Forward-looking Synthetic Aperture Radar Research Articles

Ground-Moving Target Imaging and Velocity Estimation Based on Mismatched Compression for Bistatic Forward-Looking SAR

Bistatic forward-looking synthetic aperture radar (BFL-SAR) is a kind of bistatic SAR system that can image forward-looking terrain in the flight direction of an aircraft. Until now, BFL-SAR imaging theories and methods have been researched for stationary targets. Unlike the stationary target, the motion of a ground-moving target (GMT) induces unknown range cell migration and additional modulation of the azimuth signal. Thus, to finely image the GMT, one must obtain its velocity parameters accurately, but they are usually unknown. In this paper, a novel GMT imaging and velocity estimation method, which is based on mismatched compression, is proposed for BFL-SAR without a priori knowledge of the GMT's velocity parameters. The main idea behind mismatched compression is to use a presumed azimuth reference function for performing correlated operation with the azimuth signal of the GMT. In general, the Doppler parameters of the presumed azimuth reference function are different from those of the GMT's azimuth signal because the velocity parameters of the GMT are unknown. Therefore, the correlation operation referred to earlier is actually mismatched compression, and the resulting image is shifted and defocused. The shifted and defocused image is utilized to get the real Doppler and velocity parameters of the GMT. The advantage of this method is that not only the GMT can be well focused but also the GMT's velocity can be simultaneously obtained. In addition, this method needs only monochannel antenna. The proposed BFL-SAR GMT imaging and velocity estimation method is validated by numerical simulations.

A New Imaging Algorithm for Forward-Looking Missile-Borne Bistatic SAR

Bistatic synthetic aperture radar (BiSAR), which can break through the limitations of the conventional monostatic SAR on forward-looking imaging, continues to gain in significance due to a variety of applications, like the missile terminal guidance. To improve Doppler resolution and to avoid azimuth ambiguities of the missile-borne SAR, an extended nonlinear chirp scaling (NLCS) algorithm for the forward-looking missile-borne bistatic SAR (FLMB-SAR) is proposed in this paper. Based on the novel algorithm and the bistatic configuration with a stationary transmitter and a forward-looking missile-borne receiver, the missile-borne SAR not only can image the forward-looking terrain of the targets but also plays sufficient role in navigation. Finally, the simulation results are exhibited to validate the correctness of the analysis and prove the two-dimensional (2-D) imaging ability of the FLMB-SAR.

A Minimum-Entropy Based Residual Range Cell Migration Correction for Bistatic Forward-Looking SAR

For bistatic forward-looking synthetic aperture radar (BFSAR), motion errors induce two adverse effects on the echo, namely, azimuth phase error and residual range cell migration (RCM). Under the presumption that residual RCM is within a range resolution cell, residual RCM can be neglected, and azimuth phase error can be compensated utilizing autofocus methods. However, in the case that residual RCM exceeds the range resolution, two-dimensional defocus would emerge in the final image. Generally speaking, residual RCM is relatively small and can be neglected in monostatic SAR, while the unique characteristics of BFSAR makes the residual RCM exceeding range resolution cell inevitable. Furthermore, the excessive residual migration is increasingly encountered as resolutions become finer. To cope with such a problem, minimum-entropy based residual RCM correction method is developed in this paper. The proposed method eliminates the necessity of the parametric model when estimating the residual RCM. Moreover, it meets the practical needs of BFSAR owing to no requirement of exhaustive computation. Simulations validate the effectiveness of the proposed method.

Focusing translational‐variant bistatic forward‐looking synthetic aperture radar data using extended azimuth non‐linear chirp scaling algorithm

Translational-variant bistatic forward-looking synthetic aperture radar (TV-BFSAR) introduces three difficulties for the imaging algorithm, namely, large range cell migration (RCM), severe range–azimuth coupling and large azimuth spatial variance. To address these problems, an extended azimuth non-linear chirp scaling (EANLCS) algorithm is proposed. The essence is to remove the linear RCM and Doppler centroid and compensate the residual RCM and range–azimuth coupling terms with secondary range compression. Thereafter, the EANLCS operation is applied to eliminate the azimuth-dependent frequency-modulation rate and cubic phase terms. Experimental results demonstrate the effectiveness and superiority of the proposed algorithm in focusing TV-BFSAR data.

