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Physics based low dimensional approaches are playing an increasingly important role in our understanding of turbulent flows. They provide an avenue for us to understand the connection between coherent structures and the overall dynamics of the flow field. As such these approaches are fundamental to the implementation of physics based active control methodologies. In this paper we review applications of various low dimensional approaches (including Proper Orthogonal Decomposition (POD), Linear Stochastic Estimation (LSE), Conditional Averages and Wavelets) to turbulent shear layers and connect the results to simulation tools. The applications of all these methods to the 2D shear layer suggest a kind of universal behavior of both the large scale structure extracted and the background turbulence, irrespective of the technique (filtering method) used. A review of the application of POD and LSE to the axisymmetric jet at Reynolds numbers between 100,000 and 800,000 and Mach numbers ranging from very low to 0.6 suggest a universal behavior where the dynamics can be described with relatively low dimensional information (1 POD mode and 5 or 6 Fourier azimuthal modes) over the Reynolds/Mach number range studied. These results provide physical justification for simulation tools such as VLES, LES and SDM since such computational methods involve different levels of low-dimensional modeling.

An azimuthal-rotation sample holder, with improvements in the rotation-angle accuracy and rotation-axes displacement during azimuthal rotation, was developed for molecular-orientation analysis using scanning transmission X-ray microscopy., In this study, an azimuthal-rotation sample holder compatible with scanning transmission X-ray microscopy was developed. This holder exhibits improvement in the accuracy of rotation angles and reduces the displacement of the rotation axes during azimuthal rotation by using a crossed roller bearing. To evaluate the performance of the holder, the authors investigated the dependence of the optical density around the C K-edge absorption of π-orbital-oriented domains in natural spherical graphite on the rotational angle by using linearly horizontally and vertically polarized undulator radiation. Based on the dependence of the optical density ratio between C 1s → π* and 1s → σ* excitation on the polarization angle of the X-rays, the average two-dimensional orientation angle of the π orbital in each position in a natural spherical graphite sample was visualized.

Distance cartograms can be viewed as a special type of azimuthal map projection. Area cartograms are a generalization of equal area maps in the sense that conventional equal area maps are a special case of area cartograms in which the physical earth surface is the propeerty that is preserved. This can be shown by examination of the map projection equations.

This paper studies the optimal detection performance of the standard frequency diverse array (FDA) radar and FDA multi-input multioutput (FDA-MIMO) radar in Gaussian clutter and noise. Array signal processing scheme at the receiver is firstly designed to obtain the array steering vector containing range, azimuth, and frequency increment. For the two array configurations, namely, collocated transmit-receive and collocated transmit distributed receive, the likelihood ratio test statistics and the test statistic distributions are derived in the Neyman–Pearson sense. It is then investigated how the number of array elements influences the detection performance of various radar systems at low signal-to-noise ratio (SNR). Several numerical simulations are carried out to demonstrate that the performance improvement is hard for MIMO and FDA-MIMO by only increasing the number of transmit elements, while it is achievable for the FDA. The paper finally makes a comparative analysis for detection performances of five radar configurations under different SNRs.

SUMMARY The horizontal-to-vertical spectral ratio (HVSR) of ambient noise is commonly used to infer a site's resonance frequency (${f_{0,site}}$). HVSR calculations are performed most commonly using the Fourier amplitude spectrum obtained from a single merged horizontal component (e.g. the geometric mean component) from a three-component sensor. However, the use of a single merged horizontal component implicitly relies on the assumptions of azimuthally isotropic seismic noise and 1-D surface and subsurface conditions. These assumptions may not be justified at many sites, leading to azimuthal variability in HVSR measurements that cannot be accounted for using a single merged component. This paper proposes a new statistical method to account for azimuthal variability in the peak frequency of HVSR curves (${f_{0,HVSR}}$). The method uses rotated horizontal components at evenly distributed azimuthal intervals to investigate and quantify azimuthal variability. To ensure unbiased statistics for ${f_{0,HVSR}}$ are obtained, a frequency-domain window-rejection algorithm is applied at each azimuth to automatically remove contaminated time windows in which the ${f_{0,HVSR}}$ values are statistical outliers relative to those obtained from the majority of windows at that azimuth. Then, a weighting scheme is used to account for different numbers of accepted time windows at each azimuth. The new method is applied to a data set of 114 HVSR measurements with significant azimuthal variability in ${f_{0,HVSR}}$, and is shown to reliably account for this variability. The methodology is also extended to the estimation of a complete lognormal-median HVSR curve that accounts for azimuthal variability. To encourage the adoption of this statistical approach to accounting for azimuthal variability in single-station HVSR measurements, the methods presented in this paper have been incorporated into hvsrpy, an open-source Python package for HVSR processing.

Methods for measuring of sea currents are currently being actively developed. Significant success has been achieved at the middle incidence angles, when the scattering mechanism is presumed to be resonant. This paper presents results from experiments with Doppler radar installed on the bridge over a river. The measurements were carried out at small incidence angles, when the backscattering mechanism is quasi-specular. The dependences of the main parameters of the Doppler spectrum on the azimuth angle were constructed for three scheme of measurements. It is shown that the use of the kurtosis coefficient and the Doppler spectrum width ΔF 42 to describe the Doppler spectrum make it possible to determine the direction of wave propagation relative to the direction of the current. The paper tabulates detailed experimental data for interested scientists, discusses the results, and outlines future plans for experiments.

A novel low-order integrated guidance and control (LOIGC) design model is deduced, and an original diving integrated guidance and control design approach is proposed in this paper. An analytical model between three-channel body rates and components of acceleration of the hypersonic vehicle in the ballistic frame is derived, and the commanded body rates can be directly obtained by analytic calculation. The LOIGC design model is conducted based on three dimensional (3D) relative dynamics between the hypersonic vehicle and target, and the direct relation with respect to line-of-sight (LOS) angles and control surface fin deflections of the hypersonic vehicle is established. Based on the LOIGC model, the design of six-degree-of-freedom (6DOF) guidance and control system for diving hypersonic vehicles can be converted into an output tracking problem of a low-order nonlinear system, and the commanded control surface fin deflections can be directly obtained by controlling derivatives of elevation and azimuth angles of line-of-sight between the hypersonic vehicle and target. In this paper, the system order and tuning parameters of the 6DOF guidance and control system are both decreased, and the design procedure of 6DOF guidance and control system is simplified. The process of calculating commands of angle of attack and bank angle based on desired guidance overloads, and the tracking loops with respect to Euler angles and body rates of the rotational system can also be omitted. In addition, the newly proposed method can improve the utilization of velocity measurements in the body frame of the hypersonic vehicle. Finally, the effectiveness and robustness of the newly proposed integrated guidance and control design approach are verified and investigated using a 6DOF generic hypersonic vehicle model.

