Articles de revues : « Spectral Doppler measurements »

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Moisseev, Dmitri N., et V. Chandrasekar. « Nonparametric Estimation of Raindrop Size Distributions from Dual-Polarization Radar Spectral Observations ». Journal of Atmospheric and Oceanic Technology 24, n^o 6 (juin 2007) : 1008–18. http://dx.doi.org/10.1175/jtech2024.1.

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This paper presents a method to retrieve raindrop size distributions (DSD) from slant profile dual-polarization Doppler spectra observations. It is shown that using radar measurements taken at a high elevation angle raindrop size distributions can be retrieved without making an assumption on the form of a DSD. In this paper it is shown that drop size distributions can be retrieved from Doppler power spectra by compensating for the effect of spectrum broadening and mean velocity shift. To accomplish that, spectrum deconvolution is used where the spectral broadening kernel width and wind velocity are estimated from spectral differential reflectivity measurements. Since convolution kernel is estimated from dual-polarization Doppler spectra observations and does not require observation of a clear-air signal, this method can be used by most radars capable of dual-polarization spectra measurements. To validate the technique, sensitivity of this method to the underlying assumptions and calibration errors is evaluated on realistic simulations of radar observations. Furthermore, performance of the method is illustrated on Colorado State University–University of Chicago–Illinois State Water Survey radar (CSU–CHILL) measurements of stratiform precipitation.

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Chaxel, Y., E. M. Griffin et N. P. Meredith. « Spectral analysis techniques for doppler lidar measurements ». Advances in Space Research 21, n^o 10 (janvier 1998) : 1441–44. http://dx.doi.org/10.1016/s0273-1177(97)00657-1.

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Панчук, В. Е., В. Л. Афанасьев, А. Г. Пельменев, М. В. Юшкин, Г. С. Жуклевич et С. В. Ларионов. « Doppler measurements of stars : evolution of accuracy and some perspectives ». Научные труды Института астрономии РАН, n^o 4 (31 décembre 2021) : 129–35. http://dx.doi.org/10.51194/inasan.2021.6.4.006.

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Перечислены основные этапы повышения точности доплеровских измерений спектров звезд и связанные с этимтехнологические прорывы. Упоминаются методы интерференционной спектроскопии, ориентированные как на сни-жение стоимости массовых доплеровских измерений, так и на увеличение спектрального разрешения до значений,превосходящих спектральное разрешение согласованного спектрографа высокого разрешения. The main stages of increasing the accuracy of Doppler measurements of stellar spectra and related technological break-throughs are listed. Methods of interference spectroscopy are mentioned, which are aimed at both reducing the cost of massDoppler measurements and increasing the spectral resolution to values exceeding the spectral resolution of a conventionalhigh-resolution spectrograph.

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Gómez Vega, Carlos A., Carlos A. Gutiérrez, Jose J. Jaime Rodriguez, Javier Vazquez Castillo, Daniel U. Campos Delgado, Jose M. Luna Rivera et Miguel A. Díaz Ibarra. « Doppler spectrum measurements of vehicular radio channels using a narrowband sounder ». Revista Facultad de Ingeniería Universidad de Antioquia, n^o 93 (23 août 2019) : 32–40. http://dx.doi.org/10.17533/10.17533/udea.redin.20190405.

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This paper describes the implementation of a narrowband sounder for Doppler power spectrum (DPS) measurements of vehicular communication channels. The narrowband channel sounder is implemented using general purpose equipment, making such measurement platform easy to replicate for didactic and research purposes. To demonstrate the practical value of this framework, a measurement campaign was conducted to obtain empirical information about the spectral characteristics of vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) multipath radio channels in the 700 MHz band. The collected data wasprocessed to compute the average Doppler shift and the Doppler spread of the measured channels. The obtained results show that the spectral properties of frequency-dispersive vehicular radio channels can be effectively analyzed using narrowband sounding principles.

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Cifuentes-Lorenzen, Alejandro, James B. Edson, Christopher J. Zappa et Ludovic Bariteau. « A Multisensor Comparison of Ocean Wave Frequency Spectra from a Research Vessel during the Southern Ocean Gas Exchange Experiment ». Journal of Atmospheric and Oceanic Technology 30, n^o 12 (1 décembre 2013) : 2907–25. http://dx.doi.org/10.1175/jtech-d-12-00181.1.

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Abstract Obtaining accurate measurements of wave statistics from research vessels remains a challenge due to the platform motion. One principal correction is the removal of ship heave and Doppler effects from point measurements. Here, open-ocean wave measurements were collected using a laser altimeter, a Doppler radar microwave sensor, a radar-based system, and inertial measurement units. Multiple instruments were deployed to capture the low- and high-frequency sea surface displacements. Doppler and motion correction algorithms were applied to obtain a full 1D (0.035–1.3 ± 0.2 Hz) wave spectrum. The radar-based system combined with the laser altimeter provided the optimal low- and high-frequency combination, producing a frequency spectrum in the range from 0.035 to 1.2 Hz for cruising speeds ≤3 m s−1 with a spectral rolloff of f−4 Hz and noise floor of −20/−30 dB. While on station, the significant wave height estimates were comparable within 10%–15% among instrumentation. Discrepancies in the total energy and in the spectral shape between instruments arise when the ship is in motion. These differences can be quantified using the spectral behavior of the measurements, accounting for aliasing and Doppler corrections. The inertial sensors provided information on the amplitude of the ship’s modulation transfer function, which was estimated to be ~1.3 ± 0.2 while on station and increased while underway [2.1 at ship-over-ground (SOG) speed; 4.3 m s−1]. The correction scheme presented here is adequate for measurements collected at cruising speeds of 3 m s−1 or less. At speeds greater than 5 m s−1, the motion and Doppler corrections are not sufficient to correct the observed spectral degradation.

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Mattesini, Paolo, Alessandro Ramalli, Lorena Petrusca, Olivier Basset, Herve Liebgott et Piero Tortoli. « Spectral Doppler Measurements With 2-D Sparse Arrays ». IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 67, n^o 2 (février 2020) : 278–85. http://dx.doi.org/10.1109/tuffc.2019.2944090.

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Wang, Yadong, Tian-You Yu, Alexander V. Ryzhkov et Matthew R. Kumjian. « Application of Spectral Polarimetry to a Hailstorm at Low Elevation Angle ». Journal of Atmospheric and Oceanic Technology 36, n^o 4 (avril 2019) : 567–83. http://dx.doi.org/10.1175/jtech-d-18-0115.1.

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AbstractSpectral polarimetry has the potential to be used to study microphysical properties in relation to the dynamics within a radar resolution volume by combining Doppler and polarimetric measurements. The past studies of spectral polarimetry have focused on using radar measurements from higher elevation angles, where both the size sorting from the hydrometeors’ terminal velocities and polarimetric characteristics are maintained. In this work, spectral polarimetry is applied to data from the 0° elevation angle, where polarimetric properties are maximized. Radar data collected by the C-band University of Oklahoma Polarimetric Radar for Innovations in Meteorology and Engineering (OU-PRIME) during a hailstorm event on 24 April 2011 are used in the analysis. The slope of the spectral differential reflectivity exhibits interesting variations across the hail core, which suggests the presence of size sorting of hydrometeors caused by vertical shear in a turbulent environment. A nearby S-band polarimetric Weather Surveillance Radar-1988 Doppler (KOUN) is also used to provide insights into this hailstorm. Moreover, a flexible numerical simulation is developed for this study, in which different types of hydrometeors such as rain and melting hail can be considered individually or as a combination under different sheared and turbulent conditions. The impacts of particle size distribution, shear, turbulence, attenuation, and mixture of rain and melting hail on polarimetric spectral signatures are investigated with the simulated Doppler spectra and spectral differential reflectivity.

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Anandan, V. K., C. J. Pan, T. Rajalakshmi et G. Ramachandra Reddy. « Multitaper spectral analysis of atmospheric radar signals ». Annales Geophysicae 22, n^o 11 (29 novembre 2004) : 3995–4003. http://dx.doi.org/10.5194/angeo-22-3995-2004.

