727 research outputs found

    Satellite remote sensing of surface winds, waves, and currents: Where are we now?

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    This review paper reports on the state-of-the-art concerning observations of surface winds, waves, and currents from space and their use for scientific research and subsequent applications. The development of observations of sea state parameters from space dates back to the 1970s, with a significant increase in the number and diversity of space missions since the 1990s. Sensors used to monitor the sea-state parameters from space are mainly based on microwave techniques. They are either specifically designed to monitor surface parameters or are used for their abilities to provide opportunistic measurements complementary to their primary purpose. The principles on which is based on the estimation of the sea surface parameters are first described, including the performance and limitations of each method. Numerous examples and references on the use of these observations for scientific and operational applications are then given. The richness and diversity of these applications are linked to the importance of knowledge of the sea state in many fields. Firstly, surface wind, waves, and currents are significant factors influencing exchanges at the air/sea interface, impacting oceanic and atmospheric boundary layers, contributing to sea level rise at the coasts, and interacting with the sea-ice formation or destruction in the polar zones. Secondly, ocean surface currents combined with wind- and wave- induced drift contribute to the transport of heat, salt, and pollutants. Waves and surface currents also impact sediment transport and erosion in coastal areas. For operational applications, observations of surface parameters are necessary on the one hand to constrain the numerical solutions of predictive models (numerical wave, oceanic, or atmospheric models), and on the other hand to validate their results. In turn, these predictive models are used to guarantee safe, efficient, and successful offshore operations, including the commercial shipping and energy sector, as well as tourism and coastal activities. Long-time series of global sea-state observations are also becoming increasingly important to analyze the impact of climate change on our environment. All these aspects are recalled in the article, relating to both historical and contemporary activities in these fields

    Beam scanning by liquid-crystal biasing in a modified SIW structure

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    A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium

    BDS GNSS for Earth Observation

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    For millennia, human communities have wondered about the possibility of observing phenomena in their surroundings, and in particular those affecting the Earth on which they live. More generally, it can be conceptually defined as Earth observation (EO) and is the collection of information about the biological, chemical and physical systems of planet Earth. It can be undertaken through sensors in direct contact with the ground or airborne platforms (such as weather balloons and stations) or remote-sensing technologies. However, the definition of EO has only become significant in the last 50 years, since it has been possible to send artificial satellites out of Earthโ€™s orbit. Referring strictly to civil applications, satellites of this type were initially designed to provide satellite images; later, their purpose expanded to include the study of information on land characteristics, growing vegetation, crops, and environmental pollution. The data collected are used for several purposes, including the identification of natural resources and the production of accurate cartography. Satellite observations can cover the land, the atmosphere, and the oceans. Remote-sensing satellites may be equipped with passive instrumentation such as infrared or cameras for imaging the visible or active instrumentation such as radar. Generally, such satellites are non-geostationary satellites, i.e., they move at a certain speed along orbits inclined with respect to the Earthโ€™s equatorial plane, often in polar orbit, at low or medium altitude, Low Earth Orbit (LEO) and Medium Earth Orbit (MEO), thus covering the entire Earthโ€™s surface in a certain scan time (properly called โ€™temporal resolutionโ€™), i.e., in a certain number of orbits around the Earth. The first remote-sensing satellites were the American NASA/USGS Landsat Program; subsequently, the European: ENVISAT (ENVironmental SATellite), ERS (European Remote-Sensing satellite), RapidEye, the French SPOT (Satellite Pour lโ€™Observation de laTerre), and the Canadian RADARSAT satellites were launched. The IKONOS, QuickBird, and GeoEye-1 satellites were dedicated to cartography. The WorldView-1 and WorldView-2 satellites and the COSMO-SkyMed system are more recent. The latest generation are the low payloads called Small Satellites, e.g., the Chinese BuFeng-1 and Fengyun-3 series. Also, Global Navigation Satellite Systems (GNSSs) have captured the attention of researchers worldwide for a multitude of Earth monitoring and exploration applications. On the other hand, over the past 40 years, GNSSs have become an essential part of many human activities. As is widely noted, there are currently four fully operational GNSSs; two of these were developed for military purposes (American NAVstar GPS and Russian GLONASS), whilst two others were developed for civil purposes such as the Chinese BeiDou satellite navigation system (BDS) and the European Galileo. In addition, many other regional GNSSs, such as the South Korean Regional Positioning System (KPS), the Japanese quasi-zenital satellite system (QZSS), and the Indian Regional Navigation Satellite System (IRNSS/NavIC), will become available in the next few years, which will have enormous potential for scientific applications and geomatics professionals. In addition to their traditional role of providing global positioning, navigation, and timing (PNT) information, GNSS navigation signals are now being used in new and innovative ways. Across the globe, new fields of scientific study are opening up to examine how signals can provide information about the characteristics of the atmosphere and even the surfaces from which they are reflected before being collected by a receiver. EO researchers monitor global environmental systems using in situ and remote monitoring tools. Their findings provide tools to support decision makers in various areas of interest, from security to the natural environment. GNSS signals are considered an important new source of information because they are a free, real-time, and globally available resource for the EO community

