Search

Being capable of providing weather-independent, day-and-night, forward-looking, and high-resolution images, bistatic forward-looking synthetic aperture radar (BFSAR) is a promising sensing technique in applications such as the scene-matching-aided navigation for recently emerging high-speed maneuvering platforms (HMPs). Because of the high speed and the great maneuverability of HMPs and the bistatic forward-looking configuration, conventional image formation algorithms, such as polar format algorithm (PFA), are no longer suitable for HMP-borne BFSAR (HMP-BFSAR). Hence, in this paper, we propose a novel PFA for HMP-BFSAR image formation. In the proposed PFA, a range model, termed as quasi-continuous -move range model, is established by taking the maneuvers of the receiver during pulse propagation into account instead of adopting stop-and-go approximation. Moreover, to take advantage of the collected $k$ -set efficiently, an affine mapping, termed as $k$ -set affine mapping, is conceived to transform the parallelogram-shaped $k$ -set support region to a horizontal and quasi-rectangular one. Furthermore, to compensate for the defocus effect induced by wavefront curvature, a closed-form refocus filter based on the implicit function theorem is derived. Both point target simulation and distributed target simulation are presented in this paper. The simulation results show that the proposed PFA significantly outperforms the conventional PFA in terms of focusing quality and computational efficiency when applied to HMP-BFSAR image formation. [ABSTRACT FROM AUTHOR]

This paper proposed a 3-D surface deformation retrieval algorithm for synthetic aperture radar interferometry (InSAR) based on a Global Navigation Satellite System (GNSS) as a transmitter and a fixed receiver (GNSS-based InSAR), where simultaneous multiple GNSS transmitters and a repeat-pass concept were adopted. This paper consists of three parts. First, the interferometric phase model under repeat-pass concept was established for both general and bistatic permanent scatterer (PS) cases in the GNSS-based InSAR. Second, the 3-D deformation retrieval algorithm was presented, and the position dilution of precision was analytically derived to evaluate the performance of measured 3-D deformation. Third, using a designed transponder onboard displacement device as the bistatic PS, the feasibility of 3-D deformation retrieval using GNSS-based InSAR was confirmed by repeat-pass experiments, where four simultaneously available Beidou-2 Inclined Geosynchronous Orbit satellites were used as transmitters, and 16 repeat-pass data sets were collected. Using the proposed algorithm, the final experimental results suggested that the GNSS-based InSAR could obtain deformation estimations with better accuracy than at 5 mm in all three directions. Thus, huge potential exists for applications such as landslide prediction and infrastructure safety monitoring. [ABSTRACT FROM AUTHOR]

This paper presents a novel imaging algorithm which has the capacity of processing multireceiver synthetic aperture sonar (SAS) data based on Vu’s spectrum. There are two important issues when developing the fast Fourier-based imaging algorithms. One is the point target reference spectrum (PTRS) with analytical expression in the 2-D frequency domain; the other is the space variance of the PTRS. Vu has solved the first issue based on the Lagrange inversion theorem. However, the second issue has not been solved yet. Our approach in this paper is to apply Vu’s spectral result in an indirect range-Doppler (R-D) imaging algorithm. For each transmitter/receiver pair, the close-form azimuth modulation is derived first, and then, range/azimuth coupling can be obtained via the difference between the PTRS and the azimuth modulation. Unlike traditional R-D algorithm, which utilizes the interpolation operation to eliminate the range cell migration (RCM), the RCM correction (RCMC) in this paper is carried out through two steps, i.e., bulk RCMC and differential RCMC. Bulk RCMC accounts for the RCMC in one reference range. Differential RCMC accomplished with a range-dependent subblock postprocessing method is used to compensate for the space variant deviation phase error. Via the proposed method, the echoed signal corresponding to each transmitter/receiver pair is processed first. Following which, a high-resolution SAS complex image is obtained by coherent superposition of all the coarse subimages. Numerical simulations and real data processing results show that the proposed method has the similar focusing capabilities with the accurate back projection algorithm across the whole swath. Although the presented method has the disadvantage of latency, it is very easy to make a complete phase center approximation correction, allowing for the tandem nature of the collected data. Besides, the proposed algorithm can handle general case with explicit point of stationary phase (PSP) no matter how complicated the PSP is, and it is also suitable for the imagery of tandem synthetic aperture radar. [ABSTRACT FROM PUBLISHER]

