[jayhuang]

Some additional information to augment the info provided by the other replies. One more recent example would be the RADARSAT Constellation Mission (RCM):

http://www.asc-csa.gc.ca/eng/satellites/radarsat/default.asp

Having a constellation of satellites basically allows for 24/7 coverage of any given surface of the Earth, given the spherical nature of the Earth.

Full disclosure: I worked on RCM (linked above)