Satellites are electronic devices placed into orbit to monitor and examine regions which we may not be able observe from earth. These are used routinely to monitor atmospheric ozone, ultraviolet radiation, and many other constituents. Satellite altimetry and radar interferometry are used to measure magnetic field intensity and track Antarctic ice flow. Geosynchronous satellites--whose orbit is 24 hours--are used for communication. Arthur C. Clarke proposed the concept of geosynchronous satellites in 1945.