    Validation of wind measurements from a multirotor RPAS-mounted ultrasonic wind sensor using a ground-based LiDAR system

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    The aim of this research was to establish the validity of wind measurements from on board a multirotor Remotely Piloted Aircraft System (RPAS) for the purposes of wind monitoring applications. A custom-built hexacopter RPAS recorded wind speed and direction by means of an onboard ultrasonic wind sensor, whilst operating in the inherently highly stochastic nature of open field atmospheric conditions. Experimental data were collected during open field hovering flights subject to different ambient conditions with free stream horizontal wind speeds reaching up to 12 m/s. Flights were conducted at different altitudes above ground level and in proximity to a Light Detection and Ranging (LiDAR) remote wind measurement unit that was used as a low-resolution reference meteorological station. Very good correlation was obtained between the RPAS and LiDAR unit for both wind speed and wind direction measurements across all hovering flight altitudes. The RPAS-based wind speed measurements were found to have a consistent 1 m/s positive offset, whilst the RPAS-based wind direction readings had a 6.16โ—ฆ negative offset. These were potentially caused by differences in the localized wind fields between the LiDAR and RPAS measuring positions, as well as by localized RPAS rotor-induced air flows for wind speed measurements and potential slight misalignments in the instrumentsโ€™ reference datum for wind direction readings.peer-reviewe

    Selected Problems of High-Resolution Automotive Imaging Radar

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    This thesis aims at two selected problems in the development of high-resolution au- tomotive imaging radar: 1) The feasibility of using sub-THz for the next generation of automotive radar; 2) The development of the physics-based image segmentation approach on the automotive radar imagery. The wide range of feasibility studies on the use of sub-THz frequencies for auto- motive radar have been undertaken in the Microwave Integrated Systems Laboratory (MISL) at the University of Birmingham, and the candidate is in charge of the included study on the theoretical modelling and experimental verification of the attenuation through the vehicle infrastructures which is the first part of this thesis. The importance of this work is related to the fact that automotive radar is placed within the car infras- tructure. Therefore, it would be a potential show-stopper in the development of this innovation if attenuation within the car bumper or badge is prohibitively high. Both theoretical modelling and experimental measurement are conducted by considering the impact factors on the propagation properties of the sub-THz signal such as the incident angle, frequency, characteristic parameters of materials, and the thicknesses of infrastructure layers. The transmissivity of multilayered structure has been modelled and good agreement with the results of measurements was demonstrated, so that the developed approach can be used in further studies on propagation through car infrastruc- ture. The published results on transmissivity and complex permittivity of automotive paints are valuable for researchers in either field of THz technology or automotive radar. The image segmentation on automotive radar maps aims at identifying the passable and impassable areas for path planning in autonomous driving. Contrary to traditional radar, radar clutter is regarded as the physical meaningful information, which can deliver valuable feature information for surface characterization, and enable the full scene reconstruction of automotive radar maps. The proposed novel segmentation algorithm is a hybrid method composed of pre-segmentation based on image processing methods, and the region classification using the multivariate Gaussian distribution (MGD) classifier developed based on the statistical distribution feature parameters of radar returns of various areas. Moving target indication (MTI) is implemented for the first time based on frame-to-frame context association. The end-to-end segmentation framework is therefore achieved robustly with good segmentation performance, and automatically without human intervention