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Abstract. Multitaper spectral analysis using sinusoidal taper has been carried out on the backscattered signals received from the troposphere and lower stratosphere by the Gadanki Mesosphere-Stratosphere-Troposphere (MST) radar under various conditions of the signal-to-noise ratio. Comparison of study is made with sinusoidal taper of the order of three and single tapers of Hanning and rectangular tapers, to understand the relative merits of processing under the scheme. Power spectra plots show that echoes are better identified in the case of multitaper estimation, especially in the region of a weak signal-to-noise ratio. Further analysis is carried out to obtain three lower order moments from three estimation techniques. The results show that multitaper analysis gives a better signal-to-noise ratio or higher detectability. The spectral analysis through multitaper and single tapers is subjected to study of consistency in measurements. Results show that the multitaper estimate is better consistent in Doppler measurements compared to single taper estimates. Doppler width measurements with different approaches were studied and the results show that the estimation was better in the multitaper technique in terms of temporal resolution and estimation accuracy.

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Bascom, Peter A. J., K. Wayne Johnston, Richard S. C. Cobbold et Matadial Ojha. « Defining the limitations of measurements from Doppler spectral recordings ». Journal of Vascular Surgery 24, n^o 1 (juillet 1996) : 34–45. http://dx.doi.org/10.1016/s0741-5214(96)70142-8.

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Lee, Bor-Ray, Huihua Kenny Chiang, Yi-Hong Chou, Cheng-Deng Kuo, Jia-Horng Wang et San-Kan Lee. « Implementation of spectral width Doppler in pulsatile flow measurements ». Ultrasound in Medicine & ; Biology 25, n^o 8 (octobre 1999) : 1221–27. http://dx.doi.org/10.1016/s0301-5629(99)00083-6.

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Lanchester, B. S., M. Galand, S. C. Robertson, M. H. Rees, D. Lummerzheim, I. Furniss, L. M. Peticolas, H. U. Frey, J. Baumgardner et M. Mendillo. « High resolution measurements and modeling of auroral hydrogen emission line profiles ». Annales Geophysicae 21, n^o 7 (31 juillet 2003) : 1629–43. http://dx.doi.org/10.5194/angeo-21-1629-2003.

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Abstract. Measurements in the visible wavelength range at high spectral resolution (1.3 Å) have been made at Longyearbyen, Svalbard (15.8 E,78.2 N) during an interval of intense proton precipitation. The shape and Doppler shift of hydrogen Balmer beta line profiles have been compared with model line profiles, using as input ion energy spectra from almost coincident passes of the FAST and DMSP spacecraft. The comparison shows that the simulation contains the important physical processes that produce the profiles, and confirms that measured changes in the shape and peak wave-length of the hydrogen profiles are the result of changing energy input. This combination of high resolution measurements with modeling provides a method of estimating the incoming energy and changes in flux of precipitating protons over Svalbard, for given energy and pitch-angle distributions. Whereas for electron precipitation, information on the incident particles is derived from brightness and brightness ratios which require at least two spectral windows, for proton precipitation the Doppler profile of resulting hydrogen emission is directly related to the energy and energy flux of the incident energetic protons and can be used to gather information about the source region. As well as the expected Doppler shift to shorter wavelengths, the measured profiles have a significant red-shifted component, the result of upward flowing emitting hydrogen atoms.Key words. Ionosphere (auroral ionosphere; particle precipitation) – Magnetospheric physics (auroral phenomena)

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Fang, Ming, Bruce Albrecht, Eunsil Jung, Pavlos Kollias, Haflidi Jonsson et Ivan PopStefanija. « Retrieval of Vertical Air Motion in Precipitating Clouds Using Mie Scattering and Comparison with In Situ Measurements ». Journal of Applied Meteorology and Climatology 56, n^o 3 (mars 2017) : 537–53. http://dx.doi.org/10.1175/jamc-d-16-0158.1.

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AbstractFor the first time, the Mie notch retrieval technique is applied to airborne cloud Doppler radar observations in warm precipitating clouds to retrieve the vertical air velocity profile above the aircraft. The retrieval algorithm prescribed here accounts for two major sources of bias: aircraft motion and horizontal wind. The retrieval methodology is evaluated using the aircraft in situ vertical air velocity measurements. The standard deviations of the residuals for the retrieved and in situ measured data for an 18-s time segment are 0.21 and 0.24 m s−1, respectively; the mean difference between the two is 0.01 m s−1. For the studied cases, the total theoretical uncertainty is less than 0.19 m s−1 and the actual retrieval uncertainty is about 0.1 m s−1. These results demonstrate that the Mie notch technique combined with the bias removal procedure described in this paper can successfully retrieve vertical air velocity from airborne radar observations with low spectral broadening due to Doppler fading, which enables new opportunities in cloud and precipitation research. A separate spectral peak due to returns from the cloud droplets is also observed in the same radar Doppler spectra and is also used to retrieve vertical air motion. The vertical air velocities retrieved using the two different methods agree well with each other, and the correlation coefficient is as high as 0.996, which indicates that the spectral peak due to cloud droplets might provide another way to retrieve vertical air velocity in clouds when the Mie notch is not detected but the cloud droplets’ spectral peak is discernable.

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Liu, Liping, et Jiafeng Zheng. « Algorithms for Doppler Spectral Density Data Quality Control and Merging for the Ka-Band Solid-State Transmitter Cloud Radar ». Remote Sensing 11, n^o 2 (21 janvier 2019) : 209. http://dx.doi.org/10.3390/rs11020209.

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The Chinese Ka-band solid-state transmitter cloud radar (CR) can operate in three different work modes with different pulse widths and coherent integration and non-coherent integration numbers to meet the requirement for long-term cloud measurements. The CR was used to observe cloud and precipitation data in southern China in 2016. In order to resolve the data quality problems caused by coherent integration and pulse compression, which are used to detect weak cloud in the cloud radar, this study focuses on analyzing the consistencies of reflectivity spectra using the three modes and the influence of coherent integration and pulse compression, developing an algorithm for Doppler spectral density data quality control (QC) and merging based on multiple-mode observation data. After dealiasing Doppler velocity and artefact removal, the three types of Doppler spectral density data were merged. Then, Doppler moments such as reflectivity, radial velocity, and spectral width were recalculated from the merged reflectivity spectra. Performance of the merging algorithm was evaluated. Three conclusions were drawn. Firstly, four rounds of coherent integration with a pulse repetition frequency (PRF) of 8333 Hz underestimated the reflectivity spectra for Doppler velocities exceeding 2 m·s−1, causing a large negative bias in the reflectivity and radial velocity when large drops were present. In contrast, two rounds of coherent integration affected the reflectivity spectra to a lesser extent. The reflectivity spectra were underestimated for low signal-to-noise ratios in the low-sensitivity mode. Secondly, pulse compression improved the radar sensitivity and air vertical speed observation, whereas the precipitation mode and coherent integration led to an underestimation of the number concentration of big raindrops and an overestimation of the number concentration of small drops. Thirdly, a comparison of the individual spectra with the merged reflectivity spectra showed that the Doppler moments filled in the gaps in the individual spectra during weak cloud periods, reduced the effects of coherent integration and pulse compression in liquid precipitation, mitigated the aliasing of Doppler velocity, and removed the artefacts, yielding a comprehensive and accurate depiction of most of the clouds and precipitation in the vertical column above the radar. The recalculated moments of the Doppler spectra had better quality than those merged from raw data.

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Dérian, Pierre, Christopher F. Mauzey et Shane D. Mayor. « Wavelet-Based Optical Flow for Two-Component Wind Field Estimation from Single Aerosol Lidar Data ». Journal of Atmospheric and Oceanic Technology 32, n^o 10 (octobre 2015) : 1759–78. http://dx.doi.org/10.1175/jtech-d-15-0010.1.

