Range Of Elevation Angles Research Articles

Topology-optimized terahertz real-time reconfigurable multifunctional liquid-crystal-coding metasurface

In this paper, a reflective terahertz reconfigurable multifunctional coding metasurface based on topology optimization using liquid crystal (LC) is presented. First, the LC metasurface unit is topologically optimized using the NSGA-II multi-objective optimization algorithm. By applying the bias voltage to dynamically adjust the dielectric constant of the LC, the metasurface unit is capable of 2-bit coding at the frequency of 0.67 THz. Then, based on the designed metasurface unit, the array arrangements of the coding metasurface are reversely designed, which makes the metasurface realize flexible control and dynamic switching between beam assignment and vortex beam generation. The results show that for the beam assignment, the single-beam and multi-beam control can be realized with the reflected electromagnetic waves at continuous arbitrary angles in the range of elevation angle of 40° and azimuth angle of 360°. Moreover, the elevation angle and azimuth angle of each beam in the multi-beam can be controlled independently. Using the Fourier convolution addition operation, the control range of the single-beam elevation angle can be expanded. For the vortex beam, the single vortex beam and the multi-vortex beam can be generated in the range of elevation angle of 40° and azimuth angle of 360° with topological charges l=±1, ±2, ±3 at arbitrary angles. This provides flexibility and diversity in the generation of vortex beams. Therefore, the proposed terahertz LC metasurface can realize flexible control of reconfigurable functions and has certain application prospects in terahertz communication, phased array radar, and vortex radar.

Assessing the Impact of Environmental Conditions on Reflectance Values in Inland Waters Using Multispectral UAS Imagery

This study investigates the impact of environmental conditions on reflectance values obtained from multispectral Unmanned Aerial System (UAS) imagery in inland waters, focusing on sun glint, cloud glint, wind-generated waves, and cloud shading projections. Conducted in two reservoirs with differing water qualities, UAS platforms equipped with MicaSense Altum and DJI Phantom 4 Multispectral sensors were used to collect multispectral images. The results show that sun glint significantly increases reflectance variability as solar elevation rises, particularly beyond 54°, compromising data quality. Optimal flight operations should occur within a solar elevation angle range of 25° to 47° to minimize these effects. Cloud shading introduces complex variability, reducing median reflectance. Wind-generated waves enhance sun glint, increasing variability across all spectral bands, while cloud glints amplify reflectance non-uniformly, leading to inconsistent data variability. These findings underscore the need for precise correction techniques and strategic UAS deployment to mitigate environmental interferences. This study offers valuable insights for improving UAS-based monitoring and guiding future research in diverse aquatic environments.

Devising an analytical method for estimating aircraft positioning accuracy by an infocommunication network of optoelectronic stations

The object of this study is the monitoring processes of airspace in the visible and infrared frequency ranges using an infocommunication network of optoelectronic tracking stations. The problem addressed is the assessment of accuracy in positioning aerial vehicles in airspace, considering random and systematic errors in video surveillance. The proposed method allows for the analytical evaluation of variance in the coordinates of aerial vehicles in airspace at any given moment, depending on the variance in errors from all components of the video surveillance process. The following results are reported: – mathematical models for both open and covert video surveillance by the infocommunication network of optoelectronic stations based on the trajectories of aerial vehicles, which enable the estimation of their coordinates for each moment of video surveillance. – analytical relationships between the variance of geolocation of aerial vehicles and the variance of errors in all components of the video surveillance process. A key feature of the method is its practical independence on the type and size of aerial vehicles. The method requires the availability of metrological characteristics of the optoelectronic station instruments and synchronized measurements of azimuth, elevation angle (for covert), and slant range (for open video surveillance). The numerical values of the root mean square deviations of the positioning errors of aerial vehicles under various video surveillance conditions range from 0.1 to 0.35 meters, confirming the effectiveness of using the infocommunication network of optoelectronic stations as an instrumental tool for high-precision trajectory measurements.

