Satellites in tandem reveal 30 years of Antarctic ice flow

ESA’s first Earth observing satellites, ERS-1 and ERS-2, were launched in 1991 and 1995, respectively. At the time, these two satellites were Europe’s most sophisticated Earth observation satellites ever developed and launched.

Shortly after ERS-2 had been placed in orbit, ESA manoeuvred the two ERS satellites into a novel tandem formation, allowing them to observe the same area of Earth just 24 hours apart. 

This orbital arrangement was maintained for nine months. The tandem mission provided scientists with an unprecedented volume of closely spaced observations and a unique opportunity to track changes occurring over very short timescales.

While a further tandem campaign was carried out in 2008 with ERS-2 and the Envisat satellite, the most recent repeat of the concept involved the Copernicus Sentinel-1C and Sentinel-1D radar satellites.

During Sentinel-1D’s commissioning phase, it was placed temporarily in close formation with Sentinel-1C, to achieve a one-day repeat-pass interval for the constellation. Importantly, this tandem configuration supported cross-calibration of the two satellites and verified their interferometric synthetic aperture radar performance.

In parallel, ESA maximised the scientific return from Sentinel-1A before its recent retirement by operating it with Sentinel-1C in its standard six-day repeat-pass configuration.

ESA’s Sentinel-1 System Manager, Dirk Geudtner, said, “The near-simultaneous observations from the three Sentinel-1 satellites provided a rare opportunity to monitor glacier and ice-sheet dynamics across different timescales.

“In particular, by imaging the same region of Antarctica just one day apart, the Sentinel-1C and Sentinel-1D satellites recreated the observation time interval that made the original ERS-1–ERS-2 tandem mission a breakthrough for measuring glacier motion and mapping grounding lines.”

A grounding line is the critical boundary where a glacier or ice sheet stops resting on bedrock and begins to float on the ocean, forming an ice shelf. It acts as an anchor point, controlling how quickly ice flows from the continent into the sea.

Both the flow velocity of glaciers and ice streams, and changes in the position of the grounding line are key indicators of how ice sheets are responding to climate warming. Together, these measurements help scientists estimate changes in ice mass balance and quantify the amount of ice being discharged into the ocean, improving projections of future sea-level rise.

Comparisons between the measurements from the ERS tandem phase and those from the Sentinel-1C–Sentinel-1D tandem configuration reveal how Antarctica has changed over the past three decades - as the side-by-side interferograms below show.

As an example, changes of ice velocity and surface deformation of the Scar Inlet Ice Shelf, the southern remnant of the former Larsen-B Ice Shelf, and over the fast-flowing Evans Ice Stream in West Antarctica can be seen in the side-by-side interferograms.

The Sentinel-1C–Sentinel-1D interferogram on the right reveals major fractures and rifts in the ice shelf that were not present in the corresponding 1995 ERS-1–ERS-2 interferogram on the left.

The weakening of the ice shelf has contributed to the acceleration and thinning of tributary glaciers, increasing ice discharge and driving the inland migration of the grounding line. This change is a key indicator of the dynamic response and stability of grounded ice masses.

The graph below shows the substantial increase in Leppard Glacier’s ice-flow velocity between 1995 and 2026 using one-day interferometric synthetic aperture radar observations from both tandem phases.

Prof. Emeritus Helmut Rott, who having worked extensively with ERS-1–ERS-2 tandem interferometric synthetic aperture radar data, said, “Detailed observations of such features and their temporal change, as provided by the ERS-1–ERS-2 and Sentinel-1C–Sentinel-1D one-day repeat data, opens excellent opportunities for improved understanding and predictions of processes of ice-shelf weakening and disintegration, grounding line migration and loss of grounded ice.”

The rare combination of observations from all three Sentinel-1 satellites also highlights the ice-flow velocity over the Evans Ice Stream, in West Antarctica.

Evans Ice Stream, the principal ice stream draining the southern Antarctic Peninsula into the Ronne Ice Shelf, transports ice at speeds of around 3–4 metres per day.

Because of its fast flow and well-characterised dynamics, it has been proposed as the calibration and validation site for polar ice-sheet products from the upcoming Copernicus ROSE-L mission.

The ice stream occupies a deep trough bounded by steep valley walls, with pronounced shear margins that appear as tightly packed fringes in the radar interferograms. These features provide valuable information on ice deformation and help scientists assess how ice streams respond to climate change.

Because the ice flows so rapidly, the standard six-day Sentinel-1A–Sentinel-1C interferogram, acquired on 22 and 28 March 2026, contains highly compressed fringes, making it difficult to measure absolute ice-flow velocity accurately. In contrast, the one-day Sentinel-1C–Sentinel-1D interferogram from 22 and 23 March 2026 produces a much more widely spaced fringe pattern, enabling far more precise measurements of ice-flow velocity.

In addition, the ‘double-difference’ interferogram, derived from two Sentinel-1C–Sentinel-1D pairs, clearly reveals the tidal ice deformation and shear zones.

Thomas Nagler, CEO at ENVEO and member of ESA’s Mission Advisory Group for the Sentinel-1 Next Generation and the ROSE-L missions, said, “Reducing the repeat-pass interval from six days to just one day enables monitoring shear zones and fast-moving glaciers and ice stream by means of interferometry with greater accuracy.”

ESA’s Sentinel-1 and Sentinel-1 Next Generation Project Manager, Thibaut Decoopman, said, “Sentinel-1 has established a benchmark for high-quality, end-to-end radar interferometric performance.

“Building on this success, Sentinel-1 Next Generation will secure the continuity of C-band observations while providing enhanced performance to address the evolving needs of users and Copernicus services.”

Note: This research was supported by ESA SUPSAR-Ice Velocity and ESA’s Climate Change Initiative projects.