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AbstractA motion estimation algorithm was applied to image sequences produced by a horizontally scanning elastic backscatter lidar. The algorithm, a wavelet-based optical flow estimator named Typhoon, produces dense two-component vector flow fields that correspond to the apparent motion of microscale aerosol features. To validate the efficacy of this approach for the remote measurement of wind fields in the lower atmosphere, an experiment was conducted in Chico, California, in 2013 and 2014. The flow fields, estimated every 17 s, were compared with measurements from an independent Doppler lidar. Time series of wind speed and direction, statistical assessment of the 10-min averages, and examples of wind fields are presented. The comparison of 10-min averages at 100 m AGL reveals excellent correlations between estimates from the Typhoon algorithm and measurements from the Doppler lidar. Power spectra and spectral transfer functions are computed to estimate the filtering effects of the algorithm in the spatial domain.

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Kobayashi, Takahisa, et Ahoro Adachi. « Retrieval of Arbitrarily Shaped Raindrop Size Distributions from Wind Profiler Measurements ». Journal of Atmospheric and Oceanic Technology 22, n^o 4 (1 avril 2005) : 433–42. http://dx.doi.org/10.1175/jtech1705.1.

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Abstract An efficient iterative retrieval method for arbitrarily shaped raindrop size distributions (ITRAN) is developed for Doppler spectra measured with a wind profiler. A measured Doppler spectrum is a convolution of the precipitation spectrum and the turbulent spectrum. Deconvolution of the Doppler spectra is achieved through repeated convolutions. The developed method assumes no prior shape of drop size distributions and automatically obtains raindrop size distributions; additionally, it can be applied to large data volumes. Furthermore, it is insensitive to initial values. The method was applied to both simulated and observed spectra. Derived drop size distributions agree with simulated values. Narrower turbulent spectral widths yield better results. Integral values of median volume diameter (D0), liquid water content (LWC), and radar reflectivity factor are estimated with errors of less than 10%. Accurate vertical profiles of raindrop size distributions result when this method is applied to wind profiler data. The technique performed very well with most observed spectra. Some recovered spectra departed from the corresponding measured spectra, for cases in which a clear-air peak could not be accurately reproduced because of uncertainties in the location of the minimum position between the clear-air echo and the precipitation echo. Statistical relationships between LWC and integral rainfall parameters yield interesting features. The median volume diameter is statistically independent of the LWC and is associated with the large variability of the total number of drops, NT, between events. Vertical profiles from one event show a clear inverse relationship between NT and D0

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Ueda, Hiromasa, Tetsuo Fukui, Mizuo Kajino, Mitsuaki Horiguchi, Hiroyuki Hashiguchi et Shoichiro Fukao. « Eddy Diffusivities for Momentum and Heat in the Upper Troposphere and Lower Stratosphere Measured by MU Radar and RASS, and a Comparison of Turbulence Model Predictions ». Journal of the Atmospheric Sciences 69, n^o 1 (1 janvier 2012) : 323–37. http://dx.doi.org/10.1175/jas-d-11-023.1.

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Abstract Recently, middle- and upper-atmosphere Doppler radar (MU radar) has enabled the measurement of middle-atmosphere turbulence from radar backscatter Doppler spectra. In this work, eddy diffusivities for momentum Km in the upper troposphere and lower stratosphere during clear-air conditions were derived from direct measurements of the Reynolds stress and vertical gradient of mean wind velocity measured by MU radar. Eddy diffusivity for heat Kh below 8 km was determined from measurements of temperature fluctuations by the Radio Acoustic Sounding System (RASS) attached to the MU radar. The eddy diffusivity for momentum was on the order of 10 m2 s−1 in the upper troposphere and decreased gradually in the stratosphere by an order of magnitude or more. The eddy diffusivity for heat was almost of the same order of magnitude as Km. Estimates of eddy diffusivity from the radar echo power spectral width give fairly good values compared with the direct measurement of Km. Applicability of three turbulence models—the spectral width method, the k–ɛ model modified for stratified flows, and the algebraic stress model—were also examined, using radar observation values of turbulent kinetic energy k and turbulent energy dissipation rate ɛ together with atmospheric stability observations from rawinsonde data. It is concluded that the algebraic stress model shows the best fit with the direct measurement of Km, even in the free atmosphere above the atmospheric boundary layer once k and ɛ values are obtained from observations or a model.

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Yeo, Sunmi, Changhan Yoon, Ching-Ling Lien, Tai-Kyong Song et K. Kirk Shung. « Monitoring of Adult Zebrafish Heart Regeneration Using High-Frequency Ultrasound Spectral Doppler and Nakagami Imaging ». Sensors 19, n^o 19 (22 septembre 2019) : 4094. http://dx.doi.org/10.3390/s19194094.

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This paper reports the feasibility of Nakagami imaging in monitoring the regeneration process of zebrafish hearts in a noninvasive manner. In addition, spectral Doppler waveforms that are typically used to access the diastolic function were measured to validate the performance of Nakagami imaging. A 30-MHz high-frequency ultrasound array transducer was used to acquire backscattered echo signal for spectral Doppler and Nakagami imaging. The performances of both methods were validated with flow and tissue-mimicking phantom experiments. For in vivo experiments, both spectral Doppler and Nakagami imaging were simultaneously obtained from adult zebrafish with amputated hearts. Longitudinal measurements were performed for five zebrafish. From the experiments, the E/A ratio measured using spectral Doppler imaging increased at 3 days post-amputation (3 dpa) and then decreased to the value before amputation, which were consistent with previous studies. Similar results were obtained from the Nakagami imaging where the Nakagami parameter value increased at 3 dpa and decreased to its original value. These results suggested that the Nakagami and spectral Doppler imaging would be useful techniques in monitoring the regeneration of heart or tissues.

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Prunier, Fabrice, Roger Gaertner, Liliane Louedec, Jean-Baptiste Michel, Jean-Jacques Mercadier et Brigitte Escoubet. « Doppler echocardiographic estimation of left ventricular end-diastolic pressure after MI in rats ». American Journal of Physiology-Heart and Circulatory Physiology 283, n^o 1 (1 juillet 2002) : H346—H352. http://dx.doi.org/10.1152/ajpheart.01050.2001.

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The spectral Doppler mitral flow pattern, alone or combined with tissue Doppler mitral annulus velocity, can be used to predict left ventricular (LV) filling pressure in humans, whereas invasive hemodynamic measurements are still required in the rat. This study was undertaken to assess whether LV end-diastolic pressure (LVEDP) can be estimated using Doppler echocardiography in the rat after myocardial infarction (MI). Thirty-seven rats (23 rats with MI after left coronary artery ligation and 14 sham-operated rats) were evaluated 3 mo after surgery with echo-Doppler and invasive hemodynamic measurements. Pulse wave spectral Doppler at the mitral valve tip was used to measure the E wave, the E wave deceleration time (DT), and the A wave; spectral Doppler tissue imaging was used to measure the early diastolic lateral mitral annulus velocity ( E a). We found weak correlations between LVEDP and the peak velocity of the early mitral inflow ( E), E/peak velocity of the late mitral inflow, and DT, and strong correlations with E a and especially with E/ E a [ R 2= 0.89, LVEDP (in mmHg) = 0.987 E/ E a − 4.229]. Longitudinal followup of a subgroup of rats with MI revealed a marked rise of E/ E a between days 7 and 21 in rats with heart failure only. We conclude that Doppler echocardiography can be used for serial assessment of LV diastolic function in rats with MI.

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Prikner, K. « Simultaneous conjugate ground-satellite ULF pulsation measurements at the middle latitudes ». Annales Geophysicae 15, n^o 6 (30 juin 1997) : 645–55. http://dx.doi.org/10.1007/s00585-997-0645-6.