Focusing one‐stationary bistatic forward‐looking synthetic aperture radar with squint minimisation method

This study deals with the imaging problem of one-stationary bistatic forward-looking synthetic aperture radar (BFSAR). This BFSAR has extensive potential applications, such as self-navigation and self-landing in the airport. However, the forward-looking (or squint) angle is usually large in one-stationary BFSAR. It causes large range migration and strong range dependence on the secondary range compression term. Therefore it is difficult to use traditional imaging algorithms to focus one-stationary BFSAR. To solve this problem, an imaging algorithm based on squint minimisation method is proposed in this study. The squint minimisation operation can shear the high forward-looking angle data spectrum and decrease the coupling between range and azimuth. Then, the authors derive a modified range Doppler algorithm (RDA) to focus the sheared data. Numerical simulations show that by combining squint minimisation method and modified RDA, the raw data of one-stationary BFSAR can be well imaged.

A raw data simulator for Bistatic Forward-looking High-speed Maneuvering-platform SAR

As a basic tool for signal processing in Synthetic Aperture Radar (SAR), raw data simulation plays a significant role in those complicated configurations in which the real raw data is difficult to obtain. For the new and special configuration in Bistatic Forward-looking High-speed Maneuvering-platform SAR (BFHM-SAR), the double square roots and high-order terms in range history caused by high velocities and accelerations from both platforms make it difficult to gain the corresponding 2-dimensional frequency spectrum (2-D FS) effectively. In addition, space variances in phase terms of 2-D FS owing to curvilinear trajectories and different traveling directions of two platforms influence the accuracy of echo data significantly. In this paper, a fast and effective scene raw data simulator is developed based on the effectual access of 2-D FS with high precision and quantificational analysis and correction of phase space variance of it. As the inverse process of imaging, raw signal can be reconstructed and generated by this method more efficiently than traditional approaches nearly without sacrificing accuracy. Plenty of simulation results for scattering targets and real scene verify its validity and effectiveness. Moreover, our method can be applied for handling the simulation in the conventional airborne platform of bistatic SAR besides BFHM-SAR.

Bistatic forward-looking SAR ground moving target detection and imaging

Bistatic forward-looking synthetic aperture radar (BFSAR) is a kind of bistatic SAR (BiSAR) system that can image the forward-looking terrain in the flight direction of a receiver. Current literature and reports about BFSAR imaging are mainly concentrated on stationary scene imaging. In this paper, the ground moving target (GMT) theories for BFSAR are discussed. Firstly, the BFSAR signal models of a general GMT in both time-domain and Doppler-domain are shown. Meanwhile, the echo characteristics of the GMT are analyzed, which include the migration characteristic, the Doppler characteristic, the coupling characteristic, etc. After the above analysis, a GMT detection and imaging method for BFSAR is presented. The main purpose of the GMT detection method is to find out the difference between the GMT and the stationary background in an appropriate domain. However, for BFSAR, it is hard to do this since the relative movements between the BFSAR sensors and the GMT are strongly coupled, i.e., their respective influences cannot be easily separated. Different from the existing detection methods which use Doppler frequency (Doppler centroid) in Doppler-domain or space-time characteristics in space-time domain to differentiate the GMT from stationary background, the main idea of the method proposed in this paper is to distinguish the GMT from stationary background by their different Doppler frequency rates (DFRs) in DFR-domain. However, in order to do this, the stationary background’s DFRs must be equalized and the GMT’s DFR should be distinct from the same DFR of the stationary background. Then, the product second-order ambiguity function (PSAF) is used to detect and focus the GMT. Numerical simulations and experimental data processing verify the validity of the GMT theories and the method shown in this paper.

