Optical Ground Research Articles

High-Resolution Mapping of Aerosol Optical Depth and Ground Aerosol Coefficients for Mainland China

Aerosols play an important role in climate change, and ground aerosols (e.g., fine particulate matter, abbreviated as PM2.5) are associated with a variety of health problems. Due to clouds and high reflectance conditions, satellite-derived aerosol optical depth (AOD) products usually have large percentages of missing values (e.g., on average greater than 60% for mainland China), which limits their applicability. In this study, we generated grid maps of high-resolution, daily complete AOD and ground aerosol coefficients for the large study area of mainland China from 2015 to 2018. Based on the AOD retrieved using the recent Multi-Angle Implementation of Atmospheric Correction advanced algorithm, we added a geographic zoning factor to account for variability in meteorology, and developed an adaptive method based on the improved full residual deep network (with attention layers) to impute extensively missing AOD in the whole study area consistently and reliably. Furthermore, we generated high-resolution grid maps of complete AOD and ground aerosol coefficients. Overall, compared with the original residual model, in the independent test of 20% samples, our daily models achieved an average test R2 of 0.90 (an improvement of approximately 5%) with a range of 0.75–0.97 (average test root mean square error: 0.075). This high test performance shows the validity of AOD imputation. In the evaluation using the ground AOD data from six Aerosol Robotic Network monitoring stations, our method obtained an R2 of 0.78, which further illustrated the reliability of the dataset. In addition, ground aerosol coefficients were generated to provide an improved correlation with PM2.5. With the complete AOD data and ground coefficients, we presented and interpreted their spatiotemporal variations in mainland China. This study has important implications for using satellite-derived AOD to estimate aerosol air pollutants.

Calculation and Analysis of Induced Current in Optical Fiber Composite Overhead Ground Wire

In this paper, the mechanism of electromagnetic induction current of overhead ground wire is analyzed, the models of overhead ground wire with different voltage levels are established on ATP-EMTP, the electromagnetic induction current of optical fiber composite overhead ground wire (OPGW) is calculated, and the factors affecting the induced current on OPGW are analyzed, including the change in loads of transmission lines, the value of grounding resistance and the location of lines. The results of this paper provide a reference for the actual measurement and the use of this induced current for induction power supply.

A reliability‐based optimal μ‐PMU placement scheme for efficient observability enhancement of smart distribution grids under various contingencies

A reliability-based optimal μ-PMU (micro-phasor measurement unit) placement scheme is suggested for efficient observability enhancement of smart distribution grids at steady-state and contingencies conditions. This article introduces a unique method for the μ-PMU allocation in reconfigurable smart distribution grids in which communication system requirements and zero injection nodes (ZINs) are considered. The original objective function and limitations are proposed aiming at minimizing the capital cost, including communication links and installation costs of μ-PMU, optical power ground wire cost, power losses cost, and reliability cost as well as obtaining the maximum number of measurement redundancy constrained to full system observability in the presence of ZINs and tie switches. The suggested method is formulated as a mixed-integer linear programming problem applied to find optimal μ-PMU locations considering the cost of communication infrastructure and co-optimize the system switching plan simultaneously. In this regard, CPLEX—a high-performance mathematical solver—is used to solve the proposed mixed-integer linear optimization problem to reach the global optimality. The simulations are performed on 33, 69, and 85-bus radial distribution networks, and comprehensive simulation studies show the effectiveness of the suggested method.

A PHY Layer Security of a Jamming-Based Underlay Cognitive Satellite-Terrestrial Network

In this work, we investigate the physical layer security of a jamming-based underlay cognitive satellite-terrestrial network consisting of a radio frequency link at the first hop and an optical feeder at the second hop. Particularly, one secondary user (SU) is transmitting data to an end-user optical ground station through the aid of a relay satellite, in the presence of an active eavesdropper at each hop. Moreover, another SU located in the first hop is acting as a friendly jammer and continuously broadcasting an artificial noise that cannot be decoded by the wiretapper so as to impinge positively on the system’s secrecy. Owing to the underlying strategy, the SUs are permanently adjusting their transmit powers in order to avoid causing harmful interference to primary users. The RF channels undergo shadowed-Rician and Rayleigh fading models, while the optical link is subject to Gamma-Gamma turbulence with pointing error. Closed-form and asymptotic expressions for the intercept probability (IP) are derived considering two different scenarios regardless of the channel’s conditions, namely (i) absence and (ii) presence of a friendly jammer. The effect of various key parameters on IP, e.g., sources’ transmit power, artificial noise, maximum tolerated interference power, and fading severity parameters are examined. Precisely, we aim to answer the following question: could a friendly jammer further enhance the security of such a system even in a low SNR regime? All the derived results are corroborated by Monte Carlo simulations and new insights into the considered system’s secrecy are gained.