Summary In this paper, a gamma detection system consisting of a cesium-137 gamma source and seven gamma detectors is used to evaluate the azimuthal density of cement sheath in horizontal wells. In order to eliminate the impact to gamma detection information caused by casing eccentricity and formation density changes in horizontal wells and obtain correct density results, this paper give the theoratical response of gamma detectors at different source distances and azimuths and raise an inverse method based on Newton iteration and truncated singular values Regularization.Simulation examples with different conditions are designed to verify the inversion results

The dispersion characteristics of the background ionosphere and the random fluctuations of the ionospheric irregularities are an important source of phase error that seriously damages the quality of radar images. To mitigate the ionospheric distortions of P-band spaceborne synthetic aperture radar (SAR) images, a super-resolution deep learning method is proposed in this paper. Different from the traditional imaging method based on the prior knowledge of imaging, the method proposed in this paper directly trains the ionospheric implicit imaging model of spaceborne SAR without complicated iterative processes. First, the phase errors, which are caused by the dispersion and the scintillation in the range and azimuth directions, respectively, are analyzed. Second, an improved u-net structure that embeds the residual network between the encoder and decoder is briefly proposed. Finally, the range doppler algorithm is used to preprocess radar image as the input of the convolutional neural network (CNN) and is compare with the predicted output of the CNN. Experimental results prove the effectiveness of the proposed method in radar image focusing.

A long synthetic aperture passive localization method for two Frequency shift keying (2FSK) signal via azimuth chirp-rate contour is proposed in this paper. By introducing synthetic aperture radar (SAR) imaging technology into passive localization, Doppler frequency change rate of received signal, which is called as azimuth chirp-rate in this paper, is estimated by azimuth focusing. Then, a grid map is formed on the ground and azimuth chirp-rate of each point is calculated to get an azimuth chirp-rate contour map. In the contour map, signal emitter is located in an azimuth chirp-rate curve in which the azimuth chirp-rate value is equal to its estimate. The azimuth chirp-rate contour map of a ground area varies with position of sensor. Therefore, two different azimuth chirp-rate curves can be obtained through different periods of a trajectory and the intersection of the two curves gives estimate of the emitter location.

Similar to the conventional squint mode synthetic aperture radar (SAR) imaging processing, the estimation of Doppler centroid is one key problem for the low-squint-mode (within the range of [−5°, 5°]) multichannel in an azimuth high-resolution and wide-swath (HRWS) SAR system. In this paper, based on the electromagnetic scattering model, a novel weighted Doppler centroid estimation approach is proposed for the multichannel in the azimuth HRWS (MC-HRWS) SAR system. First, Maxwell’s equations are employed to derive the echo model for the multichannel SAR system. Then, an improved backscattering model is presented based on the small perturbation method and the available echo model, which is adopted to produce the weights for Doppler centroid estimation. More importantly, in order to improve the precision of Doppler centroid estimation, a weighted ambiguity-free Doppler centroid estimation approach is proposed, where the range-variant characteristic of Doppler centroid is employed to resolve the ambiguity number of Doppler centroid, and the echoes from different range bins with different signal-to-noise ratios are considered. In addition, the Cramer–Rao low bound of the estimated Doppler centroid is also discussed in this paper. The effectiveness of the proposed Doppler centroid estimation approach is verified via simulated and real measured low-squint-mode MC-HRWS SAR data.

Despite its success with regular close-range and remote-sensing images, the scale-invariant feature transform (SIFT) algorithm is essentially not invariant to illumination differences due to the use of gradients for feature description. In planetary remote sensing imagery, which normally lacks sufficient textural information, salient regions are generally triggered by the shadow effects of keypoints, reducing the matching performance of classical SIFT. Based on the observation of dual peaks in a histogram of the dominant orientations of SIFT keypoints, this paper proposes an illumination-invariant SIFT matching method for high-resolution planetary remote sensing images. First, as the peaks in the orientation histogram are generally aligned closely with the sub-solar azimuth angle at the time of image collection, an adaptive suppression Gaussian function is tuned to level the histogram and thereby alleviate the differences in illumination caused by a changing solar angle. Next, the suppression function is incorporated into the original SIFT procedure for obtaining feature descriptors, which are used for initial image matching. Finally, as the distribution of feature descriptors changes after anisotropic suppression, and the ratio check used for matching and outlier removal in classical SIFT may produce inferior results, this paper proposes an improved matching procedure based on cross-checking and template image matching. The experimental results for several high-resolution remote sensing images from both the Moon and Mars, with illumination differences of 20°–180°, reveal that the proposed method retrieves about 40%–60% more matches than the classical SIFT method. The proposed method is of significance for matching or co-registration of planetary remote sensing images for their synergistic use in various applications. It also has the potential to be useful for flyby and rover images by integrating with the affine invariant feature detectors.

The zero-Doppler centroid control in geosynchronous synthetic aperture radar (GEO SAR) is beneficial to reduce the imaging complexity (reduces range-azimuth coupling in received data), which can be realized by adjusting the radar line of sight (RLS). In order to maintain the zero-Doppler centroid throughout the whole orbit of the GEO SAR satellite, the RLS needs to be adjusted in real-time. Due to the ultra-long synthetic aperture time of GEO SAR, the RLS variation during the synthetic aperture time cannot be neglected. However, in the previous related papers, the real-time variation of RLS during the synthetic aperture time was not taken into account in the calculation of Doppler parameters, which are closely related to the RLS, resulting in inaccurate calculation of Doppler parameters. Considering this issue, an accurate Doppler model (the model of relative motion between satellite and ground target) of GEO SAR is proposed in this paper for the accurate calculation of Doppler parameters (Doppler centroid and Doppler bandwidth and other parameters). Finally, simulation experiments are designed to confirm the effectiveness and necessity of the proposed model. The results indicate that the RLS variation during the synthetic aperture time has a considerable effect on Doppler parameters performance of the GEO SAR, and refers to a more stable azimuth resolution performance (the resolution is kept near a relatively stable value at most positions of the elliptical orbit) compared with the case that does not consider the real-time zero-Doppler centroid control.

