# GNSS Profile from the Greenland Korth Expeditions in the Context of Satellite Data
## 6 Updated Estimate of Mass Change in the Greenland Ice Sheet from Satellite Gravimetry  
The GRACE satellites started observing the Earth's gravity field in 2002, and during the first few years in orbit the GRACE measurements of the time-variable gravity were validated by independent terrestrial, mainly seasonal hydrological data [35,47,48], and only afterwards first estimates of Antarctic and Greenland mass loss based on estimated linear trend started to appear [49, 50, 51, 52]. In Figure 9a-d we present maps for the linear trend fitted to the time series of GRACE monthly fields. The results in this section are derived from monthly gravity solutions corrected for the glacial isostatic adjustment effect (Section 5). The particular displayed periods were selected to highlight the changing location of the maximum mass loss over time. Over 2002-2007 the maximum of the mass loss stayed in south-east Greenland, and then it moved to the entire west coast and peaked over 2007-2012. In the final years of GRACE in orbit, 2012-2017, the maximum mass loss was confined to the central west. These three approximately homogeneous phases produce an average trend map shown in Figure 9d. Figure 9: Trend in the equivalent water height over Greenland derived from GRACE monthly solutions (UT CSR RL6, corrected for GIA, 200 km Gaussian smoothing) for the periods: (**a**) 4/2002–3/2007, (**b**) 4/2007–3/2012, (**c**) 4/2012–3/2017, (**d**) the whole flight of GRACE, 4/2002–3/2017. In Figure 10 we present the time evolution of mass balance for the Greenland ice sheet derived from satellite observations. As a reference, we show the reconciled estimate of the Greenland mass rate from the Ice Sheet Mass Balance Inter-Comparison Exercise (IMBIE), a collaboration between scientists supported by ESA and NASA [53]. The IMBIE-2019 ice sheet mass balance estimate (in blue) is based on 26 individual groups of satellite measurements coming from altimetry, gravimetry and the input-output method [6; 8]. The longest record starting in 1992 is derived from the input-output method, which is based on estimating the balance between input (surface mass balance of snowfall, rainfall vs. sublimation, runoff) and output (ocean ice discharge). Figure 10 shows a balanced mass budget until around 2002, when a mass loss started to prevail. Monthly gravity fields from GRACE are available for its 15-year operational period covering 4/2002-6/2017. In order to proceed with successful GRACE observations, a continuation mission GRACE Follow-On (GRACE-FO) was organized and has started to provide monthly solutions since 6/2018 [54; 55]. Due to technical problems with launching the two GRACE-FO satellites, a one-year gap in the time series of monthly gravity fields produced by GRACE and GRACE-FO occurred. This one-year gap may be completed by gravity fields derived from the GNSS tracking of three Swarm satellites, in low-Earth orbit since 11/2013 [56; 57]. Indeed, it was demonstrated that precise satellite positions from the geodetic-quality GNSS receivers (one receiver aboard each Swarm satellite) could be processed to produce monthly gravity fields that, although with a reduced spatial resolution of 1300 km and more noise, are able to produce reasonable estimates also for the mass trends in Greenland [58; 59; 60; 61]. We processed the monthly gravity solutions of GRACE, GRACE-FO and Swarm similarly to the IMBIE approach; the linear trend was fitted within a 36-month moving window using a weighted least squares [6]. To reduce the higher noise in the Swarm monthly gravity fields, for computing the linear trend we used a 48-month moving window. As shown in Figure 10, our GRACE estimated mass loss rates (in red) accompanied with an uncertainty band agree well with the reference IMBIE-19 curve. In accordance with other gravimetry solutions shown in the IMBIE-19 paper [6], the maximum yearly mass loss from GRACE was recorded in 2011, followed by a return to moderate mass balance values over 2012-2017.