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71.
72.
为了提高我国卫星导航系统GEO卫星定轨能力,针对我国卫星导航系统所特有的新数据和新方法,利用CAPS系统的C波段转发式测距数据进行了单GEO卫星定轨实验.在分析该方法技术特点的基础上,从定轨残差和轨道重叠弧段比较等方面,对定轨结果进行了分析.由于设备转发时延是影响该方法定轨精度的重要因素,因此同时还分析了系统偏差对定轨精度的影响,得出了一些有益的结论. 相似文献
73.
北斗GEO卫星轨道算法研究 总被引:1,自引:0,他引:1
通过实际采集的北斗广播星历数据分析了GEO广播星历的相关轨道根数特性,北斗GEO卫星广播星历拟合坐标旋转法,给出了适用于北斗GEO卫星的轨道计算方法,通过理论分析和试验验证了北斗GEO法的可用性与正确性。 相似文献
74.
There is an urgent need for the development of a method that can undertake rapid, effective, and
accurate monitoring and identification of fog by satellite remote sensing, since heavy fog can cause
enormous disasters to China’s national economy and people's lives and property in the urban and coastal
areas. In this paper, the correlative relationship between the reflectivity of land surface and clouds in
different time phases is found, based on the analysis of the radiative and satellite-based spectral
characteristics of fog. Through calculation and analyses of the relative variability of the reflectivity in the
images, the threshold to identify quasi-fog areas is generated automatically. Furthermore, using the
technique of quick image run-length encoding, and in combination with such practical methods as
analyzing texture and shape fractures, smoothness, and template characteristics, the automatic
identification of fog and fog-cloud separation using meteorological satellite remote sensing images are
studied, with good results in application. 相似文献
75.
76.
S. Alan Stern 《Icarus》2009,199(2):571-573
In this Note, I present first-order scaling calculations to examine the efficacy of impacts by Kuiper Belt debris in causing regolith exchange between objects in the Pluto system. It is found that ejecta can escape Nix and Hydra with sufficient velocity to reach one another, as well as Charon, and even Pluto. The degree of ejecta exchanged between Nix and Hydra is sufficient to cover these bodies with much more material than is required for photometrically change. In specific, Nix and Hydra may have exchanged as up to 10s of meters of regolith, and may have covered Charon to depths up to 14 cm with their ejecta. Pluto is likely unaffected by most Nix and Hydra ejecta by virtue of a combination of dynamical shielding from Charon and Pluto's own annual atmospheric frost deposition cycle. As a result of ejecta exchange between Nix, Hydra, and Charon, these bodies are expected to evolve their colors, albedos, and other photometric properties to be self similar. These are testable predictions of this model, as is the prediction that Nix and Hydra will have diameters near 50 km, owing to having a Charon-like albedo induced by ejecta exchange. As I discuss, this ejecta exchange process can also be effective in many KBOs and asteroids with satellites, and may be the reason that very many KBO and asteroid satellite systems have like colors. 相似文献
77.
We have reanalyzed the high-resolution spectrum of Titan between 2.87 and 3.12 μm observed with NIRSPEC/Keck II on 2001 Nov. 21 in southern summer, using updated CH3D and C2H6 line-by-line models. From new synthetic spectra, we identify all but a few of the previously unidentified significant absorption spectral features in this wavelength range as due to these two species, both of which had been previously detected by Voyager and ground-based observations at other wavelengths. We also derive opacities and reflectivities of haze particles as functions of altitude for the 2.87-2.92 μm wavelength range, where Titan's atmosphere is partially transparent down to the surface. The extinction per unit altitude is observed to increase from 100 km (∼8 mbar) toward lower altitude. The derived total optical depth is approximately 1.1 for the 2.87-2.92 μm range. At wavelengths increasing beyond 2.92 μm the haze layers become much more optically thick, and the surface is rapidly hidden from view. These conclusions apply to equatorial and southern-temperate regions on Titan, excluding polar regions. We also find it unlikely that there is a large enhancement of the tropospheric CH4 mole fraction over the value reported from analysis of the Huygens/GCMS observations. 相似文献
78.
