next up previous contents
Next: Cellular Radio Up: Computer Communications Previous: Computer Communications

Geosynchronous Satellites

A geosynchronous satellite remains above the same spot on the earth by orbiting at roughly 36,000 kilometers up; it allows communication between any two points in its footprint (all the places it can broadcast to). For example, the recently launched AsiaSat-1 has a footprint extending over China, Japan, and most of the Pacific Rim countries. Earth stations beam (uplink) microwaves to the satellite and the satellite beams (downlinks) them back to earth. (Microwaves are poorly named; they are so named because they are the shortest radio waves, but radio waves are longer than most other electromagnetic waves, as for example, light.) Microwaves allow a communications capacity of about 1/4 megabyte per second, but with 1/4 second round trip time lag because they must travel to space and back.

As the technology has improved, receivers have shrunk; currently receivers can be less than 1 meter wide (an arm's length) and are expected to shrink further. These receivers are affordable by individuals and are growing ever cheaper. There are now 1,400 satellites of all types in orbit [6].

Gregory J. E. Rawlins