Of course, one assumes that these are for use with geosynchronous satellites. But, one has to keep in mind that not all satellites are geosynchronous. There are quite a few satellites in Low Earth Orbit (LEO), such as the Iridium satellites used for world wide cell-phone like coverage, which orbit at an altitude of 485 miles (780 km).

Compare this to a geosynchronous satellite which has an orbit of 22,236 miles (35,786 km). Then, realize that radio signals have square law properties, which means that if you double the distance they have to travel, the power density drops by a factor of four. This means that a LEO satellite can work with MUCH weaker radio signals than a geosynchronous satellite, which almost always require high gain antennas (parabolic dish antennas or likewise). The reason that not all satellites are LEOs instead of geosyncs are that LEOs spend a large amount of their time below the horizon, and are inaccessible. The Iridium system handles this problem by using an array of 66 satellites so that one is always visible from any point on Earth.

http://en.wikipedia.org/wiki/Iridium_%28satellite%29
http://en.wikipedia.org/wiki/Geosynchronous#Orbital_characteristics

This brings up another interesting problem that geosynchronous satellites have. Since geosyncs appear to stay in one position that’s located 22,236 miles above the equator, and since the satellite has to be above the horizon (since radio waves can’t go through dirt), this means that it may be difficult to access geosync satellites from the polar regions (A slight hill, or some trees may be enough to block the signal.). To solve this problem, Russia, which has quite a few areas at high latitudes, developed a satellite orbit to solve this problem. This orbit is known as a molniya orbit (after the satellite series of the same name).

http://en.wikipedia.org/wiki/Molniya_orbit

Dave

P.S. Let’s see if this works. I’ve been having trouble with comments on this blog, so I had to hack the page a bit to enable the comments block to show up.