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Wideband multiple-input-multiple-output (MIMO) imaging radar can achieve high-resolution imaging with a specific multi-antenna structure. However, its imaging performance is severely affected by the array errors, including the inter-channel errors and the position errors of all the transmitting and receiving elements (TEs/REs). Conventional calibration methods are suitable for the narrow-band signal model, and cannot separate the element position errors from the array errors. This paper proposes a method for estimating and compensating the array errors of wideband MIMO imaging radar based on multiple prominent targets. Firstly, a high-precision target position estimation method is proposed to acquire the prominent targets’ positions without other equipment. Secondly, the inter-channel amplitude and delay errors are estimated by solving an equation-constrained least square problem. After this, the element position errors are estimated with the genetic algorithm to eliminate the spatial-variant error phase. Finally, the feasibility and correctness of this method are validated with both simulated and experimental datasets.

An improved features from accelerated segment test (FAST) corner-detection method for suppressing SAR image speckle noise is proposed here. The direction of the corner candidate is first computed by the law of universal gravitation and is introduced into corner detection to divide the 16 pixels around the corner candidate into two groups without repeatedly searching. Then, the similarity between the corner candidate and its surrounding pixels is computed to suppress the points affected by SAR speckle noise. Finally, the phenomenon of corner density is eliminated based on non-maximum suppression. The experimental results show that authors’ method has better suppressing effect compared with FAST-8 and SIFT corner-detection methods.

Multiple-input multiple-output (MIMO) radar is a novel radar system which can achieve more individual observation echoes than the number of the actual antennas by transmitting orthogonal waveforms. At present, pattern theory is usually utilised to evaluate the azimuth imaging indicator of the MIMO imaging radar system. Although it cannot describe the two-dimensional performance, it is quite effective to analyse the azimuth resolution of far-field targets. However, the pattern based on the far-field condition becomes invalid for targets in the near-field area, thus it cannot explain the appearance of near-field azimuth grating lobes. In this article, spatial spectrums and generalised ambiguity functions (GAFs) of near-field targets are calculated. By describing the near-field GAF (NGAF) approximately, the relationship between the distortions of NGAF and the spatial spectrum is studied qualitatively, which can explain the appearance of azimuth near-field grating lobes. In addition, the calibration method of amplitude and phase errors by an isolated corner reflector is presented in this article. Finally, a simulation and an experiment are carried out to verify the conclusions.

Bistatic radar system has some advantages over conventional monostatic radar system. However, time and frequency synchronisation is a crucial issue for bistatic radar. This study proposes an integrated time–frequency synchronisation method for cooperative bistatic radar based on GPS disciplined frequency standard. The method uses a high-precision GPS-disciplined frequency source as the frequency reference of the transmitter and receiver, and the frequency synchronisation is realised by full-coherent frequency synthesis. Using GPS time information and PPS signal to generate the PRT trigger signal, and using the local oscillator frequency source to maintain the PRT trigger signal and triggering again within a fixed time, time synchronisation is realised. Through the experiments, the effectiveness of the time-frequency synchronisation method is verified, and it can well meet the imaging requirements of bistatic radar.

Multiple-input multiple-output (MIMO) radar plays a more and more important role in radar imaging, target detection, and other fields owing to its unique array structure. Transmitting orthogonal signals by different transmitters, a MIMO radar can significantly enhance the azimuth resolution. It forms multiple observation channels from different transmit and receive array elements, achieving an equivalent large aperture in azimuth direction. However, the amplitude and phase errors in each channel are introduced, which will lead to inferior imaging performance of the system, especially in azimuth direction. To solve this problem, a method to estimate the amplitude and phase errors of each channel based on a strong scatterer is proposed here. Peak amplitude of one-dimensional range between channel is estimated using constrained least square (CLS) based on ideal point target characteristics. It proved to be valid by a simulation and an imaging experiment.

The ability of multiple-input multiple-output (MIMO) radar to provide enhanced performance has been verified in both theoretical and practical level. Through applying orthogonal waveforms, this type of system can realize far more observation channels than the actual number of transmit and receive (T/R) elements. Generally, the azimuth one-dimensional (1-D) generalized ambiguity function (GAF) and resolution of a MIMO radar system are analyzed by pattern theory, which can only analyze the GAF along the equidistant curve to the radar's center. However, the two-dimensional (2-D) GAF of wideband MIMO radar system shows that its sidelobes are not along the equidistant curve accurately due to the coupling effect between the range-dimension and the azimuth one. Obviously, the traditional pattern theory becomes invalid in this case. To tackle this problem, the 2-D GAF and the spatial spectrum of far-field targets are derived in this paper. By analyzing the 2-D GAF, existence of three individual directions of sidelobes is revealed and it is proved that every sidelobes is along a straight line in the pseudopolar coordinate system. Furthermore, a method to calculate the 1-D GAF and resolution in the arbitrary direction is proposed. Finally, a simulation and two experiments have been carried out to validate the theoretic results.

