Earth’s Time-Variable Gravity from GRACE Follow-On K-Band Range-Rates and Pseudo-Observed Orbits

Abstract

During its science phase from 2002–2017, the low-low satellite-to-satellite tracking mission Gravity Field Recovery And Climate Experiment (GRACE) provided an insight into Earth’s time-variable gravity (TVG). The unprecedented quality of gravity field solutions from GRACE sensor data improved the understanding of mass changes in Earth’s system considerably. Monthly gravity field solutions as the main products of the GRACE mission, published by several analysis centers (ACs) from Europe, USA and China, became indispensable products for quantifying terrestrial water storage, ice sheet mass balance and sea level change. The successor mission GRACE Follow-On (GRACE-FO) was launched in May 2018 and proceeds observing Earth’s TVG. The Institute of Geodesy (IfE) at Leibniz University Hannover (LUH) is one of the most recent ACs. The purpose of this article is to give a detailed insight into the gravity field recovery processing strategy applied at LUH; to compare the obtained gravity field results to the gravity field solutions of other established ACs; and to compare the GRACE-FO performance to that of the preceding GRACE mission in terms of post-fit residuals. We use the in-house-developed MATLAB-based GRACE-SIGMA software to compute unconstrained solutions based on the generalized orbit determination of 3 h arcs. K-band range-rates (KBRR) and kinematic orbits are used as (pseudo)-observations. A comparison of the obtained solutions to the results of the GRACE-FO Science Data System (SDS) and Combination Service for Time-variable Gravity Fields (COST-G) ACs, reveals a competitive quality of our solutions. While the spectral and spatial noise levels slightly differ, the signal content of the solutions is similar among all ACs. The carried out comparison of GRACE and GRACE-FO KBRR post-fit residuals highlights an improvement of the GRACE-FO K-band ranging system performance. The overall amplitude of GRACE-FO post-fit residuals is about three times smaller, compared to GRACE. GRACE-FO post-fit residuals show less systematics, compared to GRACE. Nevertheless, the power spectral density of GRACE-FO and GRACE post-fit residuals is dominated by similar spikes located at multiples of the orbital and daily frequencies. To our knowledge, the detailed origin of these spikes and their influence on the gravity field recovery quality were not addressed in any study so far and therefore deserve further attention in the future. Presented results are based on 29 monthly gravity field solutions from June 2018 until December 2020. The regularly updated LUH-GRACE-FO-2020 time series of monthly gravity field solutions can be found on the website of the International Centre for Global Earth Models (ICGEM) and in LUH’s research data repository. These operationally published products complement the time series of the already established ACs and allow for a continuous and independent assessment of mass changes in Earth’s system.

WHU‐GRACE‐GPD01s: A Series of Constrained Monthly Gravity Field Solutions Derived From GRACE‐Based Geopotential Differences

“…The Gravity Recovery and Climate Experiment (GRACE) and its successor GRACE Follow‐On (GFO) missions have enabled breakthroughs in monitoring the Earth's gravity field variations from space, especially for tracking surface mass variations which are mainly attributed to the hydrological cycle, ice sheet mass change, sea‐level rise, and postglacial rebound (J. Chen et al., 2022; Landerer et al., 2020; Tapley et al., 2004, 2019). The commonly used GRACE/GFO Level‐2 monthly gravity field solutions are routinely computed from Level‐1B observations, including precise orbits, intersatellite range rates, attitude data, and accelerometer measurements of the GRACE/GFO satellites (Q. Chen et al., 2016; Dahle et al., 2019; Koch et al., 2021; Kvas et al., 2019; Pie et al., 2021; Save et al., 2012), which is widely used to quantify the Earth's surface mass variations. Three official processing centers within the GRACE/GFO Science Data System (SDS; i.e., the Center for Space Research (CSR; Bettadpur, 2018), Jet Propulsion Laboratory (JPL; Yuan, 2018), and German Research Center for Geosciences (GFZ; Dahle et al., 2018)) generate GRACE/GFO Level‐2 monthly gravity field solutions for users in an operational manner.…”