The reception of indirect signals, either in the form of non-line-of-sight (NLOS) reception or multipath interference, is a major cause of GNSS position errors in urban environments. We explore the potential of using dual-polarisation antenna technology for detecting and mitigating the reception of NLOS signals and severe multipath interference. The new technique computes the value of the carrier-power-to-noise-density (C/N0) measurements from left-hand circular polarised outputs subtracted from the right-hand circular polarised C/N0 counterpart. If this quality is negative, NLOS signal reception is assumed. If the C/N0 difference is positive, but falls below a threshold based on its lower bound in an open-sky environment, severe multipath interference is assumed. Results from two experiments are presented. Open-field testing was first performed to characterise the antenna behaviour and determine a suitable multipath detection threshold. The techniques were then tested in a dense urban area. Using the new method, two signals in the urban data were identified as NLOS-only reception during the occupation period at one station, while the majority of the remaining signals present were subject to a mixture of NLOS reception and severe multipath interference. The point positioning results were dramatically improved by excluding the detected NLOS measurements. The new technique is suited to a wide range of static ground applications based on our results. 相似文献
Tephra shards for electron probe microanalysis are most efficiently extracted from peat using acid digestion, which removes organic material that hinders density separation methods. However, strong acids are known to alter glass chemical compositions, and several studies have examined how acid digestion affects rhyolitic volcanic glass. The focus on rhyolitic tephra in these studies leaves considerable uncertainty, as the dissolution rates of natural glasses (including tephra) are determined by the chemical composition and surface area/volume ratio, both of which vary in tephra deposits. Here, we use duplicate samples of basaltic, trachydacitic and rhyolitic tephra to examine physical and geochemical alteration following acid digestion. Scanning electron microscope imagery reveals no discernible degradation of glass surfaces, and electron probe microanalysis results from duplicate samples are statistically indistinguishable. These findings suggest the acid digestion protocol for organic peats does not significantly alter glass geochemistry regardless of shard morphologies or geochemical compositions. 相似文献
Water resources in semi-arid regions like the Mediterranean Basin are highly vulnerable because of the high variability of weather systems. Additionally, climate change is altering the timing and pattern of water availability in a region where growing populations are placing extra demands on water supplies. Importantly, how reservoirs and dams have an influence on the amount of water resources available is poorly quantified. Therefore, we examine the impact of reservoirs on water resources together with the impact of climate change in a semi-arid Mediterranean catchment. We simulated the Susurluk basin (23.779-km2) using the Soil and Water Assessment Tool (SWAT) model. We generate results for with (RSV) and without reservoirs (WRSV) scenarios. We run simulations for current and future conditions using dynamically downscaled outputs of the MPI-ESM-MR general circulation model under two greenhouse gas relative concentration pathways (RCPs) in order to reveal the coupled effect of reservoir and climate impacts. Water resources were then converted to their usages – blue water (water in aquifers and rivers), green water storage (water in the soil) and green water flow (water losses by evaporation and transpiration). The results demonstrate that all water resources except green water flow are projected to decrease under all RCPs compared to the reference period, both long-term and at seasonal scales. However, while water scarcity is expected in the future, reservoir storage is shown to be adequate to overcome this problem. Nevertheless, reservoirs reduce the availability of water, particularly in soil moisture stores, which increases the potential for drought by reducing streamflow. Furthermore, reservoirs cause water losses through evaporation from their open surfaces. We conclude that pressures to protect society from economic damage by building reservoirs have a strong impact on the fluxes of watersheds. This is additional to the effect of climate change on water resources. 相似文献
