At much lower altitudes, between the two Van Allen belts, we find the MEO
(Medium-Earth Orbit) satellites. As viewed from the earth, these drift slowly in
longitude, taking something like 6 hours to circle the earth. Accordingly, they
must be tracked as they move through the sky. Because they are lower than the
GEOs, they have a smaller footprint on the ground and require less powerful
transmitters to reach them. Currently they are used for navigation systems rather
than telecommunications, so we will not examine them further here. The constellation
of roughly 30 GPS (Global Positioning System) satellites orbiting at about
20,200 km are examples of MEO satellites.
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