地心變動定義為地球質量中心 (center of mass, CM) 相對於幾何中心 (center of figure, CF) 之位置變化，一般可以一階項之球諧係數表示。地心變動主要成因來自於地球系統中之質量重新分佈，如海洋、大氣、陸上水文、冰川及冰原間之質量交換，以及地球內部質量變化如冰後回彈 (Post-Glacial Rebound, PGR) 及地震。地心變動反應出地球質量平衡以及固體地球與地球系統中移動質量間之交互作用，因此精確估計地心變動有助於了解地球內部變化。先前已有許多研究利用衛星觀測地面固定站進行地心變動估計，但受到觀測點位分布不均勻以及海洋區域無資料之影響導致結果精度較差。GRACE (Gravity Recovery and Climate Experiment) 自2002年發射升空後，成功地提供高精度之月平均重力場解，其與氣象模型或其他衛星觀測量結合可精確推算地心變動，但依然受到模型精度無法評估之限制。本研究結合GRACE與測高資料，經由兩種計算方法精確估計地心變動，方法分別為Swenson等人 [2008]方式和疊代方式。兩方法計算成果與先前研究相符，而疊代方法之成果精度較佳。由於衛星測高觀測量是以ITRF (International Terrestrial Reference Frame) 為參考框架，其與CF間之差異對地心變動之計算產生影響，因此本研究另外使用海洋模型取代測高資料進行計算，其結果顯示ITRF與CF之差異並無造成地心變動計算明顯改變，而使用不同範圍之海洋資料將導致地心變動估計成果不同，當以海洋模型取代測高資料時，地心變動趨勢及相位有明顯差異。本研究亦考慮洩漏效應 (leakage effects) 對於地心變動計算之影響，成果藉由移除海岸線300 公里內之海洋資料來消除。另外，PGR一階項改正量對於地心變動計算之影響僅反應於趨勢訊號上，影響量約佔改正量之80%。
 

Geocenter variation, defined as the motion of center of mass (CM) of the Earth relative to the center of figure (CF), is usually represented by degree one spherical harmonic coefficients. The geocenter variation is mainly caused by the mass redistribution in the Earth system including the mass exchange between ocean, atmosphere, ground hydrology, mountain glacier and ice sheets, and the mass change inside of the Earth like Post-Glacial Rebound (PGR) and earthquakes. Accurate Estimation of geocenter variation has a contribution to understand the variation in the Earth since geocenter variation reflects the Earth mass balance and the interaction between the solid Earth and transported mass in the Earth system. Several studies for estimating geocenter variation have been done, but the results were limited as a result of uneven distribution of measurements or shortage of data in oceans. GRACE has provided monthly gravity solutions with high precision since launched in 2002, which can be used for estimating geocenter variation by combining with climate models or other satellite measurements, but the results are still limited because of the uncertainties of climate models. In this study, geocenter variation is determined by two novel methods using the combination of GRACE and altimetry data including the method used by Swenson et al. [2008] and an iterative algorithm. Both results agree well with previous researches and the accuracy of the result derived by the iterative method is higher. Since the altimetry measurements refer to ITRF (International Terrestrial Reference Frame), the displacement between the origins of ITRF and CF would affect the estimation of geocenter variation. Therefore, ocean models are used for geocenter variation determination instead of altimetry data. The result shows the displacement between the origins of ITRF and CF would not change the estimation of geocenter variation significantly but the different covering areas of altimeters or ocean models would change the estimation of geocenter variation in terms of trends and phases. In addition, leakage effects are also considered in the computation by removing ocean data near coastlines within 300 km distance. Different degree one coefficients of PGR models only change the trends of estimated geocenter variation with 80% of their values.
 

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