This paper addresses some important aspects for the spaceborne/stationary bistatic synthetic aperture radar (SAR) (SS-BiSAR) imaging with the transmitter, TerraSAR-X, operated in staring spotlight (ST) mode. With the large integration time reaching 7.5 s and the azimuth steering span reaching \pm 2.2^0, several significant effects occur, including troposphere delay, precision phase and time synchronization, the curved orbit effect, azimuth spectrum aliasing problem, and efficient frequency domain focusing algorithm. To circumvent the main effects, corresponding solutions are proposed, including a precise synchronization strategy with troposphere delay correction based on the direct signal from the transmitter and a modified and integrative bistatic polar format algorithm (PFA). This paper covers the theoretical development, implementation, and analysis of the SS-BiSAR PFA based on 2-D fast Gaussian gridding nonuniform fast Fourier transform with wavefront curvature correction. Furthermore, the high-resolution ST-mode SS-BiSAR image processed by the proposed algorithm is acquired, and the differences of scattering behaviors between monostatic and bistatic SAR images are analyzed in detail. [ABSTRACT FROM AUTHOR]

Due to the high-altitude illumination and the separation of the receiver and transmitter, geosynchronous (GEO) spaceborne–airborne bistatic synthetic aperture radar (BiSAR) is more flexible and accessible in remote sensing applications. In this paper, the Doppler characteristics of GEO BiSAR with a squint receiver are analyzed. It is found that the Doppler spectrum is generally aliased in GEO BiSAR regarding the low pulse repetition frequency (PRF) adopted by the GEO SAR. In order to suppress the ambiguity without adjusting the PRF of GEO SAR, the azimuth multichannel receiving technique is applied to the receiver and then the multichannel transfer function for GEO BiSAR is derived. However, the whole bandwidth of the imaging scene is much larger than that of the center point, which requires extra receiving channels to suppress the ambiguity and thereby increasing the system complexity. A two-step nonlinear chirp scaling (NLCS) method is proposed to obtain the well-focused image with reduced receiving channels. First, a preprocessing step is conducted to achieve space-variant range cell migration correction. After that, the first-step NLCS processing is applied to equalize the 2-D space-variant Doppler centroid and thereby the Doppler bandwidth is decreased, i.e., the required number of receiving channels for reconstruction is reduced. Then, the unambiguous spectrum is reconstructed based on the proposed multichannel transfer function. Finally, the second-step NLCS processing is carried out to equalize the 2-D space-variant high-order Doppler parameters and obtain the well-focused image. The simulation results validate the effectiveness of the proposed method. With the proposed two-step NLCS method, the well-focused image for GEO BiSAR is obtained and the required number of receiving channels can be decreased, which is beneficial to reducing the system complexity and hardware cost. [ABSTRACT FROM AUTHOR]

This paper presents an efficient multiple-input multiple-output (MIMO) borehole radar imaging method based on a high degree of freedom for subsurface sensing. The variable separations between the different transmitter and receiver pairs in MIMO borehole radar are considered and introduced as imaging coefficients into the expanded unified MIMO sample set, which gives the MIMO sample set desired characteristics of a high degree of freedom. By exploiting the high degree of freedom in the unified MIMO expanded sample set, the sample interpolation and energy migration of target reflections can be done in once efficient imaging processing for all transmitters of the MIMO radar system, and finally, accurate target image with low sidelobe level (SL) can be delivered. The computational cost of the proposed method will barely increase with the number of transmitters in the MIMO radar survey, which enables the proposed method to handle MIMO borehole radar imaging in an accurate and efficient manner. The imaging properties of the proposed method are proven to be superior to the conventional imaging methods in SL and computational cost, which is suitable for large MIMO borehole imaging surveys in subsurface sensing applications. [ABSTRACT FROM AUTHOR]

