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

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 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)