Page - 74 - in Contributions to GRACE Gravity Field Recovery - Improvements in Dynamic Orbit Integration, Stochastic Modelling of the Antenna Offset Correction, and Co-Estimation of Satellite Orientations

the acceleration due to conservative forces derived from the background models are integrated in the determination of the GRACE dynamic orbits (eq. (5.2.2)). Due to the integration, noise at lower frequencies is amplified, while noise at higher frequencies is damped. Figure 6.8b shows the estimated arc-wise variance factorsσ2m for the ll-SST observations. Their mean is 1, due to the normalization applied in eq. (6.5.29). A systematic increase of the arc-wise variance factors can be observed during the third quarter of the month. According to Flechtner (2010), GRACE-A was commanded into attitude hold mode (AHM) on June 17, 2010, during which the pointing towards GRACE-B is not enforced as strictly, and can undergo larger variations. The time period that GRACE-A spent in AHM is shaded with a grey backdrop, and corresponds well with a period of increased arc-wise variance factors. This demonstrates how a drop in data quality was automatically detected through VCE. The arc-wise variance factors increase in value, meaning the weight of the corresponding arcs in the LSA decreases accordingly. Figures 6.8d and 6.8f show the arc-wise variance factors for the POD observations on GRACE-A and GRACE-B. As the GPS observations are not affected by the AHM, their arc-wise variance factors do not show a corresponding change during this period. Figures 6.8c and 6.8e show the PSDs for the GRACE-A and GRACE-B POD observa- tions. The PSDs are estimated separately for each coordinate axis in the orbit system, giving the three curves shown. 6.6 Complete solution With the dynamic orbits determined, a functional model set up, and the stochastic characteristics of the signals estimated, a complete GRACE gravity field solution can be determined. For ITSG-Grace2016, this comprises both monthly and daily gravity fields. An ITSG-Grace2016 monthly solution is computed from all data available for that specific calendar month, beginning at 00:00:00 UTC on the 1st day of the month, and up to, but not including, 24:00:00 UTC on the last day of the month. For months with large data gaps no attempt is made to combine data from multiple months. The resulting gravity field is then the temporal mean of the time-variable gravity signal for that month. Together with the static field and long-term time-variable signal, which were introduced as a priori reference fields in the dynamic orbit integration, this gives the full mean potential for the month. This corresponds to the required step of adding the adjusted parameters to the Taylor point in any LSA, with xˆ=x0+∆xˆ. Monthly gravity fields are computed up to D/O 120. Expansions up to D/O 90 and 60 are then determined through truncation of the full normal equation system. All three solutions are then provided to the user community. Daily gravity field solutions are determined using GRACE observations from 00:00:00 UTC of that day up to, but not including, 24:00:00 UTC on the same day. In addition to the GRACE observations, additional information in the form of the stochastic information on the temporal variability of gravity due to geophysical processes is Chapter6 ITSG-Grace201674