On-situ monitoring of sleet-thawing for OPGW based on long distance BOTDR

With the absence of on-situ temperature monitoring of optical fiber composite overhead ground wire (OPGW) for the process of sleet-thawing, early temperature warning and safety control of direct current (DC) in sleet-thawing process is difficult. Here we propose a Brillouin optical time-domain reflectometry (BOTDR) with broadband receiving for fast measurement and with distributed Raman amplification for long distance measurement of about 100 km. A field experiment for on-situ temperature monitoring of sleet-thawing of OPGW is also reported, which shows uneven change of temperature along the OPGW. The difference between the maximum and the minimum temperature change can be greater than 40 °C.

System Stability Improvement and Cost-Effective Solution by Accelerated Distance Protection Using Direct Fiber Optic Signal

This paper presents a simplified method of system stability improvement and cost-effective solution by accelerated distance protection using direct fiber optic signal between the (end-to-end) relays. Transmission line protection is one of the important priority of protection, as their correct and selective tripping and fast fault clearance will avoid instability in the grid, which otherwise may result in cascade tripping leading to instability of the grid. This paper proposes a new approach by using optical ground wire (OPGW) direct relay-to-relay teleportation in association with modern numerical distance relay for accelerated distance protection, instead of carrier aided power line communication protection scheme. This methodology enhances the tripping function faster. Thus, quick isolation of the fault improves the stability of the grid. In this new technique, there is significant reduction in circuit complexity which reduces in cost for PLCC equipments and scheme and also saving the control room area.

Research and Development of a Transportable Optical Ground Station in NICT: The Results of the First Performance Test

In recent years, the necessity of free-space optical (FSO) communications has increased as a method for realizing high-speed communications between satellites and the ground. However, one disadvantage of FSO communications is the significant influence of the atmosphere. Specifically, FSO communications cannot be utilized under certain atmospheric conditions, especially in the presence of clouds. One of the solutions to this problem is the site diversity technique, which makes it possible to select a given ground station with better atmospheric conditions among a number of fixed ground stations. The other solution is to prepare a ground station that can be moved to a place with better atmospheric conditions. We applied the latter method and developed a transportable optical ground station in NICT. We utilize a realistic telescope diameter, which is about 30 cm at the maximum, capable of being set up quickly, and with a pointing accuracy of about 100 µrad. In addition, it is necessary to prepare a fine-pointing optical system that performs tracking with about 1/10 of the pointing accuracy of the telescope. In this paper, we report the results of the first performance test of the transportable optical ground station in NICT.

Influence on Short-circuit of Substation Grounding Grids by the Use of OPGW Cables

Optical Ground Wire (OPGW) are widely used on power systems. However, the usage of such cables during a transient request may lead to an unusual distribution of current along the grounding electrodes. Thus, it is interesting observe if these cables would case decreased or increased of safety on the vicinity of substation. On the light of such possibility, this paper analyzes the influence of the use of such cables on the intensity of fault currents that flow through the interconnected substation ground grids. For the analysis a comparison between the conventional ground wires set and a system considering the presence of an OPGW are studied. For the modeling of these systems, an EMTP-type software is used as a computational tool for the generation of sensitivity analyzes. The results ilustrate that the use of OPGW significantly reduces the grid currents of the interconnected substations. The studies with this thematic founding in literature approach the subject using only conventional ground wires.