Based on the permanent scatterer (PS) technique, deformation measurements were taken in ground-based synthetic aperture radar (GB-SAR) applications. To realize long-term and real-time deformation m...

The discontinuous mode of ground-based synthetic aperture radar (GB-SAR) is suitable for monitoring creep landslides. However, the instrument needs to be installed and disassembled repeatedly, which could inevitably cause repositioning error, and severely affect the accuracy of deformation measurements. This paper performs a detailed theoretical analysis of the repositioning error based on the Taylor expansion of a ternary function, and it can be built as a linear multi-parameter model. Simulations are made to validate the effectiveness of this model compared with two common first-order and second-order models. Then a compensation method based on the permanent scatterer (PS) technique is proposed. Two experiments of discontinuous monitoring are discussed. The first one is an equivalent discontinuous experiment, which utilizes two corner reflectors to evaluate the compensation accuracy. The other one is a discontinuous experiment taken on a steep mountain. Compared with the common methods, the proposed method can better compensate for the error phase and benefit high-precision deformation monitoring.

Phase analysis based on high-quality pixel (HQP) is crucial to ensure the measurement accuracy of ground-based SAR (GB-SAR). The amplitude dispersion (ADI) criterion has been widely applied to identify pixels with high amplitude stability, i.e., permanent scatterers (PSs), which typically are point-wise scatterers such as stones or man-made structures. However, the PS number in natural scenes is few and limits the GB-SAR applications. This paper proposes an improved method to take HQP selection applied for natural scenes in GB-SAR interferometry. In order to increase the spatial density of HQP for phase measurement, three types of HQPs including PS, quasi-permanent scatter (QPS), and distributed scatter (DS), are selected with different criteria. The ADI method is firstly utilized to take PS selection. To select those pixels with high phase stability but moderate amplitude stability, the temporal phase coherence (TPC) is defined. Those pixels with moderate ADI values and high TPC are selected as QPSs. Then the feasibility of the DS technique is explored. To validate the feasibility of the proposed method, 2370 GB-SAR images of a natural slope are processed. Experimental results prove that the HQP number could be significantly increased while slightly sacrificing phase quality.

Entomological radars are important for scientific research of insect migration and early warning of migratory pests. However, insects are hard to detect because of their tiny size and highly maneuvering trajectory. Generalized Radon–Fourier transform (GRFT) has been proposed for effective weak maneuvering target detection by long-time coherent detection via jointly motion parameter search, but the heavy computational burden makes it impractical in real signal processing. Particle swarm optimization (PSO) has been used to achieve GRFT detection by fast heuristic parameter search, but it suffers from obvious detection probability loss and is only suitable for single target detection. In this paper, we convert the realization of GRFT into a multimodal optimization problem for insect multi-target detection. A novel niching method without radius parameter is proposed to detect unevenly distributed insect targets. Species reset and boundary constraint strategy are used to improve the detection performance. Simulation analyses of detection performance and computational cost are given to prove the effectiveness of the proposed method. Furthermore, real observation data acquired from a Ku-band entomological radar is used to test this method. The results show that it has better performance on detected target amount and track continuity in insect multi-target detection.

With the development trends of multistatic spaceborne synthetic aperture radar (SAR), geosynchronous SAR (GEO SAR) employing several formation-flying small satellites also has great potential for remote sensing. The small satellites can cooperate to acquire multi-channel data for moving target detection and parameter estimation in strong clutters. However, multistatic GEO SAR has large satellite spacing and a curved trajectory, which induce the near-field effects and channels out of alignment, respectively, bringing about challenges for the spatial adaptive processing. These problems produce a high-order term in the multi-channel slant range model, making the traditional model and adaptive processing method invalid. In this paper, to meet the requirement of SAR focusing, we firstly derive a fourth-order slant range model and a third-order path difference model for multistatic GEO SAR. Secondly, based on the derived model, the principle of stationary phase and series reversion method are utilized to derive the spatial steering vector for a moving target, which is a basis of spatial adaptive processing in the range-Doppler domain. Thirdly, the time-domain match filtering is constructed based on the fourth-order slant range model to image the moving target. Additionally, the moving targets are detected in the image domain. The motion parameter is estimated by iteratively maximizing the output signal to clutter and noise ratio (SCNR) through the range of possible target velocities. Finally, considering that the GEO SAR is still in development, the computer simulations are carried out to verify the effectiveness and evaluate the performance.

The monitoring and early warning of a landslide in the reservoir area of a hydropower station are of great significance in the dam structure of the hydropower station and in the safety of people’s life and property on the reservoir bank. In this study, a new ground-based interferometric synthetic aperture radar system LKR-05-KU-S100 was used to carry out field monitoring tests on Lagu landslide and Xiaozhaju landslide of Dahuaqiao hydropower station and No. 1 landslide on the left bank of Xiaowan hydropower station on the Lancang river. The results show that, during the monitoring period, Lagu landslide of Dahuaqiao hydropower station and No. 1 landslide on the left bank of Xiaowan hydropower station are basically stable, and the deformation trend of Xiaozhaju landslide is obvious so it should undergo continuous monitoring. At the same time, the field monitoring test also shows that the new ground-based interferometric synthetic aperture radar system LKR-05-KU-S100 has the advantages of high precision and long-distance, all-day, all-weather, and large-scale monitoring and has unique advantages and broad application prospects for the overall deformation monitoring of large landslides in the reservoir area.

