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21.
A new global TEC model for estimating transionospheric radio wave propagation errors 总被引：4，自引：0，他引：4
Space-based navigation and radar systems operating at single frequencies of <10 GHz require ionospheric corrections of the
signal delay or range error. Because this ionospheric propagation error is proportional to the total electron content of the
ionosphere along the ray path, a user friendly TEC model covering global scale and all levels of solar activity should be
helpful in various applications. Since such a model is not available yet, we present an empirical model approach that allows
determining global TEC very easily. Although the number of model coefficients and parameters is rather small, the model describes
main ionospheric features with good quality. Presented is the empirical approach describing dependencies on local time, geographic/geomagnetic
location and solar irradiance and activity. The non-linear approach needs only 12 coefficients and a few empirically fixed
parameters for describing the broad spectrum of TEC variation at all levels of solar activity. The model approach is applied
on high-quality global TEC data derived by the Center for Orbit Determination in Europe (CODE) at the University of Berne
over more than half a solar cycle (1998–2007). The model fits to these input data with a negative bias of 0.3 TECU and a RMS
deviation of 7.5 TECU. As other empirical models too, the proposed Global Neustrelitz TEC
Model NTCM-GLis climatological, i.e. the model describes the average behaviour under quiet geomagnetic conditions. During severe space
weather events the actual TEC data may deviate from the model values considerably by more than 100%. A preliminary comparison
with independent data sets as TOPEX/Poseidon altimeter data reveals similar results for NeQuick and NTCM-GL with RMS deviations
in the order of 5 and 11 TECU (1 TECU = 1016 electrons/m2) for low and high-solar activity conditions, respectively. The more extended data base of ionosphere information that accumulates
in the coming years will help in further improving the set of coefficients of the model. 相似文献
22.
Mohammad A. Hoque Aftab A. Khan M. Shamsudduha Muhammad S. Hossain Tariqul Islam Shahid H. Chowdhury 《Environmental Geology》2009,56(8):1687-1695
This study investigated the relationship between near-surface lithology and the spatial variability of As concentrations using
sediment grain-size analysis and electromagnetic induction survey in the southeast Bangladesh. It has been observed that the
aquifers overlain by finer sediments have higher concentrations of As in groundwater, whereas As concentrations are remarkably
low in aquifers having permeable sandy materials or thinner silt/clay layer at the surface. The near-surface lithology acts
as a controlling factor for spatial distributions of groundwater As within the very shallow depths (<15 m). Shallow alluvial
aquifers can provide low-As drinking water in many areas of the country when tube wells are properly installed after investigation
of the overlying near-surface sediment attributes and hydraulic properties. 相似文献
23.
Zusammenfassung Eine Markov'sche Kettenanalyse wird in der Untersuchung von zwei stratigraphischen Ausschnitten aus der Kreide des Benue-Troges in Nigeria eingesetzt. Vier Faziesabschnitte bestehend aus grobkörnigem, mittelkörnigem und feinem Sandstein sowie Tonstein, lassen sich trennen. Matrizen zu den Faziesübergängen werden nach einem eingeschlossenen und einem mehrstufigen Markovmodell gebildet. Daraus lassen sich unabhängige Probematrizen ableiten. Bei einem Vergleich dieser zwei Serien von Matrizen und der Anwendung von zwei statistischen Tests zu diesen Matrizen stellt man fest, daß die untersuchten Formationsausschnitte Markov'sche Eigenschaften aufweisen, die eine nach oben feiner werdende Zyklizität repräsentieren. Der Fazieszyklus läßt sich als eine Faziesabfolge von Flußrinnen, Gleithängen und Überflutungsebenen eines Flußsystems interpretieren. Mit der Methode einer Fazieszählung läßt sich die Zusammensetzung eines Modalzyklus ermitteln; eine übergeordnete Matrize zeigt wichtige Zusammenhänge auf, die in der eingeschlossenen Matrize nicht sichtbar sind. Im allgemeinen nehmen die Chi-Quadratzahlen bei der Abnahme der Zahl der Nullen in der Ubergangsmatrize zu. Deshalb hat die Wahl der Zählmethode Einfluß auf die Chi-Quadratzahlen. Die Zufälligkeit im Auftreten eines Faziestypes innerhalb eines Zyklus wird durch die Entropie ausgedrückt, die von den Markovmatrizen abgeleitet werden kann. Zwei Arten von Entropien werden für jede Fazies berechnet: die Entropie nach der Ablagerung und vor der Ablagerung bilden zusammen eine Entropieserie. Die Entropie des Gesamtsystems läßt sich ebenfalls berechnen. Diese Werte werden graphisch dargestellt und mit den idialisierten Diagrammen von Hattori verglichen, wobei sich zeigt, daß es sich hier im wesentlichen um einen abgeschnittenen, asymmetrischen Zyklus (Typ A-4 von Hattory) handelt, der in einem Flußsystem abgelagert wurde. Die Untersuchung zeigt weiterhin, daß die Anwendung der Entropiefunktion zusammen mit der Markov'schen Kettenanalyse charakteristische Züge einer Sedimentserie erkennen läßt, die als diagnostischer Hinweis bei der Bewertung von Ablagerungsbedingungen in einer stratigraphischen