Impact of the atmospheric refraction on the precise astrometry with adaptive optics in infrared     

Krzysztof G. Hełminiak 《New Astronomy》2009,14(6):521-527

We study the impact of the atmospheric differential chromatic refraction on the measurements and precision of relative astrometry. Specifically, we address the problem of measuring the separations of close pairs of binary stars with adaptive optics in the J and K bands.We investigate the influence of weather conditions, zenithal distance, star’s spectral type and observing wavelength on the astrometric precision and determine the accuracy of these parameters that is necessary to detect exoplanets with existing and planned large ground based telescopes with adaptive optics facilities. The analytical formulae for simple monochromatic refraction and a full approach, as well as moderately simplified procedure, are used to compute refraction corrections under a variety of observing conditions.It is shown that the atmospheric refraction must be taken into account in astrometric studies but the full procedure is not necessary in many cases. Requirements for achieving a certain astrometric precision are specified.  相似文献   

Investigation of the 16-year and 18-year ZTD Time Series Derived from GPS Data Processing     

Zofia Bałdysz  Grzegorz Nykiel  Mariusz Figurski  Karolina Szafranek  Krzysztof KroszczyńSki 《Acta Geophysica》2015,63(4):1103-1125

The GPS system can play an important role in activities related to the monitoring of climate. Long time series, coherent strategy, and very high quality of tropospheric parameter Zenith Tropospheric Delay (ZTD) estimated on the basis of GPS data analysis allows to investigate its usefulness for climate research as a direct GPS product. This paper presents results of analysis of 16-year time series derived from EUREF Permanent Network (EPN) reprocessing performed by the Military University of Technology. For 58 stations Lomb-Scargle periodograms were performed in order to obtain information about the oscillations in ZTD time series. Seasonal components and linear trend were estimated using Least Square Estimation (LSE) and Mann—Kendall trend test was used to confirm the presence of a linear trend designated by LSE method. In order to verify the impact of the length of time series on trend value, comparison between 16 and 18 years were performed.  相似文献   

LAGEOS Sensitivity to Ocean Tides     

Krzysztof Sośnica 《Acta Geophysica》2015,63(4):1181-1203

Satellite Laser Ranging (SLR) to LAGEOS has a remarkable contribution to high-precise geodesy and geodynamics through deriving and validating various global geophysical models. This paper validates ocean tide models based on the analysis of satellite altimetry data, coastal tide gauges, and hydrodynamic data, i.e., CSR3.0, TOPEX4.0, CSR4.0A, FES2004, GOT00.2, and the CSRC Schwiderski model. LAGEOS orbits and SLR observation residuals from solutions based on different ocean tide models are compared and examined. It is found that LAGEOS orbits are sensitive to tidal waves larger than 5 mm. The analysis of the aliasing periods of LAGEOS orbits and tidal waves reveals that, in particular, the tidal constituent S₂ is not well established in the recent ocean tide models. Some of the models introduce spurious peaks to empirical orbit parameters, which can be associated with S₂, S_a, and K₂ tidal constituents, and, as a consequence, can be propagated to fundamental parameters derived from LAGEOS observations.  相似文献   

Stability of the Triangular Libration Points in the Unrestricted Planar Problem of a Symmetric Rigid Body and a Point Mass     

Krzysztof Goździewski 《Celestial Mechanics and Dynamical Astronomy》2003,85(1):79-103

In papers (Godziewski and Maciejewski, 1998a, b, 1999), we investigate unrestricted, planar problem of a dynamically symmetric rigid body and a sphere. Following the original statement of the problem by Kokoriev and Kirpichnikov (1988), we assume that the potential of the rigid body is approximated by the gravitational field of a dumb-bell. The model is described in terms of a 2D Hamiltonian depending on three parameters.In this paper, we investigate the stability of triangular equilibria permissible by the dynamics of the model, under the assumption of low-order resonances. We analyze all resonances of order smaller than four, and we examine the stability with application of theorems by Markeev and Sokolsky. These are the possible following cases: the non-diagonal resonance of the first order with two null characteristic frequencies (unstable); resonances of the first order with one nonzero frequency (diagonal and non-diagonal, stable and unstable); the second-order resonance, which is non-diagonal and stable, and the third-order resonance which is generically unstable, except for three points in the parameters' space, corresponding to stable equilibria.We discuss a perturbed version of Kokoriev and Kirpichnikov model, and we find that if the perturbation is small and depends on the coordinates only, the triangular equilibria persist, except if for the unperturbed equilibria the first-order resonance occurs. We show that the resonances of the order higher than two are also preserved if the perturbation acts.  相似文献   

Book review     

Peter Molnar  John D. Weeks  Claude J. Allegre  Yehuda Bock  Stephen K. Dobos  Leszek Czechowski  Kacper R. Rybicki  Krzysztof Haman 《Pure and Applied Geophysics》1987,125(4):679-698

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Book review     

Jeremy Bloxham  David Gubbins  Victor C. Li  Leszek Czechowski  Jan Błęcki  Anil Suri  Pierre De Mey  R. Balcer  Maciej Sadowski  Krzysztof E. Haman 《Pure and Applied Geophysics》1989,130(1):127-148

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On the informative value of the largest sample element of log-Gumbel distribution     

Witold G. Strupczewski  Krzysztof Kochanek  Vijay. P. Singh 《Acta Geophysica》2007,55(4):652-678