Through-the-wall radar imaging is a powerful tool for mapping buildings’ interiors and hidden objects behind the walls. Through-the-wall imaging systems require large linear arrays of directive antennas to form a large aperture for obtaining images with a high cross-range resolution. However, the low mobility and limited field of view of the conventional systems limit their imaging capability. The concept of all-directions through-the-wall imaging has recently been proposed to enhance the mobility, cross-range resolution, and field of view of the through-the-wall imaging systems. In this technique, the large linear array of directive antennas is replaced by a dense 2-D synthetic array formed by small moving transceivers utilizing omnidirectional antennas. The 2-D synthetic array provides 360° high cross-range resolution images. This paper focuses on the implementation of a system realizing all-directions through-the-wall imaging and measurement results. A bi-static frequency-modulated continuous wave (FMCW) radar system utilizing a simple wireless synchronization scheme and wideband omnidirectional antennas is fabricated and an image formation technique compatible with bi-static FMCW imaging system is presented. Measurement results show that the imaging system can provide 360° high-resolution image of objects and walls in a short time. [ABSTRACT FROM AUTHOR]

A newly designed grounded electrical-source airborne transient electromagnetics (GREATEM) system was introduced recently. Detailed data preprocessing techniques to acquire the high-precision measured magnetic field are discussed here. Different from the previous work in which the reconstruction of the underground structure is performed in 1-D, we interpret the GREATEM data in 3-D by the volume integral equation (VIE) method in the frequency domain. Therefore, the VIE in the forward electromagnetic scattering model is formulated in the low-frequency regime. It is solved by using the stabilized biconjugate gradient fast Fourier transform (BCGS-FFT) method. In the nonlinear inversion, the Born iterative method (BIM) and the conjugate gradient method are adopted to minimize the cost function. A synthetic model of GREATEM survey is used to validate the proposed 3-D forward and inversion algorithms. Then, the field data from two GREATEM surveys are used to test the effectiveness and accuracy of the proposed inversion algorithm. The reconstructed conductivity structures are consistent with geological drilling results, confirming the potential of our method for solving the 3-D GREATEM inversion problems in geophysical engineering applications. This paper represents the first application of the BCGS-FFT and BIM algorithms to a GREATEM system. [ABSTRACT FROM AUTHOR]

In geosynchronous spaceborne–airborne bistatic synthetic aperture radar (GEO-BiSAR) system, the airborne platform achieves high-resolution imaging by passively receiving the signal from the interested scenario. In this paper, the Doppler characteristics of GEO-BiSAR and the individual contribution of the transmitter and the receiver are first analyzed. The airborne receiver is found to be the dominant contributor for the total Doppler bandwidth, which will lead to Doppler spectrum aliasing regarding the low pulse repetition frequency (PRF) adopted by the GEO-SAR. In order to suppress the Doppler ambiguity without adjusting the PRF of GEO-SAR, azimuth multichannel receiving technique is introduced to the airborne receiver. The multichannel transfer function is derived based on the method of series reversion and the spectrum reconstruction algorithm is then modified for multichannel GEO-BiSAR. Moreover, the reconstruction performance is closely related to the corresponding spacing between each channel (i.e., channel configuration). Therefore, the channel configuration design for GEO-BiSAR aims at optimizing the azimuth ambiguity-to-signal ratio with a satisfactory level of signal-to-noise ratio scaling factor by adjusting the channel configuration. The channel configuration design is modeled as a constrained single objective optimization problem (CSOP). Then, a channel configuration design method based on differential evolution and feasibility rule is proposed to solve the CSOP and obtain the channel configuration for the receiver with the optimal reconstruction performance. Finally, simulations results are presented to verify the effectiveness of the proposed method, and characteristics of channel configuration are analyzed in detail, which can be a practical guide for the implementation of multichannel GEO-BiSAR systems. [ABSTRACT FROM AUTHOR]

Due to its flexibility and low cost, the bistatic forward-looking synthetic aperture radar (BFSAR) which employs side-looking transmitter and forward-looking receiver has been studied in recent years. Sparsity-based techniques have been applied in the field of BFSAR imaging and show great potential. In sparsity-based BFSAR imaging, compensation of the motion errors is crucial to get a well-focused image. For fields that admit a sparse representation, we propose a sparsity-based imaging approach integrated with motion error estimation and compensation in this paper. First, a novel joint phase-amplitude compensation-based motion error correction scheme is developed to cope with the spatial variance of motion error. Then, an inversion observation model of the range-Doppler algorithm combined with motion error correction is derived, based on which a joint problem of BFSAR imaging and motion error estimation is formulated as a sparse recovery problem and solved in an iterative way, where in each iteration, both image formation and motion error correction are carried out. Experiments on both the simulated and real BFSAR data show that the proposed method can obtain a more accurate estimation result, and generate better focused images compared with the existing methods. [ABSTRACT FROM AUTHOR]

