Data For: Grounding zone of Helheim Glacier, Greenland, from terrestrial radar interferometry

Helheim Glacier, in East Greenland, is a major outlet glacier that discharges about 33 Gigatons of ice per year into the ocean. Helheim has been undergoing a retreat since 2003. Using a portable terrestrial radar interferometer that scans the glacier every 2 minutes, we characterize its regime of tidal flexure at the ice front margin in relation to oceanic tide, bed slope, and iceberg calving events. The data reveals the presence of a central bed ridge that acts as a fulcrum, with one side of the glacier moving in phase with the tide, while the other is moving out of phase with a reduced amplitude. We detect seawater intrusions of one full thickness beneath the glacier front at high tide. Following tabular calving events, most ice flexing ceases, indicating that the ice front becomes grounded and that calving events involve ice blocks that are already afloat. 

Methods
The GPRI complex images in slant-range geometry were combined to generate interferograms and geocoded into an Earth-fixed grid. The pointing angle of the GPRI was refined by adjusting image features in the geocoded datga with a reference Landsat image. Phase unwrapping was conducted from a stationary point on the glacier side margin. Here, we form interferograms using data acquired 16-minutes apart and we difference tthe interferograms every 6 hours. We extend the analysis over a period of 6 days. From the differential interferograms, we locate the transition boundary where the ice surface is first displaced vertically, or first fringe of displacement, which is our proxy for the grounding line. By repeating the exercise over time, we measure how the grounding line migrates back and forth with tide.