Effect of cloud seeding on aerosol properties and particulate matter variability in the United Arab Emirates

In this study, satellite data and those obtained from air quality monitoring stations were used to evaluate the effect of cloud seeding missions conducted from January 2017 to March 2017 in the United Arab Emirates (UAE). To determine PM variability during and after the cloud seeding missions, information on daily mean particulate matter (PM10 and PM2.5) concentrations obtained from 20 air quality ground-based stations was analyzed from January 2017 to April 2017. The stations were divided according to four distinct geographic zones, which include petrochemical and non-petrochemical industrial, residential, and low populated areas. Moreover, data from the MAIAC 1-km product (MCD19A2) obtained using the Moderate Resolution Imaging Spectroradiometer onboard the Terra satellite were used to identify the correlation between aerosol optical depth and ground PM concentrations. The results showed that among the areas, the petrochemical and non-petrochemical industrial areas had the highest PM2.5 levels. Urban areas located adjacent to deserts had significantly higher PM10 concentrations than other urban areas. The PM concentration was high during the period of cloud seeding missions. This finding indicated that silver iodine crystals fired during the missions might have increased the concentration of PM in the air. Moreover, cloud seeding missions might have a more significant effect in increasing PM10 concentrations compared with PM2.5 concentrations. Hence, local weather conditions might have affected the decay of silver iodine particles during the missions. Long-range air mass transport may have an influence on the natural and anthropogenic particle concentrations in land and water bodies surrounding the UAE.

Adaptive Optics pre-compensated laser uplink to LEO and GEO.

We present the results from a Monte Carlo computer simulation of adaptive optics (AO) pre-compensated laser uplink propagation through the Earth's atmospheric turbulence from the ground to orbiting satellites. The simulation includes the so-called point-ahead angle and tests several potential AO mitigation modes such as tip/tilt or full AO from the downlink beam, and a laser guide star at the point ahead angle. The performance of these modes, as measured by metrics relevant for free-space optical communication, are compared with no correction and perfect correction. The aim of the study is to investigate fundamental limitations of free-space optical communications with AO pre-compensation and a point-ahead angle, therefore the results represent an upper bound of AO corrected performance, demonstrating the potential of pre-compensation technology. Performance is assessed with varying launch aperture size, wavelength, launch geometry, ground layer turbulence strength (i.e. day/night), elevation angle and satellite orbit (Low-Earth and Geostationary). By exploring this large parameter space we are able examine trends on performance with the aim of informing the design of future optical ground stations and demonstrating and quantifying the potential upper bounds of adaptive optics performance in free-space optical communications.

Impact of Wind Gust on High-Speed Characteristics of Polarization Mode Dispersion in Optical Power Ground Wire Cables.

Polarization mode dispersion is recognized as a key factor limiting optical transmission systems, particularly those fiber links that run at bit rates beyond 10 Gbps. In-line test and characterization of polarization mode dispersion are thus of critical importance to evaluate the quality of installed optical fibers that are in use for high-speed signal traffics. However, polarization-based effects in optical fibers are stochastic and quite sensitive to a range of environmental changes, including optical cable movements. This, in turn, gives rise to undesired variations in light polarization that adversely impair the quality of the signal transmission in the link. In this work, we elaborate on experimental testing and theoretical analysis to asses changes of polarization mode dispersion in optical fibers that are caused by environmental variations, here wind gusts in particular. The study was performed on commercially harnessed optical fibers installed within optical power ground wire cables, taking into account different weather conditions. More specifically, we showed that changes caused by wind gusts significantly influence the differential group delay and the principal state of polarization in those optical fibers. For this, we experimentally measured a number of parameters to characterize light polarization properties. Measurements were carried out on C-band operated fiber-optic link formed by 111-km-long power ground wire cables and 88 spectral channels, with a test time step of 1 min during 12 consecutive days. Variations in differential group delay allowed for sensitive testing of environmental changes with measured maxims up to 10 ps under the worst wind conditions. Moreover, measured parameters were used in a numerical model to assess the quality of transmitted high-bit-rate optical signals as a function of wind conditions. The analysis revealed a negligible impact of wind on a 10 Gbps transmission, while substantial influence was noticed for higher bit rates up to 100 Gbps. These results show promises for efficient sensing of environmental changes and subsequent monitoring of the quality of recently used fiber-optic link infrastructures.

