of the range history and two-dimensional spectrum for forward-looking bistatic SAR, respectively. The spectrum is used for high-order phase compensation, range compression, secondary range compression, range cell migration correction, and azimuth compression. Especially, the proposed algorithm is able to focus the translational variant bistatic configuration where the transmitter and receiver platforms are moving in parallel tracks with different velocities and high squints. Finally, simulations validate the effectiveness of the algorithm.

Multiple Beams Spaceborne SAR Imaging

High azimuth resolution and wide swath coverage pose contradicting requirements in conventional spaceborne synthetic aperture radar (SAR) systems. In order to overcome these inherent limitations, the technique of displaced phase centers multiple azimuth beams (MAB) has been proposed in recent years. The principle of MAB SAR is discussed here and then the problems related with this mode, such as the differential path error and nonuniform sampling error, are analyzed when the Earth rotation is considered. The corresponding solutions to these problems are presented.

Efficient DPCA SAR imaging with fast iterative spectrum reconstruction method

The displaced phase center antenna (DPCA) technique is an effective strategy to achieve wide-swath synthetic aperture radar (SAR) imaging with high azimuth resolution. However, traditionally, it requires strict limitation of the pulse repetition frequency (PRF) to avoid non-uniform sampling. Otherwise, any deviation could bring serious ambiguity if the data are directly processed using a matched filter. To break this limitation, a recently proposed spectrum reconstruction method is capable of recovering the true spectrum from the nonuniform samples. However, the performance is sensitive to the selection of the PRF. Sparse regularization based imaging may provide a way to overcome this sensitivity. The existing time-domain method, however, requires a large-scale observation matrix to be built, which brings a high computational cost. In this paper, we propose a frequency domain method, called the iterative spectrum reconstruction method, through integration of the sparse regularization technique with spectrum analysis of the DPCA signal. By approximately expressing the observation in the frequency domain, which is realized via a series of decoupled linear operations, the method performs SAR imaging which is then not directly based on the observation matrix, which reduces the computational cost from O(N 2) to O(N log N) (where N is the number of range cells), and is therefore more efficient than the time domain method. The sparse regularization scheme, realized via a fast thresholding iteration, has been adopted in this method, which brings the robustness of the imaging process to the PRF selection. We provide a series of simulations and ground based experiments to demonstrate the high efficiency and robustness of the method. The simulations show that the new method is almost as fast as the traditional mono-channel algorithm, and works well almost independently of the PRF selection. Consequently, the suggested method can be accepted as a practical and efficient wide-swath SAR imaging technique.

THE LOW BACKSCATTERING TARGETS CLASSIFICATION IN URBAN AREAS

Abstract. The Polarimetric and Interferometric Synthetic Aperture Radar (POLINSAR) is widely used in urban area nowadays. Because of the physical and geometric sensitivity, the POLINSAR is suitable for the city classification, power-lines detection, building extraction, etc. As the new X-band POLINSAR radar, the china prototype airborne system, XSAR works with high spatial resolution in azimuth (0.1 m) and slant range (0.4 m). In land applications, SAR image classification is a useful tool to distinguish the interesting area and obtain the target information. The bare soil, the cement road, the water and the building shadow are common scenes in the urban area. As it always exists low backscattering sign objects (LBO) with the similar scattering mechanism (all odd bounce except for shadow) in the XSAR images, classes are usually confused in Wishart-H-Alpha and Freeman-Durden methods. It is very hard to distinguish those targets only using the general information. To overcome the shortage, this paper explores an improved algorithm for LBO refined classification based on the Pre-Classification in urban areas. Firstly, the Pre-Classification is applied in the polarimetric datum and the mixture class is marked which contains LBO. Then, the polarimetric covariance matrix C3 is re-estimated on the Pre-Classification results to get more reliable results. Finally, the occurrence space which combining the entropy and the phase-diff standard deviation between HH and VV channel is used to refine the Pre-Classification results. The XSAR airborne experiments show the improved method is potential to distinguish the mixture classes in the low backscattering objects.

Target Localization and Tracking in Noncoherent Multiple-Input Multiple-Output Radar Systems

For a noncoherent multiple-input multiple-output (MIMO) radar system, the maximum likelihood estimator (MLE) of the target location and velocity, as well as the corresponding Cramer-Rao lower bound (CRLB) matrix, is derived. MIMO radar's potential in localization and tracking performance is demonstrated by adopting simple Gaussian pulse waveforms. Due to the short duration of the Gaussian pulses, a very high localization performance can be achieved, even when the matched filter ignores the Doppler effect by matching to zero Doppler shift. This leads to significantly reduced complexities for the matched filter and the MLE. Further, two interactive signal processing and tracking algorithms, based on the Kalman filter and the particle filter (PF), respectively, are proposed for noncoherent MIMO radar target tracking. For a system with a large number of transmit/receive elements and a high signal-to-noise ratio (SNR) value, the Kalman filter (KF) is a good choice; while for a system with a small number of elements and a low SNR value, the PF outperforms the KF significantly. In both methods, the tracker provides predictive information regarding the target location, so that the matched filter can match to the most probable target locations, reducing the complexity of the matched filter and improving the tracking performance. Since tracking is performed without detection, the presented approach can be deemed as a track-before-detect approach. It is demonstrated through simulations that the noncoherent MIMO radar provides significant tracking performance improvement over a monostatic phased array radar with high range and azimuth resolutions. Further, the effects of coherent integration of pulses are investigated for both the phased array radar and a hybrid MIMO radar, where only the pulses transmitted and received by colocated transceivers are coherently integrated and the other pulses are combined noncoherently. It is shown that the hybrid MIMO radar achieves significant tracking performance improvement when compared with the phased array radar, by using the extra Doppler information obtained through coherent pulse integration.