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Abstract. The spectra of three series of simultaneous ground (Budkov Observatory) and satellite (Freja) ULF measurements have been processed. The following features were studied in the FFT spectra, power-spectra and cross-spectra of the six-minute samples, recorded during quasi-conjugate approaches of Freja to the local Budkov magnetic field line: (a) the Doppler frequency shift (about 1mHz and less) in the predominant and some particular spectral components on Freja; (b) the amplitude relations between the ground and satellite (ratios 0.3 to 0.8); (c) phase shifts between particular signal components on the spacecraft above the ionosphere (in the azimuthal y-direction on Freja) and on the ground (in the meridional x-direction at Budkov) have been estimated (phase lags on the ground ≤ 0.9rad) and (d) also dynamical characteristics of the fundamental wave modes in the (x,y)-plane, perpendicular to the magnetic field line, have been estimated for the predominant spectral components, considering standing oscillation along the field line. Azimuthal wave numbers were about 2.0, propagating northwest at ~400km/s.

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Mashnich, G. « Spectral Observations of Filament Activation ». Proceedings of the International Astronomical Union 8, S300 (juin 2013) : 447–48. http://dx.doi.org/10.1017/s1743921313011496.

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AbstractStudies of solar filament (prominence) activation and eruption are often based on measurements of intensity fluctuations in various solar emission bands and rarely on Doppler velocity measurements. The goal of this paper is to analyze the process of quiescent filament activation, using spectral data, and its associated events in solar UV band. Motions have been examined in a small southern fragment of a quiescent, extended filament in the northern hemisphere prior to and during its activation on June 14 2012. A part of the fragment disappeared after the filament activation.

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Engler, N., R. Latteck, B. Strelnikov, W. Singer et M. Rapp. « Turbulent energy dissipation rates observed by Doppler MST Radar and by rocket-borne instruments during the MIDAS/MaCWAVE campaign 2002 ». Annales Geophysicae 23, n^o 4 (3 juin 2005) : 1147–56. http://dx.doi.org/10.5194/angeo-23-1147-2005.

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Abstract. During the MIDAS/MaCWAVE campaign in summer 2002 we have observed turbulence using Doppler beam steering measurements obtained from the ALWIN VHF radar at Andøya/Northern Norway. This radar was operated in the Doppler beam steering mode for turbulence investigations during the campaign, as well as in spaced antenna mode, for continuously measuring the background wind field. The real-time data analysis of the Doppler radar backscattering provided the launch conditions for the sounding rockets. The spectral width data observed during the occurrence of PMSE were corrected for beam and shear broadening caused by the background wind field to obtain the turbulent part of the spectral width. The turbulent energy dissipation rates determined from the turbulent spectral width vary between 5 and 100mW kg-1 in the altitude range of 80-92km and increase with altitude. These estimations agree well with the in-situ measurements using the CONE sensor which was launched on 3 sounding rockets during the campaign.

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Thurnherr, A. M. « The Finescale Response of Lowered ADCP Velocity Measurements Processed with Different Methods ». Journal of Atmospheric and Oceanic Technology 29, n^o 4 (1 avril 2012) : 597–600. http://dx.doi.org/10.1175/jtech-d-11-00158.1.

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Abstract In a paper published in 2002 in this journal, K. Polzin et al. derive corrections for spectra of vertical shear calculated from lowered acoustic Doppler current profiler (LADCP) velocity data. To illustrate and validate the corrections, they use velocities derived with a specific implementation of the shear method for LADCP processing that is no longer supported or widely used. In several recent publications, spectral corrections specific to this old processing method have been applied without modification to LADCP data processed with the more modern and much more widely used velocity-inversion method, which is associated with significantly less damping at high vertical wavenumbers than the older method. The purpose of this work is to derive and validate spectral corrections appropriate for different LADCP processing methods.

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Hofmann, J., F. L. Deubner, B. Fleck et W. Schmidt. « Polarimetric Measurements of the Fine Structure of a Sunspot Penumbra ». International Astronomical Union Colloquium 141 (1993) : 44–47. http://dx.doi.org/10.1017/s0252921100028748.

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AbstractObservations of a fairly regular sunspot penumbra near disk center have been carried out using a Stokes V polarimeter at the German Vacuum-Tower-Telescope in Izaña, Tenerife. We obtained two-dimensional spectral data by scanning the limb oriented sector of the penumbra in steps of 1” with the slit parallel to the solar limb. From Stokes V and Stokes I spectra of four magnetic sensitive lines (Fe I 5247.1, Cr I 5247.6, Fe I 5250.2, and Fe I 5250.7) obtained with 1024 × 1024 pixel CCDs we have determined Doppler shifts and several magnetic parameters. The results of a correlation analysis point out a clear relation between continuum brightness and magnetic field inclination in the sense of steeper field lines in the bright penumbral filaments. There is also a doubtless connection between intensity and Doppler shift. We did find indications for a correlation between field strength and brightness which is not uniform across the penumbra, though.

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Ferrone, Alfonso, Anne-Claire Billault-Roux et Alexis Berne. « ERUO : a spectral processing routine for the Micro Rain Radar PRO (MRR-PRO) ». Atmospheric Measurement Techniques 15, n^o 11 (14 juin 2022) : 3569–92. http://dx.doi.org/10.5194/amt-15-3569-2022.

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Abstract. The Micro Rain Radar PRO (MRR-PRO) is a K-band Doppler weather radar, using frequency-modulated continuous-wave (FMCW) signals, developed by Metek Meteorologische Messtechnik GmbH (Metek) as a successor to the MRR-2. Benefiting from four datasets collected during two field campaigns in Antarctica and Switzerland, we developed a processing library for snowfall measurements named ERUO (Enhancement and Reconstruction of the spectrUm for the MRR-PRO), with a twofold objective. Firstly, the proposed method addresses a series of issues plaguing the radar variables, including interference lines and power drops at the extremes of the Doppler spectrum. Secondly, the algorithm aims to improve the quality of the final variables by lowering the minimum detectable equivalent attenuated reflectivity factor and extending the valid Doppler velocity range through dealiasing. The performance of the algorithm has been tested against the measurements of a co-located W-band Doppler radar. Information from a close-by X-band Doppler dual-polarization radar has been used to exclude unsuitable radar volumes from the comparison. Particular attention has been dedicated to verifying the estimation of the meteorological signal in the spectra covered by interferences.

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Monselesan, D. P., R. J. Morris, P. L. Dyson et M. R. Hyde. « Southern high-latitude Digisonde observations of ionosphere E-region Bragg scatter during intense lacuna conditions ». Annales Geophysicae 22, n^o 8 (7 septembre 2004) : 2819–35. http://dx.doi.org/10.5194/angeo-22-2819-2004.

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Abstract. During summer months at solar cycle minimum, F-region lacuna and slant-Es conditions (SEC) are common features of daytime ionograms recorded around local magnetic noon at Casey, Antarctica. Digisonde measurements of drift velocity height profiles show that the occurrence of lacuna prevents the determination of F-region drift velocities and also affects E-region drift velocity measurements. Unique E-region spectral features revealed as intervals of Bragg scatter superimposed on typical background E-region reflection were observed in Digisonde Doppler spectra during intense lacuna conditions. Daytime E-region Doppler spectra recorded at carrier frequencies from 1.5 to 2.7MHz, below the E-region critical frequency foE, have two side-peaks corresponding to Bragg scatter at approximately ±1-2Hz symmetrically located on each side of a central-peak corresponding to near-zenith total reflections. Angle-of-arrival information and ray-tracing simulations show that echo returns are coming from oblique directions most likely resulting from direct backscatter from just below the total reflection height for each sounding frequency. The Bragg backscatter events are shown to manifest during polar lacuna conditions, and to affect the determination of E-region background drift velocities, and as such must be considered when using standard Doppler-sorted interferometry (DSI) techniques to estimate ionospheric drift velocities. Given the Doppler and spatial separation of the echoes determined from high-resolution Doppler measurements, we are able to estimate the Bragg scatter phase velocity independently from the bulk E-region motion. The phase velocity coincides with the ExB direction derived from in situ fluxgate magnetometer records. When ionospheric refraction is considered, the phase velocity amplitudes deduced from DSI are comparable to the ion-acoustic speed expected in the E-region. We briefly consider the plausibility that these previously unreported polar cap E-region Bragg scatter Doppler spectral signatures, observed at Casey in December 1996 during SEC/lacuna conditions may be linked to ionosphere irregularities. These irregularities may possibly be generated by primary plasma waves triggered by current-driven instabilities, that is to say, a hybrid of the "modified two-stream" and "gradient drift" instability mechanisms.