    The NASA CYGNSS SmallSat Constellation

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    The NASA Cyclone Global Navigation Satellite System (CYGNSS) is a constellation of eight microsatellites in low earth orbit at ~525 km altitude and 35 deg inclination. CYGNSS was launched in December 2016 for a planned 2 year mission and 7 of the 8 spacecraft continue to operatue nominally as of May 2023. Each microsatellites carries a bistatic radar receiver to measure reflected GPS signals from the Earth surface. The measurements can be converted to surface wind speed and latent and sensible heat flux over the ocean, and to surface soil moisture and wetland extent over land. Measurements penetrate through all levels of precipitation as well as moderate to heavy vegetation due to the low microwave frequency used by GPS. The number of satellites in the constellation results in sub-daily refresh rates which supports imaging of short time scale weather events such as hurricane rapid intensification, flood inundation dynamics, and sudden soil saturation after major rain events. CYGNSS satellites uses a single string design architecture to reduce the complexity and recurring cost of each unit. Mission redundancy is obtained at the constellation level. Data products are produced by combining measurements from all satellites in such a way that the sampling requirements can be met using only a subset of the satellites. Constellation-level redundancy also permits individual satellites to be switched from their nominal science data taking mode to various engineering test and calibration modes while the overall mission is still able to meet its science requirements

    Reconstructing the history of the Antarctic ice sheet using internal reflecting horizons from radio-echo sounding

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    Understanding the contribution of the Antarctic Ice Sheet (AIS) to past and future sea-level rise has emerged as a scientific priority over the last four decades. Whilst our knowledge of ice-dynamical changes occurring as a result of current anthropogenic forcing has improved considerably since the start of the satellite era, significantly less is known about the evolution of the AIS during the pre-industrial Holocene (the last ~11.7 thousand years; ka). Quantifying these changes is crucial, however, as this time period corresponds to a time when the ice sheet was retreating from its maximal extent at the Last Glacial Maximum (LGM; ~20 ka) and environmental conditions were similar to today. Therefore, improving our understanding of this period may provide a long-term context to the decadal changes observed in recent times and how these may evolve in the future. Whilst point-based geochronological measurements of ice and sediment cores, or surface exposure dating, can be used to assess past ice-sheet changes over the AIS, it remains unclear how representative they are of a wider region. A complementary and spatially extensive resource across the ice sheet are Internal Reflecting Horizons (IRHs) as imaged by Radio-Echo Sounding (RES) techniques, which provide a cumulative record of accumulation, basal melt and ice dynamics that, if dated precisely at ice cores, can be used to inform numerical ice-sheet models projecting past and future changes on large spatial scales. The aim of this thesis is therefore to develop and extend age-depth models from IRHs across the AIS to assess the past stability of the ice sheet. In this thesis, an age-depth model of Pine Island Glacier spanning the LGM and Holocene periods is derived from spatially extensive IRHs. The connection between RES profiles and the WAIS Divide ice core enables the direct dating of the IRHs, and reveals that they match large peaks in sulphate concentrations which are unparalleled in the 68,000 year-old record, thus suggesting that the cause of these IRHs is from past explosive volcanic eruptions. By connecting this IRH stratigraphy with a previously developed age-depth model across the Institute and Mรถller Ice Streams (IMIS), I show that a precisely dated age-depth model now exists over 20% of the West Antarctic Ice Sheet (WAIS). One of these IRHs, precisely dated at ~4.7 ka, is then used as input into a one-dimensional ice-flow model to estimate past accumulation rates during the mid-Holocene over the catchments encompassing Pine Island Glacier, Thwaites Glacier, and IMIS, together representing 30% of the WAIS. The inferred mid-Holocene accumulation estimates are then compared with modern rates derived from climate models and observational measurements to show that accumulation rates were 18% greater during the last five millennia compared to the present over the Amundsen-Weddell-Ross Divide. These results match previous findings from isolated ice-core measurements and spatially targeted studies over the divide, and correspond to periods of grounding line retreat and readvance during the Holocene over the WAIS. Together, these show the potential for extracting further IRH information from other sectors of the AIS in order to build an age-depth model of the ice sheet. However, the underlying RES data necessary for this work were, until recently, relatively inaccessible to the wider scientific community, thus restricting the extraction and interpretation of age-depth models across the AIS. This motivated the release of ~300,000 line-km of RES profiles acquired by the British Antarctic Survey between 2004 and 2020. In addition to standardising and releasing these data, this thesis shows that large sections of continuous englacial layering exist widely across both East and West Antarctica, suggesting that, together with previously developed age-depth models of both regions and nearby ice-core stratigraphies, these newly released RES datasets will be critical in our aim to build an ice-sheet wide age-depth model of Antarctica, as motivated by the AntArchitecture Initiative. Together, the findings from this thesis reveal the spatially extensive nature of IRHs across West and East Antarctica and demonstrate how these can be used to infer past ice-sheet changes. This thesis also highlights the need to extract further age-depth models, particularly across East Antarctica, in order to provide important boundary conditions such as past accumulation rates and ice-elevation change which can be used by numerical ice-sheet models to help improve predictions of past and future ice-sheet change and ensuing sea-level rise contributions