Enhancing Satellite Link Security Against Drone Eavesdropping Through Cooperative Communication

ABSTRACTIntegrated satellite terrestrial networks (ISTNs) are emerging as a promising next‐generation communication technology, for example, B5G and 6G, with low‐earth orbit (LEO) satellites playing a growing role. However, the complex and unique characteristics of ISTNs make them more susceptible to cyberattacks. Recently, the use of drones for public and private services has increased the risk of eavesdropping on LEO satellite links. Such scenario presents an extremely challenging environment due to dynamic nature of LEO satellite and drone along with atmospheric attenuation at sub‐THz frequencies. This study proposes a novel adaptive power‐bandwidth cooperative scheme designed to mitigate the likelihood of eavesdropping attacks on LEO satellite links communicating with a ground station when a drone is within the line of sight. The mathematical algorithm dynamically adapts the resources to maximize the normalized secrecy capacity in this challenging scenario while maintaining a reasonable signal‐to‐noise ratio (SNR) at the legitimate receiver. The adaptive scheme involves strategic cooperation with a nearby terrestrial third party to amplify and forward the satellite signal to the ground station receiver. The simulation results demonstrate the effectiveness of the proposed algorithm, showing significant improvements (> 70%) compared to the non‐adaptive scheme over a wide range of elevation angles.

Far field ring beam generation based on 3-bit encoded metasurface

Abstract The encoding metasurface establishes a bridge between the physical and digital worlds, ushering in a new era of manipulating electromagnetic waves and realizing programmable metamaterials through digital coding sequences. This "digital metasurface," relying on binary logic, significantly simplifies the design process, thereby enhancing the flexibility and efficiency of controlling electromagnetic waves. While most encoding metasurfaces control beamforming for pencil-shaped beams, we propose a 3-bit encoding metasurface with a "well structure" in the microwave band.The 3-bit encoding metasurface features a more extensive encoding sequence, offering increased degrees of freedom and flexibility in manipulating the direction of electromagnetic wave propagation. Its symmetric design features polarization-insensitive characteristics, suitable for generating annular beams by varying gradient-encoded numbers radially. This approach enables the production of linearly polarized omnidirectional radiation within the desired elevation angle range. Additionally, adjustments in quantity and orientation of the produced annular beams are achieved through Fourier convolution theorem. This type of annular beams holds promise for applications in various fields, including wireless radio broadcasting and wireless local area networks.

Stellar scintillation statistics and the impact of aperture averaging on space-to-ground optical communications.

Statistical probability distributions characterizing received optical power fluctuations, or scintillation, enable performance predictions of space-to-ground optical communication systems. In this paper, we present measurements of stellar scintillation over a wide range of elevation angles and turbulence conditions collected simultaneously with a 5 cm and 40 cm telescope aperture, which allows a comparison between minimal and significant aperture averaging conditions. The measured data is compared to a reasonable set of candidate probability distribution functions (PDFs), including lognormal, which is most often cited in the literature for weak to moderate scintillation. For scintillation indices (SIs) less than about 0.2, the Nakagami-m distribution provides the best representation of the collected data for both apertures and imposes a greater lasercom link penalty than a lognormal distribution, which has been inaccurately implemented as the default probability distribution in the literature. For larger values of the SI, the scintillation is best characterized by a Gamma-Gamma distribution. Additionally, the measured temporal covariance for weak to moderate scintillation conditions is found to be in reasonably good agreement with theoretical predictions.

Improvement Analysis of a Height‐Deviation Compensation‐Based Linear Interpolation Method for Multi‐Station Regional Troposphere

AbstractIn network real‐time kinematic positioning of multi‐reference station, the spatial and temporal distribution of tropospheric delay is affected by both horizontal and elevation. The traditional modeling strategy of regional troposphere takes more consideration of the horizontal factor, and the incomplete consideration of the elevation factor will lead to the problem of reduced modeling accuracy, especially in the face of the scene with large regional height deviation. Based on the traditional linear interpolation method (LIM), a simple and effective height‐deviation compensation‐based linear interpolation method (HCLIM) for regional tropospheric is proposed. The modeling accuracy of troposphere and the positioning accuracy of user RTK in large height deviation region are significantly improved. The method was verified based on six experimental subnets with large height deviations from a provincial continuously operating GNSS reference stations network in central China. The results showed that: For GPS satellite modeling, compared with the traditional LIM method, the average modeling accuracy improvement rate of HCLIM method is (84.5%, 75.5%, 59.3%, 26.7%) in the elevation angle range of (10–30°/30–40°/40–50°/50–90°). For BDS satellite, the average modeling accuracy improvement rate of HCLIM method in the above four elevation angles is (83.3%, 70%, 50%, 23.5%). For the positioning performance of user RTK, The horizontal positioning accuracy and RTK fixing rate were similar under the two methods, while HCLIM method showed only slight improvement. However, in the U direction, LIM method showed obvious systematic bias, while HCLIM method showed consistent positioning accuracy, which was improved to 82.8% compared with LIM method.

