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排序方式: 共有114条查询结果,搜索用时 15 毫秒
71.
SBAS orbit and satellite clock corrections for precise point positioning   总被引:2,自引:0,他引:2  
The quality of real-time GPS positions based on the method of precise point positioning (PPP) heavily depends on the availability and accuracy of GPS satellite orbits and satellite clock corrections. Satellite-based augmentation systems (SBAS) provide such corrections but they are actually intended to be used for wide area differential GPS with positioning results on the 1-m accuracy level. Nevertheless, carrier phase-based PPP is able to achieve much more accurate results with the same correction values. We applied SBAS corrections for dual-frequency PPP and compared the results with PPP obtained using other real-time correction data streams, for example, the GPS broadcast message and precise corrections from the French Centre National d’Etudes Spatiales and the German Deutsches Zentrum für Luft- und Raumfahrt. Among the three existing SBAS, the best results were achieved for the North American wide area augmentation system (WAAS): horizontal and vertical position accuracies were considerably smaller than 10 cm for static 24-h observation data sets and smaller than 30 cm for epoch-by-epoch solutions with 2 h of continuous observations. The European geostationary navigation overlay service and the Japanese multi-functional satellite augmentation system yield positioning results with biases of several tens of centimeters and variations larger by factors of 2–4 as compared to WAAS.  相似文献   
72.
Many brightest cluster galaxies (BCGs) at the centres of X-ray selected clusters exhibit clear evidence for recent star formation. However, studies of BCGs in optically selected clusters show that star formation is not enhanced when compared to control samples of non-BCGs of similar stellar mass. Here, we analyse a sample of 113 BCGs in low-redshift  ( z < 0.1)  , optically selected clusters, a matched control sample of non-BCGs, and a smaller sample of BCGs in X-ray selected clusters. We convolve the Sloan Digital Sky Survey images of the BCGs to match the resolution of the Galaxy Evolution Explorer ( GALEX ) data and we measure UV-optical colours in their inner and outer regions. We find that optically selected BCGs exhibit smaller scatter in optical colours and redder inner  NUV − r   colours than the control galaxies, indicating that they are a homogenous population with very little ongoing star formation. The BCGs in the X-ray selected cluster sample span a similar range in optical colours, but have bluer  NUV − r   colours. Among X-ray selected BCGs, those located in clusters with central cooling times of less than 1 Gyr are significantly bluer than those located in clusters where the central gas cooling times are long. Our main conclusion is that the location of a galaxy at the centre of its halo is not sufficient to determine whether or not it is currently forming stars. One must also have information about the thermodynamic state of the gas in the core of the halo.  相似文献   
73.
This study focused on clogging processes and on the benthic microalgal and meiofaunal assemblage in the sandy littoral zone of Lake Tegel, which are significantly involved in bank filtration, in a long-term. Our approach combined field studies and “in situ” experiments to highlight the structure of the biological active filter zone as well as the mechanisms and effects of clogging in the interstices that influence the infiltration process.Campaigns to measure “in situ” infiltration rates and hydraulic potential were conducted monthly from March 2004 to April 2005. Meiofaunal abundances and fine particulate organic matter (FPOM) were determined every 6 weeks in freeze cores down to depths of 50 cm. In parallel, concentrations of carbon, nitrogen and chlorophyll a were measured in samples of unfrozen sediment cores, that were divided in 1-cm steps down to depths of ≥10 cm. Similar sediment profiles were generated for analysis of colloidal carbohydrates, extracellular polymeric substances (EPS) and proteins between December 2005 and June 2006. Electron microscopy was used to visualize biofilm structure. Long-term experiments with natural FPOM and melamine resin particles as fluorescent tracers were performed to study “in situ” particle retention and transport, respectively. Additionally seston input was quantified during a 1-week period in April 2005.Infiltration rates showed a high temporal and spatial variability, but were not correlated with hydraulic conductivities as hydraulic gradients changed a lot. Likewise a correlation between infiltration rates and hydraulic potentials was not observed, indicating clogging processes. These are triggered to a high extend by biological compounds. In addition, seston input and intermittent gas intrusion are considered to reduce the hydraulic conductivity considerably. No significant “in situ” transport of inert natural fluorescent tracers was observed. However, a complete and permanent clogging of the sandy sediment does not occur, and daily infiltration rates of 0.7-27 L m−2 h−1 (mean 9 L m−2 h−1) guarantee a sufficient water supply by bank filtration for decades.  相似文献   
74.
A comparison study for the solar radiative flux above clouds is presented between the regional climate model system BALTEX integrated model system (BALTIMOS) and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite observations. For MODIS, an algorithm has been developed to retrieve reflected shortwave fluxes over clouds. The study area is the Baltic Sea catchment area during an 11-month period from February to December 2002. The intercomparison focuses on the variations of the daily and seasonal cycle and the spatial distributions. We found good agreement between the observed and the simulated data with a bias of the temporal mean of 13.6 W/m2 and a bias of the spatial mean of 35.5 W/m2. For summer months, BALTIMOS overestimates the solar flux with up to 90 W/m2 (20%). This might be explained by the insufficient representation of cirrus clouds in the regional climate model.  相似文献   
75.
