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In desert environments with low water and salt contents, rapid thermal variations may be an important source of rock weathering. We have obtained temperature measurements of the surface of rocks in hyper-arid hot and cold desert environments at a rate of 1/s over several days. The values of temperature change over 1-second intervals were similar in hot and cold deserts despite a 30 °C difference in absolute rock surface temperature. The average percentage of the time dT/dt > 2 °C/min was ~ 8 ± 3%, > 4 °C/min was 1 ± 0.9%, and > 8 °C/min was 0.02 ± 0.03%. The maximum change over a 1-second interval was ~ 10 °C/min. When sampled to simulate data taken over intervals longer than 1 s, we found a reduction in time spent above the 2 °C/min temperature gradient threshold. For 1-minute samples, the time spent above any given threshold was about two orders of magnitude lower than the corresponding value for 1-second sampling. We suggest that a rough measure of efficacy of weathering as a function of frequency is the product of the percentage of time spent above a given threshold value multiplied by the damping depth for the corresponding frequency. This product has a broad maximum for periods between 3 and 10 s. 相似文献
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崔照明 《广东海洋大学学报》1991,(1)
本文分析了各种无线电导航设备的特性及其在南海海区应用的优劣,并考虑到GPS对导航领域的影响,根据南海区渔船的实际需要,提出了南海区渔船选择无线电导航仪器的意见,供各类渔船在选用无线电导航仪器时作参考。 相似文献
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基于GPS信标观测数据重建电离层电子密度的时变三维电离层层析问题的数学基础,在简化电离层电子密度随时间变化的条件下,导出了三维平行束层析重建公式. 在二维情况下,本文导出的三维平行束层析重建公式与Yeh等给出的二维电离层层析重建公式相同. 文中还讨论了有限视角和有限接收机孔径对重建图像的影响. 根据文中给出的三维平行束层析重建公式,在这些非理想条件下,对一个脉冲源函数进行CT重建的结果表明,基于GPS信标观测数据重建电离层电子密度的时变三维电离层层析是可行的. 相似文献
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Exploration activities on the lunar surface will require precise knowledge of the position of a robotic or manned vehicle. This paper discusses the use of radio beacons as method to determine the position of a mobile unit on the surface. Previous concepts consider the installation of such equipment by the robot itself. A novel idea is discussed here, namely to use miniaturized radio beacons which are deployed (released) during the descent of the lander on the surface. This idea has three major advantages compared to previous proposals: (i) it avoids the time costly and energy consuming installation of the equipment by a rover. (ii) The impact velocities of the probes are in reasonable range since the probes are deployed at low altitude from the main lander that approaches its final landing site. (iii) The probes can take reconnaissance pictures during their free-fall to the surface. This method will therefore deliver charts of the proximity of the landing area with higher resolution than those done by orbital means. Such information will enable scientists and mission operators to precisely plan robotic excursions (and later Extra Vehicular Activity) through the identification of hazardous areas and spots of interest.The paper will study the feasibility of this system from different aspects. The first section will outline the application scenario and the potential outcome of such a system for the coming phase of lunar exploration. A technological readiness review was done to evaluate if the payload instrumentation for these high velocity impacting probes is available. The second section presents the simulation of the impact process of a preliminary probe model in nonlinear transient dynamic finite element analysis using the Lagrangian hydrocode LS-DYNA. The purpose of this simulation was to evaluate if the beacon is able to communicate with the mobile unit even when buried into the soil.The integration of this payload into coming lunar missions will contribute to the international efforts of lunar exploration with a landing site ad hoc navigation system for robotic or manned excursions. 相似文献
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