ISAR IMAGING OF NON-UNIFORM ROTATION TARGETS WITH LIMITED PULSES VIA COMPRESSED SENSING

This research introduces compressed sensing (CS) prin- ciple into inverse synthetic aperture radar (ISAR) imaging of non- uniform rotation targets, and high azimuth resolution can be achieved with limited number of pulses. Firstly, the sparsity of the echoed signal of radar targets with non-uniform rotation in certain matching Fourier domain is analyzed. Then the restricted isometry property (RIP) and incoherence of partial matching Fourier matrices are checked, following which an ISAR imaging method based on CS for both random sparse aperture and short aperture cases is proposed. In particular, consid- ering the dependence of the sparse dictionary on the relative rotation parameter, a parameter estimation method by the optimal search in fractional Fourier domain is presented. Simulation experiments verify the efiectiveness as well as superiority of the proposed imaging method over traditional methods in terms of imaging performance.

Entropy-based motion error correction for high-resolution spotlight SAR imagery

Spotlight synthetic aperture radar (SAR) is an effective way to obtain finer azimuth resolution than the achievement in strip-map mode with the same physical antenna. In general, for spotlight SAR imaging, high azimuth resolution requires long synthetic aperture length. However, in practical airborne applications, because of the inevitable atmospheric turbulence during the long flight trajectory, more complicated motion error is induced that severely degrades the focusing quality of the SAR image. It makes motion compensation (MOCO) for high-resolution spotlight (HR-Spotlight) SAR imagery more difficult than that for other SAR systems. To tackle the HR-Spotlight SAR data contaminated by the complicated motion error, a novel MOCO algorithm based on entropy minimisation is proposed, which is named by minimum entropy MOCO (ME-MOCO). In this approach, by fitting a polynomial to the motion error, the entropy of a focused image is utilised as the optimisation function of the polynomial coefficients. Attributed to damped Newton method, a modified strategy is designed, which results in that the data-driven ME-MOCO algorithm estimates high order polynomial parameters accurately and efficiently. Besides, the proposed algorithm is efficient for the capability of exploiting the fast Fourier transform through the processing chain. Real data experiments and comparisons demonstrate the effectiveness and superiority of the proposal.

Bistatic Forward-Looking SAR: Results of a Spaceborne–Airborne Experiment

Forward-looking radar imaging continues to gain in significance due to a variety of convenient applications, like landing assistance for aircraft in poor visibility conditions. Synthetic aperture radar (SAR) techniques are typically used to achieve a high azimuth resolution, but conventional monostatic SAR is not applicable in forward direction because of azimuth ambiguities and poor Doppler resolution. To improve the Doppler resolution and to avoid azimuth ambiguities, bistatic SAR configurations can be used to obtain high-resolution radar images. This is demonstrated for the first time in a spaceborne-airborne SAR experiment by using TerraSAR-X as the illuminator and the Phased Array Multifunctional Imaging Radar as the receiver. For convenience, the receiver's SAR antenna was mounted on the aircraft's loading ramp and looked backward. Due to identical image properties and the same challenges for forward- and backward-looking sensors, this configuration also demonstrates the feasibility of forward-looking bistatic SAR. This letter describes the experimental setup, analyzes the performance, and presents the imaging results.

Analysis of wide-angle radar imaging

In wideband radar imaging, it is necessary to increase the target observation angle, that is, the imaging time to obtain high azimuth resolution. For wide-angle targets, however, migration through resolution cell will occur and the Doppler information of the scatterers will be time varying, which can lead to image defocusing in both the range and azimuth domain if the range–Doppler (RD) method is applied. Aimed at this problem, this study builds up the imaging model for steadily moving targets with a constant rotation axis. Then it compares the RD algorithm, the Keystone algorithm and the polar format algorithm with theoretical analysis and deduces the maximum observation angles that fit for these methods, respectively. Additionally, this study proposes a new algorithm for imaging of targets with large observation angles based on the complex inverse Radon transform. Instead of applying the Taylor's series approximation, this method strictly follows the signal model for scatterers with a wide observation angle and utilises curvilinear integral with phase information for image formation. Therefore it is valid in wide-angle scenarios. Finally, imaging results for both the simulated and measured data are given to analyse the performance of the available algorithms and to prove the validity of the proposed algorithm.