Azimuthal Direction Research Articles

Bidirectional high sidelobe suppression silicon optical phased array

An optical phased array (OPA), the most promising non-mechanical beam steering technique, has great potential for solid-state light detection and ranging systems, holographic imaging, and free-space optical communications. A high quality beam with low sidelobes is crucial for long-distance free-space transmission and detection. However, most previously reported OPAs suffer from high sidelobe levels, and few efforts are devoted to reducing sidelobe levels in both azimuthal ( φ ) and polar ( θ ) directions. To solve this issue, we propose a Y-splitter-assisted cascaded coupling scheme to realize Gaussian power distribution in the azimuthal direction, which overcomes the bottleneck in the conventional cascaded coupling scheme and significantly increases the sidelobe suppression ratio (SLSR) in the φ direction from 20 to 66 dB in theory for a 120-channel OPA. Moreover, we designed an apodized grating emitter to realize Gaussian power distribution in the polar direction to increase the SLSR. Based on both designs, we experimentally demonstrated a 120-channel OPA with dual-Gaussian power distribution in both φ and θ directions. The SLSRs in φ and θ directions are measured to be 15.1 dB and 25 dB , respectively. Furthermore, we steer the beam to the maximum field of view of 25°×13.2° with a periodic 2λ pitch (3.1 μm). The maximum total power consumption is only 0.332 W with a thermo-optic efficiency of 2.7 mW/π .

Allometric relations between DBH and sapwood area for predicting stand transpiration: lessons learned from the Quercus genus

Catchment-scale transpiration is commonly determined by the use of sap-flow sensors, and its quantification, which is critical for water and forest management, relies crucially on the total catchment’s sapwood area (As). Species-specific allometric relationships between the trees As and the trees diameter at breast height (DBH) are widely used for determining stand or catchment As. However, substantial differences between studies challenge the robustness of these relationships between sites displaying various topographical and environmental characteristics. Our objectives for this study are to compare the parameters of these relationships between species of the Quercus genus from different sites across the globe and to test the role of topographical factors on the As-DBH relationship in Quercus petraea. Using 145 trees sampled within a 0.455 km2 catchment, we found that topography (slope, flow accumulation, aspect, curvature, and topographic wetness index) does not modulate the As-DBH relationship in Q. petraea, within our catchment. We compared our curve parameters with those from 16 studies on oak trees and found that the As-DBH relationship is not only species-specific, but depends on the site’s conditions. The use of species-specific parameters from other sites may lead to more than 100% difference in the calculation of As, and therefore in forest transpiration. In the light of these results, we recommend building site- and species-specific As-DBH relationships for determining stand or catchment transpiration, using a minimum of nine, randomly sampled trees, and different methods and azimuthal directions for determining sapwood depth.

Geolocation Accuracy Validation of High-Resolution SAR Satellite Images Based on the Xianning Validation Field

The geolocation accuracy of Synthetic Aperture Radar (SAR) images is crucial for their application in various industries. Five high-resolution SAR satellites, namely ALOS, TerraSAR-X, Cosmo-SkyMed, RadarSat-2, and Chinese YG-3, provide a vast amount of image data for research purposes, although their geometric accuracies differ despite similar resolutions. To evaluate and compare the geometric accuracy of these satellites under the same ground control reference, a validation field was established in Xianning, China. The rational function model (RFM) was used to analyze the geometric performance of the five satellites based on the Xianning validation field. The study showed that each image could achieve sub-pixel positioning accuracy in range and azimuth direction when four ground control points (GCPs) were placed in the corners, resulting in a root mean square error (RMSE) of 1.5 pixels. The study also highlighted the effectiveness of an automated GCP-matching approach to mitigate manual identification of points in SAR images, and results demonstrate that the five SAR satellite images can all achieve sub-pixel positioning accuracy in range and azimuth direction when four GCPs are used. Overall, the verification results provide a reference for SAR satellite systems’ designs, calibrations, and various remote sensing activities.

Laminar unsteady magnetohydrodynamic flow of Newtonian nongray optically thin fluid through porous medium between two concentric vertical cylinders

AbstractThis paper addresses the effects of porous medium in magnetohydrodynamic unsteady flows of the viscous nongray optical thinner liquid between two infinitely concentric perpendicular cylinders under the influence of time reliant periodical pressure gradients with respect to the an azimuthal direction applied magnetic domain. The effects of the thermally radiating as well as periodical walls temperatures are also taken into consideration. The governing nondimensional equations were altered into ordinary differential equations and these were then resolved in the expressions of the modified Bessels functions. The impacts of the different parameters on the velocity, temperature, skin frictions, and the rates of temperature transfers near the surfaces of the cylinders were evaluated computationally as well as these are discussed graphically. This was found that, the velocity of the fluid augments by an amplifying in penetrability of the absorbent medium. The buoyancy forces reasons the liquid flow to accelerated, and thereby causing the mass efflux to enhance proportionally. The reductions into fluid temperature are straight proportionate to the diminution into the thermal diffusivity. The current computations were reliable with those of the existing study in the limiting cases.

