# GNSS Profile from the Greenland Korth Expeditions in the Context of Satellite Data
###### Abstract  
Over the last two decades, a small group of researchers repeatedly crossed the Greenland interior skiing along a 700-km long route from east to west, acquiring precise GNSS measurements at exactly the same locations. Four such elevation profiles of the ice sheet measured in 2002, 2006, 2010 and 2015 were differenced and used to analyze the surface elevation change. Our goal is to compare such locally measured GNSS data with independent satellite observations. First, we show an agreement in the rate of elevation change between the GNSS data and satellite radar altimetry (ERS, Envisat, CryoSat-2). Both datasets agree well (2002-2015), and both correctly display local features such as an elevation increase in the central part of the ice sheet and a sharp gradual decline in the surface heights above Jakobshav Glacier. Second, we processed satellite gravimetry data (GRACE) in order for them to be comparable with local GNSS measurements. The agreement is demonstrated by a time series at one of the measurement sites. Finally, we provide our own satellite gravimetry (GRACE, GRACE-FO, Swarm) estimate of the Greenland mass balance: first a mild decrease (2002-2007: \(-210\pm 29\) Gt/yr), then an accelerated mass loss (2007-2012: \(-335\pm 29\) Gt/yr), which was noticeably reduced afterwards (2012-2017: \(-178\pm 72\) Gt/yr), and nowadays it seems to increase again (2018-2019: \(-278\pm 67\) Gt/yr). Greenland ice sheet; GNSS; gravimetry; altimetry; GRACE; Swarm 2021 11 1115 122  
in Antarctica and Greenland contain about 70% of all freshwater on Earth and over 99% of freshwater ice; if melted completely, the Greenland ice sheet may cause the global sea level to rise by 7 m (e.g., [4]). Another unique aspect of the Greenland ice sheet is the ice-drilling projects; starting in 1966, they have been helping in attempts to reconstruct the historical record of climatic and environmental conditions on Earth back to more than 100,000 years ago [5]. Since the 1990s, there have been several space missions whose aim was to observe the state and physical conditions of Greenland. Both ground-based and satellite observations revealed that ice loss in Greenland has accelerated over the last two decades (Section 6). As this is an important issue for the Earth as a planet, the current state of the Greenland ice sheet and its possible future climatic evolution are of great concern (for a review, see, e.g., [6; 7; 8]). In our paper, we want to compare locally measured GNSS positions with independent observations from satellites. Ground-based GNSS data provide reliable and precise information about the space coordinates of the measurement site at the uncertainty level of a few centimeters (Section 2). On the other hand, satellite geodetic techniques cover broader regions of the surface at once, and thus each technique has its own advantages and disadvantages [7; 9]. For the comparison with the altitude differences obtained from GNSS, we will use satellite radar altimetry data (Section 4) and satellite gravimetry data from the GRACE mission (Section 5). Laser and radar _satellite altimetry_ has better spatial resolution than satellite gravimetry, but it falls short of the high temporal resolution of GRACE. To convert the volume changes into mass changes, altimetry requires assumptions about the firn density. Because of radar signal penetration into the snow, altimetry has problems with the areas of large slopes, such as those around the ice sheet margin. _Satellite gravimetry_ provides direct measurements of mass and mass changes, but these are integrated vertically and one cannot distinguish between the individual sources, whether they are in the atmosphere, hydrosphere or cryosphere, or from the underlying solid Earth. Besides, the spatial resolution of satellite gravity is limited (250-500 km), so one cannot isolate a precise location of smaller anomaly sources (e.g., a glacier). In polar regions, a particular effect is present, called _glacial isostatic adjustment_ or _postglacial rebound_ (e.g., [9]).