Serie gewertet werden können.AbstractMarkov chain analysis is used in the study of two composite stratigraphic sections from the Cretaceous of the Benue trough of Nigeria. Four facies states, consisting of coarse sandstone, medium sandstone, fine sandstone and shale, are recognized in the sections. Faciès transition matrices are structured on embedded Markov and multistory Markov model, from which independent trials matrices are derived. By comparing these two sets of matrices, and applying two statistical tests to the matrices, it is established that the stratigraphic sections examined exhibit a Markovian property with a fining-upward cyclicity. The cycle is interpreted in terms of in-channel, point-bar and overbank facies association in a fluvial system. Methods of faciès count determine the composition of the modal cycle in a succession; multistory matrix reveals important relationships between facies states that are not apparent in the embedded matrix. Generally, Chi-square values increase with decrease in the number of zeros in the transition matrix; therefore, choice of count method has an effect on Chi-square value. The randomness in the occurrence of facies within a cycle is evaluated in terms of entropy which can be calculated from the Markov matrices. Two types of entropies are calculated for every facies state: entropy after deposition and entropy before deposition, which together form an entropy set; the entropy for the whole system is also calculated. These values are plotted and compared withHattori's idealized plots which indicate that the sequence is essentially a truncated asymmetric cycle (type A-4 ofHattori), and was deposited in a fluvial environment. The study further suggests that use of the entropy function together with Markov chain analysis reveals some characteristic features in a succession which can be used as diagnostic evidence in evaluating depositional environment of a stratigraphic sequence.
Résumé On utilise une analyse par chaîne de Markov pour étudier deux sections stratigraphiques composites du Crétacé dans l'auge de Benue au Nigeria. Ces sections comportent quatre facies: grès grossier, grès moyen, grès fin et schiste. Les matrices de transition des facies sont structurées d'après un modèle de Markov emboîté et un modèle à étages multiples, à partier de quoi on a dérivé des matrices d'essais indépendantes. En comparant ces deux ensembles de matrices et en appliquant deux tests statistiques à ces matrices, on a établi que les sections stratigraphiques examinées ont une propriété Markovienne avec une cyclicité des matériaux par affinement vers le haut. Le cycle est interprété comme l'association des facies de chenal, de berge (»point-bar«) et de rive (»overbank«) dans un système fluvial. Les méthodes de comptage du facies déterminent la composition du cycle modal dans une séquence. La matrice à étages multiples révêle des relations importantes entre les facies, qui ne sont pas apparentes dans la matrice emboîtée. En général, les valeurs chi-carré augmentent lorsque le nombre de zéros décroît dans la matrice de transition. Par conséquent, le choix de la méthode de comptage a un effet sur la valeur chi-carré. Le caractère plus ou moins aléatoire de l'apparition des facies à l'intérieur d'un cycle s'évalue en termes d'entropie que l'on peut calculer à partir des matrices de Markov. On calcule deux sortes d'entropies pour chaque faciès: l'entropie après dépôt et l'entropie avant dépôt, qui forment ainsi un ensemble d'entropies. On calcule aussi l'entropie du système entier. Ces valeurs sont reportées et comparées avec les tracés idéaux d'Hattori, ce qui indique que la séquence est essentiellement un cycle assymétrique tronqué (type A4 d'Hattori), et qu'elle s'est déposée dans un milieu fluvial. De plus l'étude suggère que l'usage de la fonction d'entropie, conjointement à l'analyse par chaîne de Markov, révèle certains traits caractéristiques dans une succession. Ces traits peuvent être utilisés comme arguments diagnostiques dans la détermination du milieu de dépôt d'une séquence stratigraphique.
Benue, , . 4 , , . . . , , , . , , . . , . . , . : . . , , Hattory: , — — 4 Hattory —, . , , , .相似文献
24.
With the increasing number of precise navigation and positioning applications using Global Navigation Satellite Systems (GNSS)
such as the Global Positioning System (GPS), higher order ionospheric effects and their correction become more and more important.
Whereas the first-order error can be completely eliminated by a linear combination of dual- frequency measurements, the second-
and third-order residual effects remain uncorrected in this approach. To quantify the second-order residual effect, a simple
formula has been derived for GNSS users in Germany. Our proposed correction algorithm reduces the second-order effects to
a residual error of fractions of 1 mm up to 2 mm at a vertical total electron content level of 1018 electrons/m2 (100 TECU), depending on satellite azimuth and elevation angles. The correction formula can be implemented in real-time applications
as it does not require the knowledge of the geomagnetic field or the electron density distribution in the ionosphere along
the signal path. It is expected that the correction will enable more accurate positioning using the line-of-sight carrier-phase
measurements. 相似文献
25.