Unsteady MHD Casson fluid flow with Dufour and Soret’s effects due to a rotating cone

This study discusses the unsteady Casson fluid flow over a rotating cone. The effects of Dufour and Soret, MHD, mixed convection, thermal radiation and chemically reactive species are also considered. The modeled governing equations are solved by using the Homotopy technique. We start the analysis by comparing the current results with the existing literature work on a limited intellect. The effects of different physical parameters on concentration, velocity and temperature are analyzed and presented graphically. The effects of the skin friction, Nusselt number and Sherwood number for various parameters are presented in a tabular form. The skin friction graph in azimuthal and tangential directions is a reverse function of the unsteady parameter.

Dual vortex retarder Mueller matrix ellipsometry

We propose a dual vortex retarder Mueller matrix ellipsometry (DVRMME) that can extract 16 Mueller matrix elements by analyzing a single intensity image. Two customized vortex quarter wave retarders (VQWRs) with different orders are employed to realize the spatial modulation in the polarization state generation (PSG) and polarization state analyzing (PSA) arms. Due to the spatial polarization modulation by VQWRs, a pattern with regularly varying intensity along the azimuth direction is formed, and the sample's Mueller matrix can be obtained by processing the modulated intensity image. The ellipsometry's working principle has been elaborated by the Stokes-Mueller formalism, and its main error sources are presented and analyzed, and then a modulation vector calibration method is proposed to reduce the measurement error. To verify the validity of the scheme, the DVRMME has been constructed and demonstrated in experiment. The Mueller matrices of air, polarizer and retarder were measured, and their RMS errors of 16 Mueller matrix elements were respectively less than 0.0110, 0.0094 and 0.0180, and their maximum absolute errors were respectively less than 0.0196, 0.0230 and 0.0352.

Parametric scatterer extraction method for space‐target inverse synthetic aperture radar image CLEAN

AbstractOwing to the time variance of the inverse synthetic aperture radar (ISAR) imaging plane of space targets, scatterers with large heights relative to the imaging plane migrate through several Doppler cells. This defocussing in the azimuth direction of each scatterer cannot be compensated via the traditional migration through the resolution cell (MTRC) compensation method because the height of each scatterer is quite different and independently distributed in the ISAR image. In this study, a parametric azimuth‐defocussing model‐based CLEAN method is proposed for the space‐target ISAR image. First, high‐order defocus parameters and their influences on the image of space targets were analysed at different arc segments. Subsequently, a mathematical azimuth‐defocussing expression of the ISAR image scatterers was derived after MTRC and azimuth high‐order phase compensation, and an approximate defocus expression based on the Fresnel integral was provided for computational efficiency. This model can estimate the complex amplitude, coordinates and defocus parameters of each scatterer in the image domain by applying a sequential quadratic programing algorithm. Finally, the validity and stability of the proposed algorithm for the isolated and coupled scatterer extraction were verified at different signal‐to‐noise ratios, and different defocus parameters were verified through simulations.

A Modified Keystone-Based Forward-Looking Arc Array Synthetic Aperture Radar 3D Imaging Method

An arc array synthetic aperture radar (AA-SAR) is a new type of omnidirectional observation and imaging system. Based on linear array 3D imaging, this paper introduces a keystone algorithm combined with the arc array SAR 2D imaging method and proposes a modified 3D imaging algorithm based on keystone transformation. The first step is to discuss the target azimuth angle, retain the far-field approximation method of the first-order term, analyze the influence of the forward motion of the platform on the along-track position, and realize the two-dimensional focusing of the target slant range-azimuth direction. The second step is to redefine a new azimuth angle variable in the slant-range along-track imaging and use the keystone-based processing algorithm in the range frequency domain to eliminate the coupling term generated by the array angle and the slant-range time. The corrected data are used to perform along-track pulse compression to obtain the focused image of the target and realize the three-dimensional imaging of the target. Finally, in this article, the spatial resolution of the AA-SAR system in the forward-looking state is analyzed in detail, and the change in the spatial resolution of the system and the effectiveness of the algorithm are verified through simulation.

