Page - 108 - 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

0.00.20.40.6 PDFof σˆAOC,A 0 1 2 3 4 5 6 GRACE-A 0.0 0.2 0.4 0.6 PDFof σˆAOC,B GRACE-B Figure 8.5: Kernel density estimate of PDFs of estimated AOC variance factors σˆAOC,A/B for complete GRACE time series. Each barb represents one variance factor for one month. 8.3.2 Arc-wise Variance Factors The AOC covariance matrices, scaled by their variance factors, are expected to model some of the increased noise in observations with large opening angles. It is thus expected that the increase in the arc-wise variance factors for the stationary noise com- ponent previously observed for these time periods should, to some extent, disappear. One such period is the time that GRACE-A spent in attitude hold mode in June 2010, mentioned before in section 6.5 and seen in the arc-wise variance factors displayed in fig. 6.8b. Figure 8.6a illustrates the arc-wise variance factors for the same data in June 2010, using the improved stochastic model. The arc-wise variance factors for the stochastic model considering only the estimated stationary covariance function (in brown) again shows the increase in arc-wise variance factors during the period of increased opening angles (darker shaded area). Introducing the a priori AOC covariance information for this month (in blue), the estimated arc-wise variance factors are noticeably closer to 1 during this period. The same effect can be observed in other periods of increased opening angles, for example the orbit swap manoeuvre in December 2005 when both GRACE-A and GRACE-B were commanded to attitude hold mode. In addition the 1° pitch bias that keeps the KBR horns in alignment was removed, leading to very large angles during the periods when a KBR link was established. Figure 8.6b shows that during this period the arc-wise variance factors for the processing using the AOC covariance information (in blue) are again much improved. Chapter8 Star Camera Observations and Uncertainties108