TwinSAR-L (Terrain Wide-swath Interferometric L-band SAR mission) is an innovative space-borne bistatic SAR mission for global dynamics, which will be launched in 2020. This paper investigates ionospheric effects on phase and Faraday rotation of interferometry for TwinSAR-L systems. This ionospheric offset arises from different incident angles along each path in inhomogeneous ionosphere. Plus, the inhomogeneity of ionospheric TEC will cause different range delay and defocusing due to dispersion and azimuth shift. The analysis in this paper will be important for TwinSAR-L mission and potential Tandem-L mission in the future.

A radar direction-finding technique based on time-modulated arrays (TMAs) is proposed in this paper. The carrier frequency of signals radiated/received by TMAs with phase center motion suffers from the Doppler effect and the frequency shifting is direction-dependent. This interesting property of TMAs can be used for direction-finding. A radar direction-finding system using TMA is designed in this paper. Performance of the novel bearing technique is deeply discussed and compared with conventional direction-finding techniques. Both numerical and experimental results demonstrate that this technique is effective and provides reconfigurable angle-searching range and bearing accuracy. The flexibility allows the proposed direction-finding technique to be qualified for different detection requirements just by changing time schemes of TMAs.

In this paper, we present two approaches to calculate the voltage of a tilted coil in the cylindrically multilayered medium. The calculation of electromagnetic (EM) fields can be solved using a set of pseudo-analytical formulas. These formulas contribute to the forward modeling and inversion of traditional azimuthal resistivity Logging-While-Drilling (LWD) measurements or the new extra-deep azimuthal resistivity tool (EDAR). We assume the transmitter is an axial-coil and therefore only zero-order harmonic components of electric fields need to be considered because of axial symmetry. A set of novel general reflection and transmission coefficients proposed in this paper are presented as the ratios of the modified Bessel functions, and thereby the numerical overflow problems can be obviated. Numerical examples are presented to verify the validity and the stability of the proposed formulas.

In the previous paper, we introduced a numerical method which uses an interpolation of the big known reference function to obtain a reference function for any distance and radiation pattern width bypassing the Fourier transform stage. Interpolation has an algorithmic complexity O(N), while the FFT has a complexity O(NlogN). But modern FFT libraries are highly optimized to meet the modern CPUs and GPUs feature. So, it is not only the algorithmic complexity which the final performance depends on but also satisfying the architectural requirements such as data memory arrangement and vector extensions. This paper reveals some approaches and steps to gain the maximal algorithm performance on the modern CPUs.

There is phase distortion in squint Frequency Modulated Continuous Wave Synthetic Aperture Radar (FMCW-SAR), and the phase distortion may affect the imaging process. In this paper, the echo signal model of squint FMCW-SAR is established, and the phase error caused by squint is analysed. Then in this paper, an improved Range-Doppler (RD) imaging algorithm based on FMCW-SAR is proposed from the point of view of real-time processing. The algorithm has the characteristics of small computation and high precision, and is suitable for real-time imaging. Finally, the feasibility and validity of the algorithm are verified by simulation analysis.

This paper is the second of two focusing on the analytical solutions for light transport in infinite homogeneous tissue with an azimuth-dependent (m-dependent) anisotropic scattering kernel by two approaches, Case’s singular eigenfuncions (CSEs) expansion and Fourier transform, and proving the consistence of the two solutions theoretically. In this paper, the analytical solution for the m-dependent truncated scattering kernel was derived via the Fourier transform and inversion, and expanded with the m-dependent generalized singular eigenfuncions (GSEs). Two kinds of GSEs that are defined by Ganapol in the case m=0 are extended to arbitrary azimuthal orders and proven to be consistent with CSEs both in expression forms and in intrinsic behaviors. By applying the Fourier transform inversion on the solution for the three-term recurrences, the Green’s function of radiance distributions is obtained successfully, and it conforms perfectly to the CSEs solution in the limit, which has already been discussed in our first accompanying paper. Meanwhile, as a byproduct, a series of identities about the m-dependent Chandrasekhar orthogonal polynomials were presented and will be greatly helpful for further studies.

In this paper we apply the Doppler beam sharpening technique of azimuth refinement to the emerging area of low-THz radar. This improves the image quality and thus aids object classification in low-THz radar imaging systems, for example for autonomous platforms. The paper briefly explains the theory behind the process of Doppler beam sharpening; this is then experimentally tested and verified against two closely space corner reflector reference targets.

This paper proposes a new method to attain explicit expressions of some quantities associated with performance analysis of the maximum likelihood (ML) direction-of-arrival (DOA) algorithm in the presence of an additive Gaussian noise on the antenna elements. The motivation for the paper is to make quantitative analysis of the ML DOA algorithm in the case of multiple incident signals. We present a simple method to derive closed-form expression of estimation error of DOA estimate based on the Taylor series expansion. We address the performance analysis of the ML angle-of-arrival (AOA) estimation algorithm in this paper. We discuss the estimation of azimuth of multiple incident signals using bistatic multiple input multiple output (MIMO) radar model. Based on the Taylor series expansion and approximation, we get explicit expressions of estimation error of all incident signals. The validity of the derived expressions is shown by simulation results.

When the airborne or missile borne radar works at the forward-looking imaging mode, the common techniques for improving azimuth resolution are invalid, because the difference between the Doppler frequencies of targets in different azimuths is very small. Meanwhile, other real beam sharpening methods only have very limited effect on azimuth resolution enhancement. For the problem of forward-looking imaging of ship targets at sea surface, a L 1/2 regularization based azimuth resolution enhancement algorithm is proposed in this paper. This algorithm can make full use of the obvious sparsity in the imaging area. The linear observation signal model for forward-looking imaging is built, and the iterative calculation process of L 1/2 regularization and the detailed steps of multi-channel radar forward-looking imaging are provided in this paper. Finally, the effectiveness of the proposed algorithm is tested and verified with simulation data and real data.