The upcoming L -band geosynchronous differential interferometric synthetic aperture radar (GEO D-InSAR) system has the capability to monitor rapid deformations due to its excellent revisit capability. However, because of its low working frequency, the random ionospheric scintillation signal will degrade the deformation retrieval accuracy by giving rise to extra interferometric phase errors and obvious decorrelations in GEO D-InSAR interferograms. In this paper, aiming at impacts of ionospheric scintillations on GEO D-InSAR system, we theoretically establish its interferometric phase error and decorrelation models by using the scintillation statistical parameters directly. Simulations based on the scintillation sampling model, the Cornell university scintillation model, the phase screen mode, and the ionospheric scintillation signal acquired by the ground-based global positioning system receiver are carried out to verify the proposed model. Moreover, quantitative analyses of the ionospheric scintillation interferometric phase error and decorrelation impacts under different scintillation cases are obtained. The results verify that the proposed models and the analyses are effective. Meanwhile, they also suggest that the generated defocusing decorrelation dominates the ionospheric scintillation impacts on GEO D-InSAR, which can induce a coherence loss of more than 0.1 in the interferogram when only one image of the interferometric pair suffers the weak ionospheric scintillation.

When multiple ground-based differential interferometric radar (GB-DInRad) systems are utilised together to resolve three-dimensional (3D) deformation, the resolving accuracy is related with the measurement geometry of these radar systems. This Letter focuses on how to determine an optimal geometry. Based on the resolving equation of the 3D deformation, the Geometric Dilution of Precision (GDOP) is defined. Then the minimum value of the GDOP is calculated and a detailed geometrical analysis is made to determine the geometrical conditions of the minimum GDOP. The results prove that the optimal geometry could be obtained when the unit measurement vectors of these radar systems construct a regular pyramid with a specific structure. A numerical simulation is utilised to validate the analysis conclusion.

GEO SAR can achieve better revisit time and observation performances when using high squint mode. However, high squint mode also causes huge range walk and severe range-azimuth coupling problems as well as severe 2-D spatial variance of the focusing parameters and range cell migration (RCM), which restrict the available scene size and increase the difficulty of GEO SAR imaging. To achieve high squint mode GEO SAR imaging, the effects of squint mode on imaging processing—especially the azimuth-variant RCM—are first analyzed in detail, and the relationship between the azimuth-variant RCM and the azimuth-variant focus parameters is discussed. Then, in this paper, a 2-D nonlinear chirp scaling algorithm (2-D NCSA) based on optimal azimuth polynomial compensation is proposed to realize high squint GEO SAR imaging. In the proposed algorithm, the optimal azimuth polynomial compensation which consists of a time-domain compensation and a frequency compensation removes the range walk, weakens the azimuth-variant RCM, and simplifies the imaging processing, while the 2-D NCSA which consists of a range NCSA and a higher order azimuth NCSA solves the 2-D spatial variance of focusing parameters and achieves high squint GEO SAR imaging. Computer simulations for different orbital positions validate the effectiveness of the proposed algorithm under high squint conditions.

The detection of a power transmission tower in a synthetic aperture radar (SAR) image has been widely studied. However, few works consider the geometric features of the power transmission tower. In a high-resolution SAR image, the geometric features of the power transmission tower are more obvious and can be used to further reduce the false-alarm probability. In this letter, a new power transmission tower detection method is proposed, which takes into account the geometric features of the target and obtains lower false-alarm probability than the traditional methods. First, the polar coordinate semivariogram is proposed, which has the advantages of low computational complexity and high sensitivity to the shape of the targets. Then, a three-layer neural network is employed to detect the power transmission tower, taking the geometric features as the input vector. Finally, the validity of the proposed method is illuminated by the experimental measurement results of the airborne data with 0.5-m resolution.

Photodynamic therapy (PDT) is limited by its penetration depth because of the selection of photosensitizer and light source. In this study, Ce-doped TiO 2 (TiO 2 :Ce) was synthesized by a modified sol-gel method to act as a photosensitizer activated by low-dose X-rays, less harmful than ionizing radiation, to generate intracellular reactive oxygen species (ROS). The A549 cell line was used as the in vitro and in vivo model to evaluate the efficacy of X-ray-irradiated TiO 2 :Ce to induce cell death. The cell viability significantly decreased with TiO 2 :Ce exposure under low-dose X-ray irradiation. An LDH assay showed that intracellular TiO 2 :Ce under X-ray irradiation was cytotoxic to A549 cancer cells. TiO 2 :Ce produced significant ROS under low-dose X-ray irradiation and promoted apoptosis/necrosis of A549 cancer cells. TiO 2 :Ce can enhance the efficacy of X-ray-induced PDT, and tumor growth was inhibited in vivo . The mechanisms underlying ROS generation by TiO 2 :Ce activated by X-ray irradiation to induce cell toxicity, thereby inhibiting tumor growth, is discussed.

