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621.
The celestial mechanics approach: theoretical foundations 总被引:4,自引:3,他引:1
Gravity field determination using the measurements of Global Positioning receivers onboard low Earth orbiters and inter-satellite
measurements in a constellation of satellites is a generalized orbit determination problem involving all satellites of the
constellation. The celestial mechanics approach (CMA) is comprehensive in the sense that it encompasses many different methods
currently in use, in particular so-called short-arc methods, reduced-dynamic methods, and pure dynamic methods. The method
is very flexible because the actual solution type may be selected just prior to the combination of the satellite-, arc- and
technique-specific normal equation systems. It is thus possible to generate ensembles of substantially different solutions—essentially
at the cost of generating one particular solution. The article outlines the general aspects of orbit and gravity field determination.
Then the focus is put on the particularities of the CMA, in particular on the way to use accelerometer data and the statistical
information associated with it. 相似文献
622.
Detailed measurements of three-dimensional beach cusp morphology were made on a steep gradient, low energy, microtidal beach in Perth, Western Australia. During the field campaign a variety of wave conditions and tidal ranges were experienced, and these differing hydrodynamic conditions were reflected in a consistent pattern of morphological changes to the beach cusp system. A useful parameter to delineate between trends of cusp destruction and re-formation appeared to be the surf similarity parameter ξ = tan β/√I0/L0, where H0 is offshore wave height, L0 is deep water wave length and tan β is beach gradient. For ξ < 1·2 the beach cusps were planed off, whereas cusp morphology was enhanced when ξ > 1·2. A small storm was experienced at the start of the field campaign period and resulted in considerable erosion of the beach face. The cusp morphology across the lower beachface was destroyed, but a subtle remnant of the pre-storm cusp morphology was preserved on the upper beachface. When cusps reformed after the storm, under the influence of declining wave conditions, they appeared at the same location and with the same dimensions as the pre-storm cusp morphology. Hence, it is considered that the cusp re-formation was controlled more by the antecedent morphology than the hydrodynamic conditions. This indicates that positive feedback between swash hydrodynamics and beachface morphology, necessary to form beach cusps, does not require a large variation in relief. © 1997 John Wiley & Sons, Ltd. 相似文献
623.
Numerical integrations of 99 orbits centered on that of comet P/Scotti (P/2000 Y3), and of the nominal orbit, were made 4000 days backwards in time, and 73000 days into the future. The integrations show that this comet has been transferred into its present orbit as recently as 1998. The future orbital evolution indicates a stable period for almost 150 years, when another close encounter with Jupiter may lead to further drastic changes of the present orbit. 相似文献