1 TheincoherentscattertechniqueTheuseofincoherentscatterradarsasapowerfulground baseddiagnostictoolforstudyingthenear EarthspaceenvironmentbeganwiththefirsttheoreticalpredictionsbyGordon( 1 958) ,andthefirstobservationsbyBowles( 1 958)afewmonthslater .Thereares…  相似文献   
76.
As seen in many of the satellite images from the tsunami in the Indian Ocean which struck in 2004, there is a distinctive limit between an area with sand coverage, vegetation destruction, and soil erosion on the one hand, and the unaffected natural vegetation on the other. This distinction provides a good landmark to map the inundation width, delimited on the landward side by a trimline. In this study, older trimlines, dating back about 300?years, from tsunamis that occurred throughout the world were documented. We discuss the origin and chronology of trimline modification and extinction, both of which depend on local topography, rock type, and climate.  相似文献   
77.
Soil organic matter (SOM) is important for soil fertility and for the global C cycle. Previous studies have shown that during SOM formation no new compound classes are formed and that it consists basically of plant- and microorganism-derived materials. However, little data on the contribution from microbial sources are available. Therefore, we investigated previously in a model study the fate of C from 13C-labelled Gram-negative bacteria in soil (Kindler, R., Miltner, A. Richnow, H.H., Kästner, M., 2006. Fate of gram negative bacterial biomass in soil – mineralization and contribution to SOM. Soil Biology and Biochemistry 38, 2860–2870) and showed that 44% of the bulk 13C remained in the soil. Here we present the corresponding data on the fate of amino acids hydrolysed from proteins, which are the most abundant components of microbial biomass. After 224 days incubation, the label in the total amino acids in the soil amended with 13C-labelled cells decreased only to >95%. The total amino acids therefore clearly showed a lower turnover than the bulk 13C and a surprisingly stable concentration. Proteins therefore have to be considered as being stabilised in soil in dead, non-extractable biomass or cell fragments by known general stabilisation mechanisms. The label in the amino acids in a fraction highly enriched in living microbial biomass decreased to a greater extent, i.e. to 25% of the initially added amount. The amino acids removed from this fraction were redistributed via the microbial food web to non-living SOM. All amino acids in the microbial biomass were degraded at similar rates without a change in isotopic signature. The nuclear magnetic resonance (NMR) spectra of the soils were very similar and indicate that the residues of the degraded microbial biomass were very similar to those of the SOM and are a significant source for the formation of the SOM.  相似文献   
78.
International Journal of Earth Sciences - Detrital zircon provenance studies are an established tool to develop palaeogeographic models, mostly based on zircon of siliciclastic rocks and isotope...  相似文献   
79.
The LiDAR point clouds captured with airborne laser scanning provide considerably more information about the terrain surface than most data sources in the past. This rich information is not simply accessed and convertible to a high quality digital elevation model (DEM) surface. The aim of the study is to generate a homogeneous and high quality DEM with the relevant resolution, as a 2.5D surface. The study is focused on extraction of terrain (bare earth) points from a point cloud, using a number of different filtering techniques accessible by selected freeware. The proposed methodology consists of: (1) assessing advantages/disadvantages of different filters across the study area, (2) regionalization of the area according to the most suitable filtering results, (3) data fusion considering differently filtered point clouds and regions, and (4) interpolation with a standard algorithm. The resulting DEM is interpolated from a point cloud fused from partial point clouds which were filtered with multiscale curvature classification (MCC), hierarchical robust interpolation (HRI), and the LAStools filtering. An important advantage of the proposed methodology is that the selected landscape and datasets properties have been more holistically studied, with applied expert knowledge and automated techniques. The resulting highly applicable DEM fulfils geometrical (numerical), geomorphological (shape), and semantic quality properties.  相似文献   
80.
Episodic and localized illite mineralization is documented in the hydrothermally altered Soultz-sous-Forêts granite (Upper Rhine Graben, France). Separated grain-size fractions of altered granite and argillite vein samples contain mixtures of 2M1 and 1M trans-vacant illite varieties. The platy pseudohexagonal 2M1 illite phases dominate the vein fillings, whereas the 1M illite occurs largely as a fibrous pore-filling variety, which is particularly abundant in the granite matrix. Multiple phases of fluid injections into the granite body have resulted in different illite assemblages, each sample containing a mixture of polytype generations formed during different crystal growth events. On the basis of mineralogical and K–Ar isotopic constraints, the ages of these vein-mineralizing events are determined by plotting the K–Ar values of the various grain-size fractions against polytype abundance and the fitted volume-weighted crystallite thickness distributions. The results suggest a Permian age for the formation of the studied argillite veins, characterized by successive injections of hydrothermal fluids. Secondary episodes of illite crystallization occurred during Jurassic and Cretaceous (or even younger times) in both the veins and the granite matrix. There are indications that the polytype structure and composition of illite were strongly influenced by variations in fluid chemistry and the degree of fluid–rock interaction as the granite was progressively sealed during post-Variscan, episodic hydrothermal activity.  相似文献   
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