Comparative analysis of different empirical mode decomposition-kind algorithms on sea-level inversion by GNSS-MR

Abstract The rising sea level caused by global climate change might impact the human living environment. Global navigation satellite systems (GNSS)-multipath reflection (MR) technology holds significant potential for monitoring tide level changes. GNSS-MR technology typically employs low-order polynomials to extract the signal-to-noise ratio (SNR) residuals containing GNSS interference signals. It utilizes Lomb-Scargle (LSP) spectral analysis or empirical mode decomposition (EMD) to obtain the dominant frequency of the SNR residuals, which is then converted into tidal heights. However, as the satellite elevation angle increases, the GNSS interference signals decrease and the traditional method does not adapt well to the extraction of SNR residuals under such conditions. A series of improved EMD-kind algorithms, namely ensemble empirical mode decomposition (EEMD), complementary ensemble empirical mode decomposition (CEEMD), complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN), and improved complete ensemble empirical mode decomposition with adaptive noise (ICEEDMAN), have been proposed to address the shortcomings of EMD algorithms such as end effect and mode aliasing. However, these improved EMD-kind algorithms have yet to be reported in sea level inversion. This study investigates the mitigation effects of EMD-kind algorithms on GNSS-MR direct signal and noise to improve the stability and accuracy of an SNR residual sequence with high satellite elevation angles. Experimental data from the HKQT station for one week and the SC02 station for one year are utilized to validate the effectiveness and accuracy of these algorithms in extracting SNR residuals. Compared to the traditional polynomial method, the experimental results demonstrate that all EMD-kind algorithms effectively address the distortion issue in traditional inversion methods under long periods, higher satellite elevation angles, and low GNSS receiver sampling rates. Among these algorithms, the results from the experiments show that ICEEMDAN consistently provides the best inversion accuracy. The results of the comparative analysis show that ICEEMDAN effectively reduces non-interference signals in SNR residuals at higher satellite elevation angles, expanding the useable range of satellite elevation angles and improving the utilization and temporal resolution of GNSS data inversion. Hence, it is an effective and appropriate approach to improving the accuracy of GNSS-MR tide level monitoring.

Wind pressure distribution variation law and interference effect of heliostats

The heliostat is different from the ordinary low building because of the special shape of the heliostat. The wind tunnel pressure test is carried out the model of heliostats in the range of azimuth angles are between 0° to 180° and the range of elevation angles are between 0° to 90. The wind pressure time history of each measuring point on the mirror panel are obtained. On this basis, the mean wind pressure distribution of the mirror panel under typical working conditions is obtained, and then the maximum (minimum) value of the mean wind pressure under all working conditions and corresponding working conditions and measuring points are obtained. Then 10 representative measuring points are selected to study the variation law of wind pressure with wind direction angle and pitch angle respectively, and then the variation law of the mean wind pressure of 10 measuring points under all working conditions is obtained. Finally, the interference effect of the mean wind pressure of heliostats is studied, and the variation law of the interference effect of the mean wind pressure is obtained, and the maximum value, minimum value and corresponding working conditions of the interference effect are obtained. The results show that the maximum value of the mean wind pressure of heliostats under all working conditions is appeared at the measurement point of the lower edge of the mirror panel and on working condition 15-60 (wind direction angle - elevation angle), and the minimum value is appeared at the measurement point of the upper left corner of the mirror panel and on working condition 150-20. The variation law of the mean wind pressure of 10 measuring points under all working conditions is similar, and the position of measuring points has little effect on the variation law of the mean wind pressure under all working conditions. Only the working condition of the maximum value and minimum value are affected by the position of the measuring points. The mean wind pressure distribution under the most unfavorable working condition of heliostats is obtained, the maximum (minimum) value of the interference effect and corresponding working conditions are obtained. Which can be a reference for structural design and research.

Can the Orbital Debris Disease Be Cured Using Lasers?

