NASA's Earth-observation programme, part of its Earth Science Enterprise programme and the first observing system to provide integrated measurements of the Earth's processes. The satellites have been Terra, launched on 18 December 1999 to observe the land, oceans, and the Earth's atmosphere; Aqua, launched on 4 May 2002 to study the interaction of oceans with land; and Aura, launched on 15 July 2004 to study the ozone, air quality, and climate.

Data are received from a coordinated series of polar-orbiting and low-inclination satellites that provide long-term observations of the land, clouds, oceans, polar ice sheets, atmosphere, and biosphere. Scientific interpretation of the EOS data provides improved weather forecasts, better predictions of climate change, practical information for farmers and fishermen, and other enhanced knowledge about the global environment.

http://eospso.gsfc.nasa.gov/ Huge resource with information about the many projects which make up NASA's Earth Science Enterprise. There are superb image galleries organized by theme and by source, as well as a wide selection of documents, from tutorials to technical reports.