The earliest communications satellites orbited only a few hundred miles above the earth. They were low earth-orbit (LEO) satellites. Because of their low orbits, LEO satellites took only about 90 minutes to complete one revolution.

This made communication spotty and inconvenient, because a satellite was in range of any given ground station for only a few minutes at a time. Because of this, GEO satellites became predominant.

However, GEO satellites have certain limitations. A geostationary orbit requires constant adjustment, because a tiny change in altitude will cause the satellite to get out of sync with the earth’s rotation.

Geostationary satellites are expensive to launch and maintain. When communicating through them, there is always a delay because of the path length. It takes high transmitter power, and a sophisticated, precisely aimed antenna, to communicate reliably.

These problems with GEO satellites have brought about a revival of the LEO scheme. Instead of one single satellite, the new concept is to have a large fleet of them.

Imagine dozens of LEO satellites in orbits such that, for any point on the earth, there is always at least one satellite in range. Further, suppose that the satellites can relay messages throughout the fleet. Then any two points on the surface can always make, and maintain, contact through the satellites.

A LEO system employs satellites in orbits strategically spaced around the globe. The satellites are placed in polar orbits (routes that pass over or near the earth’s geographic poles) because such orbits optimize the coverage of the system.

A LEO satellite wireless communications link is easier to access and use than a GEO satellite link. A small, simple antenna will suffice, and it doesn’t have to be aimed in any particular direction.

The transmitter can reach the network using only a few watts of power. The propagation delay is much shorter than is the case with a geostationary link, usually much less than 0.1 second.

No comments:

Post a Comment