In this paper, we present a set of compact formulations to model the response of the electromagnetic well-logging tools in eccentric multilayered medium. Different from previous models, the coil antennas are imbedded in an insulating protection layer (IPL) rather than expose to the borehole directly, which agrees much better with some real tools, e.g., the array induction logging tool. The effects of both metal mandrel and an IPL are considered in eccentric scenarios. Our formulations are derived on the basis of the ratios of cylindrical functions, which obviate the notorious overflow issues during numerical computation. Numerical results show that the effect of IPL gradually appears with the increase of the eccentric distance. [ABSTRACT FROM AUTHOR]

In general, autofocus methods integrated with frequency-domain imaging algorithms are instrumental to obtain a well-focused bistatic forward-looking synthetic aperture radar (BFSAR) image in the presence of motion errors. Nevertheless, before applying autofocus methods to correct the azimuth phase errors, range cell migration (RCM) should be eliminated by the RCM correction (RCMC) procedure in frequency-domain imaging algorithms. With motion errors being taken into account, there always exists some residual nonsystematic RCM (NsRCM), which refers to the residual migration components after the RCMC procedure. For the conventional side-looking SAR, NsRCM is caused by motion errors. On the other hand, NsRCM of BFSAR is originated from motion errors before RCMC and the NsRCM amplified by the RCMC procedure. In this paper, we analyze the different types of NsRCM in BFSAR imaging and their relationship. Based on the analyses, we propose an autofocus NsRCM correction scheme for BFSAR imagery using frequency-domain imaging algorithms that can eliminate the range-dependent NsRCM. The proposed scheme consists of three steps. First, for the BFSAR data after range compression and RCMC, a division procedure is carried out in the azimuth direction. The subblocks with the highest signal-to-clutter ratio along the range direction are selected after the azimuth segmentation procedure. Second, for the selected subblocks, the total NsRCM is estimated based on the minimum-entropy criterion. Based on the estimation results, different parts of the NsRCM are obtained by solving an ordinary differential equation. Third, a two-step compensation of the NsRCM is executed to reach the spatially variant correction. Simulations and experimental results are provided to demonstrate that our proposed scheme is effective for BFSAR imaging. [ABSTRACT FROM AUTHOR]

In synthetic aperture radar (SAR) imagery, images of ground moving targets (GMTs) are smeared, distorted, and shifted. Current GMT imaging methods are mostly based on range-Doppler algorithms, which have two main drawbacks: 1) coupling between range cell migration correction (RCMC) and Doppler parameter estimation and 2) cross terms degrade the performance of the nonlinear estimation methods. In this paper, an optimal 2-D spectrum matching method for SAR GMT imaging is proposed. The main innovation or advantage of this method is that the GMT imaging problem is transformed into a constrained optimization problem, and differential evolution is applied to guarantee a high-processing efficiency. As they are associated with the Doppler centroid variation compensation and range shift compensation processing, all GMT point scatterers can be well focused and well located. Compared with the current methods, the improvements exhibited by this method include three main benefits: 1) RCMC and Doppler parameter estimation can be simultaneously accomplished; 2) both along- and cross-track GMT velocities can be simultaneously estimated; and 3) this method can be applied to both monostatic and bistatic SARs. Numerical simulations and experimental data processing have verified the effectiveness and robustness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper proposes an extended nonlinear chirp scaling (CS) image formation algorithm for the bistatic synthetic aperture radar systems with the squinted transmitter and a fixed receiver. Since the transmitter with the squint mode was adopted in the system, two main problems, i.e., the spatial variance of the frequency-modulation rate and cubic phase terms, were introduced in the image formation algorithm. The former problem was solved by the linearity approximation of parameter p and deduced q (the second- and third-order coefficients of CS factors in range, which could be used to remove the spatial variation and high-order phase in the range direction) along the range domain while the latter one was compensated by a cubic analytical phase term in the frequency domain. A corresponding experimental hardware system and the bistatic experiments were also described in this paper. Both the simulation and experimental results validated the proposed algorithm. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing is an essential way to improve the production of unconventional shale oil and gas. It is important to characterize the produced fractures using either acoustic or electromagnetic (EM) methods. Conventional EM solvers in the low-frequency range face significant challenges by such multiscale problems where the fracture width is orders of magnitude smaller than its diameters. Furthermore, the cased borehole environment is extremely difficult to simulate with conventional EM solvers due to meshing difficulties and the multiscale nature of the problem. In this paper, we develop a hybrid distorted Born approximation and 3-D mixed ordered stabilized biconjugate gradient fast Fourier transform (DBA-BCGS-FFT) method to simulate the very challenging 2-D, 2-D-axisymmetric, and 3-D hydraulic fracture models under both open and cased borehole environments. Numerical examples show that this method has orders of magnitude higher efficiency than the finite element method. The capabilities of the DBA-BCGS-FFT method for the induction tool fracture mapping are demonstrated by comparing with laboratory experimental results and other reference results. [ABSTRACT FROM AUTHOR]