This paper proposes a Doppler beam sharpening (DBS) imaging method to achieve the high azimuth resolution in the forward-squint region based on fast iterative adaptive approach (F-IAA). In conventional DBS imaging, fast Fourier transform (FFT) method is adopted to obtain the estimation of Doppler center frequencies of targets, but it suffers from the low resolution and high sidelobes. Though a weighted least-squares based, nonparametric iterative adaptive approach (IAA) is proposed to instead of FFT method, it can not be applied to engineering applications because of its large computational complexity. In this paper, a F-IAA method, using the Gohberg-Semencul (GS)-type factorization of IAA covariance matrix, is introduced to reduce the computational complexity of brute force IAA method. Compared with the brute force IAA method, the F-IAA method mainly reduces the computational complexity of the operation of IAA covariance matrix and the inverse operation of covariance matrix. The effect is testified in simulation.

Comparing with the traditional mobile communication system, there is a strong line-of-sight (LOS) component in satellite to mobile communication. In fact, LOS and diffuse scattered component are both influenced by shadowing and obstacles. In this paper, a theoretical model for a land mobile satellite channel is analyzed in various shadowing situations. Since LOS and diffuse scattered component are both considered, the angles of arrival are not uniformly distributed in a given azimuth sector. So a kind of asymmetrical Doppler power spectrum is proposed in the paper. Then, in order to effectively respond to the actual channel environment, the different state length \( L_{frame} \) in each state is presented and simulated, the minimum state length should be shorter in the environment, which the channel changes intensely. Finally, numerical simulations in four-state Markov model indicate that the proposed asymmetrical Doppler power spectrum and the various \( L_{frame} \) are reasonable.

This paper focuses on the novel approaches for inverse synthetic aperture radar (ISAR) imaging of maneuvering target. It is based on the assumption that the azimuth echoes in a range bin are modeled as multi-component cubic phase signal (CPS) after the translational motion compensation, and the ISAR image quality will be deteriorated by the conventional Fourier transform in the azimuth focusing. In this paper, two novel algorithms for parameters estimation of multi-component CPS based on the local polynomial Wigner distribution (LPWD) and integrated high-order ambiguity function (IHAF) are proposed, and the high-quality instantaneous ISAR image can be obtained by the estimated CPS parameters. The results of simulated and real aircraft data are provided to validate the effectiveness of the proposed methods.

This paper is concerned with modelling the effects of swirling flow on turbomachinery noise. We develop an acoustic analogy to predict sound generation in a swirling and sheared base flow in an annular duct, including the presence of moving solid surfaces to account for blade rows. In so doing we have extended a number of classical earlier results, including Ffowcs Williams & Hawkings’ equation in a medium at rest with moving surfaces, and Lilley’s equation for a sheared but non-swirling jet. By rearranging the Navier–Stokes equations we find a single equation, in the form of a sixth-order differential operator acting on the fluctuating pressure field on the left-hand side and a series of volume and surface source terms on the right-hand side; the form of these source terms depends strongly on the presence of swirl and radial shear. The integral form of this equation is then derived, using the Green’s function tailored to the base flow in the (rigid) duct. As is often the case in duct acoustics, it is then convenient to move into temporal, axial and azimuthal Fourier space, where the Green’s function is computed numerically. This formulation can then be applied to a number of turbomachinery noise sources. For definiteness here we consider the noise produced downstream when a steady distortion flow is incident on the fan from upstream, and compare our results with those obtained using a simplistic but commonly used Doppler correction method. We show that in all but the simplest case the full inclusion of swirl within an acoustic analogy, as described in this paper, is required.

Micro Inertial Measurement Unit (MIMU) can navigate automatically without any external information but it is unable to complete a long-time high-accuracy navigation task because its error will accumulate with time increase. So it needs assistance of other navigation methods. Because polarization navigation has good stability, well invisibility, and the yaw error is not divergent. So it can be used to adjust the output of MIMU. This paper presents a novel polarization navigation sensor, which uses division-of-amplitude method. The sensor obtains the azimuth angle by measuring the Stokes vector. This paper designs the structure of the polarized light collection device, and discusses the data acquisition program. The result of experiment shows that the sensor can collect the information of polarized light and the azimuth angle can be gotten effectively.

Pre-stack one-way wave equation (OWE) is a useful tool for seismic imaging and modeling. Since the idea of OWE appeared in the 1970s, geophysicists have made great effort to improve the accuracy of the one-way wave equation extrapolators. In this paper, we present the idea of solving OWE using the Fourier integral method, which represents OWE as a Fourier integral equation and solves it in dual spaces (both space and wave-number domains). By doing this, we can propagate wave-fields up to nearly 90^o angle from the vertical direction in the presence of lateral velocity variations. The proposed method is stable, does not suffer from the numerical dispersion, and overcomes the azimuthal anisotropy problem when extended to three dimensions. The computation cost of the Fourier integral method is too high and was considered impractical for a conventional computer. In this paper, we take advantages of the Graphic Processing Unit (GPU) and use the matrix multiplication technique to accelerate the algorithm. The speedup ratio we obtained is tens of hundreds times so that the method can be applied to a real project for pre-stack depth imaging.

WindSat has provided an opportunity to investigate the first spaceborne passive fully polarimetric observations of the Earth's surface. In this paper, Arctic sea ice was investigated. The passive polarimetric data are provided in the form of the modified Stokes vector consisting of four parameters. The first two components of the modified Stokes vector are the vertically and horizontally polarized brightness temperatures, which have been continuously measured by various radiometers over the last three decades. The third and fourth Stokes components provide in formation on the degree of polarization of the emission. In this paper, three types of analysis are carried out: spatial (maps considering different azimuth angle intervals), temporal (time series of daily averaged Stokes components over a small selected azimuth angle range), and azimuthal (variations w.r.t. the azimuth angle over selected study areas). Analysis has shown the highest brightness temperature variations for the 37-GHz third Stokes component (>; 2 K) during summer. The next highest signals were observed for the 10.7-GHz third and fourth Stokes components (>; 1 K) during summer as well. The 37-GHz fourth Stokes component exhibited the least variability (>; 1 K). Spikes of up to 2 K were identified in the time series of the 37-GHz third Stokes component during mid-January 2004 (winter) over first-year ice regions. The near-surface air temperature of the European Center for Medium-Range Weather Forecasts model data and the Special Sensor Microwave/Imager National Aeronautics and Space Administration Team ice concentrations revealed that, during these events, the surface temperatures reached near melting levels and the retrieved ice concentrations were reduced to about 80%. Moreover, these observations also showed clear evidence of first harmonic azimuthal dependence. Geophysical parameters, such as temperature and ice leads, are likely to be the causes. The larger signals which occurred during summer were identified as being related to the ice surface temperatures being near melting.