Ionospheric correction using NTCM driven by GPS Klobuchar coefficients for GNSS applications 总被引：1，自引：1，他引：0
Global Navigation Satellite Systems (GNSS) require mitigation of ionospheric propagation errors because the ionospheric range errors might be larger than tens of meters at the zenith direction. Taking advantage of the frequency-dispersive property of ionospheric refractivity, the ionospheric range errors can be mitigated in dual-frequency applications to a great extent by a linear combination of carrier phases or pseudoranges. However, single-frequency GNSS operations require additional ionospheric information to apply signal delay or range error corrections. To aid single-frequency operations, the global positioning system (GPS) broadcasts 8 coefficients as part of the navigation message to drive the ionospheric correction algorithm (ICA) also known as Klobuchar model. We presented here an ionospheric correction algorithm called Neustrelitz TEC model (NTCM) which can be used as complementary to the GPS ICA. Our investigation shows that the NTCM can be driven by Klobuchar model parameters to achieve a significantly better performance than obtained by the mother ICA algorithm. Our research, using post-processed reference total electron content (TEC) data from more than one solar cycle, shows that on average the RMS modeled TEC errors are up to 40% less for the proposed NTCM model compared to the Klobuchar model during high solar activity period, and about 10% less during low solar activity period. Such an approach does not require major technology changes for GPS users rather requires only introducing the NTCM approach a complement to the existing ICA algorithm while maintaining the simplicity of ionospheric range error mitigation with an improved model performance. 相似文献
26.
Numerical study on wave-induced filtration flow across the beach face and its effects on swash zone sediment transport 总被引：2，自引：0，他引：2
A numerical model, coupling an analysis of beach groundwater flow with an analysis of swash wave motion over a uniform slope, is presented. Model calculations are performed to investigate the variations of swash-induced filtration flows across the beach face for different input parameters. Swash zone sediment transport under the influence of such filtration flow across the beach face is investigated through modification of effective weight of sediment particle and modification of swash boundary layer thickness. These effects are quantified based on a bed load transport model with a modified Shields parameter. 相似文献
27.
Ahmed Naser Hoque Muhammad Al-Amin Pradhan Biswajeet Arabameri Alireza 《Natural Resources Research》2021,30(5):3315-3337
Natural Resources Research - Groundwater is one of the most dynamic and renewable natural resources found in the earth’s crust. A spatio-temporal assessment of groundwater potential zone... 相似文献
28.
Banville Simon Sieradzki Rafal Hoque Mainul Wezka Kinga Hadas Tomasz 《GPS Solutions》2017,21(4):1817-1828
GPS Solutions - Higher-order ionospheric effects, if not properly accounted for, can propagate into geodetic parameter estimates. For this reason, several investigations have led to the development... 相似文献
29.
M. Sohel Rana B.Sc. A. T. M. Rafiqul Hoque M. Sc. S. M. Asadul Hossain B.Sc. 《International Journal of Environmental Science and Technology》2007,4(3):395-400
In this paper, experimentally the pressure distribution over wedge and sphere surface in uniform flow has been investigated. Fluid flow over a smooth wedge surface was investigated experimentally to determine the pressure distribution at different values of Reynolds numbers and wedge angles as well as pressure distributions around the sphere of different size are reported for different Reynolds numbers. The variation of static pressures is larger near the wedge vertex and gradually decreases along the length of the wedge surface. At the forward stagnation point the pressure distribution depends on the size of spheres. Separation of flow takes place at an angle of 78° from forward stagnation point for all sizes of spheres. At the rear stagnation point of the sphere the pressure distribution predicts negative pressures. Experimental results provide useful information of interest to potential industrial application. It helps in determining the shape of various wedge and sphere surfaces used in industries for cooling or heating of different wedge surfaces. In the present experiment, it has been found that the pressure near vertex lower as the included angle of the wedge decreases and at lower values of Reynolds number. 相似文献
30.
Arsenic is a carcinogen known for its acute toxicity to organisms.Geothermal waters are commonly high in arsenic,as shown at the Bjarnarflag Power Plant,Iceland(～224 μg/kg of solvent).Development of geothermal energy requires adequate disposal of arsenic-rich waters into groundwater/geothermal systems.The outcome of arsenic transport models that assess the effect of geothermal effluent on the environment and ecosystems may be influenced by the sensitivity of hydraulic parameters.However,previous such studies in Iceland do not consider the sensitivity of hydraulic parameters and thereby the interpretations remain unreliable.Here we used the Lake Myvatn basaltic aquifer system as a case study to identify the sensitive hydraulic parameters and assess their role in arsenic transport.We develop a one-dimensional reactive transport model(PHREEQC ver.2.),using geochemical data from Bjarnarflag,Iceland.In our model,arsenite(H_3 ASO_3)was predicted to be the dominant species of inorganic arsenic in both groundwater and geothermal water.Dilution reduced arsenic concentration below～5 μg/kg.Adsorption reduced the residual contamination below～0.4μg/kg at 250 m along transect.Based on our modelling,we found volumetric input to be the most sensitive parameter in the model.In addition,the adsorption strength of basaltic glass was such that the physical hydrogeological parameters,namely:groundwater velocity and longitudinal dispersivity had little influence on the concentration profile. 相似文献