Prototype of a Computer Vision-Based CubeSat Detection System for Laser Communications

Up to now, CubeSat nano-satellites have strong limitations in communication data rates ( $$\sim \hbox {100}$$ kbps) and bandwidth due to the strictness of CubeSat standard. However, if they could be endowed with optical communications (data rates up to 1 Gbps in optimal state), CubeSat applications would exponentially increase. Nonetheless, laser communications face some important drawbacks as the development of a very strict and accurate tracking mechanism. This work proposes an on-board fine pointing system to locate an optical ground station beacon using an embedded system complying with the restrictive CubeSat standard. Such on-board fine pointing system works based on computer vision. The experimental prototype is implemented in Matlab/Simulink, within a Raspberry Pi 3B. The main outcome is the usage of off-the-shelf components (COTS), obtaining an efficient tracking with low power consumption in very noisy and reflective environments. The developed system proves to be fast, stable and strong. It also satisfies the strict size and power consumption restrictions of CubeSat standard.

Comparison of Optical Transport Technologies for Centralized Radio Access Network Using Optical Ground Wire

Communication networks for wide-scale distributed energy resources (DERs) including photovoltaics (PVs), wind, storage and battery systems and electric vehicles (EVs) will be indispensable in future power grids. In this paper, we compare optical fronthaul networks using existing optical ground wires (OPGWs) for centralized radio access network (C-RAN) architecture to realize cost effective wireless communication network expansion including low population area. We investigate the applicability of optical data transport technologies of physical layer split (PLS), analog radio-on-fiber (ARoF), and common public radio interface (CPRI). The deployment costs of them are comparatively analyzed. It was shown that physical layer split and analog radio-on-fiber with subcarrier multiplexing (SCM) result in lower cost than other technologies.

The Surface Brightness of MegaConstellation Satellite Trails on Large Telescopes

On large telescopes, the trails produced by MegaConstellation satellites will be significantly defocused due to their close proximity. As a result, their apparent surface brightness will be, under a range of conditions, almost constant during their apparent sweep across the focal plane. This paper derives a few simple relationships and evaluate the impact of such trails on operations of large optical ground based facilities.

Discussion on The Application of Overhead Power Communication Optical Cable

Overhead optical cable is an important framework for the power communication network. The common types of optical cables erected with power lines of 35 kV and above include optical fiber composite ground wire (OPGW), non-metallic self-supporting optical cable (ADSS) and optical fiber composite phase wire (OPPC), different types of optical cables have their own characteristics, and are suitable for different situations. This article starts with the structure of the optical cable, analyzes the characteristics of different types of optical cables, and further analyzes and compares from the perspective of electrical, economic, and maintenance. For the problems encountered during the use of different types of optical cables, the safety, The technical regulations are more comprehensively explained. This article is for operation and designer reference.

Optimum Power Allocation Based on Channel Conditions in Optical Satellite Downlinks

The integration of optical satellite links in the next-generation networks and in the fifth generation cellular systems has been proposed in order to guarantee the handling of the extreme data traffic growth and the high-pitched demand for networks’ resources. The optical satellite communication downlink is studied and more specifically, a geostationary satellite with multiple transmitters and an optical ground station with multiple receiving terminals are considered. In this contribution a novel power allocation methodology is proposed for the downlink. The allocation methodology takes into account the scintillation effects due to atmospheric turbulence and maximizes the ergodic network capacity under total expected power and peak power constraints. The analytical optimizing schemes are based on convex optimization methods and have been inspired by waterfilling algorithm. We present emulated power allocation results using real experimental downlink data from ARTEMIS optical satellite campaign and then we investigate the performance of the proposed algorithm with extended numerical results and comparison with other allocation policies. In particular, the new power allocation strategy achieves the highest spectral efficiency, according to the power constraints, for various channel conditions and attenuation profiles and also surpasses two simple baseband allocation methods by intelligently taking advantage of the number of channels and the total expected power.

