Low-earth-orbit satellites

The initial communications satellites orbited only a few hundred miles above earth. They were low-earth-orbit (LEO) satellites. Due to their low orbits, LEO satellites took only around 90 minutes to complete one revolution. This made communication spotty and inconvenient, as a satellite was in range of any given ground station for only a few minutes at a time. Due to this, GEO satellites became predominant.However, GEO satellites have certain limitations. A geostationary orbit needs adjustment constantly, as a tiny change in altitude will cause the satellite to get out of sync with the earth’s rotation. The geostationary satellites are very expensive to launch and maintain. While communicating through them, there is always a delay due to the path length. It takes much high transmitter power, and a sophisticated, which is precisely aimed antenna, to communicate reliably.

These problems with GEO satellites have brought about a revival of LEO scheme. 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, assume that the satellites can relay messages throughout fleet. Then any 2 points on surface can always make, and maintain, contact through satellites.

A LEO system employs satellites in orbits strategically spaced around globe. The satellites are placed in polar orbits (routes which pass over the earth’s geographic poles) as such orbits optimize coverage of 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 does not have to be aimed in any particular direction. The transmitter can reach network by using only a few watts of power. The propagation delay is much shorter than is the case with the geostationary link, usually less than 0.1 second.