On 21 November 2020, Sentinel-6 Michael Freilich will be launched into orbit as the latest in a long series of ocean-monitoring satellites. It will then begin its five-and-a-half year mission to collect data about global sea levels, atmospheric temperature and humidity, contributing to our knowledge about climate change and supporting weather forecasting.
It is widely accepted that climate change is contributing to sea level rise. With 10% of the world’s population living in coastal zones less than 10 metres above sea level, we need to know both the extent and speed of change, and that is best provided by measurements made across a long time period. Fortunately, satellites have been gathering this data since 1992 and now Sentinel-6 will extend that evidence base. It will also provide the most accurate measurements of sea level variation ever, especially around coastal regions.
Here are 7 essential facts about Sentinel-6 – the latest member of the Copernicus programme – and why this satellite is so important to our understanding of climate change.
1. Why does Sentinel-6 carry the name Michael Freilich?
In January 2020, the European Space Agency (ESA) proposed that Sentinel-6 should be named in honour of the recently retired Dr. Michael Freilich, who headed NASA’s Earth Science division from 2006 to 2019. The move was highly unusual, as only once before has a mission been named after a living person.
Freilich was an unwavering advocate for pushing forward the boundaries of satellite measurements of the oceans. In addition, he championed collaborative investigations of the Earth – and Sentinel-6 is a shining example of such a partnership.
2. Who is behind Sentinel-6?
Sentinel-6 has been jointly developed and funded by ESA, NASA, the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) and the US National Oceanic and Atmospheric Administration (NOAA), with funding support from the European Commission and technical support from France’s National Centre for Space Studies (CNES).
This is the first joint Earth science satellite mission between NASA and ESA. It is also the first involvement in Copernicus, the European Union’s Earth observation programme, by a country from outside the EU.
There was a much earlier Earth science collaboration between the US and Europe, however, when NASA and CNES jointly launched the TOPEX/Poseidon satellite in 1992.
3. What will Sentinel-6 measure?
Sentinel-6’s core services will be:
- marine meteorology – atmospheric processes and their interaction with the ocean
- operational oceanography – measurements of seas and oceans, providing current state, historic data and forecasting
- sea level, coastal regions and climate monitoring
- radio occultation (see below)
- research and remote sensing science.
The primary instrument on board Sentinel-6 will be a radar altimeter (POSEIDON-4) to measure ocean topography – providing data on sea surface height and wave height. It will be the most accurate such mission ever launched and therefore the reference mission for all other altimetry missions. (This altimeter is the shallow cone-shaped instrument seen on the underside of the satellite.)
Another key instrument is the Global Navigation Satellite System – Radio Occultation (GNSS-RO). This will track radio signals from orbiting navigation satellites. When one of these crosses the horizon (from Sentinel-6’s viewpoint), the signal is refracted by the atmosphere, changing its frequency and path. By measuring this effect, scientists can measure minute changes in the atmosphere’s density, temperature and moisture content.
Sentinel-6 will also measure wind speed and provide data to map the tectonic features of the sea floor.
4. How will Sentinel-6 contribute to our understanding of climate change?
Sea level rise is due to a combination of meltwater from glaciers and ice sheets and absorption of heat trapped by rising greenhouse gases. Between 1993 and 2018, global sea level rose by an average of 3.2mm. Now it is averaging 4.8mm per year. Rising sea levels will not only change coastlines but also increase flooding from tides and storm surges.
Satellites have been monitoring rises in sea level continuously since the launch of the TOPEX/Poseidon satellite in 1992, which was followed by the JASON series of satellites. The measurements made by Sentinel-6 will be higher resolution than those of its predecessors, letting it capture smaller sea level variations near coastlines, which affect navigation and commercial fishing. It will also provide data on smaller, more complicated ocean features and map rivers and lakes for hydrology purposes.
Local sea levels can be measured by in situ gauges, but these may not be calibrated identically around the world. Now, for the first time there will be a single, highly accurate source for measurements globally.
As with other Sentinel missions, Sentinel-6 will have a ‘twin’; Sentinel-6B is scheduled to launch in 2025, extending the series of measurements by another 5 years.
5. How will Sentinel-6 help with weather forecasting?
Both the measurements of sea surface conditions and observations of the atmosphere will help to improve weather forecasts. For example, information on atmospheric temperature and humidity, plus the temperature of the upper layer of the ocean, will enhance models that track the formation and evolution of hurricanes.
Another example is the El Niño phenomenon, which disrupts weather patterns worldwide, causing droughts in some areas and intense rainfall in others. Sentinel-6 will act as an early warning system of El Niño by detecting the bulge in surface waters caused by a surge of warm waters into the cold waters of the eastern central Pacific.
6. How much of the Earth will Sentinel-6 orbit?
Sentinel-6 will travel in a low Earth orbit chosen to minimize any interference in measurements from tide changes. In fact, for the first year it will fly in tandem with the JASON-3 satellite, following just 30 seconds behind it. Both follow the same orbit as their predecessors. Using identical orbits allows scientists to cross-calibrate the data and ensure continuity of measurements.
The orbit will allow it to map 95% of the Earth’s ice-free oceans every 10 days.
7. Why does Sentinel-6 look like a Swiss chalet or garden shed?
Simply because that is the most efficient shape for what it is designed to do, in terms of the positions of the payloads and the solar panels.
In addition, the fixed solar panels ensure stability of the whole platform, which is crucial in the process of measuring accurately the range from satellite to the surface and avoid aging of moving parts on the platform itself.