Dual-Channel Forward-Looking SAR Micro-Motion Targets Detection

It is aimed at the problem of forward-looking SAR micro-motion target detection in this paper. The traditional dual-channel SAR DPCA cancellation principle is analyzed and a dual-channel DPCA model of forward-looking SAR is proposed, the amplitude and phase characters of the signal after DPCA cancellation is studied, the distance domain of the micro-motion target is determined, then the micro-motion target detection is probability under different echo SNR with Monte-Carlo Operation, when the echo SNR is bigger than -5dB,and the target detection probability is greater than 90%.

Analysis of Forward-Looking Synthetic Aperture Radar Imaging Characteristics of Rotating Target

In this paper,it is established the geometric and radar echo model of the rotating target with the Forward-looking synthetic aperture radar firstly,then the Doppler frequency History Formula is Deduced and Pointed out that the Formula has a sinusoidal characteristics. It is simulated the imaging of the stationary target and the rotating target with different rotation speed, the imaging result can be expressed as the forms of curve and the curve circle nomadic type, when these features appear in the imaging results, it can initially determine that the rotating target is presence.

Focusing bistatic forward-looking synthetic aperture radar based on modified Loffeld’s bistatic formula and chirp scaling algorithm

A focusing solution for bistatic forward-looking synthetic aperture radar (BFSAR) is presented. Forward-looking imaging is highly desirable in some potential applications, such as self-landing in bad weather, military surveillance, and navigation. Unfortunately, monostatic synthetic aperture radar reaches its limit when it is used in a forward-looking configuration. BFSAR can provide a high-resolution image in the forward-looking direction. However, due to the special forward-looking geometry, many proposed methods of deriving a bistatic point target reference spectrum (BPTRS) cannot handle the BFSAR data well. A modified Loffeld's bistatic formula (MLBF) for a forward-looking configuration is proposed first; it can get an accurate BPTRS of BFSAR. Then, a chirp scaling algorithm (CSA) based on MLBF is derived. CSA without interpolation allows high performance. Numerical simulations show that the proposed focusing solution can handle the BFSAR data well and achieve a high- resolution focused image. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. (DOI: 10.1117/1.JRS.8.083586)

MIMO-Based Forward-Looking SAR Imaging Algorithm and Simulation

Multiple-input multiple-output (MIMO) radar imaging can provide higher resolution and better sensitivity and thus can be applied to targets detection, recognition, and tracking. Missile-borne forward-looking SAR (MFL-SAR) is a new and special MIMO radar mode. It has advantage of two-dimensional (2D) imaging capability in forward direction over monostatic missile-borne SAR and airborne SAR. However, it is difficult to obtain accurate 2D frequency spectrum of the target echo signal due to the high velocity and descending height of this platform, which brings a lot of obstacles to imaging algorithm design. Therefore, a new imaging algorithm for MFL-SAR configuration based on the method of series reversion is proposed in this paper. This imaging method can implement range compression, secondary range compression (SRC), and range cell migration correction (RCMC) effectively. Finally, some simulations of point targets and comparison results confirm the efficiency of our proposed algorithm.

Property Analysis of MIMO-Based Missile-Borne Forward-Looking SAR

As a special multiple-input multiple-output (MIMO) radar networking mode, missile-borne forward-looking synthetic aperture radar (MFL-SAR) has many potential applications. This paper describes and analyzes properties of this new configuration. Range history and Doppler history are analyzed and derived using the designed geometric configuration. Then the expressions of range and Doppler resolution are determined based on the validity of two-dimensional (2D) resolution imaging capability. To help to design the proper system and motion parameters of this configuration, key parameters affecting the imaging ability are found out. Due to high velocities and accelerations of both transmitter and receiver, high-order terms in the slant range equation should be kept to reduce the approximation error. The range resolution and Doppler resolution of MFL-SAR are both space-variant and time-variant owing to the complexity of this configuration. The tiny changes of 2D resolution during the synthetic aperture time should be considered when designing the imaging algorithm of MFL-SAR.

