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Stephen J Frasier - One of the best experts on this subject based on the ideXlab platform.
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sea Surface Velocity vector retrieval using dual beam interferometry first demonstration
IEEE Transactions on Geoscience and Remote Sensing, 2005Co-Authors: Jakov V Toporkov, Dragana Perkovic, Gordon Farquharson, Mark A Sletten, Stephen J FrasierAbstract:The dual-beam interferometer consists of two interferometric synthetic aperture radars (InSARs), one squinted at 20/spl deg/ forward of broadside, and the other 20/spl deg/ aft, to allow measurement of vector Surface Velocity with only a single aircraft pass. Estimates of Surface Velocity vectors in the coastal region during high tidal flow are presented. The data were gathered over the barrier islands west of Fort Myers, Florida, as part of a March 2004 deployment. Whereas no detailed bathymetry data were available, high-quality aerial photography appears to be a useful tool in inferring bottom topography and possible current obstructions. The retrieved Velocity field clearly follows the expected outflow pattern. While comparisons with tidal current magnitudes predicted by the U.S. National Ocean Service do reveal discrepancies of up to 0.5 m/s, these differences are most likely due to the contribution of ocean Surface waves to the overall InSAR Velocity measurement. Velocity retrievals for the same area based on the data from different tracks show good consistency. The results constitute the first demonstration of vector retrieval of the Surface Velocity field with a single-pass InSAR system and confirm the robustness of the dual-beam interferometry principle.
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sea Surface Velocity vector retrieval using dual beam interferometry first demonstration
International Geoscience and Remote Sensing Symposium, 2005Co-Authors: Jakov V Toporkov, Dragana Perkovic, Gordon Farquharson, Mark A Sletten, Stephen J FrasierAbstract:The dual-beam interferometer consists of two interferometric synthetic aperture radars (InSARs), one squinted at 20° forward of broadside, and the other 20° aft, to allow measurement of vector Surface Velocity with only a single aircraft pass. Estimates of Surface Velocity vectors in the coastal region during high tidal flow are presented. The data were gathered over the barrier islands west of Fort Myers, FL, as part of a March 2004 deployment. Whereas no detailed bathymetry data were available, high-quality aerial photography appears to be a useful tool in inferring bottom topography and possible current obstructions. The retrieved Velocity field clearly follows the expected outflow pattern. While comparisons with tidal current magnitudes predicted by the U.S. National Ocean Service do reveal discrepancies of up to 0.5 m/s, these differences are most likely due to the contribution of ocean Surface waves to the overall InSAR Velocity measurement. Velocity retrievals for the same area based on the data from different tracks show good consistency. The results constitute the first demonstration of vector retrieval of the Surface Velocity field with a single-pass InSAR system and confirm the robustness of the dual-beam interferometry principle.
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dual beam interferometry for ocean Surface current vector mapping
IEEE Transactions on Geoscience and Remote Sensing, 2001Co-Authors: Stephen J Frasier, Adriano CampsAbstract:The recent use of along-track interferometry (ATI) in synthetic aperture radar (SAR) has shown promise for synoptic measurement of ocean Surface currents. ATI-SARs have been used to estimate wave fields, currents, and current features. This paper describes and analyzes a dual-beam along-track interferometer to provide spatially resolved vector Surface Velocity estimates with a single pass of an aircraft. The design employs a pair of interferometer beams, one squinted forward and one squinted aft. Each interferometric phase is sensitive to the component of Surface Doppler Velocity in the direction of the beam. Therefore, a proper combination of these measurements provides a vector Surface Velocity estimate in one pass of the aircraft. The authors find that precise measurements dictate widely spaced beams and that the spatial resolution for the squinted SAR is essentially identical to the sidelooking case. Practical instrument design issues are discussed, and an airborne system currently in development is described. Through computer simulation, they observe the azimuthal displacement of interferometric phases by moving Surfaces identical to those of conventional SAR and find that such displacement can bias the estimated Surface Velocity.
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dual beam interferometry for ocean Surface current vector mapping
IEEE Transactions on Geoscience and Remote Sensing, 2001Co-Authors: Stephen J Frasier, Adriano CampsAbstract:The recent use of along-track interferometry (ATI) in synthetic aperture radar (SAR) has shown promise for synoptic measurement of ocean Surface currents. ATI-SARs have been used to estimate wave fields, currents, and current features. This paper describes and analyzes a dual-beam along-track interferometer to provide spatially resolved vector Surface Velocity estimates with a single pass of an aircraft. The design employs a pair of interferometer beams, one squinted forward and one squinted aft. Each interferometric phase is sensitive to the component of Surface Doppler Velocity in the direction of the beam. Therefore, a proper combination of these measurements provides a vector Surface Velocity estimate in one pass of the aircraft. The authors find that precise measurements dictate widely spaced beams and that the spatial resolution for the squinted SAR is essentially identical to the sidelooking case. Practical instrument design issues are discussed, and an airborne system currently in development is described. Through computer simulation, they observe the azimuthal displacement of interferometric phases by moving Surfaces identical to those of conventional SAR and find that such displacement can bias the estimated Surface Velocity.