Automatic change detection of land cover features using high-resolution satellite images, is a challenging problem in the field of intelligent remote sensing data interpretation, and is becoming more and more effective for its applications viz. urban planning and monitoring, disaster assessment etc. In the present study, a change in detection approach based on the image morphology that analyses change in the local image grids is proposed. In this approach, edges from both the images are extracted and grid wise comparison is made by probabilistic thresholding and power spectral density analysis for identifying change area. One of the advantages of the proposed methodology is that the temporal images used in the change analysis need not be radiometrically corrected as analysis is based on edge extractions. The grid-based analysis further reduces the error, which might have been introduced by image mis-registration. The proposed methodology is validated by finding the temporal changes in the linear land cover features in parts of Kolkata city, India using three different image data-sets from LISS IV, Cartosat-1 and Google earth having varied spatial resolutions of 5.8 m, 2.5 m and about 1 m, respectively. The overall accuracy in identifying changes is found to be 64.82, 73.86 and 80.93% for LISS IV, Cartosat-1 and Google earth data-set, respectively. 相似文献
Despite many years of study, the processes involved in the development of the continental margin of southern Africa and the distinctive topography of the hinterland remain poorly understood. Previous thermochronological studies carried out within a monotonic cooling framework have failed to take into account constraints provided by Mesozoic sedimentary basins along the southern margin. We report apatite fission track analysis and vitrinite reflectance data in outcrop samples from the Late Jurassic to Early Cretaceous sedimentary fill of the Oudtshoorn, Gamtoos and Algoa Basins (Uitenhage Group), as well as isolated sedimentary remnants further west, plus underlying Paleozoic rocks (Cape Supergroup) and Permian‐Triassic sandstones from the Karoo Supergroup around the Great Escarpment. Results define a series of major regional cooling episodes. Latest Triassic to Early Jurassic cooling which began between 205 and 180 Ma is seen dominantly in basement flanks to the Algoa and Gamtoos Basins. This episode may have affected a wider region but in most places any effects have been overprinted by later events. The effects of Early Cretaceous (beginning between 145 and 130 Ma) and Early to mid‐Cretaceous (120–100 Ma) cooling are both delimited by major structures, while Late Cretaceous (85–75 Ma) cooling appears to have affected the whole region. These cooling events are all interpreted as dominantly reflecting exhumation. Higher Late Cretaceous paleotemperatures in samples from the core of the Swartberg Range, coupled with evidence for localised Cenozoic cooling, are interpreted as representing Cenozoic differential exhumation of the mountain range. Late Cretaceous paleotemperatures between 60°C and 90°C in outcropping Uitenhage Group sediments from the Oudtshoorn, Gamtoos and Algoa Basins require burial by between 1.2 and 2.2 km prior to Late Cretaceous exhumation. Because these sediments lie in depositional contact with underlying Paleozoic rocks in many places, relatively uniform Late Cretaceous paleotemperatures across most of the region, in samples of both basin fill and underlying basement, suggest the whole region may have been buried prior to Late Cretaceous exhumation. Cenozoic cooling (beginning between 30 and 20 Ma) is focussed mainly in mountainous regions and is interpreted as representing denudation which produced the modern‐day relief. Features such as the Great Escarpment are not related to continental break up, as is often supposed, but are much younger (post‐30 Ma). This history of post‐breakup burial and subsequent episodic exhumation is very different from conventional ideas of passive margin evolution, and requires a radical re‐think of models for development of continental margins. 相似文献
Igneous and sedimentary rocks recently dredged and cored from the steep western slope of the Beata Ridge provide important data on the composition, age and details of crustal evolution of the rock-types responsible for recorded compressional wave velocities. The sedimentary rock samples also provide new data concerning the age and depositional environment of overlying sedimentary reflectors.
The deepest (4,100 m) dredge haul contains deeply weathered coarsegrained igneous rocks. Nine other hauls, distributed between 4,000–2,300 m, contain holocrystalline basalts and diabases. The compressional wave velocity of air-dried samples of two holocrystalline basalts and a diabase at atmospheric pressure ranges from 5.0–5.6 km/sec. Sampling in depths less than 2,300 m shows that the crest of the Beata Ridge is capped by Quaternary deposits underlain by consolidated carbonate sediment of at least Middle Eocene age. The faunal assemblages of the Mid-Eocene samples are the product of normal accumulation in a shallow shelf environment.
The dredging results coupled with previously published seismic reflection and refraction data, suggest that the 5.4–5.7 km/sec crust is composed of a layer of basalt and diabase which outcrops below 2,300 m, on a fault-generated escarpment that was produced in the Late Cretaceous-Early Tertiary. The shallow shelf samples of Eocene age indicate that the Beata Ridge was higher in the Early Tertiary and has subsided subsequently to its present depth. 相似文献