# GNSS Profile from the Greenland Korth Expeditions in the Context of Satellite Data
## 4 Greenland Korth Expeditions and Satellite Altimetry  
In this section we will compare the GNSS data measured along the Greenland Korth Expeditions profile (Section 2) with satellite altimetry observations. Similarly to [PERSON] et al. [17] and [PERSON] et al. [20], at each measurement site three height differences were computed out of the Greenland Korth Expeditions height measurements in the years  
Figure 7: Surface elevation change from satellite radar altimetry displayed as five-year differences: (a) Dec/2006-Jan/2002, (b) Dec/2010-Jan/2006, (c) Dec/2015-Jan/2011. Panel (d): Surface elevation changes derived from the GNSS data along the Greenland Korth Expeditions profile and satellite radar altimetry (RA). The Greenland Korth Expeditions site numbers are added. Panel (e): Differences between the GNSS and RA observations compared to the uncertainty in satellite altimetry data (3-sigma). 2002, 2006, 2010 and 2015. For this study we selected only sites where measurements from all four campaings were available. In Figure 7a-c, the first three panels show maps of the surface elevation change from radar altimetry for 5 year periods corresponding to those defined by the GNSS data. The altimetry data were computed merging the observations from several satellites (ERS-1, ERS-2, Envisat, Cryosat-2, Sentinel-3) [27,28]. Already these maps demonstrate that the overall trend in the surface elevation loss is increasingly negative. During the period of 2002-2006, some areas in southern Greenland increased in elevation (see yellow shading, Figure 7a), whereas for the 2010-2015 period, decreased surface elevations prevailed (see blue shading, Figure 7c). Figure 7d compares the surface elevation differences obtained from GNSS and satellite altimetry for the studied period of 2002-2015. Both datasets clearly show the same behavior, namely that with time one can observe larger drops in surface elevation. (This is not due to glacial isostatic adjustment as will be evident later in Section 5). In the central part of the Greenland North Expeditions profile, at sites No. 11-28 (see Figures 5b and 6b), there was an increase in surface elevations for the period of 2002-2006. The largest drop in surface elevations took place at Greenland North Expeditions sites No. 31-35 near the western coast (Figure 7d). This is the area of _Jakobshavn Glacier_, one of the fastest moving glaciers in Greenland and Antarctica (Figure 6b). Jakobshavn Glacier drains 6.5% of the Greenland ice sheet and produces around 10% of all Greenland icebergs [29]. Based on satellite observations, the speed of Jakobshavn Glacier gradually increased from 5.7 km/yr to 12.6 km/yr over the period of 1992-2012. Surprisingly, over 2016-2018 Jakobshavn slowed to near its pre-1997 speed and is even thickening [30]. One hypothesis for this behavior is that cooler ocean waters were advected to the glacier's face in 2016, with water temperatures in the vicinity of Jakobshavn Glacier now colder than they have been since the mid-1980s [30]. To assess the agreement between the GNSS and altimetry profiles, Figure 7e compares the difference between GNSS and altimetry values from Figure 7d with the uncertainty of the used satellite altimetry data. At the Greenland North Expeditions profile points, the accuracy of the ground-based GNSS measurements is higher than that of satellite altimetry, with a vertical measurement uncertainty of about \(\pm 3\) cm [11,17]. Figure 7e confirms that the agreement of the GNSS and altimetry height differences is below the altimetry uncertainty curves in the majority of the measurement sites; larger discrepancies between GNSS and altimetry are found in the areas with large slopes (in the east, sites No. 5-11) and near Jakobshavn Glacier (sites No. 31-34), owing to slopes and considerable local ice dynamics. We add a note on the Greenland North Expeditions sites in the context of the ice sheet surface properties and the slope.