Manipulation of Arbitrary Polarizations and Phases Based on Metasurfaces

AbstractPolarization and phase are two essential properties of electromagnetic (EM) waves. A transmissive metasurface is proposed that can simultaneously manipulate the polarization and phase of the EM waves. The design has a powerful capability of rotating the polarization to any desired azimuth direction for a linearly polarized (LP) wave and achieving the cross‐polarization conversion for a circularly polarized (CP) wave. Meanwhile, the transmission phase of the cross‐polarization can be dynamically controlled for the CP wave. The proposed metasurface is composed of two types of substrate‐integrated waveguides (SIW) in a cross arrangement, which has an insertion loss of less than 0.3 dB from 9.7 to 10.3 GHz. Some samples are fabricated and measurements are performed, which are in satisfactory agreement with the theoretical and simulated results, indicating the feasibility of the proposed method. It is believed that the proposed design has many potential applications in the fields of imaging, communication, and radar systems.

Two-Dimensional Barrage Jamming against SAR Using a Frequency Diverse Array Jammer

Due to the modulation of tiny frequency offset on the array elements, a frequency diverse array (FDA) jammer can generate multiple range-dimension point false targets, and many deception jamming methods against SAR using an FDA jammer have been studied. However, the potential of the FDA jammer to generate barrage jamming has rarely been reported. In this paper, a barrage jamming method against SAR using an FDA jammer is proposed. To achieve two-dimensional (2-D) barrage effect, the stepped frequency offset of FDA is introduced to generate range-dimensional barrage patches, and the micro-motion modulation is employed to increase the extent of barrage patches along the azimuth direction. Mathematical derivations and simulation results demonstrate the validity of the proposed method in generating flexible and controllable barrage jamming.

Effects of Upward Reflective Film Applied to Window Glass on Indoor and Outdoor Thermal Environments in a Mid-Latitude City

The effect of upward reflective film applied to a window on the thermal environment inside and outside the window was investigated, considering the reflection and transmission characteristics depending on the angle of incident solar radiation. In terms of controlling the amount of solar radiation on a building’s windows, it is sufficient if the reflected solar radiation returns upward and does not need to be retroreflected in the azimuthal direction. Therefore, in this study, only the incident angle was considered and treated in two dimensions. The amount of incident solar radiation on the vertical façade is greater around 9:00 and around 16:00. Therefore, it is important to take measures in the morning for east-facing windows and in the afternoon for west-facing windows. The indoor MRT in front of upward reflective film is lower than in front of a transparent window due to lower transmittance. The outdoor MRT in front of upward reflective film is suppressed to the same level as in front of a transparent window, because the downward reflectance does not increase as much as in transparent windows. Upward reflective films applied to windows can improve the indoor thermal environment without worsening the outdoor thermal environment compared to thermal barrier films.

Effects of Wave-Induced Doppler Velocity on the Sea Surface Current Measurements by Ka-Band Real-Aperture Radar with Small Incidence Angle

The Doppler shift of microwave radar sea surface echoes serves as the foundation for sea surface current field retrieval; it includes the shift caused by satellite platform motion, ocean waves, and sea surface currents. The Doppler shift caused by ocean waves is known as the wave-induced Doppler velocity (UWD), and its removal is critical for the accurate retrieval of sea surface current fields. The low-incidence Ka-band real-aperture radar rotary scan regime has the capability of directly observing wide-swath two-dimensional current fields, but as a new regime to be further explored and validated, simulation and analysis of UWD in this regime have a significant influence on the hardware design and currently observed applications of this satellite system in its conceptual stage. In this study, we simulated and investigated the impacts of radar parameters and sea-state conditions on the UWD obtained from small-incidence-angle Ka-band rotational scanning radar data and verified the simulation results with the classical analytical solution of average specular scattering point velocity. Simulation results indicate that the change in the azimuth direction of platform observation affects UWD accuracy. Accuracy is the lowest when the antenna is in a vertical side-view. The UWD increases slowly with the incidence angle. Ocean waves are insensitive to polarization in the case of small-incidence-angle specular scattering. The increase in wind speed and the development of wind waves result in a substantial increase in UWD. We classified swell by wavelength and wave height and found that UWD increases with swell size, especially the contribution of swell height to UWD, which is more significant. The contribution of the swell to UWD is smaller than that of wind waves to UWD. Furthermore, the existence of sea surface currents changes the contribution of ocean waves to UWD, and the contribution weakens with increasing wind speed and increases with wind wave development.

