Influence of time variable geopotential models on precise orbits of altimetry satellites,global and regional mean sea level trends

During the last decade a significant progress has been reached in the investigation of the gravity field of the Earth. Besides static, also time variable geopotential models have been recently created. In this paper we investigate the impact of the recent time variable geopotential models on altimetry satellite orbits and such altimetry products based on these orbits, as global and regional mean sea level trends. We show that the modeling of time variable gravity improves the orbit solutions, at least for the GRACE period where time variable gravity is sufficiently accurately observed by this mission. Our analysis includes six geopotential models jointly developed by GFZ German Research Centre for Geosciences and Space Geodesy Research Group (CNES/GRGS) Toulouse: the stationary model EIGEN-GL04S, a stationary version of EIGEN-6S (EIGEN-6S_stat), a corrected version of EIGEN-6S and three enhanced versions of EIGEN-6S called EIGEN-6S2, EIGEN-6S2A and EIGEN-6S2B. By “stationary” we mean “containing periodic parameters such as annual and semi-annual variations, but no secular (drift) terms”. We computed precise orbits for the radar altimetry satellites ERS-1, ERS-2, TOPEX/Poseidon, and Envisat over 20 years between 1991 and 2011. The orbit, single-mission and multi-mission altimetry crossover analyses show that the time variable models EIGEN-6S_corrected, EIGEN-6S2 and its two precursors EIGEN-6S2A/B perform notably better than the stationary models for the GRACE period from 2003 onwards. Thus, using EIGEN-6S2 and EIGEN-6S2A/B we have got 3.6% smaller root mean square fits of satellite laser ranging observations for Envisat, as when using EIGEN-GL04S. However, for the pre-GRACE period 1991–2003, the stationary geopotential models EIGEN-GL04S and EIGEN-6S_stat as well as EIGEN-6S2 having no drift terms for degree 3–50 at this time interval perform superior compared to EIGEN-6S_correct and EIGEN-6S2A/B which contain drifts for this period. We found, that the time variable geopotential models have a low (0.1–0.2 mm/yr) impact on our results for the global mean sea level trend. However, we found strong East/West differences up to 3 mm/yr in the regional mean sea level trends when using orbits of all four satellites based on time variable and stationary geopotential models. We show that these differences are related to the relative drifts of the centers-of-origin between the orbit solutions based on the time variable and stationary geopotential models. From the results of our detailed study, we conclude that the final version of the time variable gravity field model EIGEN-6S2 performs best for the four satellites tested. This model provides the most reliable and mission-consistent sea level estimates for the whole time period from 1992 to 2010. This model is of maximum spherical harmonic degree and order 260 and contains time series for drifts as well as annual and semiannual variations of the spherical harmonic coefficients for degree 2–50.     

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同波束VLBI通过同时观测两个探测器的多点频信号,可以得到两个探测器之间高精度的差分相位时延,日本月球探测计划SELENE充分体现了这一技术在月球探测器精密定轨中的贡献。本文针对采样返回的月球探测任务中,轨道器和返回器同时绕月飞行期间,研究利用同波束VLBI跟踪数据在探测器精密定轨和月球重力场仿真解算中的贡献。结果表明,加入同波束VLBI跟踪数据之后,探测器定轨精度有显著提高,改进超过一个量级。综合同波束VLBI跟踪数据解算得到的重力场模型相比于传统的USB双程测距测速数据,中低阶次位系数精度有明显改进,并且定轨精度有望能达到米级。
     

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考虑镍基单晶合金细观两相共格的微组织结构特点,采用周期性胞元假设,在胞元不同区域构建其三维应力应变解析模型,并针对细观胞元中位错的增殖、攀移、切割及恢复等主要机制建立其黏塑性应变模型,二者结合较全面地模拟了镍基单晶合金SC16和DD3在1123 K和1033 K下的单调拉伸、蠕变及循环特性.根据模拟结果,对黏塑性应变模型中各个位错机制所起的作用及控制方式进行了具体分析,通过与试验曲线比较,得到了较一致的结果.      

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