The ionospheric scintillation induced by local ionospheric plasma anomalies could lead to significant degradation for geosynchronous earth orbit synthetic aperture radar (SAR) imaging. As radar signals pass through the ionosphere with locally variational plasma density, the signal amplitude and phase fluctuations are induced, which principally affect the azimuthal pulse response function. In this paper, the compensation of signal amplitude and phase fluctuations is studied. First, space-variance problem of scintillation is addressed by image segmentation. Then, SPECAN imaging algorithm is adopted for each image segment, because it is computationally efficient for small imaging scene. Furthermore, an iterative algorithm based on entropy minimum is derived to jointly compensate the signal amplitude and phase fluctuations. Finally, a real SAR scene simulation is used to validate our proposed method, where both the simulated scintillation using phase screen technique and the real GPS-derived scintillation data are adopted to degrade the imaging quality.

Ground-based synthetic aperture radar (GB-SAR) technique has been widely applied for the deformation monitoring and measurement of the natural and engineered slopes. To extend the 2-D deformation measurement from the conventional 1-D measurement along the radar-target line of sight (LOS), multiple aperture interferometry (MAI) techniques based on phase differences between interferograms of the forward-looking and backward-looking subapertures are tackled in this letter. The optimal subaperture selection is analyzed considering the typical signal-to-noise ratios and correlations in GB-SAR applications. Simulations prove that the coherent integration (CIM) can be utilized to improve the measurement accuracy of the MAI method. Besides, GB-SAR experiments are carried out to validate the feasibility and effectiveness of the 2-D deformation measurement method based on MAI. Accuracy comparison of deformation measurement with the MAI and cross correlation methods is also taken. Experimental results show that the accuracy of deformation measurement along the perpendicular direction to LOS based on MAI and CIM can reach millimeter level for displaceable corner reflector.

As a novel system, multiple-input multiple-output synthetic aperture radar (MIMO-SAR) has achieved rapid development in the past years. Based on the MIMO technique, multiple transmitting and receiving antennas could be utilised to compose a specific geometrical structure and acquire a large aperture. The MIMO-SAR system can be utilised to take deformation measurement to scenes with high deformation rate and can avoid the problems of phase errors caused by the antennas' vibration on the rail. This study introduces the basic principle of the MIMO technique and the key problems in the system design. Experimental datasets are acquired with the MIMO-SAR and GB-SAR (ground-based SAR) systems simultaneously. Comparisons of the imaging results indicate that the MIMO-SAR system has a better repeat-track interferometry performance than the GB-SAR system. In addition, deformation measurement results validate the high-accuracy measurement ability of the MIMO-SAR system.

Ground-based interferometric technology plays an important role in the terrain mapping sphere because it is characterized by short observation intervals, a flexible operation environment, and high data precision. Ground-based interferometric synthetic aperture radar (GB-InSAR) has a wide beam, a scene breadth comparative to the slant range, and a large downwards-looking angle. The observation scenes always show the type of slope terrain with various gradients and slope orientations. These particularities cause the invalidation of the typical terrain generation method and produce poor precision analysis results using typical values. This paper first proposes a three-dimensional-coordinate generation method based on the geolocation concept. Then, the models and analyses of the error sources and their propagations are reported. The method of calculating the correlation coefficient is meticulously discussed, and a system error distribution diagram is presented that considers the spatial distribution information of the viewing scene. The result can be adapted to different viewing scenes and encompasses the performance of the whole area, and it will help with baseline optimization. The digital elevation map (DEM) generated by GB-InSAR is compared with one produced by light detection and ranging (LiDAR). The error magnitude and the similarity of the distribution between theory and reality prove the correctness and effectiveness of the presented DEM generation method, the correlation coefficient estimation formula, and the system precision analysis method.

