Data-driven multi-step AO de-aliasing (DMD) for GRACE/GRACE-FO

The GRACE-FO’s mission primary goal is the precise mapping of variations within the Earth’s hydrology by observing changes in gravitational attraction. This particular signal, however, is widely superimposed by other signals of higher amplitudes and shorter time scales like mass redistributions within atmosphere and ocean (AO). The hereby induced temporal aliasing is treated by reducing tidal background models during the data processing. The residual signal which is caused by imperfections of the background models has so far been left untreated.

We present the DMD approach for further treatment of the residual AO-signal which is somewhat similar to the so-called Wiese parametrization proposed for multi-pair missions (Wiese et al., 2011). However, contrary to Wiese, the DMD directly affects the right-hand side of the least squares adjustment. The method consists of (a) estimating short-term low-degree fields based on the full observation vector in a first step, (b) reducing it by the observations computed from the short-term solutions (thus effectively having applied an entirely data-based de-aliasing model), and finally, (c) estimating a high-degree field over the entire observation period (e.g. a month). Resubstituting the mean of (a) to (c) then yields a gravity solution for the observation interval. The functionality of the DMD is verified by simulations and allows for an improved retrieval of the residual signal above the maximum degree of the short-term fields over the observation interval. In order to counteract potential over-estimation within the low-degree part of the solution additional conditions are introduced and discussed.