In this paper, an indoor UWB communications system that applies time reversal (TR) for transmitting the desired signal is proposed. First we define equivalent channel model of TR-UWB, which is the convolution of channel impulse response and its complex conjugate time-reversed version. Then spatial, temporal and frequency characteristics of equivalent channel are analyzed and the analytical or semi-analytical results are validated by comparing measurements with simulation. (Semi) analytical expression of equivalent channel transfer function, TR UWB power delay profile (PDP), focusing gain, spatial correlation and power azimuth spectrum (PAS) is performed. Also probability density function (PDF) of TR-UWB amplitude and path-gain is derived. Analysis and simulation results of different distributions, such as uniform, Laplacian and Gaussian for PAS, are considered and presented. It is shown that uniform and truncated Laplacian distributions are appropriate fits to the measurement results for power azimuth spectrum of TR-UWB. It is seen that for distances greater than λ/2 from the authorized receiver, received signal decreases 10 dB, where λ is the wavelength of the central frequency. Finally, PDF of TR-UWB path-gain is described. Measurement results show that for small time windows, the densities of the path-gain are highly non-Gaussian. But for starting time of 10Tw or more, and window size of 2Tw or more the densities are nearly Gaussian, where Tw is the transmitted pulse duration. Copyright © 2010 John Wiley & Sons, Ltd. Extensions of TR techniques to radio electromagnetic propagation for ultra wideband communications were investigated. The main features of the current paper are: Analysis of Time Reversal UWB power delay profile is presented and a closed form is derived for focusing gain. It is shown that uniform and truncated Laplacian distributions are appropriate fits to the measurement results for PAS of TR-UWB. Closed form of spatial correlation and channel transfer function of TR-UWB are presented. PDF of TR-UWB amplitude and path gain is derived.

To improve the impaired azimuth resolution of the novel Terrain Observation by Progressive Scans (TOPS) mode, a new multi- channel single phase center multiple beam (SPCMB) TOPS mode is proposed in this paper for high-resolution wide-swath (HRWS) imaging. However, the progressive azimuth beam scanning leads to the Doppler spectrum aliasing and both beam center time and Doppler centroid varying with the target's azimuth location. Challenges may arise for processing the SPCMB-TOPS SAR data from these problems. In this paper, an e-cient full aperture imaging approach is proposed to process the raw data. The sketch of the proposed imaging approach is described in detail. Simulation results of point targets validate the proposed imaging approach.

Three-chamber configuration is the simplest type among the fluid soft actuators with multiple degrees of freedom in three-dimension space. However, the motion control of the three-chamber fluid soft actuator is a challenging task because of the strong coupling effect of the chambers which are connected together. This short paper develops a novel electrohydraulic control device with decoupling function. The supplied pressures of three cylinders in the electrohydraulic control device are linked by a swash plate mechanism. The axial displacement, the tilted angle, and the azimuth angle of the swash plate are driven by three electrical motors separately and corresponding to the stretched length, the bending angle, and the yaw angle of the three-chamber soft actuator. Thus, the three outputs of the soft actuator can be independently controlled. Theoretical analysis is implemented to provide basis for machine design. A prototype is fabricated and experiments under both of static and dynamic conditions are carried out to evaluate the performance of the proposed concept. The results show that the electrohydraulic control device can effectively drive the three-chamber soft actuator and reduce the difficulty of controller design.

Context. Photon orbital angular momentum (POAM) has been created in the laboratory, yet it is still relatively unknown. How does POAM manifest itself in astronomy? Are there any applications for measuring astrophysical POAM? Aims. In this paper, I 1) explain POAM in an astronomical context; 2) define the POAM observables for astronomy; 3) create generic systems-based calculi that describe how POAM propagates from celestial sphere to detector; 4) use the calculi with several astronomical instruments as examples of their utility; 5) demonstrate an application for astrophysical POAM measurements; and 6) relate POAM to existing astronomical instruments and concepts. Methods. Electric fields are expanded into azimuthal Fourier components, and the intensities are expanded into correlations or rancors. The source electric fields are spatially incoherent. In the systems-based calculi, the inputs are located on the celestial sphere, the system is represented by propagation through free space and instrument, and the outputs are located in a specific plane. The diffraction and point-spread function expansions are very generic and can be used with any type of instrument. I employ these examples to demonstrate the calculi (in order of increasing difficulty): free space, single telescopes, interferometers, coronagraphs, and rancorimeters. Results. The azimuthal Fourier components of the electric field correspond to POAM vortex states. Rancors contain less information than correlations, yet they are easier to measure and can be used in many applications. Propagation through an aberrated telescope applies external torque, which may be expressed in terms of Zernike polynomials. I prove that a sectored phase mask in a focal-plane coronagraph applies torque to the low-order states, producing a null. Also, I prove that a Michelson interferometer is inherently capable of filtering POAM; e.g., tracking 180 ◦ from the central fringe eliminates even states, producing a null. A limited rancorimeter can be created by placing a focal-plane wedge mask in a coronagraph. The resulting rancors can be used to perform super-Rayleigh observations of unresolved unresolved objects, such as binary stars. There are three types of source POAM: intrinsic, structure, and pointing. Instrumental POAM, which must be calibrated, includes optical aberrations and atmospheric turbulence. Conclusions. This paper represents the starting point for future research: 1) making a priori predictions about the intrinsic POAM of astronomical sources; 2) designing ground- and space-based POAM-measuring instruments; 3) understanding existing instruments in terms of POAM; 4) minimizing the effects of random noise on POAM; and 5) calibrating all types of instrumental POAM.

The multichannel in azimuth synthetic aperture radar (MC-SAR) system is very promising for high-resolution and wide-swath (HRWS) imaging, whereas Doppler centroid block estimation related to the channel phase calibration has a great influence on image quality. In order to solve the problem of the Doppler centroid block estimation, this paper proposes an effective way to improve its precision. This paper analyses the reason of the Doppler ambiguity hop when estimating the Doppler centroid in block. According to the position of the reference baseband Doppler centroid and the block estimate of baseband Doppler centroid, the Doppler ambiguity is adjusted differently. Finally, the effectiveness of the proposed method is demonstrated by real airborne data.