Three-dimensional (3D) cell cultures developed with living cells and scaffolds have demonstrated outstanding potential for tissue engineering and regenerative medicine applications. However, no suitable tools are available to monitor dynamically variable cell behavior in such a complex microenvironment. In particular, simultaneously assessing cell behavior, cell secretion, and the general state of a 3D culture system is of a really challenging task. This paper presents our development of a dual-transduction-integrated biosensing system that assesses electrical impedance in conjunction with imaging techniques to simultaneously investigate the 3D cell-culture for bone regeneration. First, we created models to mimic the dynamic deposition of the extracellular matrix (ECM) in 3D culture, which underwent osteogenesis by incorporating different amounts of bone-ECM components (collagen, hydroxyapatite [HAp], and hyaluronic acid [HA]) into alginate-based hydrogels. The formed models were investigated by means of electrical impedance spectroscopy (EIS), with the results showing that the impedances increased linearly with collagen and hyaluronan, but changed in a more complex manner with HAp. Thereafter, we created two models that consisted of primary osteoblast cells (OBs), which expressed the enhanced green fluorescent protein (EGFP), and 4T1 cells, which secreted the EGFP-HA, in the alginate hydrogel. We found the capacitance (associated with impedance and measured by EIS) increased with the increases in initial embedded OBs, and also confirmed the cell proliferation over 3 days with the EGFP signal as monitored by the fluorescent imaging component in our system. Interestingly, the change in capacitance is found to be associated with OB migration following stimulation. Also, we show higher capacitance in 4T1 cells that secret HA when compared to control 4T1 cells after a 3-day culture. Taken together, we demonstrate that our biosensing system is able to investigate the dynamic process of 3D culture in a non-invasive and real-time manner.

Recently, structural monitoring by radar remote sensing has become more necessary for both economic and security reasons. Infrastructure monitoring with no incorporated deformation sensors (e.g., old generation water dams for which regulations did not impose monitoring capabilities) is usually performed by regular in situ topographic surveys. However, these surveys cannot be performed very often, and alternative methods are desirable. A feasible nonintrusive way to monitor such a structure is with interferometric synthetic aperture radar (SAR) data that can be acquired with monostatic/bistatic sensors.

This work presents the ideal combination of space-borne and ground-based (GB) Interferometric Synthetic Aperture Radar (InSAR) applications. In the absence of early investigation reporting and specialized monitoring, the Zhongbao landslide unexpectedly occurred on 25 July 2020, forming a barrier lake that caused an emergency. As an emergency measure, the GB-InSAR system was installed 1.8 km opposite the landslide to assess real-time cumulative deformation with a monitoring frequency of 3 min. A zone of strong deformation was detected, with 178 mm deformation accumulated within 15 h, and then a successful emergency warning was issued to evacuate on-site personnel. Post-event InSAR analysis of 19 images acquired by the ESA Sentinel-1 from December 2019 to August 2020 revealed that the landslide started in March 2020. However, the deformation time series obtained from satellite InSAR did not show any signs that the landslide had occurred. The results suggest that satellite InSAR is effective for mapping unstable areas but is not qualified for rapid landslide monitoring and timely warning. The GB-InSAR system performs well in monitoring and providing early warning, even with dense vegetation on the landslide. The results show the shortcomings of satellite InSAR and GB-InSAR and a clearer understanding of the necessity of combining multiple monitoring methods.

Low signal power density limits the performance of space-surface bistatic synthetic aperture radar (SS-BiSAR) using Global Navigation Satellite System (GNSS) satellites as illuminators. To tackle this problem, in this letter, a novel bistatic SAR image enhancement technique based on repeat-pass coherent fusion is proposed. The works in this letter include three aspects. First, repeat-pass experiments are designed to ensure the best resolution. Second, a modified CLEAN technique is applied to remove the direct signal interference from the focused BiSAR images. Third, a coherence-processing method is proposed to implement coherence of each repeat-pass BiSAR image and then they are coherently fused to obtain a quality-improved BiSAR image. Twenty-two days of repeat-pass BiSAR experiments with Beidou-2/Compass-2 inclined geosynchronous orbit satellites as illuminators have been designed and conducted. The data were processed by the proposed method. The results show that the method can obtain better image quality compared with the traditional noncoherent fusion method and the single-day imaging result, which validates the proposed method and proves the huge potential in realizing local area monitoring with SS-BiSAR using GNSS satellites as illuminators.

The basic performance of space-surface bistatic synthetic aperture radar (SS-BSAR) using BeiDou satellites as transmitters of opportunity is presented. The transmitted signal, the satellite trajectory, and the observation time are analyzed to demonstrate the potential of BeiDou satellites to be used as transmitters of SS-BSAR. When a BeiDou medium earth orbit (MEO) satellite is used, the signal-to-noise ratio and the resolution are examined for different SS-BSAR cases, where the receiver is fixed on the ground or mounted on a moving platform. Since the parameters of the BeiDou geostationary earth orbit (GEO) satellite and the inclined geosynchronous satellite orbit (IGSO) satellite are different from those of the MEO satellite, the peculiarities of SS-BSAR with the GEO or IGSO satellite as transmitter are analyzed and then compared with the case of MEO satellite. In order to show the performance difference of SS-BSAR with BeiDou satellites as transmitters, comparisons with other global navigation satellite system (GNSS) transmitters are also made. The theoretical results show that SS-BSAR using BeiDou satellites as transmitters can achieve different sounding performance and can provide some new potential applications compared to other GNSS transmitters.