Ground-based high-power lasers are, in principle, able to de-orbit any kind of space debris object from the low Earth orbit (LEO) by remotely inducing laser-ablative momentum. However, the assessment of efficiency and operational safety depends on many factors, like atmospheric constraints or the risk of debris disintegration during irradiation. We analyze laser momentum for a great variety of target geometries and sizes and—for the first time in a large-scale simulation—include thermal constraints in the laser irradiation configuration. Using a coherently coupled 100 kJ laser system at 1030 nm wavelength and a 5 ns pulse duration in an optimized pointing elevation angle range, the pulse frequency should amount to less than 10 Hz to prevent fragment meltdown. For mechanically intact payloads or rocket bodies, repetition rates should be even lower. Small debris fragments sized between 10 and 40 cm can be de-orbited by employing around 100 to 400 station passes with head-on irradiation, while objects exceeding 2 m typically require far more than 1000 irradiations for de-orbit. Hence, laser-based debris removal cannot be considered a prime space sustainability measure to tackle the highest-risk large debris, yet it can provide the remediation of a multitude of small-sized debris using small networks of globally distributed laser sites.

Automatic design system of optimal sunlight-guiding micro prism based on genetic algorithm

Sunlight-guiding micro prism, one of the daylighting systems, can redirect the sunlight into the room to increase indoor daylighting. However, this passive system still has limitations; it cannot accommodate the sun's movement and might produce unwanted glare. An automatic design process is proposed to generate an optimal prism based on the user's location. It uses Python to connect genetic algorithms, 3D computer-aided design, optical simulation software, and image-processed related technology for generating geometric parameter sets, modeling, optical simulation, and data evaluation. The optimal triangular sunlight-guiding prism calculated by the light-guiding fitness function for a 24° north latitude location has 100% light-guiding efficiency at 35°–65° solar elevation angle and has more than 90% at 20° and 70°. It shows that the proposed automatic design process could provide the customized optimal design of the sunlight-guiding prism with a widely applicable solar elevation angle range based on the user's building condition.

Planar metal-only omnidirectional retroreflector using transmitarray and blazed grating for TE and TM polarizations.

Retroreflector is a device that reflects light along its incident direction over a continuous range of incident angles. The retroreflectors proposed so far are so bulky and uneven that they cannot be mounted on large structures. Recently, two-layer planar retroreflectors have been proposed in the field of optics and acoustics, but at microwave frequencies only one retroreflector with non-planar overall structure has been presented that doesn't cover omnidirectional incidence and only supports TM polarization. The term omnidirectional means the capability of retroreflectivity in all azimuth angles (φi) and a wide elevation angle (θi) range. In this paper, a novel planar and metal-only retroreflector is proposed at microwave frequencies that efficiently reflects the electromagnetic wave along its incident direction in omnidirectional angle range and supports both TE and TM polarization. The proposed structure consists of two separate cascaded layers. The top layer is a transmitarray structure in which incident waves with different angles are focused at different locations on the bottom layer. The bottom layer reflects back focused waves along its focus direction and two distinct structures are proposed to implement it. The first type is a novel spatially varying blazed grating and the second type is a spatially varying phase gradient metasurface, as used in the optics and aucostics. The first type offers omnidirectional property for both TE and TM polarization while the second type offers higher monostatic RCS for only TE polarization. Simulated and measured results of the proposed retroreflector with the first type bottom layer show an omnidirectional retroreflectivity with half-power (3-dB RCS level) elevation field of view of 60° (-30°to 30°) at frequency of 10 GHz with a normal incidence efficiency of 63%.

Planar, wide-band omnidirectional retroreflector using metal-only transmitarray structure for TE and TM polarizations

In this paper, a novel planar and wideband metal-only retroreflector is proposed that efficiently reflects the obliquely incident electromagnetic wave along its incident direction in omnidirectional angle range. The means of omnidirectional is the capability of retroreflectivity in all azimuth angles (varphi_{i} ) and in a wide elevation angle (theta_{i} ) range. The proposed structure consists of a symmetrical transmitarray structure with beam scanning capability in which a metal plate is placed instead of the feed. The transmitarray is designed by using the generalized multifocal approach in which the beam scanning capability is possible via feed displacement and the phase of the elements has azimuthal symmetry that causes omnidirectional property. Now by placing a metal plate at the location of feed displacement and provided that the focused waves are nearly perpendicular to the metal plate, the plane wave is reflected back along its incident direction i.e. the retroreflectivity property is obtained. The proposed retroreflector simulated by CST STUDIO software and fabricated with laser cut technology. The results of measurement show an omnidirectional retroreflectivity with half-power (3-dB RCS level) elevation field of view of 60° (− 30° to 30°) in the frequency range of 8.5–10 GHz (approximately 17%) for both TE and TM polarization.