    Airborne investigation of quasi-specular Ku-band radar scattering for satellite altimetry over snow-covered Arctic sea ice

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    Surface-based Ku-band radar altimetry investigations indicate the radar signal is typically backscattered from well above the snow-sea ice interface. However, this would induce a bias in satellite altimeter sea ice thickness retrievals not reflected by buoy validation. Our study presents a mechanism to potentially explain this paradox: probabilistic quasi-specular radar scattering from the snow-ice interface. We introduce the theory for this mechanism before identifying it in airborne Ku-band radar observations collected over landfast first year Arctic sea ice near Eureka, Canada, in spring 2016. Based on SAR data, this study area likely represents level first year sea ice across the Arctic. Radar backscatter from the snow and ice interfaces were estimated by co-aligning laser scanner and radar observations with in situ measurements. On average, 4-5 times more radar power was scattered from the snow-ice than the air-snow interface over first-year ice. However, return power varied by up to 20 dB between consecutive radar echoes, particularly from the snow-ice interface, depending on local slope and roughness. Measured laser-radar snow depths were more accurate when radar returns were specular, but there was no systematic bias between airborne and in situ snow depths. The probability and strength of quasi-specular returns depend on the measuring height above and slope distribution of sea ice, so these findings have implications for satellite altimetry snow depth and freeboard estimates. This mechanism could explain the apparent differences in Ku-band radar penetration into snow on sea ice when observed from the range of a surface-, airborne- or satellite-based sensor