With appropriate geometry configurations, bistatic synthetic aperture radar (SAR) can break through the limitations of monostatic SAR on forward-looking imaging. Owing to such a capability, bistatic forward-looking SAR (BFSAR) has extensive potential applications. In BFSAR, the compensation of the spatially variant motion errors is of great significance to get a well-focused image. In this paper, first, the spatial-variance properties of motion errors are analyzed analytically and quantitatively. Different from the side-looking monostatic and bistatic SAR, 2-D space-variant motion errors should be taken into consideration in BFSAR. The 2-D spatial variance of the motion errors can be categorized into two parts, range-variant motion errors of the transmitter and azimuth-variant motion errors of the receiver. Moreover, these two parts are independent of each other. Based on this property analysis, second, a motion compensation (MoCo) approach with cubic-order processing is proposed to deal with the spatially variant motion errors in BFSAR. In the cubic-order processing, the first-order MoCo is performed to correct the spatially independent motion errors on the raw data. The second-order MoCo is accomplished on the non-range-cell-migration (RCM) data to deal with the range-variant errors. After the second-order MoCo, since the signal direction of the non-RCM data coincides with the variant direction of the uncompensated phase errors, the azimuth-variant motion errors and slow time signal are coupled together. To cope with such a problem, the slow time signal is transformed into the direction perpendicular to the azimuth by a novel procedure named azimuth–slow time decoupling. At this stage, the coupling between the azimuth-variant motion errors and slow time signal has been eliminated. Azimuth-variant motion errors can be corrected precisely. Simulation and experimental results verify the effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

With the geosynchronous synthetic aperture radar (SAR) satellite as the transmitter, the unmanned aerial vehicle (UAV) can passively receive the echo within the illuminated ground area and achieve 2-D imaging of the interested target. This SAR system, known as GEO-UAV bistatic SAR, is capable of autonomously accomplishing the bistatic SAR mission in rough terrain environments by prespecifying a path for the UAV receiver. In this paper, the GEO-UAV bistatic SAR system is first investigated. The practical advantages and spatial resolution are then analyzed in detail. The spatial resolution of GEO-UAV bistatic SAR is dependent on the observation geometry, which is determined by the UAV path. Therefore, the path planning for GEO-UAV bistatic SAR aims at identifying a set of optimal paths for the UAV receiver to travel through a 3-D terrain environment that simultaneously guarantees the safety of the UAV and achieves SAR imaging with optimized performance during the flight. The path planning is modeled as a constrained multiobjective optimization problem (MOP), which accurately represents the two main aspects for the path planning problem, i.e., UAV navigation and bistatic SAR imaging. Then, a path planning method based on a constrained-adaptive-multiobjective-differential-evolution algorithm is proposed to solve the MOP and generate multiple feasible paths for the UAV receiver with different tradeoffs between navigation for UAV and bistatic SAR imaging performance. The GEO-UAV bistatic SAR mission designer can choose a path from the solution set according to the application requirements, which makes the method more pragmatic. [ABSTRACT FROM PUBLISHER]