Ultra-wideband bi-static RCS reduction with full angular stability using diffraction periodic symmetrical structures

In this paper, a planar, two-layer, and symmetrical structure is presented to obtain ultra-wideband bi-static RCS reduction with angular stability in azimuth and elevation directions. The proposed structure is composed of two layers. The bottom layer contains a periodic surface with periodicity larger than one wavelength to scatter multiple Floquet harmonic and thus bi-static RCS reduction. To achieve angular stability for the azimuth direction, the geometry of the unit cell is optimized with symmetry to its center. The top layer contains a substrate with high relative permittivity, which, according to refraction Snell’s law, increases the bandwidth of RCS reduction and contributes to angular stability regarding the elevation direction. The results show the RCS reduction by 10 dB over a wide frequency range from 4.2 GHz to 22.9 GHz (137%) at the normal incidence, and more than 80% for oblique incident waves within the elevation angle range.

Camera-Augmented Non-Contact Vital Sign Monitoring in Real Time

This study develops a camera-guided frequency-modulated continuous-wave (FMCW) radar to monitor vital signs. A red-green-blue-depth (RGB-D) camera estimates the human torso landmarks and a processing unit constantly adapts the radar beams to the direction of the subjects. To constantly optimize the regions of interest for monitoring respiratory rate (RR) and heart rate (HR), a novel method, coined “singular value-based point detection (SVPD),” is designed. Vital sign extraction is then followed as the last step. Experiments are conducted for the cases of single-subject (10 subjects, 31 scenarios, and 1550 repetitions) and dual-subject monitoring (6 subjects, 6 scenarios, and 90 repetitions). Average (RR, HR) accuracies of (97.68%, 85.88%), (90.02%, 86.05%), (96.71%, 89.50%), and (97.52%, 86.71%) are achieved for the range of distances (0.5-2.5 m), azimuth angles (0°–30°), elevation angles (−30°–+30°), and incident angles (−30°–+30°), respectively. The higher chest and upper abdomen are determined as the optimal regions for RR and HR estimation respectively, with average accuracies of 98.31% and 86.93%. Finally, the capability of dual-subject monitoring at various inter-subject distances (range of 20–70 cm) is confirmed with average accuracies of 92.26% and 73.23% for RR and HR respectively.

A Multibeam Ambient Electromagnetic Energy Harvester With Full Azimuthal Coverage

Confronting the challenge of replacing or recharging batteries for numerous low-power devices in IoT, the harvesting of ambient electromagnetic (EM) energy is considered a promising approach. Since EM energy is randomly distributed in the environment, omnidirectional antennas or multibeam antennas are better choices for their large coverages which enable them to collect as much energy as possible. However, omnidirectional antennas usually have low gains resulting in low receiving energy, and most multibeam antennas with high gains require complicated feeding networks. To address these issues, a six-beam antenna without a complex feeding network is proposed in this paper. It consists of a triangular patch array layer and a surface waveguide layer. A peak gain of 8.3 dBi, as well as a 3-dB beamwidth of 620 in H-plane and 560 in E-plane have been achieved, affording full azimuthal coverage within a certain elevation angle range. For demonstration, an EM energy harvester is constructed based on the proposed antenna. It can be observed that the implemented device can harvest almost the same amount of EM energy in different horizontal orientations, and acquire multi-fold DC power in the case of multiple EM transmitters.