    Study on Real-Time Ocean Wave Analysis Based on X-Band Radar Measurement Data

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    ํ•™์œ„๋…ผ๋ฌธ(์„์‚ฌ) -- ์„œ์šธ๋Œ€ํ•™๊ต๋Œ€ํ•™์› : ๊ณต๊ณผ๋Œ€ํ•™ ์กฐ์„ ํ•ด์–‘๊ณตํ•™๊ณผ, 2023. 2. ๊น€์šฉํ™˜.ํ•ด์–‘ ํ™œ๋™์˜ ์•ˆ์ „์„ฑ ๋ฐ ํšจ์œจ์„ ํ–ฅ์ƒํ•˜๊ธฐ ์œ„ํ•ด ์‹ ๋ขฐ๋„ ๋†’์€ ํŒŒ๋ž‘ ์ •๋ณด์˜ ํš๋“์ด ์š”๊ตฌ๋จ์— ๋”ฐ๋ผ ์ „ ์„ธ๊ณ„์ ์œผ๋กœ ๋‹ค์–‘ํ•œ ๋ฐฉ์‹์˜ ํŒŒ๋ž‘ ๊ณ„์ธก์ด ์ˆ˜ํ–‰๋˜๊ณ  ์žˆ๋‹ค. ์ด ์ค‘ ํ•ด์–‘ X-band ๋ ˆ์ด๋”๋Š” ๋„“์€ ์˜์—ญ์˜ ํŒŒ๋ž‘ ์ •๋ณด๋ฅผ ๋™์‹œ์— ๊ณ„์ธกํ•  ์ˆ˜ ์žˆ์œผ๋ฏ€๋กœ ๋‹จ์‹œ๊ฐ„์˜ ๊ณ„์ธก์„ ํ†ตํ•ด ํ†ต๊ณ„์ ์œผ๋กœ ์ˆ˜๋ ด๋„ ๋†’์€ ํ•ด์–‘ํŒŒ ์ •๋ณด๋ฅผ ์–ป์„ ์ˆ˜ ์žˆ๋‹ค๋Š” ์žฅ์ ์ด ์žˆ๋‹ค. ๋•Œ๋ฌธ์—, ๋‹ค์–‘ํ•œ ์„ ๋ฐ• ๋ฐ ํ•ด์–‘ ๊ตฌ์กฐ๋ฌผ์—์„œ ํ•ด์–‘ X-band ๋ ˆ์ด๋”๋ฅผ ํ™œ์šฉํ•˜์—ฌ ํŒŒ๋ž‘ ๊ณ„์ธก์„ ์ˆ˜ํ–‰ํ•˜๊ณ  ์žˆ์œผ๋ฉฐ, ๊ณ„์ธก ๊ธฐ๋ฒ•์˜ ๊ณ ๋„ํ™”์— ๋Œ€ํ•œ ๋‹ค์–‘ํ•œ ๋…ผ์˜๊ฐ€ ์ง„ํ–‰๋˜๊ณ  ์žˆ๋‹ค. ํ•ด์–‘ ๋ ˆ์ด๋”๋Š” ์•ˆํ…Œ๋‚˜์—์„œ ์†ก์‹ ๋œ X-band ๋งˆ์ดํฌ๋กœํŒŒ์™€ ํ•ด๋ฉด์ƒ ์ž”๋ฌผ๊ฒฐ ๊ฐ„์˜ Bragg ๊ณต์ง„ ํ˜„์ƒ์— ์˜ํ•ด ํ›„๋ฐฉ ์‚ฐ๋ž€๋˜๋Š” ์ „์ž๊ธฐํŒŒ์˜ ์„ธ๊ธฐ๋ฅผ ๊ณ„์ธกํ•œ๋‹ค. ์ด๋Ÿฌํ•œ ์›๊ฒฉ ๊ณ„์ธก ๊ณผ์ •์€ ๊ทธ๋ฆผ์ž, ๊ธฐ์šธ์ž„, ์œ ์ฒด๋™์—ญํ•™์  ํšจ๊ณผ ๋“ฑ ์ˆ˜๋งŽ์€ ๋น„๋ฌผ๋ฆฌ์  ๋ณ€์กฐ ํšจ๊ณผ๋ฅผ ์ˆ˜๋ฐ˜ํ•œ๋‹ค. ๋”ฐ๋ผ์„œ, ๋ ˆ์ด๋” ์ด๋ฏธ์ง€๋กœ๋ถ€ํ„ฐ ํŒŒ๋ž‘ ์ •๋ณด๋ฅผ ๋„์ถœํ•˜๊ธฐ ์œ„ํ•ด์„œ๋Š” ์ด๋ฏธ์ง€ ๊ฐ•๋„์— ํฌํ•จ๋œ ๋น„๋ฌผ๋ฆฌ์  ์„ฑ๋ถ„์„ ์ œ๊ฑฐํ•˜๊ณ  ์ŠคํŽ™ํŠธ๋Ÿผ์˜ ์—๋„ˆ์ง€๋ฅผ ์œ ์˜ํŒŒ๊ณ ์— ๋”ฐ๋ผ ์กฐ์ •ํ•˜๋Š” ํŒŒ๋ž‘์žฅ ์žฌ๊ตฌ์„ฑ ๊ณผ์ •์ด ์š”๊ตฌ๋œ๋‹ค. ๋ณธ ๋…ผ๋ฌธ์€ ๊ณ ๋„ํ™”๋œ ํŒŒ๋ž‘์žฅ ์žฌ๊ตฌ์„ฑ ๊ธฐ๋ฒ•์— ๋Œ€ํ•œ ์—ฐ๊ตฌ๋ฅผ ๋‹ค๋ฃจ๊ณ  ์žˆ๋‹ค. ์ œ์‹œ๋œ ์ „์ฒด ์žฌ๊ตฌ์„ฑ ์ ˆ์ฐจ๋Š” ๊ทธ๋ฆผ์ž ๊ธฐ๋ฐ˜ ์œ ์˜ํŒŒ๊ณ  ์ถ”์ •๊ณผ 3D-FFT ๊ธฐ๋ฐ˜ ํŒŒ๋ž‘์žฅ ์žฌ๊ตฌ์„ฑ์œผ๋กœ ๊ตฌ์„ฑ๋˜๋ฉฐ, ๊ฐ ํ•ด์„ ๊ณผ์ •์ด ๋†’์€ ์—ฐ์‚ฐ ํšจ์œจ์„ ์ง€๋‹ˆ๊ณ  ์žˆ๋‹ค. ๋˜ํ•œ, ๋ณธ ์—ฐ๊ตฌ์—์„œ๋Š” ํŒŒ๋ž‘์žฅ ์žฌ๊ตฌ์„ฑ ๊ธฐ๋ฒ•์˜ ์ •ํ™•๋„ ํ–ฅ์ƒ์„ ์œ„ํ•ด ๊ทธ๋ฆผ์ž ๋ฐœ์ƒ์˜ ๊ณต๊ฐ„ ํ†ต๊ณ„์  