The L-band geosynchronous synthetic aperture radar (GEO SAR) is very susceptible to ionosphere as the significant increases of its integration time and wide swath, leading to image drifts and degradations. This paper demonstrates an experimental study of analyzing ionospheric impacts on GEO SAR, including both background ionosphere and ionospheric scintillation. The experiment consists of two parts. One is the global positioning system (GPS) data recording in which we employ GPS satellites to probe ionosphere and collect the transionosphere GPS signals. Then the recorded signals are used to create the data basis on which simulations are based. The other is the reconstruction of the signal distortions based on the GPS data. Then the two parts are combined to generate the ionosphere-impacted GEO SAR signals. But GEO SAR has very different orbit trajectories from GPS. Thus, in the real operation, the transformation of the temporal–spatial frame between GPS and GEO SAR should be first performed before the focusing and the evaluation are carried out. In cases of current GEO SAR configurations, the background ionosphere will induce image drifts but can be corrected through image registration techniques. The image is also likely to get defocused in azimuth when the second and higher derivatives of total electron content exceed thresholds which are dependent on GEO SAR configurations and the corresponding integration time. Comparatively, scintillations will mainly affect the focusing in azimuth, especially for integrated sidelobe ratios (ISLRs). But scintillations rarely occur over China mainland, and it is suggested to avoid the GEO SAR working during its occurrence.

The interest of applying ground-based synthetic aperture radar (GB-SAR) in terrain displacement monitoring has increased for decades. Due to the large illuminating coverage and suitable working bandwidth, GB-SAR can achieve multitemporal surface deformation maps of the entire terrain with high spatial resolution and submilimetric accuracy. Unfortunately, focusing GB-SAR data in an efficient way is accompanied with a series of complicated engineering problems and should be solved. In this paper, a novel imaging algorithm tailored for GB-SAR data is proposed. First, the main characteristics of GB-SAR data are analyzed, and a signal model is then built up. Second, corresponding to this model, an imaging algorithm is put forward, which is based on keystone formatting and range-Doppler domain blocking. This algorithm is phase-preserving and can be widely applied in near-field, far-field, and wide view angle scenarios. Moreover, its computational cost is quite low, which includes only one time of linear interpolation and several times of fast Fourier transforms and complex multiplications. Finally, the algorithm is extensively validated both with numerical simulations and real GB-SAR data.

The development of strategies of bone tissue engineering and regenerative medicine has been drawing considerable attention to treat bone critical-size defects (CSDs). Notably, new strategies and/or treatment approaches always require appropriate tools to track the healing process so as to evaluate their success. In this paper, we present the development of a novel approach for the non-invasive, yet real-time, monitoring and assessment of bone CSDs treated with biomaterials and biomedical approaches. For this, we employed the technique of electrical impedance spectroscopy (EIS) to quantitatively monitor and assess the changes in electrical impedance, and thus the regeneration process. In our in vitro tests, we examined the biochemical changes of the fracture area and investigated the influence of collagen and hydroxyapatite on the changes in electrical impedance by EIS, thus inferring the changes in bone regeneration and structure. Based on this success, we further demonstrated, in real time, the process of regeneration of the traumatic area in an in vivo rabbit model. Our electrical-impedance data of the experiment groups, i.e., the ones treated with natural coral and bone morphogenetic protein-2 (BMP-2), revealed that each group has its unique impedance graph characteristics, which are directly associated with the degree of regeneration. For comparison, we also employed radiography, gross anatomy, and histological analyses in examination. Our results illustrate that EIS holds considerable potential as a non-invasive tool for monitoring, in real time, the healing of bone CSDs by allowing for quantitatively characterizing the changes of both hydroxyapatite and collagen.

GEO SAR is a new type of space-borne SAR, which runs on the geosynchronous orbit and has the advantages of short revisit time and wide coverage. However, due to the high geosynchronous orbit, the system characteristics and design methods of GEO SAR are quite different from those of the traditional low-earth-orbit SAR (LEO SAR), which are introduced in this chapter. Firstly, the system characteristics of GEO SAR are analyzed. The differences between GEO SAR and LEO SAR are introduced briefly, and then the coverage and revisit abilities are compared with the traditional LEO SAR. After that, a curvature circle motion model (CMM) is introduced to describe the curved satellite track, the Doppler characteristics are analyzed and the work modes are compared. Secondly, the parameter design methods of resolution, power and ambiguity are presented for GEO SAR, considering the high squint observation. Thirdly, two attitude steering strategies are introduced for GEO SAR. The total zero Doppler steering (TZDS) method aims to compensate the serious effects of Earth rotation and obtain a zero Doppler centroid, and the optimized resolution steering (ORS) method attempts to reduce the required steering angle and obtain an optimal resolution feature.

The dihedral corner reflectors are the basic geometric structure of many targets and are the main contributions of radar cross section (RCS) in the synthetic aperture radar (SAR) images. In stealth technologies, the elaborate design of the dihedral corners with different opening angles is a useful approach to reduce the high RCS generated by multiple reflections. As bistatic synthetic aperture sensors have flexible geometric configurations and are sensitive to the dihedral corners with different opening angles, they specially fit for the stealth target detections. In this paper, the scattering characteristic of dihedral corner reflectors is accurately analyzed in bistatic synthetic aperture images. The variation of RCS with the changing opening angle is formulated and the method to design a proper bistatic radar for maximizing the detection capability is provided. Both the results of the theoretical analysis and the experiments show the bistatic SAR could detect the dihedral corners, under a certain bistatic angle which is related to the geometry of target structures.