Aspects of the interpretation of measured data on the gravity gradient tensor (GGT) are examined. The problem is posed in relation to the great progress achieved in recent years in the development of instrumentation and the method of GGT measurements on mobile carriers. In our opinion, the new methods of measurement and the new data obtained with their help require the development of new methods of interpretation of potential fields. The paper addresses two methods taking advantage of simultaneous measurements of all components of the GGT and anomalies of the gravitaty field V z. It is shown that the joint analysis of all GGT components can provide independent constraints on the noise level in various components. The method of tensor deconvolution proposed in the paper is a tensor analogue of the Euler method. The method is based on the calculation of invariants and is, therefore, stable with respect to the orientation uncertainties of the measuring system. The method provides means for estimating the structural index and, therefore, is particularly effective in the treatment of fields that contain isometric and/or elongated anomalies. The calculation of invariants and the tensor ratio can also be used for the development of procedures enabling automatic estimation of the axis strike azimuths of elongated anomaly-forming bodies.

When range ambiguity and range dependence coexist in forward-looking ground moving target indication (GMTI) radar, the traditional space-time adaptive processing (STAP) approaches may fail to work. To cope with this issue, a range-ambiguous clutter suppression approach based on elevation time diverse array (TDA) radar system and azimuth phase coding (APC) technique is presented in this paper. Due to the advantages of frequency scanning, the elevation TDA radar is able to cover all directions by transmitting a single time-shifted waveform. Therefore, a frequency bandpass filter is devised for each single range-bin to achieve the mainlobe echo extraction procedure. In the sequel, the azimuth phase decoding and Doppler filtering are performed to deal with the residual sidelobe ambiguous energy. With two-dimensional filtering, one can extract the unambiguous signals for each range region independently. Then, the traditional clutter compensation and STAP methods are employed to suppress the ground clutter. Finally, the effectiveness of the proposed framework in resolving range ambiguity are verified by the numerical simulation experiments.

In this paper, the monostatic (transmitter and receiver are located at the same place) and bistatic (transmitter and receiver are distinct) statistical shadowing functions from an anisotropic two-dimensional randomly rough surface are presented. This parameter is especially important in the case of grazing angles for computing the bistatic scattering coefficient in optical and microwave frequencies. The objective of this paper is to extend the previous work (Bourlier C, Berginc G and Saillard J 2002 Waves Random Media 12 145–74), valid for a one-dimensional surface, to a two-dimensional anistropic surface by considering a joint Gaussian process of surface slopes and heights separating two points of the surface. The monostatic average (statistical shadowing function average over the statistical variables) shadowing function is then performed in polar coordinates with respect to the incidence angle, the azimuthal direction and the surface height two-dimensional autocorrelation function. In addition...

This paper attempts to explain the relationships between fractured medium properties and seismic signatures and distortions induced by geology‐related influences on azimuthal AVO responses. In the presence of vertically aligned fractures, the relationships between fracture parameters (fracture density, fracture aspect ratio, and saturated fluid content) and their seismic signatures are linked with rock physics models of fractured media. The P‐wave seismic signatures studied in this paper include anisotropic parameters (δ(v),(v), and γ(v)), NMO velocities, and azimuthal AVO responses, where δ(v)is responsible for near‐vertical P‐wave velocity variations,(v)defines P‐wave anisotropy, and γ(v)governs the degree of shearwave splitting. The results show that in gas‐saturated fractures, anisotropic parameters δ(v)and(v)vary with fracture density alone. However, in water‐saturated fractures δ(v)and(v)depend on fracture density and crack aspect ratio and are also related to Vp/VSand Vpof background rocks, respectively. Differing from δ(v)and(v), γ(v)is the parameter most related to crack density. It is insensitive to the saturated fluid content and crack aspect ratio. The P‐wave NMO velocities in horizontally layered media are a function of δ(v), and their properties are comparable with those of δ(v).Results from 3‐D finite‐difference modeling show that P‐wave azimuthal AVO variations do not necessarily correlate with the magnitude of fracture density. Our studies reveal that, in addition to Poisson's ratio, other elastic properties of background rocks have an effect on P‐wave azimuthal AVO variations. Varying the saturated fluid content of fractures can lead to azimuthal AVO variations and may greatly change azimuthal AVO responses. For a thin fractured reservoir, a tuning effect related to seismic wavelength and reservoir thickness can result in variations in AVO gradients and in azimuthal AVO variations. Results from instantaneous frequency and instantaneous bandwidth indicate that tuning can also lead to azimuthal variations in the rates of changes of the phase and amplitude of seismic waves. For very thin fractured reservoirs, the effect of tuning could become dominant. Our numerical results show that AVO gradients may be significantly distorted in the presence of overburden anisotropy, which suggests that the inversion of fracture parameters based on an individual AVO response would be biased unless this influence were corrected. Though P‐wave azimuthal AVO variations could be useful for fracture detection, the combination of other types of data is more beneficial than using P‐wave amplitude signatures alone, especially for the quantitative characterization of a fractured reservoir.

As a high resolution airload with accurate rotor wake is pivotal for rotor BVI (Blade-vortex interaction) analysis, a hybrid method with combined Navier-Stokes equation, viscous wake model, and FW-H (Ffowcs Williams-Hawkings) equation is developed for BVI airload and noise in this paper. A comparison with the CFD (Computational Fluid Dynamics)/FW-H method for the AH-1/OLS (Operational Load Survey) rotor demonstrates its capability for favorable accuracy and high computation efficiency. This paper further discusses the mechanisms for the impacts of four flight parameters (i.e., tip-path-plane angle, thrust coefficient, tip Mach number, advance ratio) on BVI noise. Under the BVI condition, several BVI events concurrently occur on the rotor disk. Each interaction has a distinct radiation direction which depends on the interaction azimuth, and its noise intensity is highly associated with the characteristic parameters (e.g., miss-distance, interaction angle, vortex strength). The BVI noise is dominated by the interactions at 30–90° in azimuth on the advancing side, of which the wake angle range is from 180° to 540°. Furthermore, the tip-path-plane angle, thrust coefficient, and tip Mach number change the noise intensity mainly via miss-distance, interaction angle, and vortex strength, but for different advance ratios, the noise intensity and propagation direction are more dependent on the interaction angle and interaction azimuth.