Using Cassini images, we examine the faint material along the orbits of Methone, Anthe and Pallene, three small moons that reside between the orbits of Mimas and Enceladus. A continuous ring of material covers the orbit of Pallene; it is visible at extremely high phase angles and appears to be localized vertically to within ±25 km of Pallene's inclined orbit. By contrast, the material associated with Anthe and Methone appears to lie in longitudinally confined arcs. The Methone arc extends over ∼10° in longitude around the satellite's position, while the Anthe arc reaches ∼20° in length. The extents of these arcs are consistent with their confinement by nearby corotation eccentricity resonances with Mimas. Anthe has even been observed to shift in longitude relative to its arc in the expected manner given the predicted librations of the moon. 相似文献
79.
We present a systematic survey for satellites of Venus using the Baade-Magellan 6.5 m telescope and IMACS wide-field CCD imager at Las Campanas observatory in Chile. In the outer portions of the Hill sphere the search was sensitive to a limiting red magnitude of about 20.4, which corresponds to satellites with radii of a few hundred meters when assuming an albedo of 0.1. In the very inner portions of the Hill sphere scattered light from Venus limited the detection to satellites of about a kilometer or larger. Although several main belt asteroids were found, no satellites (moons) of Venus were detected. 相似文献
80.
On corotation torques, horseshoe drag and the possibility of sustained stalled or outward protoplanetary migration 总被引:1,自引:0,他引:1
S.-J. Paardekooper J. C. B. Papaloizou 《Monthly notices of the Royal Astronomical Society》2009,394(4):2283-2296
We study the torque on low-mass protoplanets on fixed circular orbits, embedded in a protoplanetary disc in the isothermal limit. We consider a wide range of surface density distributions including cases where the surface density increases smoothly outwards. We perform both linear disc response calculations and non-linear numerical simulations. We consider a large range of viscosities, including the inviscid limit, as well as a range of protoplanet mass ratios, with special emphasis on the co-orbital region and the corotation torque acting between disc and protoplanet.
For low-mass protoplanets and large viscosity, the corotation torque behaves as expected from linear theory. However, when the viscosity becomes small enough to enable horseshoe turns to occur, the linear corotation torque exists only temporarily after insertion of a planet into the disc, being replaced by the horseshoe drag first discussed by Ward. This happens after a time that is equal to the horseshoe libration period reduced by a factor amounting to about twice the disc aspect ratio. This torque scales with the radial gradient of specific vorticity, as does the linear torque, but we find it to be many times larger. If the viscosity is large enough for viscous diffusion across the co-orbital region to occur within a libration period, we find that the horseshoe drag may be sustained. If not, the corotation torque saturates leaving only the linear Lindblad torques. As the magnitude of the non-linear co-orbital torque (horseshoe drag) is always found to be larger than the linear torque, we find that the sign of the total torque may change even for mildly positive surface density gradients. In combination with a kinematic viscosity large enough to keep the torque from saturating, strong sustained deviations from linear theory and outward or stalled migration may occur in such cases. 相似文献
For low-mass protoplanets and large viscosity, the corotation torque behaves as expected from linear theory. However, when the viscosity becomes small enough to enable horseshoe turns to occur, the linear corotation torque exists only temporarily after insertion of a planet into the disc, being replaced by the horseshoe drag first discussed by Ward. This happens after a time that is equal to the horseshoe libration period reduced by a factor amounting to about twice the disc aspect ratio. This torque scales with the radial gradient of specific vorticity, as does the linear torque, but we find it to be many times larger. If the viscosity is large enough for viscous diffusion across the co-orbital region to occur within a libration period, we find that the horseshoe drag may be sustained. If not, the corotation torque saturates leaving only the linear Lindblad torques. As the magnitude of the non-linear co-orbital torque (horseshoe drag) is always found to be larger than the linear torque, we find that the sign of the total torque may change even for mildly positive surface density gradients. In combination with a kinematic viscosity large enough to keep the torque from saturating, strong sustained deviations from linear theory and outward or stalled migration may occur in such cases. 相似文献