Ice sheet surface velocity determination from ICESat-2 repeat tracks

Improved understanding and forecasting of sea-level rise due to the changing cryosphere require regular high-resolution observations of ice-sheet motion. We present a methodology to calculate short repeat-time ice-sheet surface velocity using the new Advanced Topographic Laser Altimeter System aboard the ICESat-2 satellite. We use cross-correlation of altimetric time series from successive ground tracks to determine along-track displacement of surface elevation features over the satellite's 91-day revisit period, allowing us to estimate surface velocity between repeats. We demonstrate our methodology on Foundation Ice Stream (FIS), Antarctica, where the ice flow direction is well-aligned with the satellite tracks. Velocity resolution is initially limited by spacing of along-track data points. The pre-processed ICESat-2 land ice height product (ATL06) has a nominal along-track resolution of 20 m, yielding an along-track velocity resolution of ~80 m/year when comparing two subsequent orbit cycles. We improve along-track velocity determination by generating an analogous land ice height product with 5-m sample spacing from ATL03 point clouds, yielding velocity resolution of ~20 m/year, and by implementing sub-sample determination of correlation peaks. The along-track velocities obtained using this technique for fast-moving regions of FIS and its ice shelf are broadly similar to along-track velocities extracted from the MEaSUREs Antarctic Surface Velocity dataset. Velocity determination for slower moving ice is less successful. Additional analysis of intersecting ground tracks to calculate a two-dimensional velocity field is forthcoming.