Through-the-wall radar imaging systems are utilized for mapping buildings' interiors and detecting static and moving objects hidden behind the walls. The current techniques rely on large linear antenna arrays with directional radiation pattern for obtaining high cross-range resolution, and their imaging capability is limited by the low array processing gain and field of view. This paper introduces a new technique for through-the-wall radar imaging in which the linear array is replaced by a dense 2-D synthetic array formed by an ad hoc network of moving transceivers with omnidirectional antennas. Applying this method enables imaging of building interiors from outside or inside with 360° field of view. As receivers move, the direct and reflected signals from a stationary transmitter are sampled at different positions within the roaming domain, and by combining such signal samples using an appropriate beam-forming technique, a large and dense array is synthesized to provide an accurate radar map of the buildings' interiors and hidden objects in all directions. To increase the system dynamic range, the direct signals between the transmitter (Tx) and receiver (Rx) antennas are reduced, utilizing orthogonal polarizations for Tx and Rx. To improve the range resolution and reduce the background noise, a new method based on the generalized pencil of function method is proposed. This method can accurately detect the locations of the reflecting points within the image which, in combination with the standard back-projection focusing, provides high-quality radar images. A finite-element method for a simple building structure and ray tracing for a large 3-D building structure are used to evaluate the performance of the proposed method. [ABSTRACT FROM PUBLISHER]

This paper considers the exploitation of navigation satellite systems as opportunity transmitters for bistatic and multistatic synthetic aperture radar (SAR). The simultaneous availability of multiple satellites over a scene of interest at different viewing angles allows multistatic SAR acquisitions using a single receiver on or near the ground. The resulting spatial diversity could be used to drastically improve image resolution or to enhance image information space. To exploit the availability of multiple satellites, two data fusion approaches are here considered. In the former, point features of the single images obtained from different perspectives are extracted and then combined, whereas in the latter, a multistatic image is first obtained by combining the single channel data at the image level and then the point features are extracted. This is achieved by considering ad hoc CLEAN-like techniques. These techniques have been developed on both the analytical and simulation levels and experimentally verified with real GNSS-based SAR imagery. The techniques described here are not limited to GNSS-based SAR but may be applied to any multistatic SAR system. [ABSTRACT FROM AUTHOR]

Inspired by a cognitive radar in exploiting its environment to update current operation parameters and frequency diverse array (FDA) in offering a range-dependent beampattern due to the employment of frequency increments across the elements, this paper proposes a moving-target tracking approach to achieve cognitive radio frequency stealth using an FDA antenna for surveillance applications. Since surveillance systems are highly visible to intercept receivers, a traditional high-gain phased-array antenna beam is replaced by a series of low-gain FDA beams with spoiled frequency increments to reduce the system visibility, without degrading the surveillance system performance. Moreover, a cognitive closed-loop update scheme is presented to update the operation parameters in real time for improved moving-target tracking performance. All of the proposed methods are verified by simulation results. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper introduces a bistatic fast backprojection synthetic aperture radar (SAR) algorithm that is available for different bistatic geometries. The proposed algorithm is tested with the bistatic CARABAS-like data, and the results indicate that the algorithm is a good candidate for bistatic SAR data processing. The image quality measurements are quite similar to the referenced values obtained with the bistatic global backprojection algorithm. That is, the peak sidelobe ratio (PSLR) is −13.7 dB in comparison to the referenced PSLR of −13.8 dB. The half-power beamwidths (HPBWs) measured on the cuts in $x$ and $y$ and the direction where the peak sidelobes locate are 2.8, 3.7, and 3.5 m, respectively, which are approximate to the referenced HPBWs. The small differences in the measured results mainly come from the interpolation step of the algorithm. [ABSTRACT FROM AUTHOR]

This paper proposes a synchronization algorithm for bistatic synthetic aperture radar (BiSAR) imaging in a spaceborne/stationary configuration. In real bistatic systems, synchronization errors are generally introduced into the received data. Additionally, the lack of precise imaging parameters, such as the position of the transmitter and the accurate sampling time, could affect the imaging quality greatly. Fortunately, the image could be well focused by the proposed algorithm in the case of lack of the accurate position of a transmitter and the sampling time. First, a preprocessing step is employed to remove synchronization errors through matching an echo signal with a direct signal. Then, a modified chirp scaling factor containing an error phase term is constructed, and the accurate position of the transmitter and the sampling time can be acquired by the phase extraction of the direct signal and the searching method based on contrast optimization. After that, the corresponding imaging process can be implemented. Finally, the proposed algorithm is validated by the simulation and experimental results, where TerraSAR-X is used as the illuminator. [ABSTRACT FROM PUBLISHER]