Focusing bistatic forward-looking synthetic aperture radar based on modified hyperbolic approximating

The conventional monostatic synthetic aperture radar (SAR) shows a limitation if a forward-looking geometry is used. However, bistatic forward-looking SAR gives a good solution, providing a high resolution image in the flight path direction. Due to the fact that the range history of bistatic SAR is a double square root, classical imaging algorithm cannot be applied to the bistatic SAR directly. Also it is difficult to deduce imaging algorithm from double square root directly. The hyperbolic approximating method can transform double square root into single one, when being used in forward-looking geometry, and the high order term error is obvious and cannot be ignored. In this paper, we propose a modified hyperbolic approximating method, which makes a cubic term approximation and improves the precision, then we apply the new method to bistatic forward-looking SAR and deduce the bistatic point spectrum. Based on the spectrum, a modified range Doppler algorithm is proposed for focusing bistatic forward-looking SAR. Finally, numerical simulation is used to compare the original hyperbolic approximating method with the modified one for veridating the proposed algorithm and processing approach.

Focusing Bistatic Forward-Looking SAR With Stationary Transmitter Based on Keystone Transform and Nonlinear Chirp Scaling

With appropriate geometry configurations, bistatic synthetic aperture radar (SAR) can break through the limitations of monostatic SAR on forward-looking imaging. Thanks to such a capability, bistatic forward-looking SAR (BFSAR) has extensive potential applications, such as self-navigation and self-landing. In the mode of BFSAR with a stationary transmitter (ST-BFSAR), the two-dimensional spatial variation makes it difficult to use traditional data focusing algorithms. In this letter, an imaging algorithm based on keystone transform and nonlinear chirp scaling (NLCS) is proposed to deal with this problem. Keystone transform is used to remove the spatial variation of range cell migration. NLCS can eliminate the variation of azimuth reference function. Numerical simulations show that by combining first-order keystone transform and azimuth NLCS operation, the raw data of ST-BFSAR can be well imaged.

FOCUSING TRANSLATIONAL VARIANT BISTATIC FORWARD-LOOKING SAR DATA BASED ON TWO-DIMENSIONAL NON-UNIFORM FFT

Forward-looking imaging has extensive potential applications, such as self-navigation and self-landing. By choosing proper geometry, bistatic synthetic aperture radar (BiSAR) can break through the limitations of monostatic SAR on forward-looking imaging and provide possibility of the forward- looking imaging. In this special bistatic conflguration, two problems involving large range cell migration (RCM) and large range-azimuth coupling are introduced by the forward-looking beam, which make it di-cult to use traditional data focusing algorithms. To address these problems, a novel Omega-K algorithm based on two-dimensional non-uniform FFT (2-D NUFFT) for translational variant (TV) bistatic forward-looking SAR (BFSAR) imaging is proposed in this paper. In this study, we derive an accurate spectrum expression based on two-dimensional principle of stationary phase (2-D POSP). 2-D NUFFT is utilized to eliminate the range-variant term, which can make full use of the data and improve the computational e-ciency as well. The experimental results, presented herein, demonstrate the efiectiveness and advantages of the proposed algorithm.

BISTATIC FORWARD-LOOKING SYNTHETIC APERTURE RADAR IMAGING BASED ON THE MODIFIED LOFFELD'S BISTATIC FORMULA

Bistatic forward-looking SAR (BFSAR) has many potential applications, such as self- landing in bad weather and military detection. Therefore, BFSAR receives considerable attention recently. The imaging algorithms for BFSAR are the di-culties of the study. The original Lofield's Bistatic Formula (LBF) can handle most general bistatic SAR conflgurations well. But in some complex bistatic geometries, such as high squint or forward-looking cases, the performance of LBF is degenerated. Some extended LBF (ELBF) methods have been developed, which improve the performance of LBF in some special geometries, but still not the forward-looking conflguration. In this paper, we modify the LBF method and try to solve the instantaneous azimuth frequencies of transmitter and receiver directly. Then, we can obtain a bistatic point target reference spectrum (BPTRS), which is accurate enough for forward-looking conflguration. A range Doppler algorithm (RDA) based on this BPTRS is derived. Finally, simulations validate the accuracy of the modifled Lofield's Bistatic Formula (MLBF) and efiectiveness of imaging algorithm.