Efficient Implementation for SBL-Based Coherent Distributed mmWave Radar Imaging

In a distributed frequency-modulated continuous waveform (FMCW) radar system, the echo data collected are not continuous in the azimuth direction, so the imaging effect of the traditional range-Doppler (RD) algorithm is poor. Sparse Bayesian learning (SBL) is an optimization algorithm based on Bayesian theory that has been successfully applied to high-resolution radar imaging because of its strong robustness and high accuracy. However, SBL is highly computationally complex. Fortunately, with FMCW radar echo data, most of the time-consuming SBL operations involve a Toeplitz-block Toeplitz (TBT) matrix. In this article, based on this advantage, we propose a fast SBL algorithm that can be used to obtain high-angular-resolution images, in which the inverse of the TBT matrix can be transposed as the sum of the products of the block lower triangular Toeplitz matrix and the block circulant matrix by using a new decomposition method, and some of the matrix multiplications can be quickly computed using the fast Fourier transform (FFT), decreasing the computation time by several orders of magnitude. Finally, simulations and experiments were used to ensure the effectiveness of the proposed algorithm.

Reduction of BER in cellular network to enhance the performance using adaptive antenna technique

In this paper, the main attention is on the Reduction of Bit Error Rate (BER) in Cellular networks to enhance the performance using an adaptive antenna method. To accomplish this, the Least Mean Square (LMS) method is adopted. The enormous progression in technology and complexity was witnessed because of the increase in demand for services. In wireless transmission, performance degradation would be observed in the signal transmission and reception due to a high degree of error occurrences. By conducting a driving test the performance of the network is evaluated by measuring the BER. From the results, it is found that the Modified Least Mean Square (MLMS) algorithm generated the lowest BER, equal to 0.0004142, which is very low compared to that of the BER from the driving test. Here in this paper, it is concluded that the MLMS method gave the improved performance along with the lowest BER value that too most effectively reduced than LMS method ensuring improved spectrum usage and optimized channel capacity. This work resulted in the enhancement of the current cellular network, and further work should be done to improve its application in both the horizontal and vertical azimuth directions.

Ship velocity estimation in SAR images using multitask deep learning

SAR satellites are used for monitoring ships worldwide. Moving ships are Doppler shifted by an amount proportional to their velocity. An offset between the ship and its wake is then produced during SAR processing. We present a novel automatic method for calculating the ship velocity. The method relies on multitask deep learning to estimate the offset and ship heading. From these parameters, the ship velocity can be obtained. A convolutional neural network is trained using a coupled loss function. The loss function allows both parameters to be estimated at the same time. We show the methods’ effectiveness for ships in Sentinel-1 SAR images. For this purpose, a large dataset of 30,000 AIS annotated SAR ship images is collected. These images have 20 × 22 m pixel resolution and ships do not have a clear wake. The AIS provides the true ship velocity and allows the method to be evaluated. As a result, we can determine the ship speed with an accuracy of 1.1 m/s. The offset disappears near the azimuth direction of the SAR image. Yet, our method is reliable except for ships sailing within 2.5 degrees of the azimuth direction.

Spoke formation in low temperature E×B plasmas: Transition from gradient-drift instability to ionization wave

Long wavelength plasma non-uniformities rotating in the azimuthal direction (“rotating spokes”) have been observed in a number of experiments on Hall thrusters or magnetron discharges. We use a two-dimensional (2D), axial-azimuthal particle-in-cell Monte Carlo collisions model to study the formation of instabilities in a direct current magnetron discharge under conditions close to recent experiments. Despite the simplified 2D geometry of the model, the simulations can reproduce the main features of the experimental results. At a given position above the cathode, corresponding to the spoke location, the simulations show large amplitude oscillations of the plasma density and a very sharp increase in the plasma potential and electron temperature at the leading edge of the spoke, as in time resolved probe measurements. Moreover, the simulations show that the instability evolves in time from a gradient-drift type of instability in the linear phase, to an ionization wave in the non-linear phase, with rotation in the +E×B direction in the first phase and in the −E×B direction in the second phase. The number of spokes is found to increase with pressure, as in experiments. The mechanisms of electron heating and the role of the B×∇B drift in electron heating and in the coherence and direction of spoke rotation are discussed.