In this paper, the formulation of Stolt migration is modified for impulse borehole radar near-field imaging in the subsurface scenarios where the transceiver is widely separated with respect to the detection range. The proposed approach consists of the following aspects. First, the locations of the transmitter and receiver in the survey are regarded as independent sample dimensions, and the original sample set is converted to an enlarged virtual sample set. The frequency–wavenumber spectrum (FWS) of the virtual sample set is available via multidimensional fast Fourier transform (FFT). Then, the relation between the angular frequency and wavenumbers of the transmitter and receiver is derived in the frame of the virtual sample set, which provides the basis for the interpolation in angular frequency and weighting process of FWS. By applying multidimensional inverse FFT (IFFT) to the interpolated and weighted FWS of the virtual sample set, the energy of target responses will focus in some profile of the IFFT result, the position of which is related with the separation between the transmitter and receiver. Finally, the desired target space can be extracted from the IFFT result. The improved Stolt migration technique is compared with the conventional Stolt migration algorithm, back-projection method, and Kirchhoff migration algorithm on synthetic data and validated by single-borehole radar experiment in the subsurface scenario. The results show that the developed Stolt migration is superior to the conventional methods in cross-range resolution, computational cost, and the ability to reconstruct locations and shapes of targets. [ABSTRACT FROM PUBLISHER]

In this paper, an omega-K imaging algorithm to focus the raw data of translational invariant (TI) bistatic synthetic aperture radar (BSAR) is proposed. The method utilizes a point target reference spectrum of generalized Loffeld's bistatic formula (GLBF). Without the bistatic deformation term, GLBF is the latest development of Loffeld's bistatic formula. It is comparable in precision with the method of series reversion (MSR), but it has a much simpler form than MSR and a similar form to a monostatic case. Based on the spatial linearization of GLBF, the Stolt transformation relationship is derived. The method can consider the linear spatial variation of Doppler parameters, which is always ignored in previous publications about bistatic omega-K algorithms. This method can handle the cases of TI BSAR with high squint angles and large bistatic degrees. In addition, a compensation method for the phase error caused by the linearization is discussed. Numerical simulations and experimental data processing verify the effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

In translationally variant (TV) bistatic synthetic aperture radar (BSAR), 2-D spatial variation is a major problem to be tackled. In this paper, a generalized Omega-K imaging algorithm to deal with this problem is proposed. The method utilizes a point target reference spectrum of the generalized Loffeld's bistatic formula (LBF) (GLBF). Without the bistatic-deformation term, GLBF is the latest development of LBF. Similar to the monostatic case, it has a much simpler form than other point target reference spectra. Based on the spatial linearization of GLBF, the Stolt mapping relationship is derived. Different from the traditional Omega-K algorithms for monostatic SAR and translationally invariant BSAR, this approach uses a 2-D Stolt frequency transformation. Through this transformation, the method can deal with the 2-D spatial variation. It can also consider the linear spatial variation of Doppler parameters, which is usually not considered in the previous publications on bistatic Omega-K algorithms. This method can handle the cases of TV-BSAR with different trajectories, different velocities, high squint angles, and large bistatic angles. In addition, a compensation method for the phase error caused by the linearization is discussed. Numerical simulations and experimental data processing verify the effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

The current procedure to detect and identify unexploded ordnance (UXO) using electromagnetic induction (EMI) time-domain sensors is based on two steps. First, data are acquired over large areas in dynamic mode, and locations of interest are flagged based on measured field amplitudes. Second, sensors return to the flagged areas for more in-depth cued interrogation, providing high-quality data for subsequent identification and classification. Flagging based on field amplitude, however, has potential drawbacks: The magnetic field in the EMI regime exhibits a 1/R^6 drop-off with range, and a deep UXO may not produce a strong response while still being potentially hazardous. To address this problem, we propose in this paper an inversion method based on Kalman and extended Kalman filters meant to do the following: 1) work with dynamic data; 2) provide both position and polarizability estimates; and 3) operate in real time (less than 100 ms in our case). Such full characterization of the target, albeit limited to within the 2.7-ms interrogation time window of the dynamic mode associated with the sensors studied here, provides useful information when deciding whether to continue with the cued interrogation. We validate the method for two popular EMI sensors, the second-version Man Portable Vector (MPV-II) and the MetalMapper, operated in very different settings: The MPV-II is used to interrogate a limited region of space atop a single target, whereas the MetalMapper is driven over long lanes along which several targets and clutter items are present. [ABSTRACT FROM AUTHOR]

