共查询到18条相似文献,搜索用时 93 毫秒
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双频GPS接收机天线相位中心的测定 总被引:1,自引:0,他引:1
GPS接收机天线的相位中心,与厂家所确定的位置有偏差,在高精度定位测量中,是不容忽视的。讨论了用“旋转天线法”绘制GPS接收天线的相对相位方向图,来确定天线相位中心的原理方法.使用这种方法,天线相位中心位置的测定精度为1.5mm左右. 相似文献
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紫金山天文台13:7米射电望远镜天线表面精度的首次射电全息测量 总被引:2,自引:0,他引:2
1992年1月,在紫金山天文台13.7米毫米波望远镜上进行了用射电全息术的相位恢复方法测量天线表面精度的尝试。利用望远镜的22GHz系统,用强水脉泽源ORION-KL作为信号源测量了天线的聚焦和偏焦方向图,采用Misell算法获得了天线口面上的相位分布,由此得到的天线表面相对于理想抛物面偏差的均方根值与1989年用经典的经纬仪带尺方法测量的结果基本相符。 相似文献
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王锦清 《中国科学院上海天文台年刊》2008,(1)
天线的远场辐射方向图(幅度图)在实际天线测量中应用极其普遍,而在一般的资料中很少论及相位特性以及相位和幅度的相互关系,该文对远场的相位以及相位和幅度之间的相互影响作了比较深入的分析,在该文中比较详细地分析了远场相位的理论与实际偏差,及引起这种相位误差的来源和测量方式,这对天线测量及评定天线的性能具有一定的理论和实际参考意义,该文也给出了佘山25m天线的测量结果。由于射电天文望远镜通常为圆形,且工作于信号的远场区域,该文就远场区的表达式在圆形口径面的情况下用标量衍射的手段作了一定的推导。 相似文献
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《天文研究与技术》2016,(3)
明安图射电频谱日像仪(Mingantu Ultrawide Sp Ectral Radioheliograph,MUSER)是新一代具有高时间、高空间、高频率分辨率的太阳专用射电望远镜,采用综合孔径原理成像,所以幅度和相位是决定最后成图质量的关键因素。天线的相位方向图会影响日像仪输出的幅度和相位,根据日像仪的馈源设计和综合孔径原理,针对明安图射电频谱日像仪天线数目多,且为户外环境,根据天文观测须经常测试天线性能的特点,给出了基于相关结果测量日像仪天线相位方向图的方法,该方法可以直接通过日像仪的相关输出结果高效准确地得到天线的相位方向图。对MUSER-I天线的相位方向图进行了测量和分析,同时分析了天线相位方向图对日像仪成像的影响,为得到高质量的太阳图像提供了参考和保障。 相似文献
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《Chinese Astronomy and Astrophysics》2022,46(4):500-514
The ring focus antenna has special electromagnetic characteristics and application fields. The phase error of ring focus antenna is analyzed theoretically and simulatedly. The phase error caused by the position deviation of the feed and the subreflector, the compensation relationship between the main reflector and subreflector, and the optical path difference caused by the operation of the antenna in holographic measurement are derived. The results will provide theoretical basis and reference for accurate surface measurement and compensation of the ring focus antenna. 相似文献
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M. Kobrick 《Earth, Moon, and Planets》1976,15(1-2):83-89
The offset of the center of mass of the Moon from its center of figure together with moment of inertia differences are explainable by a lunar crust of randomly varying thickness. The necessity of postulating a method of preferential material transport into a particular lunar hemisphere to explain the lunar asymmetry is eliminated.This paper represents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under Contract No. NAS 7-100, sponsored by the National Aeronautics and Space Administration. 相似文献
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天线有限元模型与实际天线往往存在结构参数误差和材料参数误差等,使有限元模型不能真实反映实际天线结构在各种载荷下的变形特性.针对亚毫米波天线,提出了亚毫米波天线有限元模型参数修正复合优化方法.该方法包括3步,第1步建立天线参数化有限元模型,并进行参数灵敏度分析;第2步,以第1步中得到的高灵敏度参数为优化变量建立优化函数,以初始模型与假想实际天线变形特性的差值最小为目标进行优化;第3步,基于零阶优化法和一阶优化法对天线有限元模型进行复合优化,使天线有限元模型逐渐逼近实际天线.为验证所提出参数修正优化方法的可行性,针对一台1.2 m亚毫米波天线,将对其进行假想实验获得的变形特性作为优化目标,分别对多个材料参数在不同的载荷工况下进行优化,结果表明天线有限元模型经过参数优化后与假想实际天线一致,证明了所提出的亚毫米波天线有限元模型参数优化修正方法可行,并在重力载荷下对修正优化后的天线模型的变形规律进行分析.这种优化方法可以推广应用到大口径亚毫米波天线的有限元模型修正中. 相似文献
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Eastern Aphrodite Terra and Western Aphrodite form an altimetrically prominent 14,000 km long part of the equatorial highlands on Venus. Several parallel linear discontinuities striking northwest across the general east-west regional strike of the highlands are mapped in the altimetric and radar image data of Eastern Aphrodite and identified on the basis of abrupt termination of rift-like central chasma, offset and segmentation of the center of the highlands, and radar image discontinuities in the lowlands to the north. These characteristics are similar to those of linear discontinuities previously mapped in Western Aphrodite in terms of length, orientation, and influence on the central highlands and adjacent lowlands.Altimetric profiles in directions parallel to the discontinuities are regionally symmetric, more ridge-like in Eastern Aphrodite compared to the plateau-dominated form of topography in Western Aphrodite, and are characterized by alternating paired ridge-and-trough forms near their crests and on their flanks. By mapping the center of symmetry in multiple profiles, the prominent segmentation of the highland is shown to be imparted by an offset of the regional symmetry along the mapped discontinuities. These characteristics are morphologically similar to several of the large-scale characteristics of divergent plate boundaries of Earth, including mid-ocean rise crests and rifts, offset at fracture zones and transform faults, and symmetric thermal boundary layer topography.The altitude of the surface in profiles parallel to the discontinuities decreases as the square root of distance from the symmetry axes and with a slope similar to that predicted for thermal boundary layer topography associated with rates of divergence on Venus of ~ 1 ± 0.5 cm/yr. In order to test the hypothesis that the linear discontinuities are analogous to fracture zones, the predicted altitude of the surface at great distance from the centers of symmetry of the central highland and in directions across the