Sea-ice motion is driven by wind and ocean stress, and varies in space and time. Small-scale drifts primarily affect the opening of leads, while large-scale drift primarily controls the loss of sea ice. Both the opening of leads and the loss of sea ice play a major role in the energy balance of the Arctic and Antarctic regions. Understanding sea-ice drift is therefore important for modelling and projecting regional and global climate change. Accurately modelling sea ice and its dynamics requires high-resolution vectorized observations in the polar regions. Synthetic Aperture Radar (SAR) has proven to be a useful tool in the observations of sea-ice drift. Most of the SAR-derived sea-ice-drift estimates make use of feature tracking, which depend on two SAR acquisitions. This limits the temporal resolution and has the tendency to underestimate the sea-ice drift velocity by 10-20%. Single-pass sea-ice-drift velocities can be inferred from SAR data using Doppler centroid anomaly estimation, but it is limited to the line-of-sight direction and has a resolution of several kilometers. The only single-pass interferometric observations of sea ice were made using Tandem-X Along-Track Interferometry (ATI). Its high sensitivity enables the determination of high-resolution sea-ice drift and also to estimate the rotations of individual floes. However, as with the other two methods, Tandem-X is only sensitive to the line-of-sight. One of the main objectives of Earth Explorer 10 candidate Harmony is the observation of sea-ice drift. We will present the first results of a performance analysis of Harmony's observations over sea ice. The passive instruments onboard the Harmony satellites will use Sentinel-1 as an illuminator to provide multistatic observations of the sea-ice surface. Harmony's reconfigurable constellation can either be optimized for a large line-of-sight difference (Stereo) or for range-direction sensitivy (ATI). In the Stereo configuration, it will be possible, for the first time, to obtain instantaneous sea-ice drift vectors. The ATI configuration enables Harmony to acquire high-resolution sea-ice-velocity estimates. As Sentinel-1 is operating in the wide-swath mode over most of the sea ice covered areas, the polar region is sampled once every 1-4 days.

The Gravity Recovery and Climate Experiment (GRACE) mission (launched 2002) and the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) mission (March 2009 to November 2013) collected spaceborne gravity data for the preseismic and postseismic periods of the 2011 Tohoku-Oki earthquake. In addition, the dense Japan GeoNet Global Navigation Satellite Systems (GNSS) network measured with approximately 1050 stations the coseismic and postseismic surface displacements. We use a novel combination of GNSS, GRACE, and GOCE observations for a distributed fault slip model addressing the issues with gravimetric and geometric change over consistent time windows. Our model integrates the coseismic and postseismic effects as we include GOCE observations averaged over a 2 year interval, but their inclusion reveals the gravity change with unprecedented spatial accuracy. The gravity gradient grid, evaluated at GOCE orbit height of 265 km, has an estimated formal error of 0.20 mE which provides sensitivity to the mainly coseismic and integrated postseismic-induced gravity gradient signal of -1.03 mE. We show that an increased resolution of the gravity change provides valuable information, with GOCE gravity gradient observations sensitive to a more focused slip distribution in contrast to the filtered GRACE equivalent. The 2 year averaging window of the observations makes it important to incorporate estimates of the variance/covariance of unmodeled processes in the inversion. The GNSS and GRACE/GOCE combined model shows a slip pattern with 20 m peak slip at the trench. The total gravity change (≈200 μGal) and the spatial mapping accuracy would have been considerably lower by omitting the GOCE-derived fine-scale gravity field information.