The synthetic aperture radar (SAR) image of ground moving target is easily defocused given the unknown relative motions between radar and target. In addition, the limitation of pulse repetition frequency for SAR easily induces the azimuth ambiguity (including Doppler center blur and spectrum ambiguity) of target signals, which causes the difficulty of moving target imaging. In this paper, a new efficient method for imaging and motion parameter estimation of ground moving targets with azimuth ambiguity is presented. Firstly, the time reversal process-2D scaled Fourier transform (TRP-2DSCFT) is developed to eliminate the effects of cross-track velocity and estimate the along-track velocity of target, simultaneously. Secondly, an operation based on discrete polynomial-phase transform and phase compensation function is proposed to estimate the cross-track velocity of target. Finally, a matched filter function based on estimated parameters is constructed to focus the moving targets. The well-focused result can be obtained by the presented method without the residual compensation errors. Moreover, the proposed method is computationally efficient given that the exhaustive searching steps are avoided. Additionally, the azimuth ambiguity can be effectively removed without the ambiguity number searching and pre-processing based on the prior information. The cross-terms for multiple target processing are analyzed. The effectiveness of the method is verified by both spaceborne and airborne real data results.

Inverse synthetic aperture radar (ISAR) imaging is used to produce high resolution images of moving targets from long distances which is used for automatic target recognition. Although the principle is same for a ship target on the sea and that of flying target in the sky, the former usually have more complex motions than the latter which makes the motion compensation very difficult [1]. However the change in phase chirp rate caused by complex motions of ship will deteriorate the azimuth focusing quality. In this paper, imaging of a target is shown for different ISAR techniques like Range Doppler, Range instantaneous Doppler and Range instantaneous chirp. The paper is organized as follows. In Section-I, an Introduction to ISAR imaging of ship target with motions is given. In Section-II the imaging techniques of the target with motion is given with necessary mathematical expressions. In Section-Ill the simulation results are shown. In Section IV finally the conclusion is explained.

Based on the imaging geometry of squint mode, the characteristics of echoes based on four order approximate expressions in slow time domain in squint mode are analyzed in this paper. The conclusion that the influence of range walk is much larger than range bending in high squint mode is drawn. The traditional Range-Doppler algorithm is no longer suitable for high squint SAR imaging. Based on the conclusion a modified Range-Doppler algorithm suitable for high squint SAR imaging is derived in this paper. At first range walk is corrected in range frequency domain to make the coupling between range and azimuth small. After that, range compression, secondary range compression, azimuth compression and higher order phase correction are presented in the modified algorithm. Compared with the traditional Range-Doppler algorithm, the imaging results of simulation are presented to demonstrate the accuracy and validity of the proposed algorithms in high squint SAR imaging.

This paper is focused on the detection and resolution of Unmanned Aerial Vehicles (UAVs) via a multi-channel phased-array radar with emphasis on the challenging situation where multiple targets are unresolved in the range-Doppler domain and closely spaced in angle. An adaptive processor is devised which, after triggering a detection in a given range-Doppler cell, implements a binary decision about the presence of a single or multiple targets. In the former situation, the plain monopulse algorithm is used to estimate the target azimuth whereas, in the latter case, a super-resolution technique is activated to get the angular parameters of the UAVs. The performance of the signal processor is assessed both on simulated and on measured data (endowed by ground-truth) collected by a 12-channel phased-array radar operating in the C-band. The results highlight the capabilities of the architecture to detect the presence of targets, to discriminate the single from the multiple targets scenario, and to resolve the UAVs in angle domain when they are unresolved in range and Doppler.

In this paper, we propose an imaging algorithm of m issile-borne SAR in diving and squint mode. The diving acceleration, vertical speed, and squint angle caused a large range cell migration (RCM). The algorithm divides the RCM correction (RCMC) into two steps. Firstly, correcting the main part of range walk in time domain. Secondly, correcting the remaining RCMC in range frequency-azimuth Doppler frequency domain, using an analytical form of the signal 2-D spectrum derived by the method of series reversion. Theoretical analysis and simulation results illustrate its validity, and satisfy the imaging quality of missile-borne SAR in high squint model. Index Terms - Missile-borne SAR, diving and squint mode, RCMC, series reversion. I. Introduction Compared with general airborne or space-borne SAR, missile-borne SAR has significantly different characteristics. In order to attack the target, missile-borne SAR cannot do the uniform line motion, and also it has a certain acceleration and vertical velocity in the diving flight. Moreover, it usually works in a squint mode in order to acquire a period of turning time. Therefore, the general SAR imaging algorithms are no longer suitable for missile-borne SAR (1-2). In (3), the 2-D spectrum of target echo was derived by the method of series reversion, and the RCMC, range and azimuth compression were accomplished in the 2-D frequency domain. But the algorithm was only suitable for the center single point target imaging of the scene in side-looking mode. An imaging algorithm for missile-borne SAR based on azimuth nonlinear chirp scaling (NLCS) was proposed in (4). After finished RCMC and range compression in the 2-D frequency domain by the method of series reversion, azimuth variation of Doppler FM rates for echo signal were compensated with the operation of azimuth NLCS, which could improve focusing depth and effect. However, the computation is too heavy, and it also only worked in side-looking mode. In (5), the spectral overlapping phenomenon in distance was solved by introducing a new CS factor, and the influence caused by velocity and acceleration of the missile were compensated. A deramp method was introduced to cope with the Doppler spectral overlapping phenomenon in azimuth, but this algorithm was too complex to realize. According to the characteristics of the missile-borne SAR platform motion, a space geometry relationship of the diving and squint movement is established in this paper. Then the instantaneous range between radar and target is calculated, and the point target echo signal model is derived. The Doppler frequency center is corrected in range frequency-azimuth time domain (6). The 2-D spectrum of the point target echo signal is deduced using the method of series reversion. After finishing the pulse compression processing, a focused point target image is got. At last, a missile-borne SAR simulation is used to illustrate its validity of the algorithm. II. Geometric Model and Instantaneous Range

With the development of SAR imaging, the efficiency and rapidity of imaging algorithms have always been a research direction, and real-time imaging is a reflection of the speed of the imaging algorithm. For missile-borne SAR, real-time imaging is particularly important because it provides radar with a sense of the battlefield environment. In the case with a highly squinted angle, the azimuth space variance and the coupling between range and azimuth dimension will become serious in imaging, thus an azimuth frequency nonlinear chirp scaling (AFNCS) algorithm is proposed to solve this problem. Based on linear range walk correction, a novel chirp scaling algorithm is adopted to correct the range migration, and then a high-order phase filtering factor is introduced into the azimuth dimension frequency domain to decrease the azimuth space variance. In addition, combined with the SPECtral Analysis method, the image is focused on the Doppler domain. The AFNCS algorithm does not need complex mathematical calculations such as interpolation and can meet the real-time requirement of missile-borne SAR imaging. Simulation results illustrate the effectiveness of the proposed algorithm. The integration of the research in this paper and the deep learning will further pave the way for real-time SAR imaging applications in disaster monitoring, security and surveillance.