SNR Optimization for LEO Satellite at Sub-THz Frequencies

The next generation of satellite constellations would include low Earth orbit (LEO) satellites with communication link above 100 GHz in order to achieve higher efficiency and capacity of the satellite networks due to large available bandwidth. The LEO satellites require tracking by the ground station receiver; however, due to the complexity, inefficiency, and nonlinearity of the phase shifters at terahertz (THz) frequencies, the conventional concept of scanning phased array antenna is less recommended. In this work, we derive an optimization algorithm for an electronically switchable subarray of phased array at the ground station that maximizes the signal-to-noise ratio (SNR) depending on the range of elevation angles of satellite. The algorithm optimizes the SNR by adapting the interelement spacing between array elements considering the impact of molecular absorption loss and noise, thermal noise from the sky and the ground. In comparison to uniform phased array design with equal spacing, the SNR can be improved for a typical scenario by about 3 dB and thus improving the link performance. To the best of our knowledge, this LEO satellite communication algorithm at the sub-THz frequencies is for the first time to be published.

Modeling of Ka-band slant path rain attenuation for hilly tropical region

Experimental measurement of satellite signal at 20.2 GHz using GSAT-14 satellite beacon has been carried out over the hilly tropical location Umiam, Shillong during 2017–2019. Collocated rainfall measurements have been done using a laser precipitation monitor for the same duration. The data have been analyzed to understand the signature of orographic rainfall on the Ka-band signal and to achieve the highest possible reliability of signal reception for a high rainfall tropical location. Complementary cumulative distribution of experimentally obtained rain attenuation is compared with that of conventional ITU-R model calculated rain attenuation and recently developed Chang Sheng model predicted rain attenuation values. The result shows severe overestimation of the actual measurement by both the ITU-R model and Chang Sheng model. The Chang Sheng model is further modified with the orographic adjustment factor for predicting orographic rain attenuation. The model was developed based on the data of 2017–18 and validated using the measured attenuation of the year 2019. The performance of the proposed model is evaluated in terms of RMSE and correlation coefficient. The value of the correlation coefficient has been found to be 0.988, and the RMSE value has been found to be around 3.0986, which indicates the acceptable performance of the proposed model. The proposed model is further optimized for operational elevation angle range, rain rate range, and frequency range. The proposed model will help in optimizing the power utilization in the Uplink Power Control technique for rain attenuation mitigation over high rainfall hilly regions.

Performance Comparison of Quantized Control Synthesis Methods of Antenna Arrays

There is a great potential in small satellite technology for testing new sensors, processes, and technologies for space applications. Antennas need careful design when developing a small satellite to establish stable communication between the ground station and the satellite. This work is motivated by the design of an antenna array for a future rotatorless base station for the VZLUSAT group of Czech nano-satellites. The realized antenna array must cover a relatively broad range of elevation and azimuth angles, and the control must be fast enough to track the satellite in low Earth orbits. The paper deals with possibilities of synthesis of quantized control of the antenna array. It compares quantization influence for well-known deterministic synthesis methods. It shows the method for decreasing computational cost of synthesis using optimization approach and presents the multi-criteria optimization as a tool for reaching required radiation pattern shape and low sensitivity to quantization at the same time.

Retrieving the SNR metrics with different antenna configurations for GNSS-IR

Multipath, which is a major source of error for precise positioning, is the effect that occurs when Global Navigation Satellite Systems (GNSS) signals reach the receiver by reflecting from one or more surfaces. Reflected signals affect the signal-to-noise ratio (SNR) data provided by the receiver, indicating the signal strength. The structure of the antenna of the receiver and the direction in which it is oriented also change the strength of the received signal. In this study, the effect of antenna orientation and polarization on SNR data was demonstrated by using the method called GNSS-Interferometric Reflectometry (GNSS-IR), in terms of reflector height estimates. A geodetic GNSS receiver (CHC i50) and two different smartphones (Xiaomi Mi8 and Xiaomi Mi8 Lite) were used in the four-day experiments. The geodetic receiver was established as zenith-looking (ZL) in the first two days and as horizon-looking (HL) in the last two days. Smartphones were placed on the same mast with the HL receiver in the last two days. It was seen that it is more appropriate to use a 0°-60° satellite elevation angle range in the common use of all receivers’ data. In the 30°-60° range where the ZL installation receives the multipath signals weakly, it has been found that the HL receiver and smartphones have reflector height estimation accuracies with values ranging from 1.9 cm to 2.5 cm. In short, for different elevation angle ranges, accuracies below 2 cm could be obtained with each receiver. Thus, different antenna configurations may be used in GNSS-IR studies, depending on the characteristics of the study area and the surface feature to be determined.