Extremes of stream flow and precipitation are commonly modeled by heavytailed distributions. While scrutinizing annual flow maxima or the peaks over threshold, the largest sample elements are quite often suspected to be low quality data, outliers or values corresponding to much longer return periods than the observation period. Since the interest is primarily in the estimation of the right tail (in the case of floods or heavy rainfalls), sensitivity of upper quantiles to largest elements of a series constitutes a problem of special concern. This study investigated the sensitivity problem using the log-Gumbel distribution by generating samples of different sizes (n) and different values of the coefficient of variation by Monte Carlo experiments. Parameters of the log-Gumbel distribution were estimated by the probability weighted moments (PWMs) method, method of moments (MOMs) and maximum likelihood method (MLM), both for complete samples and the samples deprived of their largest elements. In the latter case, the distribution censored by the non-exceedance probability threshold, F _T , was considered. Using F _T instead of the censored threshold T creates possibility of controlling estimator property. The effect of the F _T value on the performance of the quantile estimates was then examined. It is shown that right censoring of data need not reduce an accuracy of large quantile estimates if the method of PWMs or MOMs is employed. Moreover allowing bias of estimates one can get the gain in variance and in mean square error of large quantiles even if ML method is used.  相似文献   

A generalization of the fractal/facies model     

Fred J. Molz  Tomasz J. Kozubowski  Krzysztof Podgórski  James W. Castle 《Hydrogeology Journal》2007,15(4):809-816

In order to generalize the fractal/facies concept, a new stochastic fractal model for ln(K) increment probability density functions (PDFs) is presented that produces non-Gaussian behavior at smaller lags and converges to Gaussian at larger lags. The model is based on the classical Laplace PDF. The new stochastic fractal family is called fractional Laplace motion (fLam) having stationary increments called fractional Laplace noise (fLan). This fractal is different from other fractals because the character of the underlying increment PDFs changes dramatically with lag size, which leads to lack of self-similarity. Data also appear to display this characteristic. In the larger lag size ranges, approximate self-affinity does hold. The basic field procedure for further testing of the fractional Laplace theory is to measure ln(K) increment distributions along transects, calculate frequency distributions from the data, and compare results to appropriate fLan family members. The variances of the frequency distributions should also change with lag size (scale) in a prescribed manner. There are mathematical reasons such as the geometric central limit theorem, for surmising that fLam/fLan may be more fundamental than other approaches that have been proposed for modeling ln(K) frequency distributions.  相似文献   

ADCN: An anisotropic density‐based clustering algorithm for discovering spatial point patterns with noise          下载免费PDF全文

Gengchen Mai  Krzysztof Janowicz  Yingjie Hu  Song Gao 《Transactions in GIS》2018,22(1):348-369

Density‐based clustering algorithms such as DBSCAN have been widely used for spatial knowledge discovery as they offer several key advantages compared with other clustering algorithms. They can discover clusters with arbitrary shapes, are robust to noise, and do not require prior knowledge (or estimation) of the number of clusters. The idea of using a scan circle centered at each point with a search radius Eps to find at least MinPts points as a criterion for deriving local density is easily understandable and sufficient for exploring isotropic spatial point patterns. However, there are many cases that cannot be adequately captured this way, particularly if they involve linear features or shapes with a continuously changing density, such as a spiral. In such cases, DBSCAN tends to either create an increasing number of small clusters or add noise points into large clusters. Therefore, in this article, we propose a novel anisotropic density‐based clustering algorithm (ADCN). To motivate our work, we introduce synthetic and real‐world cases that cannot be handled sufficiently by DBSCAN (or OPTICS). We then present our clustering algorithm and test it with a wide range of cases. We demonstrate that our algorithm can perform equally as well as DBSCAN in cases that do not benefit explicitly from an anisotropic perspective, and that it outperforms DBSCAN in cases that do. Finally, we show that our approach has the same time complexity as DBSCAN and OPTICS, namely O(n log n) when using a spatial index and O(n²) otherwise. We provide an implementation and test the runtime over multiple cases.  相似文献   

Statistical modelling of the main features of the <Emphasis Type="Italic">Artemisia</Emphasis> pollen season in Wrocław,Poland, during the 2002–2011 time period     

Anetta?Drzeniecka-Osiadacz  Justyna?KrynickaEmail author  Ma?gorzata?Malkiewicz  Kamilla?Klaczak  Krzysztof?Miga?a 《Theoretical and Applied Climatology》2015,119(3-4):419-432

The aim of this article is to present statistical forecasting models concerning the dynamics of Artemisia pollen seasons in Wroc?aw, including the start and end, the date of maximum pollen concentration and seasonal pollen index (SPI). For statistical evaluation, use was made of aerobiological and meteorological data from the last 10 years (2002–2011). Based on this data, agroclimatic indicators, i.e. crop heat units (CHUs), were determined for various averaging periods. The beginning of the Artemisia pollen season in the studied time period, on average, took place on 23 June. Its length usually varied between 26 and 45 days, and maximum daily concentrations occurred between 31 July and 18 August. It was found that the beginning of the pollen season depends, above all, on the values of CHUs and photothermal unit (PTU) (p?<?0.05) in the period from March to June, for various thermal thresholds. The date of maximum daily concentration correlates with sunshine duration, PTU and air temperature for June and July (p?<?0.05). On the other hand, SPI is connected with thermal variables, i.e. average, maximum and minimum air temperatures and CHUs and heliothermal unit (HTU) for July (p?<?0.05) and the beginning of spring. Based on the correlation analysis and the chosen variables, regression models for the beginning date of Artemisia pollen season and SPI were prepared, which were then verified by using leave-one-out cross-validation. A better fit between modelled and actual values was found for the analysis concerning the season start date than for the SPI.  相似文献