Automatic segmentation of optic disc in retinal fundus images using semi-supervised deep learning

Diseases of the eye require manual segmentation and examination of the optic disc by ophthalmologists. Though, image segmentation using deep learning techniques is achieving remarkable results, it leverages on large-scale labeled datasets. But, in the field of medical imaging, it is challenging to acquire large labeled datasets. Hence, this article proposes a novel deep learning model to automatically segment the optic disc in retinal fundus images by using the concepts of semi-supervised learning and transfer learning. Initially, a convolutional autoencoder (CAE) is trained to automatically learn features from a large number of unlabeled fundus images available from the Kaggle’s diabetic retinopathy (DR) dataset. The autoencoder (AE) learns the features from the unlabeled images by reconstructing the input images and becomes a pre-trained network (model). After this, the pre-trained autoencoder network is converted into a segmentation network. Later, using transfer learning, the segmentation network is trained with retinal fundus images along with their corresponding optic disc ground truth images from the DRISHTI GS1 and RIM-ONE datasets. The trained segmentation network is then tested on retinal fundus images from the test set of DRISHTI GS1 and RIM-ONE datasets. The experimental results show that the proposed method performs on par with the state-of-the-art methods achieving a 0.967 and 0.902 dice score coefficient on the test set of the DRISHTI GS1 and RIM-ONE datasets respectively. The proposed method also shows that transfer learning and semi-supervised learning overcomes the barrier imposed by the large labeled dataset. The proposed segmentation model can be used in automatic retinal image processing systems for diagnosing diseases of the eye.

Experimental and numerical analysis of damage characteristics to opgw strands under first lightning strike and continuous current

Optical fiber ground wires (OPGWs) are used to protect power transmission lines from lightning strikes. A lightning test system with a tension device was established to study the properties and influence factors of lightning damage for OPGWs under multiple consecutive lightning strikes composed of current impulse and continuous current. The lightning damage can be divided into arc sweeping damage, melting damage and ablation damage. The lightning energy injected through a W-Cu electrode was the most concentrated, leading to the most serious ablation damage and the smallest sweeping damage area. As the action integral and external tensile force increased, the lightning damage became more severe. The broken strands and ablation damage volume decreased as the discharge gap increased from 20 to 40 mm. In addition, a 3D finite element simulation of lightning melting damage was conducted considering the lightning test conditions. When 1850 K, which is close to the melting point of steel, was selected as the criterion temperature for melting damage, the deviations between the simulation and experimental results were less than 10%.

Experimental analysis of the strands breaking characteristics of optical fibre composite overhead ground wire due to simulating lightning strike

With the increasing application of OPGW (optical fibre composite overhead ground wire) in power systems, the strands in OPGW are frequently broken due to lightning strikes. In this study, ten kinds of OPGW lightning-strike tolerance was tested according to the IEC standards. The influences of the following factors, such as transfer charge, tensile force, the diameter and material of OPGW outer layer strands, were analysed. The transfer charge and tensile force are positively proportional to the degree of lightning strike damage to OPGW. The diameters of OPGW outer layer strands are positively related to the lightning strike tolerance of OPGW. Compared with OPGW with aluminium alloy outer layer strands, lightning strikes tolerance of OPGW with aluminium clad steel (AS) outer layer strands is better. It suggested that the OPGW outer layer strands diameter >3.2 mm and with AS outer-layer strands shows good lightning strike tolerance. The mechanism of OPGW strands breaking was also analysed by the infrared detection, micro-morphology detection and energy spectrum. The experimental results of this study provide significant data support for OPGW type selection for lightning protection.

Performance Analysis of DPSK Optical Communication for LEO-to-Ground Relay Link Via a GEO Satellite

Satellite optical communication has gained significant attention owing to its many quality features (e.g., a larger bandwidth, license free spectrum, higher data rate, and better security) compared to satellite microwave communication. Various experiments have been performed during many space missions to demonstrate and characterize inter-satellite links, downlinks, and uplinks. Korea has also planned to establish an experimental communication system using a geostationary earth orbit (GEO) satellite and the Geochang station as an optical ground station for low Earth orbit (LEO)-to-ground optical relay links. In this study, the performance of inter-satellite communication links and downlinks was investigated for the new Korean experimental communication system in terms of link margin, bit error rate (BER), and channel capacity. In particular, the performance of the inter-satellite links was analyzed based on the receiving apertures and the transmitting power, while that of the downlink was analyzed in terms of atmospheric turbulence conditions and transmitting power. Finally, we discussed two system parameters of receiving aperture and transmitting power to meet the three criteria of link margin, BER, and channel capacity.