Orbits are usually elliptical, but nearly all artificial satellites aim for one as circular as possible so that they can remain at the same height. They generally fall into three categories, classified as Low, Medium, and Geosynchronous Earth Orbit. The last category is the most useful for communications, though some newer systems have aimed for others.

Each of the different orbits shown in Figure 12.1 and Table 12.1 has slightly different properties. Higher orbits require more powerful rockets and take longer to reflect a signal back to Earth. Lower orbits cover a smaller area per satellite and fall back to Earth more quickly because of atmospheric drag. A very thin atmosphere extends thousands of kilometers into outer space, and friction ...

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