A Large‐Scale Magnetospheric Line Radiation Event in the Upper Ionosphere Recorded by the China‐Seismo‐Electromagnetic Satellite

AbstractThis paper reports a large‐scale magnetospheric line radiation (MLR) event during a moderate geomagnetic storm on 11 September 2018, which was well recorded by the China‐Seismo‐Electromagnetic Satellite (CSES) in the upper ionosphere. The event shows a symmetrical propagation feature at the conjugated locations between the two hemispheres, exhibiting a large spatial extension roughly from the latitudes 54°N to 53°S. The parallel structures are visible both in the electric and magnetic fields at a frequency band ranging from the local proton cyclotron frequency to ∼1.6 kHz. The wave intensity of parallel spectral lines was primarily enhanced in high latitude regions, gradually weakening at mid‐low latitudes, and then got absorbed in the equatorial region, presenting a distinct V‐shaped structure. The frequency spacings between neighboring spectral lines roughly vary from ∼80 to 110 Hz at the high latitudes and ∼80–130 Hz at the low latitudes, suggesting a slight variation feature with latitude. The parallel spectral structures of MLR drift between ∼0.39 and 0.57 Hz/s at high latitudes and ∼0.18–0.19 Hz/s at low latitudes. The wave vector analysis shows that the MLR waves are right‐hand polarized, obliquely propagating toward the Earth and in the azimuthal direction, where the Poynting flux is primarily oriented perpendicular to the ambient magnetic field. The other large‐scale MLR events all exhibit similar parallel structures and polarization characteristics, suggesting the universality of such a phenomenon. However, the azimuthal angles differ among different events, showing complex features.

A Grating-Lobes Suppression Method for Wideband MIMO Millimeter-Wave Imaging Arrays

This article proposes a novel method for eliminating the first grating-lobe of wideband multiple-input multiple-output (MIMO) millimeter-wave imaging array. It exploits the position shift phenomenon of the first grating-lobe and can achieve grating-lobes suppression without additional calculation. The far-field formula for the first grating-lobe is derived, and the cause of its shift phenomenon in both azimuth and range directions is analyzed. The proposed multiapodization (MA) method can be used to cut off the part of the shifted grating-lobes while maintaining the resolution. However, nonsparse targets still have the far-field residual grating-lobes after MA. To solve the problem, the position formula and the detailed derivation of the residual grating-lobes are given. Therefore, a pixel search (PS) method can be used to remove them. The proposed MA can also be utilized to suppress the residual grating-lobes of the near-field MIMO array. Both simulated and experimental data in different system regimes and frequencies verify its effectiveness.

Tri-Cardioid Co-Centered Co-Planar Array—Its Direction-Finding Cramér-Rao Bound and Design Guidelines

Cardioid microphones/hydrophones have a highly directional gain pattern of $[ \alpha + (1 - \alpha) \cos (\beta) ] \cos ^{k}(\beta)$, where $k$ refers to the sensor's directivity order, $\alpha$ denotes the same sensor's cardioidicity index, and $\beta$ represents an impinging signal's incident direction of arrival relative to the cardioid sensor's axis. Three such cardioids organized in orthogonal orientation in three Cartesian spatial dimensions and in spatial colocation as one sensing unit—such a 3-D triad has already attracted much recent attention in the research literature. However, not all three Cartesian coordinates have equal importance to many acoustical applications, which focus alternatively on the azimuthal direction defined on a flat plane but less on the elevation direction normal to that plane. So, this article will instead analyze a 2-D planar configuration of three colocated/cocentered cardioids differently oriented azimuthally apart by $120^\circ$. This aforesaid coplanar triplet conforms to a flat supporting surface more than a Cartesian tridimensionally perpendicular triad can. For such a coplanar triplet composed of cardioids preset at any specific $(k, \alpha)$, the triplet's polar/azimuthal direction-finding Cramér–Rao lower bounds will be analytically derived in closed forms here in this article. Those bounds will be uncovered to exhibit intricate mathematical structures, which will be dissected in detail to yield refined insights, producing simple “actionable” rules-of-thumb to guide the system engineer to choose the appropriate values for $(k, \alpha)$.

Design of a UWB Antenna with Multiple Ports on a Single Circular Radiator for Direction-Finding Applications

This paper proposes a single circular radiator with a multi-port (SRMP) antenna that can estimate the direction-of-arrival (DoA) in the azimuth and elevation directions. The proposed SRMP antenna is designed to minimize the size of the ultra-wideband system by using only one patch radiator. To verify the feasibility, the proposed antenna is fabricated, and the reflection coefficient and boresight gain are measured (-13.3 dB and 3.4 dBi at 8 GHz). Then, to observe the direction-finding performance, the DoA estimation results using the Bartlett beamformer are compared with the typical array. At all incident angles, a root-mean-square error of less than 1° is observed when the signal-to-noise ratio is higher than 6 dB.