ํŠน์„ฑ์„ ์—„๋ฐ€ํ•˜๊ฒŒ ๊ณ ๋ คํ•˜์˜€๋‹ค. ์ด๋ฅผ ์œ„ํ•ด, ์œ ์˜ํŒŒ๊ณ  ์ถ”์ • ์‹œ ํ•ด๋ฉด์˜ ๊ณต๊ฐ„์ƒ ์ž๊ธฐ์ƒ๊ด€ํ•จ์ˆ˜ ๋ฐ ํ‰๊ท ํ‘œ๋ฉด๊ฒฝ์‚ฌ์˜ ์ง๊ต์„ฑ์„ ๊ณ ๋ คํ•˜์˜€๊ณ , ํŒŒ๋ž‘์žฅ ์žฌ๊ตฌ์„ฑ ์‹œ ๊ทธ๋ฆผ์ž ๋ฐœ์ƒ์˜ ๊ณต๊ฐ„์  ํŠน์„ฑ์— ๊ธฐ์ธํ•˜๋Š” ๋ถˆ๊ท ์ผํ•œ ๋ถ„์‚ฐ ๋ถ„ํฌ์— ๋Œ€ํ•œ ๋ณด์ •์„ ์ˆ˜ํ–‰ํ•˜์˜€๋‹ค. ๋ณธ ๋…ผ๋ฌธ์—์„œ ์ œ์‹œ๋œ ๊ธฐ๋ฒ•์˜ ๊ฒ€์ฆ์„ ์œ„ํ•ด ํ•ฉ์„ฑ ๋ ˆ์ด๋” ์ด๋ฏธ์ง€์™€ ์‹คํ•ด์—ญ ๋ ˆ์ด๋” ์ด๋ฏธ์ง€์— ๋Œ€ํ•ด ํŒŒ๋ž‘์žฅ ์žฌ๊ตฌ์„ฑ์„ ์ˆ˜ํ–‰ํ•˜์˜€๋‹ค. ๋จผ์ €, ๋‹ค์–‘ํ•œ ํ•ด์ƒ ์ƒํƒœ์˜ ํ•ฉ์„ฑ ๋ ˆ์ด๋” ์ด๋ฏธ์ง€๋ฅผ ์ƒ์„ฑํ•˜์—ฌ ํ•ด์„์— ํ™œ์šฉํ•˜์˜€๊ณ , ํ•ด์ƒ ์ƒํƒœ์— ๋”ฐ๋ฅธ ์žฌ๊ตฌ์„ฑ ์ •ํ™•๋„์˜ ์˜์กด์„ฑ์„ ์‚ดํŽด๋ณด์•˜๋‹ค. ์ด๋ฅผ ํ†ตํ•ด, ๋‹ค์–‘ํ•œ ํ•ด์ƒ ์ƒํƒœ์—์„œ ๊ทธ๋ฆผ์ž ํšจ๊ณผ์— ๋Œ€ํ•œ ์—„๋ฐ€ํ•œ ๊ณ ๋ ค๋ฅผ ํ†ตํ•ด ํŒŒ๋ž‘์žฅ ์žฌ๊ตฌ์„ฑ ์ •ํ™•๋„๋ฅผ ํ–ฅ์ƒํ•  ์ˆ˜ ์žˆ์Œ์„ ํ™•์ธํ•˜์˜€๋‹ค. ๋‹ค์Œ์œผ๋กœ, ์ด์–ด๋„ ํ•ด์–‘๊ณผํ•™๊ธฐ์ง€ ๋ฐ ๊ธฐ์ƒ 1ํ˜ธ์—์„œ ๊ณ„์ธก๋œ ์‹คํ•ด์—ญ ๋ ˆ์ด๋” ์ด๋ฏธ์ง€์— ๋Œ€ํ•œ ์œ ์˜ํŒŒ๊ณ  ์ถ”์ •์„ ์ˆ˜ํ–‰ํ•˜์˜€๋‹ค. ๊ทธ ๊ฒฐ๊ณผ, ์‹คํ•ด์—ญ ๋ฐ์ดํ„ฐ์— ๋Œ€ํ•˜์—ฌ ์ •ํ™•๋„ ๋†’์€ ์œ ์˜ํŒŒ๊ณ  ์ถ”์ •์ด ๊ฐ€๋Šฅํ•จ์„ ํ™•์ธํ•˜์˜€๋‹ค.It is required to obtain reliable wave information to improve the safety and efficiency of marine activities. Various methods for wave measurements are being carried out around the world. Among them, the marine X-band radar has the advantage that it can obtain statistically converged wave information based on short-time measurement. This is because the wave radar can simultaneously measure wave elevation data in a large area. Accordingly, marine X-band radars are installed on various ships and marine platforms to perform wave measurements. Diverse discussions on X-band radar-based wave field analysis techniques are also steadily underway. In general, incoherent marine radar measures the backscattered intensity due to Bragg scattering between X-band