(29b) Satellites which look down and observe the Earth

A GOES Satellite

These include satellites which monitor the weather for the US National Oceanic and Atmospheric Administration (NOAA), such as the GOES series (Geostationary Operational Environmental Satellite), an early one of which is shown on the right. They are in synchronous orbits (r = 42,164 km), keeping the same area in view as the Earth rotates.

To this class also belong "spy satellites" which observe the ground from low circular orbits. For obvious reasons they are rarely described in public, but some are said to have sizes comparable to that of the Hubble telescope. The US Air Force furthermore sponsors polar satellites of the DMSP series (Defense Meteorological Satellite Program), scanning the cloud cover of the polar region and also studying ions and electrons of the magnetosphere.

Other satellites scan the Earth for a variety of purposes: for instance, the French SPOT series of surveillance satellites mainly serves commercial customers (one SPOT observed among other things the Soviet nuclear reactor accident in Chernobyl). NASA's Polar collected images of the polar aurora (as did the "ISIS" satellite before, and many afterwards) , and the US Landsat series launched primarily to observe vegetation. The Canadian satellite Alouette in 1962 bounced radio signals off the top of the ionosphere, and UARS (Upper Atmosphere Research Satellite), followed by the EOS series have studied the Earth atmosphere.