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Zhou, Xiao-Hui, Dong-Ping Wang et Dake Chen. « Global Wavenumber Spectrum with Corrections for Altimeter High-Frequency Noise ». Journal of Physical Oceanography 45, n^o 2 (février 2015) : 495–503. http://dx.doi.org/10.1175/jpo-d-14-0144.1.

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AbstractThe altimetry wavenumber spectra of sea surface height (SSH) provide a unique dataset for testing of geostrophic turbulence. While SSH spectral slopes of k−11/3 and k−5 are expected from theories and numerical simulations, the altimetry spectra from the original unfiltered and instrument noise–corrected data often are too shallow, falling between k−2 and k−3. In this study, the possibility that the flattened spectral slopes are partly due to contamination by unresolved high-frequency (<10 days) motions is tested. A spatiotemporal filter based on empirical orthogonal function expansion (EOF) is used to remove the temporally incoherent signals. The resulting spectral slopes are much steeper than in the previous studies. Over 70% of the revised global spectral estimates, excluding the tropics, are above k−3. Moreover, in high energy regions like the Gulf Stream and Kuroshio, the spectral slopes are about k−5, which is consistent with the classical quasigeostrophic (QG) turbulence. The spectral slopes are validated with the eddy kinetic energy (EKE) spectra from shipboard acoustic Doppler current profiler (ADCP) measurements in the high and low energy regions.

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Unal, Christine. « High-Resolution Raindrop Size Distribution Retrieval Based on the Doppler Spectrum in the Case of Slant Profiling Radar ». Journal of Atmospheric and Oceanic Technology 32, n^o 6 (juin 2015) : 1191–208. http://dx.doi.org/10.1175/jtech-d-13-00225.1.

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AbstractDoppler spectra from vertically profiling radars are usually considered to retrieve the raindrop size distribution (DSD). However, to exploit both fall velocity spectrum and polarimetric measurements, Doppler spectra acquired in slant profiling mode should be explored. Rain DSD samples are obtained from simultaneously measured vertical and slant profile Doppler spectra and evaluated. In particular, the effect of the horizontal wind and the averaging time are investigated.The Doppler spectrum provides a way to retrieve the DSD, the radial wind, and a spectral broadening factor by means of a nonlinear optimization technique. For slant profiling of light rain when the horizontal wind is strong, the DSD results can be affected. Such an effect is demonstrated on a study case of stratiform light rain. Adding a wind profiler mode to the radar simultaneously supplies the horizontal wind and Doppler spectra. Before the retrieval procedure, the Doppler spectra are shifted in velocity to remove the mean horizontal wind contribution. The DSD results are considerably improved.Generally, averaged Doppler spectra are input into this type of algorithm. Instead, high-resolution, low-averaged Doppler spectra are chosen in order to take into account the small-scale variability of the rainfall. Investigating the linear relations at fixed median volume diameter, measured reflectivity-retrieved rainfall rate, for a slant beam, the consistency of the integrated parameters is established for two averaging periods. Nevertheless, the corresponding DSD parameter distributions reveal differences attributed to the averaging of the Doppler spectra.The new aspects are to obtain the same retrieval quality as vertically profiling and highly averaged spectra in an automated way.

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Löhner-Böttcher, J., W. Schmidt, R. Schlichenmaier, H. P. Doerr, T. Steinmetz et R. Holzwarth. « Absolute velocity measurements in sunspot umbrae ». Astronomy & ; Astrophysics 617 (septembre 2018) : A19. http://dx.doi.org/10.1051/0004-6361/201832886.

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Context. In sunspot umbrae, convection is largely suppressed by the strong magnetic field. Previous measurements reported on negligible convective flows in umbral cores. Based on this, numerous studies have taken the umbra as zero reference to calculate Doppler velocities of the ambient active region.Aims. We aim to clarify the amount of convective motion in the darkest part of umbrae, by directly measuring Doppler velocities with an unprecedented accuracy and precision.Methods. We performed spectroscopic observations of sunspot umbrae with the Laser Absolute Reference Spectrograph (LARS) at the German Vacuum Tower Telescope. A laser frequency comb enabled the calibration of the high-resolution spectrograph and absolute wavelength positions for 13 observation sequences. A thorough spectral calibration, including the measurement of the reference wavelength, yielded Doppler shifts of the spectral line Ti I5713.9 Å with an uncertainty of around 5 m s−1. A bisector analysis gave the depth-dependent line asymmetry.Results. The measured Doppler shifts are a composition of umbral convection and magneto-acoustic waves. For the analysis of convective shifts, we temporally averaged each sequence to reduce the superimposed wave signal. Compared to convective blueshifts of up to −350 m s−1in the quiet Sun, sunspot umbrae yield strongly reduced convective blueshifts around −30 m s−1. We find that the velocity in a sunspot umbra correlates significantly with the magnetic field strength, but also with the umbral temperature defining the depth of the Ti I5713.9 Å line. The vertical upward motion decreases with increasing field strength. Extrapolating the linear approximation to zero magnetic field reproduces the measured quiet Sun blueshift. In the same manner, we find that the convective blueshift decreases as a function of increasing line depth.Conclusions. Simply taking the sunspot umbra as a zero velocity reference for the calculation of photospheric Dopplergrams can imply a systematic velocity error reaching 100 m s−1, or more. Setting up a relationship between vertical velocities and magnetic field strength provides a remedy for solar spectropolarimetry. We propose a novel approach of substantially increasing the accuracy of the Doppler velocities of a sunspot region by including the magnetic field information to define the umbral reference velocity.

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Hoitink, A. J. F., H. C. Peters et M. Schroevers. « Field Verification of ADCP Surface Gravity Wave Elevation Spectra ». Journal of Atmospheric and Oceanic Technology 24, n^o 5 (1 mai 2007) : 912–22. http://dx.doi.org/10.1175/jtech2000.1.

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Abstract Acoustic Doppler current profilers (ADCPs) can measure orbital velocities induced by surface gravity waves, yet the ADCP estimates of these velocities are subject to a relatively high noise level. The present paper introduces a linear filtration technique to significantly reduce the influence of noise and turbulence from energy spectra of combined orbital velocity measurements. Data were collected in 13-m-deep water with a 1.2-MHz ADCP sampling in mode 12, where a collocated wave buoy was used for verification. The surface elevation spectra derived from the filtrated and nonfiltrated measurements were compared with corresponding wave buoy spectra. In the frequency range between 0.12 and 0.5 Hz, ADCP- and wave-buoy-derived spectral estimates matched very well, even without applying the filtration technique. At frequencies below 0.12 Hz, the ADCP-derived surface elevation spectra are biased, caused by a depth-varying excess of spectral energy density in the measured orbital velocities, peaking at middepth. Internal waves may provide an explanation for the energy excess, as the experiment was conducted in the region of influence of the Rhine freshwater plume. Alternatively, infragravity waves may be the cause of the depth variation of low-frequency spectral energy density.

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Cloutier, Guy, Louis Allard et Louis-Gilles Durand. « Characterization of Blood Flow Turbulence With Pulsed-Wave and Power Doppler Ultrasound Imaging ». Journal of Biomechanical Engineering 118, n^o 3 (1 août 1996) : 318–25. http://dx.doi.org/10.1115/1.2796013.

