利用动力学方法解算GRACE时变重力场研究

本文利用动力学方法建立GRACE(Gravity Recovery And Climate Experiment)K波段距离变率(KBRR)观测、轨道观测与重力场系数的观测方程,通过GRACE Level 1B观测数据,成功解算出全球月时变重力场模型——IGG时变重力场模型,并将2008—2009年的解算结果与GRACE三大数据处理机构美国德克萨斯大学空间中心CSR(Center for Space Research)、美国宇航局喷气推进实验室JPL(Jet Propulsion Laboratory)和德国地学研究中心GFZ(GeoForschungs Zentrum)发布的最新全球时变重力场模型进行详细对比分析.结果表明:IGG结果在全球质量异常、中国及周边地区质量异常的趋势变化、全球质量异常均方差、2~60每阶位系数差值以及亚马逊流域和撒哈拉沙漠等典型区域平均质量异常等方面与CSR、JPL和GFZ解算的RL05结果较为一致.其中,IGG解算结果在2~20阶与CSR、GFZ和JPL最新解算结果基本一致,20~40阶IGG解算结果与GFZ、JPL单位最新解算结果较为接近,大于40阶IGG结果介于CSR与GFZ、JPL之间;亚马逊流域平均质量异常周年振幅IGG、CSR、GFZ和JPL获取到的结果分别为17.6±1.1cm、18.9±1.2cm、17.8±0.9cm和18.9±1.0cm等效水柱高.利用撒哈拉沙漠地区的平均质量异常做反演精度评定,IGG、CSR、GFZ和JPL的时变重力场获取到的平均质量异常均方差分别为1.1cm、0.9cm、0.8cm和1.2cm,表明IGG解算结果与CSR、GFZ和JPL最新发布的RL05结果在同一精度水平.

An investigation on GRACE temporal gravity field recovery using the dynamic approach

The Gravity Recovery and Climate Experiment (GRACE) mission can significantly improve our knowledge of the temporal variability of the Earth's gravity field. We intend to obtain monthly gravity field solutions based on dynamic approach (variational equations approach) from GPS-derived positions of GRACE satellites and K-band range-rate measurements. Moreover, these solutions will validate through GRACE RL05 products published monthly gravity field solutions by the GRACE project, which including CSR (Center for Space Research), JPL (Jet Propulsion Laboratory) and GFZ (GeoForschungsZentrum).Based on the theory of the dynamic approach, the processing began with the computation of purely dynamic orbits by fitting GNV1B orbits computed by the GRACE team using a reduced dynamic strategy. This step aimed to calibrate the GRACE A and B accelerometer data and the initial state vectors of these two satellites. Note that gravity field model parameters were not adjusted in this part. In the next step, the purely dynamic orbits were used as the reference orbit for the calculation of residual orbits, and were also applied to calculate the nominal value of K-band range-rate. In addition, the partial derivatives with respect to the initial state vectors, accelerometer parameters and geopentional coefficients were computed simultaneously with the integration of the purely dynamic orbits. Finally, normal equations of orbit positions and K-band range-rate measurements were synthesized using a fixed data weighting ratio to combine the both information matrices for the best combined solution.The monthly gravity field solution obtained though above procedures was named as the IGG temporal gravity field model. IGG temporal gravity field models were compared with GRACE RL05 products in following aspects: (i) monthly gravity field solutions in February, 2008 and September, 2009; (ii) the root mean squares of the global mass anomaly during 2008 to 2009; (iii) the trend of the mass anomaly in China and its nearby regions within 2008 to 2009; (iv) the square root degree difference variance with respect to the mean gravity field GIF48 model from 2008 to 2009; (v) time-series changes in the mean water storage in the region of the Amazon Basin and the Sahara Desert between 2008 and 2009.The results showed that IGG solutions were almost consistent with GRACE RL05 products in above aspects (i)-(iii). For aspect (iv), IGG solutions were in good agreement with latest published GRACE RL05 products at the degree from 2 to 20, and were at the same accuracy level with GFZ and JPL at the degree from 20 to 40, but were between CSR and GFZ (or JPL) at the degree exceeding 40. According to aspect (v), changes in the annual amplitude of mean water storage in the Amazon Basin were 17.6±1.1 cm for IGG, 18.9±1.2 cm for CSR, 17.8±0.9 cm for GFZ and 18.9±1.0 cm for JPL in terms of equivalent water height, respectively. The root mean squares of the mean mass anomaly in Sahara were 1.1 cm, 0.9 cm, 0.8 cm and 1.2 cm for temporal gravity field models of IGG, CSR, GFZ and JPL, respectively. Overall, it was noticeable that IGG temporal gravity field solutions were at the same accuracy level with the latest temporal gravity field solutions published by CSR, GFZ and JPL.