Surface deformation disasters, such as landslides, debris flow and collapse, do great harm to human lives and property. Ground-Based Synthetic aperture radar (GB-SAR) has been widely applied in deformation monitoring and measurement with the advantages of all-weather, all-day and high precision. Back projection (BP) algorithm, algorithm based on keystone and subblock dechirp (KSD), and far-field pseudopolar format algorithm (FPFA) are typical imaging algorithms used in GB-SAR. This paper analyzes the three algorithms from aspects of application scope, computation burden, and deformation measurement precision. Results show that KSD is most appropriate for GB-SAR.

MIMO (Multiple-Input Multiple-Output) radar is a new radar system, which can provide more observation channels than traditional array radar with the same number of the array elements. Based on far-field approximation, the usual MIMO array will lead to high azimuth grating lobes in near-field imaging. For designing near-field imaging MIMO array, this paper analyzed the reason of the grating lobes in near-field area and proposed a design method of MIMO array with low grating lobes in near-field imaging based on adding window to transmit array. Finally, the validity of the proposed method is verified by a simulation.

The passive Synthetic Aperture Radar with Global Navigation Satellite System (GNSS) employs GNSS satellites as transmitters and receivers mounted near the ground. Since GNSS constellations are designed for global, reliable and persistent operation, the most important advantage of such a system is the potential of permanent monitoring on the area which is overlooked by a fixed receiver. In the paper, an experimental prototype of GNSS-based SAR for the purpose of obtaining larger scene images as well as the development progress was introduced at the first place. And then a latest imaging experiment of GNSS-Based SAR for railway bridge was carried out and the image was obtained with high quality when it was firstly used for China Railway High-Speed (CRH) railway bridge imaging. The positive imaging result suggested that it is possible to achieve change information extraction using GNSS-based SAR for the further safety evaluation of CRH operation in China.

Multiple-Input Multiple-Output (MIMO) radar provides much more flexibility than the traditional radar thanks to its ability to realize far more observation channels than the actual number of transmit and receive (T/R) elements. In designing the MIMO imaging radar arrays, the commonly used virtual array theory generally assumes that all elements are on the same line. However, due to the physical size of the antennas and coupling effect between T/R elements, a certain height difference between T/R arrays is essential, which will result in the defocusing of edge points of the scene. On the other hand, the virtual array theory implies far-field approximation. Therefore, with a MIMO array designed by this theory, there will exist inevitable high grating lobes in the imaging results of near-field edge points of the scene. To tackle these problems, this paper derives the relationship between target’s point spread function (PSF) and pattern of T/R arrays, by which the design criterion is presented for near-field imaging MIMO arrays. Firstly, the proper height between T/R arrays is designed to focus the near-field edge points well. Secondly, the far-field array is modified to suppress the grating lobes in the near-field area. Finally, the validity of the proposed methods is verified by two simulations and an experiment.

Myocardial infarction (MI), caused by the occlusion of the left ventricular coronary artery, leads to the loss of cardiomyocytes and, potentially, heart failure. Cardiomyocytes in adult mammals proliferate at an extremely low rate and thus, a major challenge in MI treatment is supplementing exogenous cells and keeping them viable in MI areas. To address this challenge, injecting hydrogels encapsulating cells into MI areas, to compensate for the loss of cardiomyocytes, shows promise. This study synthesized two types of alginate hydrogels, based on self-crosslinking (SCL) and calcium ion crosslinking (Ca2+) in varying formulations. The hydrogels encapsulated living muscle-derived stem cells (MDSCs) and their performance was evaluated in terms of optimizing cell viability during the injection process, as well as the live/dead rate after long-term cultivation. The morphology of the hydrogel-encapsulated cells was characterized by scanning electronic microscopy (SEM) and live/dead cells were examined using an MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide staining) assay. The mechanical properties of the hydrogels were also determined via a rheometer, to identify their influence on cell viability during the injection process and with respect to long-term cultivation. The SCL hydrogel with a 0.8% alginate and 20% gelatin formulation resulted in the highest cell viability during the injection process, and the Ca2+ hydrogel composed of 1.1% alginate and 20% gelatin maintained the highest cell survival rate after two months in culture.