Flavia Tauro - One of the best experts on this subject based on the ideXlab platform.
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towards harmonisation of image velocimetry techniques for river Surface Velocity observations
Earth System Science Data, 2020Co-Authors: Matthew T Perks, Alexandre Hauet, Silvano Fortunato Dal Sasso, Elizabeth Jamieson, Jerome Le Coz, Sophie Pearce, Salvador Penaharo, Alonso Pizarro, Dariia Strelnikova, Flavia TauroAbstract:Abstract. Since the turn of the 21st Century, image based velocimetry techniques have become an increasingly popular approach for determining open-channel flow in a range of hydrological settings across Europe, and beyond. Simultaneously, a range of large-scale image velocimetry algorithms have been developed, equipped with differing image pre-processing, and analytical capabilities. Yet in operational hydrometry, these techniques are utilised by few competent authorities. Therefore, imagery collected for image velocimetry analysis, along with validation data is required both to enable inter-comparisons between these differing approaches and to test their overall efficacy. Through benchmarking exercises, it will be possible to assess which approaches are best suited for a range of fluvial settings, and to focus future software developments. Here we collate, and describe datasets acquired from six countries across Europe and Asia, consisting of videos that have been subjected to a range of pre-processing, and image velocimetry analysis (Perks et al., 2019, https://doi.org/10.4121/uuid:34764be1-31f9-4626-8b11-705b4f66b95a ). Validation data is available for 12 of the 13 case studies presented enabling these data to be used for validation and accuracy assessment.
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ice dices for monitoring stream Surface Velocity
Journal of Hydro-environment Research, 2017Co-Authors: Flavia Tauro, Salvatore GrimaldiAbstract:Abstract Non-intrusive observations are fundamental to monitor river flows and understand water processes in natural systems. Recently, the introduction of optical methods has fostered the establishment of numerous image-based techniques for characterizing the kinematics of water bodies. However, RGB image-based methods are still severely affected by illumination conditions and tracers’ visibility. In this note, the integration of particle-shaped tracers and thermal imagery is applied to characterize the Surface Velocity field of a natural stream. Specifically, the trajectories of a few grams of artificially deployed ice dices are reconstructed by analyzing thermal images with particle tracking velocimetry. Average Surface flow velocities are in agreement with benchmark values estimated with a current meter. This proof of concept experiment demonstrates the efficacy of thermal imagery for hydrological monitoring and paves the way to the integration of thermal signals in standard water gauging systems.
Adriano Camps - One of the best experts on this subject based on the ideXlab platform.
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dual beam interferometry for ocean Surface current vector mapping
IEEE Transactions on Geoscience and Remote Sensing, 2001Co-Authors: Stephen J Frasier, Adriano CampsAbstract:The recent use of along-track interferometry (ATI) in synthetic aperture radar (SAR) has shown promise for synoptic measurement of ocean Surface currents. ATI-SARs have been used to estimate wave fields, currents, and current features. This paper describes and analyzes a dual-beam along-track interferometer to provide spatially resolved vector Surface Velocity estimates with a single pass of an aircraft. The design employs a pair of interferometer beams, one squinted forward and one squinted aft. Each interferometric phase is sensitive to the component of Surface Doppler Velocity in the direction of the beam. Therefore, a proper combination of these measurements provides a vector Surface Velocity estimate in one pass of the aircraft. The authors find that precise measurements dictate widely spaced beams and that the spatial resolution for the squinted SAR is essentially identical to the sidelooking case. Practical instrument design issues are discussed, and an airborne system currently in development is described. Through computer simulation, they observe the azimuthal displacement of interferometric phases by moving Surfaces identical to those of conventional SAR and find that such displacement can bias the estimated Surface Velocity.
