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251.
252.
The Center for Orbit Determination in Europe (CODE) has been involved in the processing of combined GPS/GLONASS data during the International GLONASS Experiment (IGEX). The resulting precise orbits were analyzed using the program SORBDT. Introducing one satellites positions as pseudo-observations, the program is capable of fitting orbital arcs through these positions using an orbit improvement procedure based on the numerical integration of the satellites orbit and its partial derivative with respect to the orbit parameters. For this study, the program was enhanced to estimate selected parameters of the Earths gravity field. The orbital periods of the GPS satellites are —in contrast to those of the GLONASS satellites – 2:1 commensurable (P
Sid:P
GPS) with the rotation period of the Earth. Therefore, resonance effects of the satellite motion with terms of the geopotential occur and they influence the estimation of these parameters. A sensitivity study of the GPS and GLONASS orbits with respect to the geopotential coefficients reveals that the correlations between different geopotential coefficients and the correlations of geopotential coefficients with other orbit parameters, in particular with solar radiation pressure parameters, are the crucial issues in this context. The estimation of the resonant geopotential terms is, in the case of GPS, hindered by correlations with the simultaneously estimated radiation pressure parameters. In the GLONASS case, arc lengths of several days allow the decorrelation of the two parameter types. The formal errors of the estimates based on the GLONASS orbits are a factor of 5 to 10 smaller for all resonant terms.
AcknowledgmentsThe authors would like to thank all the organizations involved in the IGS and the IGEX campaign, in particular those operating an IGS or IGEX observation site and providing the indispensable data for precise orbit determination. 相似文献
253.
Metaheuristic techniques, which are based on ideas of Artificial Intelligence, are among the best methods for solving computationally the GPS surveying network problem. In this paper, the ant colony optimization metaheuristic, which is inspired by the behavior of real ant colonies, is developed to efficiently provide a general framework for optimizing GPS surveying networks. In this framework, a set of ants co-operate together using an indirect communication procedure to find good GPS observation schedules. A GPS surveying network can be defined as a set of stations, co-ordinated by a series of sessions formed by placing receivers on the stations. The problem is to search for the best order in which to observe these sessions to give the best schedule at minimum cost. Computational results obtained by applying the proposed technique on several networks, with known and unknown optimal schedules, prove the effectiveness of the proposed metaheuristic technique to solve the GPS surveying network problem. 相似文献
254.
In an attempt to reduce the threat of global warming, it has been proposed that the rise of atmospheric carbon dioxide concentrations be reduced by the ocean disposal of CO2 from the flue gases of fossil fuel-fired power plants. The release of large amounts of CO2 into mid or deep ocean waters will result in large plumes of acidified seawater with pH values ranging from 6 to 8. In an effort to determine whether these CO2-induced pH changes have any effect on marine nitrification processes, surficial (euphotic zone) and deep (aphotic zone) seawater samples were sparged with CO2 for varying time durations to achieve a specified pH reduction, and the rate of microbial ammonia oxidation was measured spectrophotometrically as a function of pH using an inhibitor technique. For both seawater samples taken from either the euphotic or aphotic zone, the nitrification rates dropped drastically with decreasing pH. Relative to nitrification rates in the original seawater at pH 8, nitrification rates were reduced by ca. 50% at pH 7 and more than 90% at pH 6.5. Nitrification was essentially completely inhibited at pH 6. These findings suggest that the disposal of CO2 into mid or deep oceans will most likely result in a drastic reduction of ammonia oxidation rates within the pH plume and the concomitant accumulation of ammonia instead of nitrate. It is unlikely that ammonia will reach the high concentration levels at which marine aquatic organisms are known to be negatively affected. However, if the ammonia-rich seawater from inside the pH plume is upwelled into the euphotic zone, it is likely that changes in phytoplankton abundance and community structure will occur. Finally, the large-scale inhibition of nitrification and the subsequent reduction of nitrite and nitrate concentrations could also result in a decrease of denitrification rates which, in turn, could lead to the buildup of nitrogen and unpredictable eutrophication phenomena. Clearly, more research on the environmental effects of ocean disposal of CO2 is needed to determine whether the potential costs related to marine ecosystem disturbance and disruption can be justified in terms of the perceived benefits that may be achieved by temporarily delaying global warming. 相似文献
255.
