Analysis of the inter-satellite ranging pre-fit residuals and of the impact of an IPU failure in GRACE-FO

GRACE and GRACE-FO mapped the monthly time-variable gravity fields with a spatial resolution of ~300km over the past two decades, providing information on mass transport in the Earth system. The twin spacecraft can sense the gravity variability from the low-low inter-satellite ranging system, such as the K-Band ranging (KBR) and the more precise, Laser ranging interferometer (LRI). This poster is divided into two parts, the first part (1) focuses on the features of prefit residuals, and in the second part (2) we analyzed the impact of a complete failure of the Instrument Processing Unit (IPU) in GRACE-FO:

(1) The polar-orbit configuration of GRACE/-FO can lead to uneven sampling of Earth observations, which could be manifested as north-south stripes in scientific data. Here, we use prefit residuals which are along orbit and have more flexibility to detect short-lasting signals. Prefit residuals can be deduced from inter-satellite ranging measurements and integrated orbits, with tailored processing strategy. Then we compare the residuals of GRACE-FO KBR and LRI from 2019 to 2021. According to the comparative analysis in the time, frequency and space domains, the results show that both instruments can capture most of the geophysical signals well whereas LRI prefit residuals have lower noise over the ocean area. Additionally, LRI also shows prefit residuals higher than background noise in areas where large signal trends usually occur, such as Greenland, Gulf of Alaska coast and Antarctica. But it is still difficult to capture this high-frequency variability in gravity field recovery. These results indicate that this novel perspective on GRACE-type mission data may improve our understanding of mass distribution changes at particular locations at shorter periods, especially for LRI observations.

(2) Since satellite GF-2 of GRACE-FO is already using its redundant Instrument Processing Unit, there is an elevated risk that at some point KBR and GPS observations will become unavailable for this satellite, if the redundant unit breaks as well. In order to assess the impact of such loss, we used a SLR orbit data product provided by GFZ Potsdam to replace the GPS observations from GF-2. With the help of the SLR orbit product we were able to compute the LRI light time correction, and finally derive a LRI1B dataset that would be representative for the situation of an unavailable IPU. We used the data to assess the gravity field quality. Here we show some details on the derivation of the LRI1B dataset and provide the gravity field results.