Autonomous Navigation Airborne Forward-Looking SAR High Precision Imaging with Combination of Pseudo-Polar Formatting and Overlapped Sub-Aperture Algorithm

Autonomous navigation airborne forward-looking synthetic aperture radar (SAR) observes the anterior inferior wide area with a short cross-track dimensional linear array as azimuth aperture. This is an application scenario that is drastically different from that of side-looking space-borne or air-borne SAR systems, which acquires azimuth synthetic aperture with along-track dimension platform movement. High precision imaging with a combination of pseudo-polar formatting and overlapped sub-aperture algorithm for autonomous navigation airborne forward-looking SAR imaging is presented. With the suggested imaging method, range dimensional imaging is operated with wide band signal compression. Then, 2D pseudo-polar formatting is operated. In the following, azimuth synthetic aperture is divided into several overlapped sub-apertures. Intra sub-aperture IFFT (Inverse Fast Fourier Transform), wave front curvature phase error compensation, and inter sub-aperture IFFT are operated sequentially to finish azimuth high precision imaging. The main advantage of the proposed algorithm is its extremely high precision and low memory cost. The effectiveness and performance of the proposed algorithm are demonstrated with outdoor GBSAR (Ground Based Synthetic Aperture Radar) experiments, which possesses the same imaging geometry as the airborne forward-looking SAR (short azimuth aperture, wide azimuth swath). The profile response of the trihedral angle reflectors, placed in the imaging scene, reconstructed with the proposed imaging algorithm and back projection algorithm are compared and analyzed.

Focusing of translational variant bistatic forward-looking SAR with Chirp-Z transform

The forward-looking imaging problem of synthetic aperture radar can be resolved by adopting the bistatic configuration. This paper introduces a translational variant bistatic forward-looking SAR (BFSAR) configuration with a moving transmitter and a forward-looking receiver. Focusing bistatic SAR data in frequency domain requires two-dimensional (2D) point target spectrum. However, the existence of the double square root (DSR) term in the bistatic range makes it difficult to get the exact solution for the 2D spectrum. Approximate solutions such as the method of series reversion (MSR) have been derived to get the 2D spectrum, but the accumulative error to DSR of MSR is not the smallest one. To improve the accuracy of the 2D spectrum, the method of Legendre polynomial expansion is proposed to obtain the least square approximation of the bistatic range and get a more accurate 2D spectrum. Then, the Chirp-Z transform is chosen to focus the translational variant BFSAR data based on this spectrum, and the imaging quality is compared with the traditional MSR spectrum; the simulation results verify the 2D imaging ability of the proposed method.

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.

A Frequency-Domain Imaging Algorithm for Translational Invariant Bistatic Forward-Looking SAR

With appropriate geometry configurations, bistatic Synthetic Aperture Radar (SAR) can break through the limitations of monostatic SAR for forward-looking imaging. Thanks to such a capability, bistatic forward-looking SAR (BFSAR) has extensive potential applications. This paper develops a frequency-domain imaging algorithm for translational invariant BFSAR. The algorithm uses the method of Lengendre polynomials expansion to compute the two dimensional point target reference spectrum, and this spectrum is used to perform the range cell migration correction (RCMC), secondary range compression and azimuth compression. In particular, the Doppler-centroid and bistatic-range dependent interpolation for residual RCMC is presented in detail. In addition, a method that combines the ambiguity and resolution theories to determine the forward-looking imaging swath is also presented in this paper.