The key to monitoring Earth's changing environment and to guaranteeing a consistent stream of satellite data to improve our daily lives is to take the same measurements over the course of decades. But how do you know that measurements from successive satellites, even though identical in build, are like for like?

The answer, for the Copernicus Sentinel-3 mission, is to engage in some nifty orbital flying.

Sentinel-3 is a two-satellite mission to supply the coverage and data delivery needed for Europe's Copernicus environmental monitoring programme.

Launched in 2016, Sentinel-3A has been measuring our oceans, land, ice and atmosphere to monitor and understand large-scale global dynamics and to provide critical information for marine operations, and more.