The EarthExplorer 10 mission Harmony by the European Space Agency ESA, scheduled for launch around 2029–2030, consists of two passive C-band synthetic-aperture-radar companion satellites flying in a flexible constellation with one Sentinel-1 radar satellite as an illuminator. Sentinel-1 will serve as transmitter and receiver of radar waves, and the two Harmonys will serve as bistatic receivers without the ability to transmit. During the first and last year of the 5-year mission, the two Harmony satellites will fly in a cross-track interferometric constellation, such as that known from TanDEM-X, about 350 km ahead or behind the assigned Sentinel-1. This constellation will provide 12-day repeat DEMs, among other regions, over most land-ice and permafrost areas. These repeat DEMs will be complemented by synchronous lateral terrain displacements from the well-established offset tracking method. In between the cross-track interferometry phases, one of the Harmony satellites will be moved to the opposite side of the Sentinel-1 to form a symmetric bistatic “stereo” constellation with ±~350 km along-track baseline. In this phase, the mission will provide opportunity for radar interferometry along three lines of sight, or up to six when combining ascending and descending acquisitions, enabling the measurement of three-dimensional surface motion, for instance sub- and emergence components of ice flow, or three-dimensional deformation of permafrost surfaces or slow landslides. Such measurements would, for the first time, be available for large areas and are anticipated to provide a number of novel insights into the dynamics and mass balance of a range of mass movement processes.

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Using a combination of multi-directional SAR and TIR measurements, the Harmony Earth Explorer 10 mission candidate will provide high resolution simultaneous measurements of surface stress, surface currents SST and wave spectra over oceans, 3-D deformation vectors over solid Earth, and time-series of surface elevation changes over volcanic areas and land ice masses. This will serve a series of science objectives aimed at better understating multi-scale processed and feedbacks in the Earth System.@en

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

This paper provides a compact overview of Harmony, an Earth Explorer 10 mission candidate dedicated to the observation of dynamic deformations of ice, solid earth and ocean surfaces. Harmony consists of two receive-only small Synthetic Aperture Radar (SAR) satellites using Sentinel-1 as illuminator, which will alternate close formation phases, dedicated to single-pass cross-track interferometry, with StereoSAR phases dedicated to the study of ocean surface motion and 3-D land surface deformations. In addition the payload includes a compact Thermal Infrared (TIR) camera.@en

The Antarctic Ice Sheet is an important indicator of climate change and driver of sea-level rise. Here we combine satellite observations of its changing volume, flow and gravitational attraction with modelling of its surface mass balance to show that it lost 2,720 ± 1,390 billion tonnes of ice between 1992 and 2017, which corresponds to an increase in mean sea level of 7.6 ± 3.9 millimetres (errors are one standard deviation). Over this period, ocean-driven melting has caused rates of ice loss from West Antarctica to increase from 53 ± 29 billion to 159 ± 26 billion tonnes per year; ice-shelf collapse has increased the rate of ice loss from the Antarctic Peninsula from 7 ± 13 billion to 33 ± 16 billion tonnes per year. We find large variations in and among model estimates of surface mass balance and glacial isostatic adjustment for East Antarctica, with its average rate of mass gain over the period 1992-2017 (5 ± 46 billion tonnes per year) being the least certain.

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