discontinuities was calculated on the basis of a thermal boundary layer topography model with offset of altimetric symmetry at each discontinuity. Similarity of observed Arecibo high-resolution altimetric profiles across the discontinuities with that calculated for thermal boundary layer topography offset by transform faults reveals that in terms of the sense and magnitude of regional steps in altimetry across discontinuities and the altitude of the surface, Eastern Aphrodite is similar to the known characteristics of crustal spreading at divergent boundaries. The plateau-like form of Western Aphrodite and the ridge-like form of Eastern Aphrodite are analogous respectively to the difference between areas of anomalous (Iceland) and normal crustal production along rise crests on Earth. Estimates of volumetric differences in crustal production in the environment of Venus and as it would be influenced by differences in mantle temperature beneath Western and Eastern Aphrodite imply that Eastern Aphrodite represents normal crustal production. On this basis, Western Aphrodite may be characterized by a mantle temperature that is warmer than the mantle beneath Eastern Aphrodite Terra, perhaps in association with deep convective mantle upwelling.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT. Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena). 相似文献
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Evidence of three kinds indicates a lunar compositional asymmetry: (1) mare basalts are much more abundant on the near side; (2) the incompatible rich KREEP component is mainly observed in near-side soils; and (3) materials on the far side are less dense than those of the near side, as indicated by the 2-km offset between the center of mass and center of figure. Recent models to explain the 2-km offset are based on near-side-far-side differences in the thickness of crustal units. The most widely discussed model calls for a thickness of anorthosite ~ 24 km greater on the far side than on the near side, but no satisfactory method of generating such a large difference has been proposed. We suggest that much of the offset reflects longitudinal differences in mantle composition primarily resulting from earlier (or more rapid) crystallization of the magma ocean on what is now the far-side hemisphere. As a result, the far-side mantle would be more magnesian and thus less dense than the near-side mantle. Differences in the amount of anorthosite or the amount of crustal porosity probably make relatively small contributions to the offset. We have evaluated four scenarios: (a) If the anorthositic crust initially formed a floating continent over what is now the near side, this would have provided thermal insulation that would have reduced the near-side cooling rate. (b) Crystallization of the magma ocean while the Moon was near the Earth would have resulted in heating by earthshine, thus reducing the near-side cooling rate. (c) An asymmetric bombardment could have preferentially heated the near side. (d) A suggestion by D. Stevenson, collection of metal from the magma ocean in one hemisphere would have pushed the unmelted “core” into an asymmetric position, and resulted in earlier magma ocean crystallization in the deeper hemisphere. Our assessment is that the asymmetric “core” hypothesis is the most plausible, that the floating continent mechanism is possible, and that the Earthshine and symmetric bombardment mechanisms are not viable. An attractive feature of the asymmetric- mantle model is that it also accounts for the asymmetries in the distribution of KREEP and mare basalts. More rapid crystallization of the far side would leave urKREEP, the last dregs of the magma ocean, concentrated under the near-side crust, thus leading to the observed tendency for KREEP to be found on the near side surface. Further, the concentration of urKREEP-associated radiactive elements on the near side would result in a much lower rate of conductive cooling of the near-side mantle and thus a much longer period of basalt extrusion on the near side. The formation of basalts would also be enhanced by the presence of more fusible materials in the near-side mantle. 相似文献
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By systematically searching regions around planetary nebulae (PNe) for signs of interactions of their precursors’ wind with
ambient matter we found a number of huge IRAS dust structures. Some of them may be chance projections, but a few appear to
be real, like those around NGC 6826 and NGC 2899. In the case of NGC 6826 we noticed a giant (∼2°) bipolar dust emission,
whose axis is along the proper motion of the central star. The PN itself is offset in the direction of motion both as to the
center of this ∼30 pc large dust structure and to the center of a similarly large new Hα nebula. NGC 2899 was found in the center of a 14×11 pc quadrupolar cavity, whose directions of axes coincide with the directions
of the main axes of the optical PN. In both cases, the formation of these structures appears to have commenced in the asymptotic
giant branch (AGB) phase. 相似文献