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dual beam interferometry for ocean Surface current vector mapping
IEEE Transactions on Geoscience and Remote Sensing, 2001Co-Authors: Stephen J Frasier, Adriano CampsAbstract:The recent use of along-track interferometry (ATI) in synthetic aperture radar (SAR) has shown promise for synoptic measurement of ocean Surface currents. ATI-SARs have been used to estimate wave fields, currents, and current features. This paper describes and analyzes a dual-beam along-track interferometer to provide spatially resolved vector Surface Velocity estimates with a single pass of an aircraft. The design employs a pair of interferometer beams, one squinted forward and one squinted aft. Each interferometric phase is sensitive to the component of Surface Doppler Velocity in the direction of the beam. Therefore, a proper combination of these measurements provides a vector Surface Velocity estimate in one pass of the aircraft. The authors find that precise measurements dictate widely spaced beams and that the spatial resolution for the squinted SAR is essentially identical to the sidelooking case. Practical instrument design issues are discussed, and an airborne system currently in development is described. Through computer simulation, they observe the azimuthal displacement of interferometric phases by moving Surfaces identical to those of conventional SAR and find that such displacement can bias the estimated Surface Velocity.
Dongsu Kim - One of the best experts on this subject based on the ideXlab platform.
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error analysis for electromagnetic Surface Velocity and discharge measurement in rapid mountain stream flow
Journal of Environmental Sciences-china, 2014Co-Authors: Dongsu Kim, Sungkee Yang, Wooyul JungAbstract:Abstract Fixed Electromagnetic Wave Surface Velocimetry (Fixed EWSV) has been started to be used to measure flood discharge in the mountain stream, since it has various advantages such that it works well to continuously measure stream discharge even in the night time as well as very strong weather. On the contrary, the Fixed EWSV only measures single point Surface Velocity, thus it does not consider varying feature of the transverse Velocity profile in the given stream cross-section. In addition, a conventional value of 0.85 was generally used as the ratio for converting the measured Surface Velocity into the depth-averaged Velocity. These aspects could bring in error for accurately measuring the stream discharge. The capacity of the EWSV for capturing rapid flow Velocity was also not properly validated. This study aims at conducting error analysis of using the EWSV by: 1) measuring transverse Velocity at multiple points along the cross-section to assess an error driven by the single point measurement; 2) figuring out ratio between Surface Velocity and the depth-averaged Velocity based on the concurrent ADCP measurements; 3) validating the capacity of the EWSV for capturing rapid flow Velocity. As results, the Velocity measured near the center by the fixed EWSV overestimated about 15% of the cross-sectional mean Velocity. The converting ratio from the Surface Velocity to the depth-averaged Velocity was 0.8 rather than 0.85 of a conventional ratio. Finally, the EWSV revealed unstable Velocity output when the flow Velocity was higher than 2 m/s.
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flood runoff measurements using Surface image velocimetry
Journal of Environmental Sciences-china, 2013Co-Authors: Yongseok Kim, Sungkee Yang, Dongsu KimAbstract:Abstract Surface Image Velocimetry(SIV) is an instrument to measure water Surface Velocity by using image processing techniques. Since SIV is a non-contact type measurement method, it is very effective and useful to measure water Surface Velocity for steep mountainous streams, such as streams in Jeju island. In the present study, a Surface imaging velocimetry system was used to calculate the flow rate for flood event due to a typhoon. At the same time, two types of electromagnetic Surface velocimetries (electromagnetic Surface current meter and Kalesto) were used to observe flow velocities and compare the accuracies of each instrument. The comparison showed that for Velocity distributions root mean square error(RMSE) was 0.33 and R-squared was 0.72. For discharge measurements, root mean square error(RMSE) reached 6.04 and R-squared did 0.92. It means that Surface image velocimetry could be used as an alternative method for electromagnetic Surface velocimetries in measuring flood discharge.Key words: SIV, Kalesto, electromagnetic Surface current meter, Doppler radar, Surface Velocity
James C Mcwilliams - One of the best experts on this subject based on the ideXlab platform.
