Two-Dimensional Strain in Landfast and Drifting Sea Ice Using Space- and Ground-Based Radar Interferometry

Space-based interferometric synthetic aperture radar (InSAR) has been used to quantify surface displacements across large areas of landfast sea ice for several decades, advancing understanding of landfast ice extent, stability, and variability. However, a single interferogram is only sensitive to displacement in the radar look direction; model-based interpretation is required to infer two-dimensional, horizontal motion. Additionally, the extension of space-based interferometric methods to pack ice is limited by the short (sub-minute) repeat intervals required to maintain coherence over moving ice. Here we validate an inverse modeling method for quantifying two-dimensional surface strain in landfast ice from InSAR and use this method as a framework for assessing strain in pack ice from ground-based radar interferometry, which combines short repeat intervals and a floe-centered reference frame to mitigate coherence loss due to ice drift. We utilize the Alaska Satellite Facility's hybrid pluggable processing pipeline (HyP3) to generate Sentinel 1 12-day repeat pass interferograms of Elson Lagoon, Utqiaġvik, Alaska, spanning December 2018 to May 2019. We assume any ice surface motion is the result of a linear combination of axial divergence, radial divergence, shear, tilt, and rotation. Fringe pattern orientation, reasonable bounds on resultant displacement magnitudes, and regional knowledge constrain the mode(s) most likely creating the observed signal in each interferogram, which for Elson Lagoon was radial divergence. We model horizontal displacement fields corresponding to the observed fringe pattern and most likely deformation mode(s). Our results agree well with independent, contemporaneous in situ measurements made by a laser strain observation (LSO) system. We also obtain interferograms from a GAMMA portable radar interferometer (GPRI) deployed on the Beaufort Sea during March 2020, and infer two dimensional motions from these images by adapting the method validated above to the ground-based sensor geometry. Results from both the landfast and drifting ice portions of this study demonstrate the potential of interferometry to advance understanding of in situ sea ice processes including strain accumulation and thermal expansion/contraction.