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.