The European Space Agency (ESA) has dedicated a substantial part of its programmes to observing the Earth since the launch of its first meteorological mission Meteosat in 1977. Following this mission, the subsequent series of Meteosat satellites, the ERS-1 and ERS-2 missions and, more recently, Envisat, the largest Earth observation (EO) satellite ever built, have provided a wealth of data about the Earth, its climate and changing environment.