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Blood flow turbulence downstream of a concentric 86 percent area reduction stenosis was characterized using absolute and relative Doppler spectral broadening measurements, relative Doppler velocity fluctuation, and Doppler backscattered power. Bidi-mensional mappings of each Doppler index were obtained using a 10 MHz pulsed-wave Doppler system. Calf red cells suspended in a saline solution were used to scatter ultrasound and were circulated in an in vitro steady flow loop model. Results showed that the absolute spectral broadening was not a good index of turbulence because it was strongly affected by the deceleration of the jet and by the shear layer between the jet and the recirculation zones. Relative Doppler spectral broadening (absolute broadening divided by the frequency shift), velocity fluctuation, and Doppler power indices provided consistent mapping of the centerline axial variation of turbulence evaluated by hot-film anemometry. The best agreement between the hot-film and Doppler ultrasound methods was however obtained with the Doppler back-scattered power. The most consistent bidimensional mapping of the flow characteristics downstream of the stenosis was also observed with the Doppler power index. The relative broadening and the velocity fluctuation produced artifacts in the shear layer and in the recirculation zones. Power Doppler imaging is a new emerging technique that may provide reliable in vivo characterization of blood flow turbulence.

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Wijn, P. F. F., P. van der Sar, T. H. J. M. Gootzen, M. H. J. Tilmans et S. H. Skotnicki. « Value of the spectral broadening index in continuous wave Doppler measurements ». Medical and Biological Engineering and Computing 25, n^o 4 (juillet 1987) : 377–85. http://dx.doi.org/10.1007/bf02443357.

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David, J. Y., S. A. Jones et D. P. Giddens. « Modern spectral analysis techniques for blood flow velocity and spectral measurements with pulsed Doppler ultrasound ». IEEE Transactions on Biomedical Engineering 38, n^o 6 (juin 1991) : 589–96. http://dx.doi.org/10.1109/10.81584.

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Palmer, A. J. « A spectral model for turbulence and microphysics dynamics in an ice cloud ». Nonlinear Processes in Geophysics 3, n^o 1 (31 mars 1996) : 23–28. http://dx.doi.org/10.5194/npg-3-23-1996.

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Abstract. A one-dimensional, nine-mode spectral model for temperature, velocity, and the mixing ratios of suspended and precipitating ice-particle components is shown to be consistent with ice-cloud observations. The observations include Doppler radar time-series measurements of a single winter ice cloud and direct measurements of mean particle size vs. icewater content for a set of ice clouds. Fitting of the model to the Doppler vertical-velocity measurements allows a prediction to be made of the vertical scale and turbulent Prandtl number active in the ice-cloud vertical motions. The model is then used to explore the question of how turbulence and gravity-wave motions affect the microphysical properties of an ice cloud. The model predicts interesting dynamical effects on the mixing ratios due to these motions, but no significant effects on the time-averaged microphysical quantities.

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Dufournet, Y., et H. W. J. Russchenberg. « Towards the improvement of cloud microphysical retrievals using simultaneous Doppler and polarimetric radar measurements ». Atmospheric Measurement Techniques 4, n^o 10 (14 octobre 2011) : 2163–78. http://dx.doi.org/10.5194/amt-4-2163-2011.

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Abstract. Radar-based retrievals are often employed to characterize the microphysical properties of cloud hydrometeors, i.e. their phases, habits, densities as well as their respective size and orientation distributions. These techniques are based on a synergetic use of different cloud observation sensor(s) and microphysical model(s) where the information extracted from both sensors and models is combined and converted into microphysical cloud properties. However, the amount of available information is often limited, which forces current microphysical retrieval techniques to base their algorithms on several microphysical assumptions which affect the retrieval accuracy. By simultaneously combining Doppler and polarimetric measurements obtained from fully Doppler polarimetric radars, it is possible to create spectral polarimetric parameters. Although these parameters are easily contaminated with unwanted echoes, this work shows that, from a correct radar signal processing based on filtering and averaging techniques, spectral polarimetric parameters can be correlated to microphysical cloud properties. In particular, preliminary results suggest that particle orientations and habits can be determined from the sole use of such spectral polarimetric parameters. Therefore, such additional spectral polarimetric information offers an opportunity to improve current microphysical retrievals by reducing the number of microphysical assumptions in them.

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35

Reiners, Ansgar. « Magnetic field observations of low-mass stars ». Proceedings of the International Astronomical Union 4, S259 (novembre 2008) : 339–44. http://dx.doi.org/10.1017/s1743921309030701.

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AbstractDirect measurements of magnetic fields in low-mass stars of spectral class M have become available during the last years. This contribution summarizes the data available on direct magnetic measurements in M dwarfs from Zeeman analysis in integrated and polarized light. Strong magnetic fields at kilo-Gauss strength are found throughout the whole M spectral range, and so far all field M dwarfs of spectral type M6 and later show strong magnetic fields. Zeeman Doppler images from polarized light find weaker fields, which may carry important information on magnetic field generation in partially and fully convective stars.

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Halpern, Mark. « Measurement of the Dipole Moment of the Cosmic Background Radiation at mm and Sub-mm Wavelengths ». Symposium - International Astronomical Union 124 (1987) : 63–68. http://dx.doi.org/10.1017/s0074180900159030.

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Sensitive measurements of a dipole anisotropy of the Cosmic Background Radiation at wavelengths of 1.7 and 0.8 mm are presented. Under the assumption that the dipole moment is caused by a doppler shift, these measurements are of sufficient precision to constrain spectral distortions of the CBR at short wavelengths. Measurements of the brightness and shape of diffuse galactic emission are also presented.

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Wilhelm, K., W. Curdt, A. H. Gabriel, M. Grewing, M. C. E. Huber, S. D. Jordan, M. Kühne et al. « ‘Sumer’ – Solar Ultraviolet Measurements of Emitted Radiation ». International Astronomical Union Colloquium 144 (1994) : 619–24. http://dx.doi.org/10.1017/s0252921100026178.

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AbstractThe experiment Solar Ultraviolet Measurements of Emitted Radiation (SUMER) is designed for the investigations of plasma flow characteristics, turbulence and wave motions, plasma densities and temperatures, structures and events associated with solar magnetic activity in the chromosphere, the transition zone and the corona. Specifically, SUMER will measure profiles and intensities of extreme ultraviolet (EUV) lines emitted in the solar atmosphere ranging from the upper chromosphere to the lower corona; determine line broadenings, spectral positions and Doppler shifts with high accuracy; provide stigmatic images of selected areas of the Sun in the EUV with high spatial, temporal and spectral resolution and obtain full images of the Sun and the inner corona in selectable EUV lines, corresponding to a temperature range from 104to more than 1.8 x 106K. The spatial and spectral resolution capabilities of the instrument will be considered in this contribution in some detail, and a new detector concept will be introduced.

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Stȩślicki, Marek, Janusz Sylwester, Stefan Płocieniak, Jarosław Bakała, Żaneta Szaforz, Daniel Ścisłowski, Mirosław Kowaliński, Jose Hernandez, Sergey Kuzin et Sergey Shestov. « Soft X-ray polarimeter-spectrometer SOLPEX ». Proceedings of the International Astronomical Union 11, S320 (août 2015) : 450–55. http://dx.doi.org/10.1017/s1743921316002106.

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AbstractWe present an innovative soft X-ray polarimeter and spectrometer SOLPEX. The instrument is to be mounted aboard the ISS within the Russian science complex KORTES. The measurements to be made by SOLPEX are expected to be of unprecedented quality in terms of sensitivity to detect the soft-X-ray polarization of solar emission emanating from active regions and flares in particular. Simultaneous measurements of the polarization degree and the other characteristics (eg. evolution of the spectra) constitute the last, rather unexplored area of solar X-ray spectroscopy providing substantial diagnostic potential. Second important science task to be addressed are the measurements of Doppler shifts in selected X-ray spectral emission lines formed in hot flaring sources. The novel-type Dopplerometer (flat Bragg crystal drum unit) is planned to be a part of SOLPEX and will allow to measure line Doppler shifts in absolute terms with unprecedented time resolution (fraction of a second) during the impulsive flare phases. We shall present some details of the SOLPEX instrument and discuss observing sequences in a view of science objectives to be reached.