microwaves transmitted from the antenna and ripples on the sea surface. This remote sensing process entails numerous non-physical modulation effects, such as shadowing, tilting, and hydrodynamic effects. Therefore, a series of post-processing called wave-field reconstruction is required to retrieve wave information from marine radar images. The wave-field reconstruction procedure consists of removing the non-physical components from the measured spectrum, and adjusting the total spectral energy according to the significant wave height (HS). In this study, the advanced wave-field reconstruction technique is presented. The overall reconstruction procedure is comprised of the shadowing-based HS estimation and 3D-FFT-based wave-field reconstruction, and both of each analysis process have high computational efficiency. Thats why it is suitable for real-time wave-field analysis. To enhance the wave analysis, the statistical characteristics of the shadowing effect were rigorously considered. For this purpose, the spatial autocorrelation function of the ocean surface and the orthogonality of the mean surface slope were considered for HS estimation. Moreover, the uneven variance distribution owing to the spatial dependency of the shadowing effect was mitigated during the wave-field reconstruction. Wave-field reconstruction was applied to the synthetic and real radar images to verify the presented technique. The HS estimation and 3D-FFT-based wave-field reconstruction were performed for synthetic radar images corresponding to various states, and the dependence of this technique on the sea state was examined. As a result, it was confirmed that the reconstruction accuracy could be improved through the rigorous consideration of stochastic characteristics of