Westwards component of ice surface velocity on the Greenland Ice Sheet measured in spring/summer 2012

Ice surface motion was recorded by five dual-frequency Leica SR520 GPS receivers deployed on poles drilled 2 m into the ice surface, within 700 m of Moulin L41A at 66.97N -49.27E. GPS data were post-processed kinematically (King, 2004, http://dx.doi.org/10.3189/172756504781829747) with Track v.1.27 software (Chen, 1998, Ph.D. thesis, Cambridge MA, USA) against bedrock-mounted reference stations using a precise ephemeris from the International GNSS Service )Dow et al., 2009, http://dx.doi.org/10.1007/s00190-008-0300-3). Reference stations were located 1 km from the terminus of Russell Glacier and at Kellyville, giving baseline lengths less than 41 km. Due to gaps in the time series caused by power outage, we averaged the horizontal velocities recorded at the five stations with the fewest gaps to give a single record. Positions were recorded at 30 s intervals; 1-hr means were then smoothed using a 5-point binomial filter. Since there was generally little difference in velocity between the stakes, the mean velocity across the network gives a better indication of the seasonal pattern of ice motion with fewer gaps than in the individual records. Velocities are centred differences of hourly displacements. GPS stakes required periodic re-drilling as they gradually melted out.