Assessing daily to interannual geocenter motion variations from Low Earth Orbiters

Abstract

The ever-changing fluid mass (oceans, continental water, snow, atmosphere, …) redistributions on the Earth's surface give rise to a motion of the deformable terrestrial crust, that is its geometrical center-of-figure (CF), with respect to the center-of-mass (CM) of the Earth, about which satellites naturally orbit. This motion, called “geocenter motion”, is the largest scale variability of mass within the Earth system. Yet, non-tidal geocenter motion, which reflects major water and atmosphere mass transports occurring over large regions, is traditionally neglected. However, new climate-driven precise monitoring of geocenter motion is needed. Indeed, satellite altimetry and gravimetry precise orbits connect sea level and global water budgets to the Earth’s center of mass. As such, the geocenter motion is now the leading error term in Regional Mean Sea Level and mass changes over polar ice sheets estimates. Reliable solutions of geocenter motion are thus crucial for assessing the current status of climate change and its future evolution (e.g., for the Earth’s Energy Imbalance). Global Navigation Satellite Systems (GNSS) measurement models and derived products are currently aligned to the International Terrestrial Reference Frame (ITRF) origin (which is referenced to the crust), instead of CM. Looking at sub-daily cross-track perturbations estimated with the GNSS receivers on board the Jason-3 and Sentinel-6 MF altimetry satellites during their tandem phase (December 18, 2020 – April 7, 2022) revealed consistent diurnal oscillations with an impressive temporal resolution. These could only be related to the miscentering effect of the constellation solution around the Earth’ CM. In this paper, a parametric model is derived, representing the translation of the GNSS ground station networks with respect to the center of mass of the whole Earth system. This model is estimated with GNSS-based low Earth satellite precise orbits and unambiguously validated with independent altimetry satellite missions (e.g., Sentinel-3A, Sentinel-6 MF, Jason-3). It helps to clearly identify interannual variations in the geocenter motion, as short as a day long.