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Different types of GPS clock and orbit data provided by the International GPS Service (IGS) have been used to assess the accuracy
of rapid orbit determination for satellites in low Earth orbit (LEO) using spaceborne GPS measurements. To avoid the need
for reference measurements from ground-based reference receivers, the analysis is based on an undifferenced processing of
GPS code and carrier-phase measurements. Special attention is therefore given to the quality of GPS clock data that directly
affects the resulting orbit determination accuracy. Interpolation of clock data from the available 15 min grid points is identified
as a limiting factor in the use of IGS ultra-rapid ephemerides. Despite this restriction, a 10-cm orbit determination accuracy
can be obtained with these products data as demonstrated for the GRACE-B spacecraft during selected data arcs between 2002
and 2004. This performance may be compared with a 5-cm orbit determination accuracy achievable with IGS rapid and final products
using 5 min clock samples. For improved accuracy, high-rate (30 s) clock solutions are recommended that are presently only
available from individual IGS centers. Likewise, a reduced latency and more frequent updates of IGS ultra-rapid ephemerides
are desirable to meet the requirements of upcoming satellite missions for near real-time and precise orbit determination. 相似文献
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Metamorphic evolution and geochemistry of kyanite eclogites in central Rhodope,northern Greece 总被引:1,自引:0,他引:1
In central Rhodope of northern Greece, kyanite eclogites were discovered in the area of Thermes. They are strongly overprinted and exhibit a multi-stage development of minerals and mineral assemblages formed during successive stages of the exhumation. The initial high-pressure assemblage was garnet+omphacite+kyanite+zoisite+phengite+rutile+quartz. Corundum, Fe-Mg-spinel, sapphirine and högbomite occur as products of a first, high-temperature overprint, still at high pressures, whereas various symplectites [corundum-plagioclase (pl), spinel-pl, sapphirine-pl, clinopyroxene-pl, biotite-pl, amphibole-pl] grew during subsequent stages of the exhumation. Diablastic amphibole+plagioclase formed as end-products of the amphibolitization. According to geochemical data, the protoliths of the kyanite eclogites were basalts to basaltic andesites with “volcanic arc” affinities. For the high-pressure stage of metamorphism, minimum PT conditions were around 19 kbar, 700°C, while for the initial stages of the overprint, high-pressure granulite-facies conditions prevailed (T>800°C, at P>15 kbar). The PT conditions of the amphibolite facies were 8–11 kbar, 580–690°C. The kyanite eclogites of Thermes record the highest temperatures of metamorphism within the whole of Rhodope. 相似文献
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Feeding habits of harp and hooded seals in drift ice waters along the east coast of Greenland in summer and winter 总被引:2,自引:0,他引:2
Results of analyses of stomach and intestinal contents from hooded ( Cystophora cristata ) and harp ( Phoca groenlandica ) seals captured in the pack ice belt of the Greenland Sea in summer (July-August) in 2000 and winter (February-March) in 2001 revealed that the diet of both species were comprised of relatively few prey taxa. Pelagic amphipods of the genus Parathemisto , the squid Gonatus fabricii , polar cod ( Boreogadus saida ) and capelin ( Mallotus villosus ) constituted 63-99% of the observed diet biomass in both seal species, irrespective of sampling period, but their relative contribution to the diet varied both with species and sampling period/area. For hooded seals, G. fabricii and capelin were the dominant food items in winter 2001, but the summer 2000 diet comprised a mixture of this squid and polar cod. Parathemisto was most important for the harp seals during summer 2000; in winter 2001 the contribution from krill and capelin were comparable to that of Parathemisto . Multivariate analyses revealed differences in the intestinal contents of hooded and harp seals in areas where the two species' occurrence spatially overlapped. Different foraging depths of the two species may have contributed to the observed differences in diets. 相似文献