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J. M. Ferrándiz M. E. Sansaturio R. Caballero 《Celestial Mechanics and Dynamical Astronomy》1993,57(1-2):189-202
We deal with the problem of the motion of a triaxial satellite of an oblate primary of larger mass. We show that the treatment is simplified by using a canonical set of variables previously introduced by the authors, that allows a drastic reduction in the expansions of the potential. A general method to avoid the appearance of virtual singularities when the angles between certain planes are small is designed. Our approach is applicable either to natural or artificial satellites. 相似文献
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Peter R. Leavitt Patricia R. Sanford Stephen R. Carpenter James F. Kitchell 《Journal of Paleolimnology》1994,11(2):133-149
Annual and subannual paleolimnological records of pigments and zooplankton were used to analyze three whole-lake manipulations. The relative abundance of cladoceran remains in recent seasonal laminae was significantly correlated with the relative abundance of species in the plankton (r
2=0.59,P<0.001). Comparison of food-web structure after piscivore introduction showed that there are multiple outcomes of predation and that final food-web structure depended on the strength of interaction between piscivorous and planktivorous fish. Intense predation on cyprinids (Phoxinus eos, P. neogaeus, Umbra limi) by largemouth bass (Micropterus salmoides) allowed large herbivores (Daphnia pulex) and invertebrate predators (Chaoborus punctipennis) to dominate. Analysis of fossil invertebrate morphology suggested that small grazers (Bosmina longirostris, Diaphanosoma birgei) were eliminated by invertebrate predators. Under moderate predation by rainbow trout (Oncorhynchus mykiss), cyprinids remained, only intermediate-size herbivores (D. rosea) increased in abundance, andBosmina persisted. In contrast to food-web manipulations, increased algal abundance resulting from watershed disturbance (road construction) did not alter the species composition or size-structure of fossil Cladocera. 相似文献
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本文介绍了GIS在防震减灾领域的一个应用实例,大庆油田防震减灾信息系统的建设,讨论了防震减灾信息的管理及在震害预测、震后应急反应中的应用,探讨了GIS在防震减灾领域进一步应用的发展方向。 相似文献
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针对现有拖曳式水下潜器控制机构复杂、航向与姿态不容易稳定的缺陷,提出和设计了一种具有航向与姿态稳定的多自由度可控制拖曳式水下潜器样机。该样机主要由鱼雷状浮体、固定水平主翼、转角可控制襟翼、立式翼型主体等部分组成,潜器的深度控制通过控制襟翼的偏转来诱导固定水平主翼攻角的改变来实现;潜器的横荡运动操纵以通过控制两个作为转艏控制器的导管螺旋桨的转向与转速、诱导立式翼型主体产生诱导力矩使其产生横向偏转来进行。文中所提出和设计的样机具有运动过程中自我稳定能力强、航向稳定性好、控制机构简单并具有较高实用价值的特点。 相似文献
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Snow cover strongly influences plant growth in Arctic and alpine ecosystems. Snow characteristics and snowmelt timing are likely to change in a warmer climate. We studied year rings and shoot growth of the dwarf shrub bilberry ( Vaccinium myrtillus ), and species abundances of the vegetation, in response to early or late snowmelt at a study site in the Central Alps, near Davos, Switzerland. Snowmelt was manipulated on experimental plots for 3 and 30 years. Additional plots were set up along a natural snowmelt gradient, and at high and low elevation. Growth ring data showed an increasing trend in annual growth increment over the last 20 years, especially in the extraordinarily hot summer of 2003. Comparing high and low elevation sites, growth rings were wider at low elevation, but only in cold years. In years with relatively cold summers, however, xylem ring width was greater in plots with late rather than early snowmelt along the natural snowmelt gradient, possibly indicating drought stress in early snowmelt plots. Snow cover had a strong influence on species abundances along the natural snowmelt gradient, and change (not yet significant) was beginning to be seen in plots with 30 years of snow manipulation. Our results indicate that beneficial effects of early snowmelt for shrub growth may be offset in cold summers. Although early snowmelt prolongs the growing season, harsh conditions and frost events early in the growing season may become more likely, and hamper plant growth, and this could affect plant growth in all Arctic and alpine snow-dominated ecosystems. 相似文献
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随着数字化测图广泛推广,数字产品的检查工作加大,为了减少检查工作,提高检查质量,对此开发了数字化地形图检查软件。此软件对野外采集数据处理和地形图数据检查进行了探讨,对广大软件开发者具有积极的借鉴作用。 相似文献
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研究生物操纵过程中各种食物链级的生物的作用和营养地位,采用碳氮稳定同位素的方法,研究了青草沙实证基地生态操纵池塘生态修复过程中主要种类的稳定同位素特征。青草沙实证基地生态操纵池塘人工投了多种鱼类,加上自然恢复种类,最终重建实证基地池塘中的生态系统,恢复其强大的自净能力。青草沙实证基地中,δ13 C值范围在(-21.51~-13.60);δ15 N值范围在(1.77~9.38);营养位置范围在(1~3.24)。油餐、红鳍鲌、鰕虎鱼3个物种的生态位比较接近,它们之间的营养冗余较小。实证基地池塘的生物多样性比较低,构建而成的食物网简单。通过营养级构建出的能量金字塔中,消费者营养级别最低的是底栖软体动物、甲壳动物,然后是草食性鱼类,最后是肉食性或杂食性鱼类,它们占据食物网的最顶层。这种立体复合生态操纵技术可以有效进行生态系统恢复及重建,在大型湖泊及水库的富营养化治理中起到越来越重要的作用。碳氮稳定同位素的含量可以用来研究生态系统的营养结构。通过营养级构建出的能量金字塔在人工控制的简单生态系统中具有一定的参考价值。 相似文献
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