Earth observation satellites are satellites specifically designed to observe Earth from orbit, similar to spy satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc.
Most Earth observation satellites carry instruments that should be operated at a relatively low altitude. Altitudes below 500-600 kilometers are in general avoided, though, because of the significant air-drag at such low altitudes making frequent orbit raising manoeuvres necessary. The Earth observation satellites ERS-1, ERS-2 and Envisat of European Space Agency as well as the MetOp spacecraft of the European Organisation for the Exploitation of Meteorological Satellites are all operated at altitudes of about 800 km. The Proba-1, Proba-2 and SMOS spacecraft of European Space Agency are observing the Earth from an altitude of about 700 km.