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Mauder, Matthias, Michael Eggert, Christian Gutsmuths, Stefan Oertel, Paul Wilhelm, Ingo Voelksch, Luise Wanner, Jens Tambke et Ivan Bogoev. « Comparison of turbulence measurements by a CSAT3B sonic anemometer and a high-resolution bistatic Doppler lidar ». Atmospheric Measurement Techniques 13, n^o 2 (2 mars 2020) : 969–83. http://dx.doi.org/10.5194/amt-13-969-2020.

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Abstract. Accurate measurements of turbulence statistics in the atmosphere are important for eddy-covariance measurements, wind energy research, and the validation of atmospheric numerical models. Sonic anemometers are widely used for these applications. However, these instruments are prone to probe-induced flow distortion effects, and the magnitude of the resulting errors has been debated due to the lack of an absolute reference instrument under field conditions. Here, we present the results of an intercomparison experiment between a CSAT3B sonic anemometer and a high-resolution bistatic Doppler lidar, which is inherently free of any flow distortion. This novel remote sensing instrument has otherwise very similar spatial and temporal sampling characteristics to the sonic anemometer and hence served as a reference for this comparison. The presented measurements were carried out over flat homogeneous terrain at a measurement height of 30 m. We provide a comparative statistical analysis of the resulting mean wind velocities, the standard deviations of the vertical wind speed and the friction velocity and investigate the reasons for the observed deviations based on the turbulence spectra and co-spectra. Our results show an agreement of the mean wind velocity measurements and the standard deviations of the vertical wind speed with a comparability of 0.082 and 0.020 m s−1, respectively. Biases for these two quantities were 0.003 and 0.012 m s−1, respectively. Slightly larger differences were observed for friction velocity. Analysis of the corresponding co-spectra showed that the CSAT3B underestimates this quantity systematically by about 3 % on average as a result of co-spectral losses in the frequency range between 0.1 and 5 s−1. We also found that an angle-of-attack-dependent transducer-shadowing correction does not improve the agreement between the CSAT3B and the Physikalisch-Technische Bundesanstalt (PTB) lidar effectively.

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Razenkov, Ilya I., et Edwin W. Eloranta. « Advances in atmospheric temperature profile measurements using high spectral resolution lidar ». EPJ Web of Conferences 176 (2018) : 01023. http://dx.doi.org/10.1051/epjconf/201817601023.

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This paper reports the atmospheric temperature profile measurements using a University of Wisconsin-Madison High Spectral Resolution Lidar (HSRL) and describes improvements in the instrument performance. HSRL discriminates between Mie and Rayleigh backscattering [1]. Thermal motion of molecules broadens the spectrum of the transmitted laser light due to Doppler effect. The HSRL exploits this property to allow the absolute calibration of the lidar and measurements of the aerosol volume backscatter coefficient. Two iodine absorption filters with different line widths are used to resolve temperature sensitive changes in Rayleigh backscattering for atmospheric temperature profile measurements.

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41

Vorontsov, Sergei V., et Stuart M. Jefferies. « MODELING SOLAR OSCILLATION POWER SPECTRA. II. PARAMETRIC MODEL OF SPECTRAL LINES OBSERVED IN DOPPLER-VELOCITY MEASUREMENTS ». Astrophysical Journal 778, n^o 1 (6 novembre 2013) : 75. http://dx.doi.org/10.1088/0004-637x/778/1/75.

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Dufournet, Y., et H. W. J. Russchenberg. « Towards the improvement of cloud microphysical retrievals using simultaneous Doppler and polarimetric radar measurements ». Atmospheric Measurement Techniques Discussions 4, n^o 1 (21 janvier 2011) : 527–62. http://dx.doi.org/10.5194/amtd-4-527-2011.

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Abstract. Radar-based retrievals are often employed to characterize the microphysical properties of cloud hydrometeors, i.e., their phases, habits, densities as well as their respective size and orientation distributions. These techniques are based on a synergetic use of different cloud observation sensor(s) and microphysical algorithm(s) where the information extracted from both sensors and models are combined and converted into microphysical cloud properties. However, the amount of available information is often limited, which forces current microphysical retrieval techniques to base their algorithms on several microphysical assumptions which affect the retrieval accuracy. By simultaneously combining Doppler and polarimetric measurements obtained from fully Doppler polarimetric radar, it is possible to create spectral polarimetric parameters. Although these parameters are easily contaminated with unwanted echoes, this work shows that, from a correct radar signal processing based on filtering and averaging techniques, spectral polarimetric parameters can be correlated to microphysical cloud properties. In particular, preliminary results suggest that particle orientations and habits can be easily determined from the solely use of such spectral polarimetric parameters. Therefore, such additional microphysical information offers a great opportunity to improve current microphysical models by reducing their amount of microphysical assumptions.

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Yu, Tian-You, Xiao Xiao et Yadong Wang. « Statistical Quality of Spectral Polarimetric Variables for Weather Radar ». Journal of Atmospheric and Oceanic Technology 29, n^o 9 (1 septembre 2012) : 1221–35. http://dx.doi.org/10.1175/jtech-d-11-00090.1.

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Abstract Spectral polarimetry for weather radar capitalizes on both Doppler and polarimetric measurements to reveal polarimetric variables as a function of radial velocity through spectral analysis. For example, spectral differential reflectivity at a velocity represents the differential reflectivity from all the scatterers that have the same radial velocity of interest within the radar resolution volume. Spectral polarimetry has been applied to suppress both ground and biological clutter, retrieve individual drop size distributions from a mixture of different types of hydrometeors, and estimate turbulence intensity, for example. Although spectral polarimetry has gained increasing attention, statistical quality of the estimation of spectral polarimetric variables has not been investigated. In this work, the bias and standard deviation (SD) of spectral differential reflectivity and spectral copolar correlation coefficient estimated from averaged spectra were derived using perturbation method. The results show that the bias and SD of the two estimators depend on the spectral signal-to-noise ratio, spectral copolar correlation coefficient, the number of spectrum average, and spectral differential reflectivity. A simulation to generate time series signals for spectral polarimetry was developed and used to verify the theoretical bias and SD of the two estimators.

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Lechte, C., G. D. Conway, T. Görler et C. Tröster-Schmid. « X mode Doppler reflectometryk-spectral measurements in ASDEX Upgrade : experiments and simulations ». Plasma Physics and Controlled Fusion 59, n^o 7 (24 mai 2017) : 075006. http://dx.doi.org/10.1088/1361-6587/aa6fe7.

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Walker, Andrew, Eva Olsson, Bengt Wranne, Ivar Ringqvist et Per Ask. « Accuracy of spectral Doppler flow and tissue velocity measurements in ultrasound systems ». Ultrasound in Medicine & ; Biology 30, n^o 1 (janvier 2004) : 127–32. http://dx.doi.org/10.1016/j.ultrasmedbio.2003.08.020.

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Sadrieh, Seyed Nima, Ali Broumandan et Gérard Lachapelle. « Doppler Characterization of a Mobile GNSS Receiver in Multipath Fading Channels ». Journal of Navigation 65, n^o 3 (4 avril 2012) : 477–94. http://dx.doi.org/10.1017/s037346331200015x.

