This article describes the observation techniques and suggests processing methods to estimate dynamical sea-ice parameters from data of the Earth Explorer 10 candidate Harmony. The two Harmony satellites will fly in a reconfigurable formation with Sentinel-1D. Both will be equipped with a multi-angle thermal infrared sensor and a passive radar receiver, which receives the reflected Sentinel-1D signals using two antennas. During the lifetime of the mission, two different formations will be flown. In the stereo formation, the Harmony satellites will fly approximately 300km in front and behind Sentinel-1, which allows for the estimation of instantaneous sea-ice drift vectors. We demonstrate that the addition of instantaneous sea-ice drift estimates on top of the daily integrated values from feature tracking have benefits in terms of interpretation, sampling and resolution. The wide-swath instantaneous drift observations of Harmony also help to put high-temporal-resolution instantaneous buoy observations into a spatial context. Additionally, it allows for the extraction of deformation parameters, such as shear and divergence. As a result, Harmony's data will help to improve sea-ice statistics and parametrizations to constrain sea-ice models. In the cross-track interferometry (XTI) mode, Harmony's satellites will fly in close formation with an XTI baseline to be able to estimate surface elevations. This will allow for improved estimates of sea-ice volume and also enables the retrieval of full, two-dimensional swell-wave spectra in sea-ice-covered regions without any gaps. In stereo formation, the line-of-sight diversity allows the inference of swell properties in both directions using traditional velocity bunching approaches. In XTI mode, Harmony's phase differences are only sensitive to the ground-range direction swell. To fully recover two-dimensional swell-wave spectra, a synergy between XTI height spectra and intensity spectra is required. If selected, the Harmony mission will be launched in 2028.

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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. @en