Using the active feedback control system on the elastic wave metamaterial, this research concentrates on the sound transmission with the dynamic effective model. The metamaterial is subjected to an incident pressure and immersed in the external mean flow. The elastic wave metamaterial consists of double plates and the upper and lower four-link mechanisms are attached inside. The vertical resonator is attached by the active feedback control system and connected with two four-link mechanisms. Based on the dynamic equivalent method, the metamaterial is equivalent as a single-layer plate by the dynamic effective parameter. With the coupling between the fluid and structure, the expression of the sound transmission loss (STL) is derived. This research shows the influence of effective mass density on sound transmission properties, and the STL in both modes can be tuned by the acceleration and displacement feedback constants. In addition, the dynamic response and the STL are also changed obviously by different values of structural damping, incident angle (i.e., the elevation and azimuth angles) and Mach number of the external fluid with the mean flow property. The results for sound transmission by two methods are compared, i.e., the virtual work principle for double plates and the dynamic equivalent method corresponding to a single one. This paper is expected to be helpful for understanding the sound transmission properties of both pure single- and double-plate models.

This paper presents a general bistatic SAR focusing algorithm for azimuth variant and invariant configurations. The approach used in this paper is based on Loffeld's Bistatic Formula (LBF). We considered different azimuth contributions of the transmitter and the receiver phase terms in the derivation of the Bistatic Point Target Reference Spectrum (BPTRS). An efficient focusing algorithm is implemented using Scaled Inverse Fourier Transformation (SIFT) and is verified with focusing results of azimuth variant and invariant configurations. (6 pages)

This paper determines the ray structure of a spinning acoustic mode propagating inside a semi-infinite circular cylindrical duct, and thereby determines the ray structure of the field radiated from the end of the duct. Inside the duct, but outside of a caustic cylindrical surface, the rays are piecewise linear helices; on striking the rim of the end-face of the duct, these rays produce ‘Keller cones’ of diffracted rays. The cones determine the structure of the radiated field: for example, no rays penetrate two cone-shaped far-field quiet zones centred on the duct axis; two rays pass through each point in a forward loud zone; and one ray passes through each point in a rearward loud zone. The two rays through each point in the forward loud zone interfere to produce an oscillatory directivity pattern. One quarter of the rays on each cone point back inside the duct and produce the reflected field. Thus the rim of the end-face of the duct acts as a ‘ring source’, in which the radiated and reflected fields have their origin. Every propagating duct mode determines a polar angle and an azimuthal angle; these are taken as parameters specifying the mode and are used to calculate the positions and angles of all the rays. The mathematical method on which the paper is based is Debye's approximation for the Bessel function which appears in the expression for the duct modes; the approximation shows also that the duct contains a region of smooth helical rays on which the field consists of inhomogeneous waves: this region is the inner cylinder, lying inside the annulus of piecewise linear helical rays. The results of the paper are very promising for the application of Keller's geometrical theory of diffraction to detailed calculations of the sound radiated from aeroengine ducts. An alternative description of the field, using Cargill's meridional rays, is summarized.

In this paper, experimental results and theoretical calculations are presented to study the polarimetric emission from water surfaces with directional features. It is observed that the measured Stokes parameters of corrugated fiberglass-covered water surfaces are functions of azimuth angles and agree very well with theoretical calculations. The theory, after being verified by the experimental data, was then used to calculate the Stokes parameters of periodic surfaces without fiberglass surface layer and with rms height of the order of wind-generated water ripples. The magnitudes of the azimuthal variation of the calculated emissivities at horizontal and vertical polarizations corresponding to the first two Stokes parameters are found to be comparable to the values measured by airborne radiometer and SSM/I. In addition, the third Stokes parameter not shown in the literature is seen to have approximately twice the magnitude of the azimuth variation of either Th or Tυ. The results of this paper indicate that passive polarimetry is a potential tool in the remote sensing of ocean wind vector.

Following a recent numerical investigation of tonal noise radiation through two model scale coaxial exhaust configurations (typical of small and large turbofan engines), this paper presents additional computational results in an attempt to improve agreement with the measured data. In the experiments ‘target’ spinning modes were generated with a mode synthesizer located within a uniform annular section upstream of the exhaust nozzles. The in-duct complex mode amplitude of each propagating azimuthal/radial mode was measured with a dual axial array rotated in steps over 180 o . The numerical method is based on the direct solution of the frequency-domain linearised Euler equations and uses the measured complex modal amplitudes to synthesise the modal source excitation at the in-flow boundary. The far-field sound is obtained via an open Kirchhoff surface just outside the jets. Previous comparisons of the radiated sound level directivities measured with polar and azimuthal microphone arrays with the directivities of the computed target modes revealed some discrepancies at various combinations of angle, frequency and flow condition. Additional calculations that take into account the next most dominant, non-target modes have yielded improved comparisons with the measured data. It is the purpose of the present paper to extend this study to include all the propagating modes in the comparisons with measured data for and to cover a wider range of frequencies and flow conditions typical of approach, cut-back and sideline conditions. The computed far-field radiation of (1) the target mode alone, (2) the sum of all the non-target cut-on modes and (3) all the cut-on modes are compared with the measurement. The results indicate that when the contributions from the non-target modes are included, significantly improved agreement with the measured data is usually achieved for the cases in which the directionalities of the target and non-target modes are really distinct. Even if experimentally generated modes are dominant within the duct, the far-field directivity is not necessarily dominated by these modes and comparing the computed radiation for the dominant mode with the experimental data may be misleading in terms of the prediction capability of the numerical method used.