Page - 117 - 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

0.1 1 10 100 AOCcontribution– eˆAOC 2010-06-01 00:00 2010-06-01 12:00 2010-06-02 00:00 2010-06-02 12:00 2010-06-03 00:00 0 10 20 30 GRACE-A GRACE-B Figure 8.12: Time-frequency-power diagram of disentangled residuals due to non-stationary effects for the first two days of June 2010 (top panel, colourbar identical to fig. 8.11). The bottom panel shows the opening angle β for GRACE-A and GRACE-B during the same time period. The noise due to the AOC uncertainty was shown to be non-stationary, and can thus not be described using the purely stationary noise model estimated in ITSG- Grace2016. A rigorous stochastic model for the AOC was developed through variance propagation based on full variance-covariance information for the satellite orientations. This information, with one covariance matrix per short arc per spacecraft, was then introduced into the stochastic model for the GRACE ll-SST observations. This additional information had several positive effects on the estimated stochastic model. The arc-wise variance factors for arcs with larger than normal opening angles no longer show consistently and abnormally large values. The previous stochastic model correctly identified these arcs to have inferior data quality, but the approach of simple scaling of the stationary covariance function does not do justice to the complex and time-variable characteristics of the AOC signal during these arcs. The presented augmented approach correctly models periods of increased variances and correlations in the AOC, leading to improved results in the estimation of the stationary component of the stochastic model. The estimated stationary covariance functions for months with periods of increased opening angles no longer show aliasing in the frequency band most affected by the AOC uncertainty. In turn, the remaining arcs of these months are now decorrelated 8.4 Discussion 117