the shadowing effect for all cases. Moreover, HS estimation was performed for real radar images collected from the Ieodo ocean research station and RV Gisang 1. In conclusion, a satisfactorily accurate HS estimation was also achieved.1. ์„œ๋ก  1 1.1 ์—ฐ๊ตฌ ๋ฐฐ๊ฒฝ 1 1.2 ๊ธฐ์กด ์—ฐ๊ตฌ 3 1.2.1 3D-FFT ๊ธฐ๋ฐ˜ ํŒŒ๋ž‘์žฅ ์žฌ๊ตฌ์„ฑ 3 1.2.2 ์œ ์˜ํŒŒ๊ณ  ์ถ”์ • 5 1.3 ์—ฐ๊ตฌ ๋ชฉํ‘œ ๋ฐ ์ฃผ์š” ์—ฐ๊ตฌ ๋‚ด์šฉ 7 2. ํŒŒ๋ž‘์žฅ ์žฌ๊ตฌ์„ฑ 9 2.1 ์œ„์ƒ ๋ถ„ํ•ด ํŒŒ๋ž‘์žฅ ์žฌ๊ตฌ์„ฑ ๋ฌธ์ œ 9 2.1.1 ๋ฌธ์ œ ์ •์˜ 9 2.1.2 ์ „์ฒด ํ•ด์„ ์ ˆ์ฐจ 11 2.2 ์œ ์˜ํŒŒ๊ณ  ์ถ”์ • 13 2.2.1 ๊ทธ๋ฆผ์ž ์˜์—ญ ๊ตฌ๋ถ„ 13 2.2.2 Smith ํ•จ์ˆ˜ ๊ธฐ๋ฐ˜ ํ‘œ๋ฉด ๊ฒฝ์‚ฌ ์ถ”์ • 14 2.2.3 ์ดํ‘œ๋ฉด๊ฒฝ์‚ฌ ์ถ”์ • 18 2.2.4 ์œ ์˜ํŒŒ๊ณ  ๊ณ„์‚ฐ 19 2.3 3D-FFT ๊ธฐ๋ฐ˜ ํŒŒ๋ž‘์žฅ ์žฌ๊ตฌ์„ฑ 21 2.3.1 Mean-shift ๋ณ€ํ˜• 21 2.3.2 ์—๋„ˆ์ง€ ๋ถ„ํฌ ๋ณด์ • 21 2.3.3 3์ฐจ์› ๊ณ ์† ํ‘ธ๋ฆฌ์— ๋ณ€ํ™˜(3D-FFT) 23 2.3.4 ํ•„ํ„ฐ๋ง 23 2.3.5 ๋ณ€์กฐ ์ „๋‹ฌ ํ•จ์ˆ˜(MTF) 24 3. ํ•ฉ์„ฑ ๋ ˆ์ด๋” ์ด๋ฏธ์ง€ ํ•ด์„ 26 3.1 ํ•ฉ์„ฑ ๋ ˆ์ด๋” ์ด๋ฏธ์ง€ ์ƒ์„ฑ 26 3.2 ์œ ์˜ํŒŒ๊ณ  ์ถ”์ • 30 3.2.1 ํ‰๊ท ํ‘œ๋ฉด๊ฒฝ์‚ฌ ์ถ”์ • 30 3.2.2 ์ŠคํŽ™ํŠธ๋Ÿผ ํ•ด์„ 34 3.2.3 ์œ ์˜ํŒŒ๊ณ  ์ถ”์ • ๊ฒฐ๊ณผ 36 3.3 ํŒŒ๋ž‘์žฅ ์žฌ๊ตฌ์„ฑ 38 3.3.1 ์—๋„ˆ์ง€ ๋ถ„ํฌ ๋ณด์ • 38 3.3.2 ์žฌ๊ตฌ์„ฑ ๊ฒฐ๊ณผ 40 4. ์‹คํ•ด์—ญ ๋ ˆ์ด๋” ์ด๋ฏธ์ง€ ํ•ด์„ 45 4.1 ๋ฐ์ดํ„ฐ์…‹ ์ •์˜ 45 4.1.1 ์ด์–ด๋„ ๋ฐ์ดํ„ฐ์…‹ 45 4.1.2 NIMS ๋ฐ์ดํ„ฐ์…‹ 46 4.2 ์œ ์˜ํŒŒ๊ณ  ์ถ”์ • ๊ฒฐ๊ณผ 48 4.2.1 ์ด์–ด๋„ ๋ฐ์ดํ„ฐ์…‹ ํ•ด์„ ๊ฒฐ๊ณผ 48 4.2.2 NIMS ๋ฐ์ดํ„ฐ์…‹ ํ•ด์„ ๊ฒฐ๊ณผ 53 5. ๊ฒฐ๋ก  57 6. ๊ณตํ•™์  ๊ธฐ์—ฌ ๋ฐ ํ–ฅํ›„ ์—ฐ๊ตฌ 59 ์ฐธ๊ณ ๋ฌธํ—Œ 61 ๋ถ€๋ก 66 A.1 Smith ํ•จ์ˆ˜ ์œ ๋„ 66 A.2 ํ•ด๋ฉด ํ‰๊ท ํ‘œ๋ฉด๊ฒฝ์‚ฌ์˜ ๋ฌผ๋ฆฌ์  ํŠน์„ฑ 71์„

    1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

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    A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance
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