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We consider the spatial response and sampling properties of ship-mounted four-beam and three-beam incoherent Doppler current profilers. Spatial properties are an issue because such systems cannot make a point measurement of the current field, and because the actual current field is nonuniform. We obtain expressions for the effect of beamwidth, beam separation, and block averaging on system response. We observe that spatial aliasing may occur above some critical depth, and that pulsewidth reduction cannot improve vertical resolution beyond a limit imposed by system geometry. We also find that, for the three-beam system, the along-track component is irrevocably contaminated (to some degree) by the cross-track component as a result of its nonorthogonal beam geometry. The velocity and shear estimate variances achievable with these systems are calculated in a companion paper [1]. 相似文献
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I. Gultepe G. A. Isaac P. Joe P. A. Kucera J. M. Theriault T. Fisico 《Pure and Applied Geophysics》2014,171(1-2):59-85
The objective of this work is to better understand and summarize the mountain meteorological observations collected during the Science of Nowcasting Winter Weather for the Vancouver 2010 Olympics and Paralympics (SNOW-V10) project that was supported by the Fog Remote Sensing and Modeling (FRAM) project. The Roundhouse (RND) meteorological station was located 1,856 m above sea level that is subject to the winter extreme weather conditions. Below this site, there were three additional observation sites at 1,640, 1,320, and 774 m. These four stations provided some or all the following measurements at 1 min resolution: precipitation rate (PR) and amount, cloud/fog microphysics, 3D wind speed (horizontal wind speed, U h; vertical air velocity, w a), visibility (Vis), infrared (IR) and shortwave (SW) radiative fluxes, temperature (T) and relative humidity with respect to water (RHw), and aerosol observations. In this work, comparisons are made to assess the uncertainties and variability for the measurements of Vis, RHw, T, PR, and wind for various winter weather conditions. The ground-based cloud imaging probe (GCIP) measurements of snow particles using a profiling microwave radiometer (PMWR) data have also been shown to assess the icing conditions. Overall, the conclusions suggest that uncertainties in the measurements of Vis, PR, T, and RH can be as large as 50, >60, 50, and >20 %, respectively, and these numbers may increase depending on U h, T, Vis, and PR magnitude. Variability of observations along the Whistler Mountain slope (~500 m) suggested that to verify the models, model space resolution should be better than 100 m and time scales better than 1 min. It is also concluded that differences between observed and model based parameters are strongly related to a model’s capability of accurate prediction of liquid water content (LWC), PR, and RHw over complex topography. 相似文献
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Two-way time spreading and path-loss measurements were collected in water 100 m deep, off the coast of Nova Scotia. Data were collected at frequencies of 20-22 kHz, 27-29 kHz, and 35-37 kHz using linear FM pulses 0.160 s in duration. The source-receiver was an anchored, high-frequency active sonar, and the target was a free-drifting echo repeater. Sonar and target positions were recorded using a portable tracking range. In the paper, two-way time spreading and path loss measurements are compared with modeled estimates obtained using an enhanced version of the generic sonar model (GSM). The GSM estimates of time spreading due to multipath propagation compare favorably with the experimental data. The model indicates that the path loss for individual eigenrays was extremely sensitive to fluctuations in the sound-speed profile. This led to substantial variation in the model output depending on the choice of profile. In place of the model, an empirical estimate of path loss was computed from the data. We obtained a two-way spreading loss of 2[18.4log10(R)] where R is the range from sonar to target. The data were also used to compute the standard deviation of the received echo intensity at each frequency. The standard deviation was computed two different ways. First it was computed using the peak echo level from each of the pulses at a given frequency. Then, it was computed from the total energy received from each of the pings. At all frequencies, the standard deviation was 1-2 dB lower when computed from the total received energy 相似文献
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We derive statistical lower bounds on the velocity and shear profile estimation accuracies (variances) achievable with both four-beam and three-beam Doppler current profilers. We find that performance (estimate variance) is asymptotic to a constant as SNR increases; this is evidently due to the assumed incoherent scattering properties of the medium. Curves and expressions are developed which define the tradeoffs between accuracy and resolution for two systems manufactured by AMETEK-Straza. The spatial response properties of the systems are addressed in a companion paper [3]. 相似文献
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We consider the problem of estimating structure (interface reflection coefficient values) from noisy reflection observations of a discrete, layered, lossless waveguide; this is a special case of the general acoustic reflection inverse problem. Our approach is to develop a statistical lower bound, the Cramer-Rao bound, on estimator performance. We demonstrate, by computing the bound for the one- and two-reflector cases, that the bound is not merely theoretical but also provides, through its parametric behavior, physical insight into the estimation problem. In addition, we present two estimator structures for this problem, a statistically optimal estimator, (maximum-likelihood), and a suboptimal estimator (based on the special character of the medium's response), and compare their performance. We perform Monte Carlo tests to verify that, for high signal-to-noise ratios, estimator performance is predicted by the Cramer-Rao bound. Moreover, we find that in these regions the ad hoc estimator performs comparably to the maximum likelihood. 相似文献
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