256.
遥感技术在新一轮国土资源大调查中的进展与作用 总被引:6,自引:2,他引:4
随着现代科学技术的飞速发展,遥感技术的应用领域越来越广泛,特别是在国土资源的本底调查、环境评价和科学规划等研究应用中,已成为空间多源信息获取的有力手段,为在国民经济发展中合理开发利用自然资源提供了科学的数据源和信息服务。作者在本文中就近年来遥感技术在遥感数据获取、遥感图像信息提取与分析等方面的进展作了简要阐述,并针对新一轮国土资源大调查的性质和任务,探讨了遥感、地理信息系统在新一轮国土资源大调查中的作用。 相似文献
257.
Peter Kammeyer 《Celestial Mechanics and Dynamical Astronomy》2000,77(4):241-272
This paper discusses a UT1-like quantity, UTGPS, determined daily from Earth-referenced GPS satellite orbits from the International GPS Service (IGS). For each satellite considered, the observed relation between the satellite's IGS orbit and a model of its orbit plane in inertial space is used to estimate UT1. This modeled orbit plane is initialized using the satellite's IGS orbit and the actual UT1 at an initial time. It is then propagated using standard models of gravitational forces and an empirical model representing the orbit-normal radiation pressure observed during several years of in-flight experience with the satellite. To estimate UT1, an a-priori transformation from terrestrial to true-of-date celestial coordinates is applied to the satellite's IGS orbit. The geocentric angular deviations of points of the resulting transformed orbit from the modeled orbit plane are analyzed, giving the angle between the ascending nodes of the satellite's transformed and modeled orbit planes. To this observed angle between nodes, converted to a UT1 difference, is added the a-priori UT1 value used in the transformation. From the result is subtracted a model of the angle, again converted to a UT1 difference, between the ascending nodes of the actual and modeled orbit planes. The final result is the estimate of UT1 from this satellite, and the median of the UT1 estimates from all satellites considered is UTGPS. The root-mean-square difference between UTGPS-UT1 at the beginning and at the end of an interval of one to four weeks is approximately 30 s times the square root of the interval's duration in weeks.This revised version was published online in October 2005 with corrections to the Cover Date. 相似文献
258.
水文循环的生物圈方面(BAHC计划)研究进展 总被引:12,自引:3,他引:9
在对国际地圈生物圈计划 (IGBP)的核心项目“水文循环的生物圈方面 (BAHC)”的基本目标和具体研究内容阐述的基础上 ,对 BAHC计划的国际、国内研究进展进行了详细介绍 ,并结合当前 BAHC计划的研究动态 ,对未来的 BAHC研究新的主题和内容进行了探讨。 相似文献
259.
Introduction Human-induced environmental change in China has led to different consequences due to various causes. They conver certain regions and places where their impacts may threaten the long-term or even the short-term sustainability of human-environmental relationships. Designating such areas as "critical environmental situations," Russian Geographers have developed "red data maps" showing their locations[1]. Yet, there are no fully developed conceptualizations of environmental critical… 相似文献
260.
Deep saline aquifers are one of the most suitable geologic formations for carbon sequestration. The linear and global stability analysis of the time-dependent density-driven convection in deep saline aquifers is presented for long-term storage of carbon dioxide (CO2). The convective mixing that can greatly accelerate the CO2 dissolution into saline aquifers arises because the density of brine increases upon the dissolution of CO2 and such a density difference may induce instability. The effects of anisotropic permeability on the stability criteria, such as the critical time for the appearance of convective phenomena and the critical wavelength of the most unstable perturbation, are investigated with linear and global stability analysis. The linear stability analysis provides a sufficient condition for instability while the global stability analysis yields a sufficient condition for stability. The results obtained from these two approaches are not exactly the same but show a consistent trend, both indicating that the anisotropic system becomes more unstable when either the vertical or horizontal permeability increases. 相似文献