Analysis of GRACE and GRACE-FO post-fit range-rate residuals

The monthly gravity field models at LUH are computed with a generalized dynamic orbit determination approach utilizing precise range-rate measurements derived from the K-band ranging system as main observations. A by-product of the gravity field computations are time series of range-rate residuals which represent a complex superposition of different effects, for example of instrumental, environmental and geophysical nature. An analysis of the residuals helps to gain knowledge of the different systematics that can be used to improve the computation strategy and thus the accuracy of the gravity field models.

Since a similar processing strategy was applied to the analyzed GRACE and GRACE-FO residuals, the smaller and more homogenous GRACE-FO residuals mainly reflect improvements in hardware and software.

Some noticeable abnormalities recur at certain positions and orientations of the satellites relative to the Earth and Sun. The transitions between sunlight and the Earth’s shadow are recognizable as well as the blinding of the star cameras by the Sun in specific angles. Moreover, the simultaneous blinding of both star cameras at GRACE by Sun and Moon can cause peaks in the residuals due to the transient outage of precise attitude determination.

Other amplitudes can be related, for instance, to temporary mode changes of the AOCS during maneuvers or to specific combinations of the received frequency and the temperature at the K-band ranging assembly. Besides the systematic errors, the residuals also contain geophysical signals. This applies mainly for regions with strong annual variations as well as where the models of high-frequency signals are prone to errors.