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prospects for future satellite estimation of small scale variability of ocean Surface Velocity and vorticity
Progress in Oceanography, 2019Co-Authors: Dudley B Chelton, James C Mcwilliams, Michael G Schlax, R M Samelson, Thomas J Farrar, Jeroen M Molemaker, Jonathan GulaAbstract:Abstract Recent technological developments have resulted in two techniques for estimating Surface Velocity with higher resolution than can be achieved from presently available nadir altimeter data: (1) Geostrophically computed estimates from high-resolution sea Surface height (SSH) measured interferometrically by the wide-swath altimeter on the Surface Water and Ocean Topography (SWOT) Mission with a planned launch in 2021; and (2) Measurements of ocean Surface Velocity from a Doppler scatterometer mission that is in the early planning stages, referred to here as a Winds and Currents Mission (WaCM). In this study, we conduct an analysis of the effects of uncorrelated measurement errors and sampling errors on the errors of the measured and derived variables of interest (SSH and geostrophically computed Velocity and vorticity for SWOT, and Surface Velocity and vorticity for WaCM). Our analysis includes derivations of analytical expressions for the variances and wavenumber spectra of the errors of the derived variables, which will be useful to other studies based on simulated SWOT and WaCM estimates of Velocity and vorticity. We also discuss limitations of the geostrophic approximation that must be used for SWOT estimates of Velocity. The errors of SWOT and WaCM estimates of Velocity and vorticity at the full resolutions of the measured variables are too large for the unsmoothed estimates to be scientifically useful. It will be necessary to smooth the data to reduce the noise variance. We assess the resolution capabilities of smoothed estimates of Velocity and vorticity from simulated noisy SWOT and WaCM data based on a high-resolution model of the California Current System (CCS). By our suggested minimum threshold signal-to-noise (S/N) variance ratio of 10 (a standard deviation ratio of 3.16), we conclude that the wavelength resolution capabilities of maps of Velocity and vorticity constructed from WaCM data with a swath width of 1200 km are, respectively, about 60 km and 90 km in 4-day averages. For context, the radii of resolvable features are about four times smaller than these mesoscale wavelength resolutions. If the swath width can be increased to 1800 km, the wavelength resolution capabilities of 4-day average maps of Surface Velocity and vorticity would improve to about 45 km and 70 km, respectively. Reducing the standard deviation of the uncorrelated measurement errors from the baseline value of σ spd = 0 . 50 m s−1 to a value of 0.25 m s−1 would further improve these resolution capabilities to about 20 km and 45 km. SWOT data will allow mapping of the SSH field with far greater accuracy and space–time resolution than are presently achieved by merging the data from multiple nadir altimeter missions. However, because of its much narrower 120-km measurement swath compared with WaCM and the nature of the space–time evolution of the sampling pattern during each 21-day repeat of the SWOT orbit, maps of geostrophically computed Velocity and vorticity averaged over the 14-day period that is required for SWOT to observe the full CCS model domain are contaminated by sampling errors that are too large for the estimates to be useful for any amount of smoothing considered here. Reducing the SSH measurement errors would do little to improve SWOT maps of Velocity and vorticity. SWOT estimates of these variables are likely to be useful only within individual measurement swaths or with the help of dynamic interpolation from a data assimilation model. By our criterion, in-swath SWOT estimates of Velocity and vorticity have wavelength resolution capabilities of about 30 km and 55 km, respectively. In comparison, in-swath estimates of Velocity and vorticity from WaCM data with σ spd = 0 . 50 m s−1 have a wavelength resolution capability of about 130 km for both variables. Reducing the WaCM measurement errors to σ spd = 0 . 25 m s−1 would improve the resolution capabilities to about 50 km and 75 km for Velocity and vorticity, respectively. These resolutions are somewhat coarser than the in-swath estimates from SWOT data, but the swath width is more than an order of magnitude wider for WaCM. Instantaneous maps of Velocity and vorticity constructed in-swath from WaCM data will therefore be much less prone to edge effect problems in the spatially smoothed fields. Depending on the precise value of the threshold adopted for the minimum S/N ratio and on the details of the filter used to smooth the SWOT and WaCM data, the resolution capabilities summarized above may be somewhat pessimistic. On the other hand, aspects of measurement errors and sampling errors that have not been accounted for in this study will worsen the resolution capabilities presented here. Another caveat to keep in mind is that the resolution capabilities deduced here from simulations of the CCS region during summertime may differ somewhat at other times of year and in other geographical regions where the signal variances and wavenumber spectra of the variables of interest differ from the CCS model used in this study. Our analysis nonetheless provides useful guidelines for the resolutions that can be expected from SWOT and WaCM.
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dynamically balanced absolute sea level of the global ocean derived from near Surface Velocity observations
Geophysical Research Letters, 2003Co-Authors: Pearn P Niiler, Nikolai Maximenko, James C McwilliamsAbstract:[1] The 1992–2002 time-mean absolute sea level distribution of the global ocean is computed for the first time from observations of near-Surface Velocity. For this computation, we use the near-Surface horizontal momentum balance. The Velocity observed by drifters is used to compute the Coriolis force and the force due to acceleration of water parcels. The anomaly of horizontal pressure gradient is derived from satellite altimetry and corrects the temporal bias in drifter data distribution. NCEP reanalysis winds are used to compute the force due to Ekman currents. The mean sea level gradient force, which closes the momentum balance, is integrated for mean sea level. We find that our computation agrees, within uncertainties, with the sea level computed from the geostrophic, hydrostatic momentum balance using historical mean density, except in the Antarctic Circumpolar Current. A consistent horizontally and vertically dynamically balanced, near-Surface, global pressure field has now been derived from observations.