Ambiguity function (AF) provides an effective way for analysis of the resolving performance of a radar system. With the development of ultra-wideband (UWB) and multiple-input-multiple-output (MIMO) radio techniques, recently, the combination of them-the concept of UWB MIMO radar has been proposed and received much attention. For a UWB MIMO radar, first, the signal waveform with an exponential expression which is widely adopted in narrowband cases is not appropriate anymore. Second, the MIMO radar sensors are typically organized in a certain spatial topology, which greatly impacts the performance of the MIMO system. In addition, for a typical MIMO radar application scenario, generally, there are relative motions existing between the radar sensors and the targets. The presence of the relative motions would further complicate the performance analysis of the system. In fact, for a MIMO radar, the overall performance of the system is jointly determined by the parameters, such as the signal waveform, the topology, and the relative motions. Each of the parameters impacts the resolving performance in a particular way. In order to evaluate the contribution of each individual parameter of interest, in this paper, a UWB MIMO AF is developed and factorized into several factors. By this means, each individual parameter can be evaluated via the corresponding factor to examine a certain specific characteristic of the system, instead of via the complicated whole MIMO AFs. [ABSTRACT FROM PUBLISHER]

Bistatic synthetic aperture radar (BSAR) provides strategical, technical and economical advantages in radar imaging. Motions and micro-motions of objects in an illuminated scene introduces Doppler and micro-Doppler effects in the received radar echoes. Combining the advantages introduced by the bistatic configuration and the usefulness of the micro-Doppler signature characterization will provide a powerful tool for military and civil remote sensing applications such as target recognition and classification. In this paper, a vibrating micro-Doppler signature for a BSAR system with fixed receiver is analyzed and compared to the signature obtained in a monostatic SAR system. The micro-Doppler effect is derived for a vibrating target in the bistatic SAR. The corresponding bistatic factor is shown to be a function of the bistatic acquisition geometry. Also, the effect of the target vibration on the focused image is shown to be influenced by the acquisition geometry. The derived model is useful for micro-Doppler classification. Simulations for 94 GHz and 10 GHz are given and the results confirm the derived model. [ABSTRACT FROM PUBLISHER]

The response of an electromagnetic gradiometer (EMG) system to a subsurface wire is analyzed in terms of experimental and analytical modeling results. Our objective is to explore characteristics of the response and assess the fidelity of our model. The EMG system consists of a static transmitter and a man-portable sensor, which uses a pair of receivers that yield a gradiometric measurement. Experimental results were collected over a range of wire depths from 3.4 to 8.5 m. A number of different transmitter positions were explored, and the tests studied were conducted at 200 kHz. Modeling results were consistent with the experimental results and supported a number of key findings. Results are presented showing that, in order to maximize the strength of the wire response, the transmitter should be positioned approximately 5 m off the wire axis. Furthermore, in order to avoid unwanted transmitter influence on the response, the EMG should be at least 30 m from the transmitter. Using the experimental and modeling results, we found a linear relationship between the width of the magnitude response peak and the wire depth. Based on our experimental results, the EMG is able to yield a discernible target response at a depth of at least 7 m. Lastly, an example of how the model can be used to optimize survey planning is presented. This paper illustrates how an EMG can be used to locate underground wires with applications ranging from underground utility mapping to the detection of shallow subsurface tunnels. [ABSTRACT FROM AUTHOR]

This paper presents a new bistatic point target reference spectrum. It is derived by using the 2-D principle of stationary phase which is first applied in the synthetic aperture radar (SAR) community. The spectrum contains two hyperbolic range-azimuth coupling terms and thus is very similar to the monostatic spectrum. It shows the characteristic of the conventional monostatic SAR besides an additional azimuth scaling term. Therefore, it makes the common Doppler-based monostatic processing algorithms readily suitable to handle the Bistatic SAR (BiSAR) data in the moderate-squint azimuth-variant configurations with two moving platforms. Based on the spectrum, two Doppler-based monostatic imaging algorithms [i.e., range–Doppler algorithm (RDA) and chirp-scaling algorithm (CSA)] are readily implemented to deal with the moderate-squint azimuth-variant BiSAR data. Compared to the processing procedure for the monostatic SAR, the RDA and CSA for the BiSAR need only the adjustment of Doppler parameters. Finally, the potential and limitation of the spectrum are analyzed, and the real raw data in the spaceborne/airborne configurations are used to validate the proposed spectrum and processing methods. [ABSTRACT FROM AUTHOR]