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51.
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Yanchao He Ming Xu Xianghua Jia Roberto Armellin 《Celestial Mechanics and Dynamical Astronomy》2017,128(2-3):275-294
The focus of this paper is the design and station keeping of repeat-groundtrack orbits for Sun-synchronous satellites. A method to compute the semimajor axis of the orbit is presented together with a station-keeping strategy to compensate for the perturbation due to the atmospheric drag. The results show that the nodal period converges gradually with the increase of the order used in the zonal perturbations up to \(J_{15}\). A differential correction algorithm is performed to obtain the nominal semimajor axis of the reference orbit from the inputs of the desired nodal period, eccentricity, inclination and argument of perigee. To keep the satellite in the proximity of the repeat-groundtrack condition, a practical orbit maintenance strategy is proposed in the presence of errors in the orbital measurements and control, as well as in the estimation of the semimajor axis decay rate. The performance of the maintenance strategy is assessed via the Monte Carlo simulation and the validation in a high fidelity model. Numerical simulations substantiate the validity of proposed mean-elements-based orbit maintenance strategy for repeat-groundtrack orbits. 相似文献
53.
Wang Jingai 《地理学报(英文版)》1994,(Z2)
CompilinganatlasofnaturaldisastersinChinaisabasistoresearchintoregionaldisasterssystems,torevealthetemPOralandspatialpatternofnaturaldisasters,aswellastoestablishcountermeasuresopinstnamraldisasters.TaldngtheuAtlasOfNaturalDisastersinChina"**asanexample,thisarticleinquiresintotheoreticalandpracticalproblemsaboutcompilinganatlasofregionalnaturaldisasters.ThebasictheoryofcompilinganatlasOfregionaldisastershasbeenfOundedonthecombinati0nofthesciencesofdisasters,cart0graphyandregiotalmhy.Theref… 相似文献
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To study the effect of uncertain factors on the temperature field of frozen soil, we propose a method to calculate the spatial average
variance from just the point variance based on the local average theory of random fields. We model the heat transfer coefficient
and specific heat capacity as spatially random fields instead of traditional random variables. An analysis for calculating the random
temperature field of seasonal frozen soil is suggested by the Neumann stochastic finite element method, and here we provide the
computational formulae of mathematical expectation, variance and variable coefficient. As shown in the calculation flow chart, the
stochastic finite element calculation program for solving the random temperature field, as compiled by Matrix Laboratory
(MATLAB) software, can directly output the statistical results of the temperature field of frozen soil. An example is presented to
demonstrate the random effects from random field parameters, and the feasibility of the proposed approach is proven by comparing
these results with the results derived when the random parameters are only modeled as random variables. The results show that
the Neumann stochastic finite element method can efficiently solve the problem of random temperature fields of frozen soil based
on random field theory, and it can reduce the variability of calculation results when the random parameters are modeled as spatially
random fields. 相似文献
56.
Two main volcanic events are distinguished between Saraykent and Akçakışla in the Yozgat province of central Anatolia: (1) early Late Cretaceous–Palaeocene effusive activity, that produced a sequence of intermediate to felsic ‘basal lavas’; and (2) marginally later Palaeocene explosive activity that formed a series of covering ignimbrite flows. Due to their close temporal and spatial relation, geochemical comparisons were made between the silicic members of the lavas and ignimbrites, to identify chemical groups and their relative petrogenesis. The basal lavas range from calc‐alkaline basaltic andesites to dominant rhyolites. Based on trace element correlations three main geochemical groups were identified: the Akçakışla rhyolites (present as domes); Akçakışla rhyodacites‐dacites (lava flows); and Ozan‐Saraykent rhyolites (lava flows). Large‐ion lithophile elements have been mobile in all the groups, but mainly in the Akçakışla rhyolites. Rare earth element (REE) patterns show marked similarity between the Ozan and Saraykent basal lavas. The Akçakışla dome rhyolites are more fractionated with lower LaN/YbN ratios (c.10), whereas the Akçakışla basal lavas have much higher LaN/YbN ratios (c.30). The chemical coherence and petrographic similarities between the Saraykent and Ozan lavas suggest a single suite related via fractionation. Three geochemical groups were also established for the ignimbrites: Saraykent ignimbrite; Bağlıca ignimbrite‐Toklu‐Kızıldağ crystal tuffs; and Keklikpınar ignimbrite. The ignimbrites, like the basal lavas, display a pronounced depletion in Ba on ORG‐normalized plots. Relative to the basal lavas, chondrite‐normalized patterns for the ignimbrites are different in displaying negative Eu anomalies that indicate feldspar fractionation. The lack of geochemical overlap or coherence between any of the lava and ignimbrite groups suggests that they represent distinct eruptive events and are not related in any simple volcanic development and cogenetic sense. Two geochemical features are common to all the volcanic rock groups: (1) the presence of a Nb‐Ta anomaly, which is generally accepted as a crustal signature; and (2) the relatively low Y abundances which appear characteristic for the region as a whole. These fundamental features of the local silicic volcanism largely reflect source composition and effects. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
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N2O是一种重要的温室气体,土壤是全球N2O的重要排放源。通过测定土壤源N2O中N、O同位素值,可以有效识别N2O的来源途径。本文采集了南极法尔兹半岛两个地点的海豹粪土(HS和GS)、阿德雷岛两个地点的企鹅粪土(AB和AF)以及东南极的帝企鹅粪土(DQ和DQT),在室内对所采集的样品分别在有氧和厌氧条件下进行冻融培养实验。结果表明:土壤在厌氧条件下比有氧条件下排放了更多的N2O。土壤排放的N2O与当地大气N2O相比普遍贫15N和18O。除DQT和HS外, δ15N和δ18O在有氧和厌氧培养下均呈现很好的正相关性。N2O排放量下降的同时伴随着培养瓶内剩余N2O中δ15N和δ18O 值的增加,证实N2O还原为N2的过程会引起重同位素富集。高的水分含量有利于土壤反硝化作用的进行,使释放的N2O气体富集重同位素;pH值也会影响N2O的同位素组成,低pH会引起δ15N值增加。 相似文献
59.
Wang Lifang Wu Xiangbin Zhang Baoyi Li Xuefeng Huang Anshuo Meng Fei Dai Pengyao 《Natural Resources Research》2019,28(3):587-607
Natural Resources Research - Quantitative prediction of concealed mineralization is always confronted with difficulties in comprehensive analysis between 2D and 3D data and between qualitative and... 相似文献
60.
暖温带地理景观中土壤硒的分异特征 总被引:17,自引:0,他引:17
本文按垂直自然带谱研究了暖温带地理景观中山地草旬土、山地棕壤、山地淋溶褐土和潮土中硒的分异特征。结果表明:(1)硒在土剖面上的分异,除受生物富集的影响外,淋溶和粘人作用强烈地制约着硒在剖面上的迁移;(2)不同机械组分中的硒含量以粘粒中最高,呈明显的粘粒>细粉粒>中粉粒>砂粒序列。因此,成土过程中,不同机械组分以及次生或们生矿物和垂直运动和自然分选作用对土壤硒的地理分异具有非常重要的作用。(3)易于淋溶失的有机态或钙结合的硒对所研究的土壤表层硒的贡献都大于50%。提出淋溶作用是我国低硒带形成的主要外营力。 相似文献