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Global Navigation Satellite Systems (GNSS) Doppler measurements are commonly used for velocity-based relative positioning and aiding Inertial Navigation Systems (INS) in signal degraded environments. The aim of this paper is to characterise the Doppler measurements in GNSS harsh multipath environments. In multipath fading situations such as indoor and urban canyon environments, multipath components arrive to the receiver antenna from different paths and directions. These give rise to various Doppler shifts that cause errors in the velocity solution. In this work the Doppler measurements discrepancy characterised by Doppler spread in multipath environments is investigated. By assuming a ‘sphere of scatterers’ model and considering the antenna gain pattern, the theoretical Power Spectral Density (PSD) observed by a receiver is formulated. The theoretical findings are examined using two sets of measurements in dense multipath environments. Global Positioning System (GPS) live signals using non-isotropic antennas with different orientations are used for this purpose. Different motion directions are also examined using different data sets. An Assisted GPS (A-GPS) approach is utilised where the code phase and the navigation data bits are provided by a nearby outdoor antenna. By applying a ‘Block Processing’ technique, an epoch-by-epoch Doppler and velocity estimation is implemented. Herein, the Doppler and velocity measurements accuracy in addition to the Doppler spread characterization are studied. As shown both theoretically and experimentally, in harsh multipath environments the PSD of the observed signals is a function of the scatterers' geometry and the antenna gain pattern. The Doppler estimation accuracies in multipath and multipath-free cases are compared for different ranges of Carrier-to-Noise ratio (C/N0). Theoretical and experimental results revealed inaccurate Doppler estimation and poor Doppler-derived velocity solutions in dense multipath environments.

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Kalesse, H., W. Szyrmer, S. Kneifel, P. Kollias et E. Luke. « Fingerprints of a riming event on cloud radar Doppler spectra : observations and modeling ». Atmospheric Chemistry and Physics Discussions 15, n^o 20 (22 octobre 2015) : 28619–58. http://dx.doi.org/10.5194/acpd-15-28619-2015.

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Abstract. Radar Doppler spectra measurements are exploited to study a riming event when precipitating ice from a seeder cloud sediments through a supercooled liquid water (SLW) layer. The observations were collected during the deployment of the Atmospheric Radiation Measurement Program's (ARM) mobile facility AMF2 at Hyytiälä, Finland during the BAECC (Biogenic Aerosols – Effects on Clouds and Climate Snowfall Experiment) field campaign. The presented analysis of the height evolution of the radar Doppler spectra is a state-of-the-art retrieval with profiling cloud radars in SLW layers beyond the traditional use of spectral moments. Dynamical effects are taken into account by following the particle population evolution along slanted tracks that are caused by horizontal advection of the cloud under wind shear conditions. In the SLW layer, the identified liquid peak is used as an air motion tracer to correct the Doppler spectra for vertical air motion and the ice peak is used to study the radar profiles of rimed particles. A 1-D steady-state bin microphysical model is constrained using the SLW and air motion profiles and cloud top radar observations. The observed radar moment profiles of the rimed snow can be simulated reasonably well by the model, but not without making several assumptions about the ice particle concentration and the relative role of deposition and aggregation. This suggests that in-situ observations of key ice properties are needed to complement the profiling radar observations before process-oriented studies can effectively evaluate ice microphysical parameterizations.

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Hurther, D., et U. Lemmin. « Improved Turbulence Profiling with Field-Adapted Acoustic Doppler Velocimeters Using a Bifrequency Doppler Noise Suppression Method ». Journal of Atmospheric and Oceanic Technology 25, n^o 3 (1 mars 2008) : 452–63. http://dx.doi.org/10.1175/2007jtecho512.1.

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Abstract A novel noise reduction method and corresponding technique are presented for improving turbulence measurements with acoustic Doppler velocimeters (ADVs) commonly used in field studies of coastal and nearshore regions, rivers, lakes, and estuaries. This bifrequency method is based on the decorrelation of the random and statistically independent Doppler noise terms contained in the Doppler signals at two frequencies. It is shown through experiments in an oscillating grid turbulence (OGT) tank producing diffusive isotropic turbulence that a shift in carrier frequency of less than 10% is sufficient to increase the resolved frequency range by a decade in the turbulent velocity spectra. Over this spectral range, the slope of the velocity spectra agrees well with the universal inertial range value of −5/3. The limit due to spatial averaging effects over the sample volume can be determined from the abrupt deviation of the spectral slope from the −5/3 value. As a result, the relative error of the turbulent intensity estimate and the turbulent kinetic energy (TKE) dissipation rate, measured by two different methods, does not exceed 10% in the case of isotropic turbulence. Furthermore, the bifrequency method allows accurate estimates of the turbulent microscales as shown by the good agreement of the ratio between the Taylor and Kolmogorov microscales and an Re1/4t power law. Compared to previous Doppler noise reduction methods (Garbini et al.), an increase in time resolution by a factor of 4 is achieved. The proposed method also avoids the loss of TKE energy contained in isotropic flow structures of size equal to and smaller than the sample volume. Different from Doppler noise methods proposed by Hurther and Lemmin and Blanckaert and Lemmin, this method does not require additional hardware components, electronic circuitry, or sensors because the redundant instantaneous velocity field information is captured with the same transducer. The required shift in carrier frequency is small enough for the bifrequency method to be easily implemented in commercial ADVs.

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McKenna, Mervyn Alexander, Michelle Clare Bonfield et Teresa Robinson. « Levels of agreement in the measurements of carotid artery ultrasound across a regional vascular network ». Ultrasound 26, n^o 2 (25 janvier 2018) : 101–9. http://dx.doi.org/10.1177/1742271x17751255.

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Introduction Duplex ultrasound is often the sole imaging modality used in diagnosing carotid artery disease. However, the reproducibility and repeatability of scientists in determining the peak systolic velocity and end diastolic velocity of the internal carotid artery and common carotid artery (CCA) is widely debated. Study aim To investigate intra- and inter-operator variability in diagnostic ultrasound of the carotid arteries across a centralised vascular network using a healthy test subject. To identify potential causes of variability and highlight areas for improvement. Methods Fifteen vascular scientists across four hospital Trusts in the Bristol, Bath and Weston vascular network measured the peak systolic velocity and end diastolic velocity of the internal carotid artery and common carotid artery in a subject using a single portable ultrasound machine. A double blind assessment of spectral Doppler images was performed by two vascular clinical scientists for optimal caliper placement, spectral gain and angle correction. Results were compared for intra- and inter-operator variability. Results Initial quality assessment of the Doppler images revealed that three out of 15 scientists produced suboptimal results. Box plot analysis of the common carotid artery and internal carotid artery for each scientist revealed significant variance (ANOVA p < 0.05). However, a Levene’s test revealed no single operator who consistently produced highly variable results ( p = 0.569). Conclusion This study highlights the difficulty in obtaining consistent velocity measurements from a subject. Despite the variability in absolute peak systolic velocity and end diastolic velocity, scientists were generally consistent in obtaining an optimal spectral Doppler trace. Some issues with consistency were, however, identified which were subsequently addressed.

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Shukla, Gaurav, Malay B. Chowdhuri, Kajal Shah, Nandini Yadava, Ranjana Manchanda, Kumarpalsinh A. Jadeja, Rakesh L. Tanna, Balamurali Krishna Mayya K. et Joydeep Ghosh. « Poloidal Rotation and Edge Ion Temperature Measurements Using Spectroscopy Diagnostic on Aditya-U Tokamak ». Atoms 7, n^o 3 (19 septembre 2019) : 93. http://dx.doi.org/10.3390/atoms7030093.

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The impurity ion poloidal rotation and ion temperature from the Aditya-U tokamak plasma have been measured using a high-resolution spectroscopic diagnostic. It comprises of a high resolution, 1 m, f/8.7, Czerny-Turner configuration spectrometer along with charge coupled device (CCD) detector. The system monitors the spectral line emission of C2+ impurity ions at 464.74 nm from the top port of the Aditya-U vacuum vessel with the lines of sight covering the plasma minor radius from r = 11.55 cm to 21.55 cm. The impurity ion poloidal rotation velocity and temperature have been estimated using the Doppler shift and Doppler broadening of the spectral lines respectively. The maximum poloidal rotation at a radial location of 21.55 cm in the edge of the plasma during the plasma current flat top was observed to be ~4 km/s for the analyzed discharges and the ion temperatures measured in the edge were in the range of 32–40 eV.

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