# Evaluation of the Consistency of Three GRACE Gap-Filling Data
###### Abstract  
The Gravity Recovery and Climate Experiment (GRACE) gravity mission has become a leading platform for monitoring temporal changes in the Earth's global gravity field. However, the usability of GRACE data is severely limited by 11 months of missing data between the GRACE and GRACE Follow-on (GRACE-FO) missions. To date, several approaches have been proposed to fill this data gap in the form of spherical harmonic coefficients (an expression of the Earth's gravity field, SHCs). However, systematic analysis to reveal the characteristics and consistency of the datasets produced by these latest gap-filling techniques is yet to be carried out. Here, three SHC gap-filling products are systematically analyzed and compared: (1) Combining high-low satellite-to-satellite tracking with satellite laser ranging (SLR) observations (QuantumFromters, QF), (2) SLR-based recovery incorporating the GRACE empirical orthogonal function decomposition model proposed by the Institute of Geodesy and Geoinformation at the University of Bonn (hereafter, denoted as IGG), and (3) applying the singular spectrum analysis approach (SSA). The results show that (1) the SHCs of the QF, IGG, and SSA data are consistent up to degree 12; (2) the IGG and SSA data give similar results over the 11 gap months, but the IGG shows a faster increase in the mean ocean water mass and the SSA appears to better capture the interannual variation in the terrestrial water storage; and (3) the noise level increases significantly in the high-degree terms (l > 16) of the QF data, so these data are only applicable for large-scale mass migration research. These results provide a reference for users to select a gap-filling product. Finally, we propose a new scheme based on the triple collocation method to derive a weight matrix to fuse these three datasets into a more robust solution. s 202  
field with unprecedented spatial and temporal resolution [3]. The gravity field products provided by GRACE gravity satellites have been widely used and greatly contributed to our understanding of seismic activities, terrestrial water storage, sea level changes, and polar and mountain ice melt [4; 5; 6]. The GRACE gravity satellites provide standard scientific products of Earth's monthly mean gravity field in the form of spherical harmonic coefficients (SHCs) resolved up to degrees 60 or 96. The main GRACE data processing agencies consist of the Center for Space Research (CSR) at the University of Texas at Austin, the German Research Centre for Geosciences, and the NASA Jet Propulsion Laboratory. Although the designed lifetime of the GRACE gravity satellite was only five years, the satellite has fulfilled its scientific mission, and its service was extended to June 2017. GRACE's successor satellite, GRACE-FO (GFO), was launched in May 2018 and began to deliver time-varying gravity products one month later. GFO continues to carry out the GRACE science mission, but the 11-month gap between the two missions inevitably limits our ability to systematically analyze and fully utilize the satellite observations of GRACE and GFO over the past two decades. The missing one or two months of data during the service life of GRACE are usually filled by interpolation processes, such as cubic spline interpolation [7; 8]. However, filling the 11-month gap between GRACE and GFO presents a major challenge. Gravity observations from other satellites provide promising solutions. For example, laser ranging technology (SLR) has been used to track long-term changes in the earth's gravitational field since the 1970s, when the Starlette satellite of the French National Center for Space Research and the LAGEOS satellite of NASA were deployed [9]. Although SLR yields lower-resolution gravity fields than GRACE, it has a longer operational lifetime and continues even when GRACE missions are disrupted, opening up the possibility of using SLR data to fill in the gaps between GRACE and GFO [10; 11; 12].