An atmospheric effect is a main error source that affects interferometric measurements. When a ground-based multiple-input multiple-output (GB-MIMO) radar, i.e., a specific type of GBsynthetic aperture radar (GB-SAR), was utilized to continuously monitor an open-pit mine, the interferometric phases of some interferograms were complexly space-variant due to time-variant weather conditions. The conventional method of atmospheric phase (AP) compensation was no longer applicable. This paper proposes an improved compensation method of a time-space variant AP applied to time-series GB-SAR images. The permanent scatterers (PSs) were classified into three types based on their different spatial properties: The noise-dominant PS (NPS), the deformationdominant PS (DPS), and the atmospheric effect-dominant PS (APS). The NPSs were firstly rejected based on the differential phase analysis of neighboring PSs. The DPSs were then rejected based on the cluster partition and selection. With the APSs, the space-variant AP was estimated with a spatial interpolation. To validate the feasibility of the proposed method, short-term and long-term experimental datasets were processed. Comparisons with a conventional method proved that the proposed method can well reduce AP errors and avoid the misunderstanding of motional areas.

Because of the widely dynamic range of ground targets' backscattering coefficients, original GB-SAR image has disadvantage of low contrast ration [1]. In order to enhance contrast, classical method and improved methods based on statistical property of GB-SAR images are studied. Finally, the feasibility and effectiveness of the improved methods are validated with experimental datasets.

In ground-based frequency modulated continuous wave (FMCW) synthetic aperture radar (SAR) images, false targets caused by coupling signal may appear. Traditional digital cancellation methods cannot estimate reference cancellation signal in real-time. A new method based on singular value decomposition (SVD) to suppress radar coupling signal is put forward. In slow time coupling signal is changeless while echo signal is changing, with which the reference cancellation signal can be estimated by SVD method. However, if there are too many points in range and frames in azimuth, the SVD algorithm will be too complicated to process echo data. So echo matrix partition and sub-matrixes decomposition are used in SVD processing to reduce the algorithm cost of time and space. There is a coupling signal suppression experiment using the ground-based FMCW SAR system In this paper. The validity of new method has been verified.

地球同步轨道合成孔径雷达(GEO SAR)的超长积累时间保证了其具有较高分辨率。但相反地, 多种非理想因素, 如弯曲轨道误差、大气传播效应和轨道摄动等, 也会不断积累, 导致成像质量下降, 甚至失效。本文给出了GEO SAR成像的验证实验。实验采用天地双基地SAR(SS-BISAR)配置。其中, 北斗IGSO卫星作为机会发射源;接收机由一个直达波天线和回波接收天线组成。实验分别对转发器和自然场景进行了成像处理, 结果良好, 验证了GEO SAR在几百到几千秒的积累时间下成像的可行性。

In recent years, the repeat-pass GBSAR (ground based synthetic aperture radar) system has demonstrated its capacity to acquire deformation. Nevertheless, in a variety of applications, it needs to measure the deformation with the precision up to 0.1 mm, which could not be reached by utilizing the traditional PS (permanent scatterer) algorithm in most cases. Generally, one of the main reasons could be summarized into the phase error caused by the rail determination error, because the precision of rail determination might degrade during long working hours. However, the traditional PS algorithm could not compensate for the phase error caused by the rail determination error. In order to solve the problems, we modify the conventional PS algorithm. Firstly, we deduced the transformation relationship between the rail determination error and its corresponding interferometric phase error. Then, the phase errors caused by the atmosphere and the rail determination error were jointly compensated. The experimental data, which were obtained in Fangshan District in Beijing (China), were used to test and verify the performance of the new algorithm. After the comparison between the results processed by the new algorithm and those processed by the traditional algorithm, the proposed method demonstrated its ability to obtain high-precision deformation.

Composition-graded films (CGF) of fluoroapatite (FAP) and poly(3-hydroxybutyrate- co-4-hydroxybutyrate) (PHB) were prepared via electrospinning solutions of FAP and PHB using a gradient composition of CGF. Thermal properties, mechanical properties and surface morphology of the films were investigated. Designed thermal and mechanical properties obtained by precise control of the composition gradient of the FAP/PHB CGF. The introduction of FAP in pure PHB film significantly changed the mechanical properties, such as tensile strength and extension rate of the pure PHB. Mouse fibroblast cells (L-929) were cultured on FAP/PHB uniform-films; the MTT test and cell morphology analysis indicate good biocompatibility of the modified CGF. This new method of processing makes CGF a potential candidate as an electrospinning scaffold material for tissue engineering.

A poly[N-(2-hydroxypropyl) methacrylamide] (PHPMA) hydrogel immobilized with the neurite-promoting peptide sequence of xIKVAVx was synthesized and its structure was characterized. The PHPMA-IKVAV hydrogel displayed an interconnected porous structure. The ability of the hydrogel to support axonal outgrowth in the injured adult rats cerebrum cavity and to promote tissue regeneration was evaluated. After implantation for 4, 6, 12 and 18 weeks, the brain sections were processed for histological staining. The observations of the sections showed that the polymer hydrogel provided a structural, three-dimensional continuity across the defect and favoured reorganization of local wound-repair cells, angiogenesis and axonal growth into the hydrogel scaffold. Compared with the unmodified PHPMA hydrogel, the PHPMA-IKVAV hydrogel displayed greater ability to repair tissue defects in the cerebrum nervous system.