Sentinel-3A and Sentinel-3B have been gently coaxed to flying just 30 seconds apart, which equates to a distance of 223 km. The reason for this is to understand the small differences between successive satellite’s instruments. Given the satellites’ brief separation, their measurements should be virtually the same. Even though the two Sentinel-3 satellites are identical, each carrying a radar altimeter, a radiometer and an imaging spectrometer, there’s a chance that their instruments could behave slightly differently. It is important that any differences are carefully accounted for otherwise the information they deliver could be misinterpreted as changes happening on Earth’s surface.

The mission measures the temperature, colour and height of the sea surface as well as the thickness of sea ice. These measurements are used, for example, to monitor changes in Earth’s climate and for more hands-on applications such as marine pollution. Over land, this innovative mission monitors wildfires, maps the way land is used, checks vegetation health and measures the height of rivers and lakes.