Coastal research is set to enter a new paradigm with the successful launch of the Copernicus Sentinel-6 Michael Freilich satellite, a joint US-European mission that launched on 21 November 2020. 

The mission takes over the responsibility as the world’s reference for sea-surface height measurements, a role initiated by the French-US TOPEX Poseidon satellite in 1992. It is named in honour of the former director of NASA’s Earth Science Division, a leading figure in advancing ocean observations from space. 

The spacecraft plays an important role as it ensures continuity of observations in the same orbit, against which satellite observations from different orbits, with different instruments, are compared. This enables scientists to generate a stable time series of global sea-level measurements from multiple satellite missions. Copernicus Sentinel-6 will provide a new class of measurement capability until at least 2030.

A new class for coastal measurement

The platform carries a synthetic aperture radar, a sensor capable of measuring with greater precision and better along-track sampling compared to previous missions. It will achieve a spatial resolution of 300 m – previous altimeter missions had resolution of several kilometres. 

“It’s a game-changer that will lead to significant improvements in our ability to predict future coastal change,” says Robert Weiss, a member of the Future Earth Coasts project, director of the Center for Coastal Studies at Virginia Tech’s Fralin Life Sciences Institute and professor of natural hazards, Department of Geosciences, Virginia Tech. “The new platform allows for validating methods for measuring across different components of the coastal system in an integrated fashion. I would go as far as saying that with this new platform we will enter a new scientific paradigm for study of the evolution of coastal systems.”

Copernicus Sentinel-6 ensures compatibility with previous missions because it uniquely and simultaneously provides both a low-resolution and a high-resolution observing mode. In addition, it will adopt a dedicated tandem flight for 12 months, orbiting just thirty seconds behind Jason-3 to check for and eliminate any artefacts in the data that might arise owing to the new satellite’s instruments and design.

“Copernicus Sentinel-6 includes several elements that set it apart as a climate mission,” says Craig Donlon, ESA’s mission scientist for Copernicus Sentinel-6. “Altimeter synthetic aperture radar measurement techniques will allow scientists to retrieve meaningful measurements closer to the coast.”

Assessing coastal futures

As global temperatures continue to rise, coastal areas will increasingly bear the brunt of storm surges and more frequent, intense weather extremes. Sea level is rising at 3.6 centimetres per decade and shows an accelerating trend: with every centimetre another 3 million people are put at risk of annual coastal flooding. 

Reliable, accurate and long-term observations of sea level rise and its impact in the coastal zone are vital to be able to plan and better protect the ten percent of the world’s population living less than ten metres above sea level.

To address the need for better coastal information to inform decision-making, Future Earth Coasts has a Coastal Futures initiative that draws on an assessment of the state of coastal areas and aims to inform coastal governance and management. 

“Data from Copernicus Sentinel-6 will support our Coastal Futures initiative, enabling us to trace developments along particular coastlines and align them with predictions and scenarios to inform risk assessments, for example,” says Sebastian Ferse, Executive Director of the Future Earth Coasts International Project Office. 

Using wave height from space

In the coastal zone, radar altimetry data is useful for understanding storm surges, as it can be used to determine the ‘total water level envelope’, which includes the mean rise in water levels as well as the waves riding on top. Waves can ‘over-top’ coastal defences and cause catastrophic flooding, sediment transport, damage to infrastructure and loss of life.

Significant wave height  – trough to crest of the tallest third of waves –  together with wind speed over the ocean are collectively known as ‘sea state’. Long-term data records of sea state are needed to assess change over decades, for projecting future climate change with models, and to inform decision-making. They are also used for ship routing, ship and marine infrastructure design, marine insurance, ocean wind farm development and coastal defence planning among many other applications.

Earlier this year, the ESA Climate Change Initiative (CCI) released a new global dataset of significant wave height spanning 1991-2018, as one of 23 Essential Climate Variables supported by CCI and required by the UNFCCC to support understanding of the climate system. Copernicus Sentinel-6 will extend this record.

Sea state is one of many CCI climate data records that can be visualised from a new interactive website Climate from Space, which has selectable data layers and a comparison mode, allowing users to pinch and zoom into views of two different climate variables displayed on a 3D globe or map. And next month ESA will launch a new online climate toolbox, to meet researchers’ needs for analysing, processing and visualising CCI data on the cloud next to ESA’s dedicated climate data archive (the Open Data Portal), making data downloading optional.

Despite the extra challenges that the Covid-19 crisis presented, Copernicus Sentinel-6 launched almost exactly as scheduled, thanks to the strength of the cooperation between the US and European agencies involved (ESA, NASA, EUMETSAT on behalf of the European Commission, NOAA, and France’s National Centre for Space Studies). It is a model of collaboration that will be replicated in future.