What is over the horizon communication?

Answer 1

Most communications are conducted via electromagnetic radiation. This isn't universally true; there are submarine-to-ship comm systems that use high-frequency sound waves, but this is generally true. At lower frequencies, electromagnetic waves can be "ducted" around the curve of the Earth by being reflected from the ionosphere, or refracted by several atmospheric layers, providing over-the-horizon radio signals that follow the curvature of the Earth. Commercial AM radio and the paradoxically named "high frequency" amateur radio fall into this category.

Higher frequencies - above about 50 MHz - generally are line-of-sight. The signals do not follow the curve of the Earth, and don't reflect off the ionosphere; the signals travel straight off into space. Commercial television in the VHF range starts at about 54MHz, and commercial "FM" radio starts at about 87MHz; these signals are only receivable if you have a straight shot to the transmitter.

We can communicate "over the horizon" by using repeaters; stations that receive on one frequency and broadcast the same signal - "repeating" it - at a different frequency. Many repeaters are positioned on high towers, high buildings, or high mountains to provide broad area coverage.

But if you could put your repeater in space - at a 23,000 mile synchronous orbit - you could receive the signal in about 40% of the Earth's surface at once. Science writer and SF author Arthur C. Clarke proposed a scheme in which three such satellites, located 120 degrees apart around the globe, could provide coverage of nearly the entire world outside of the polar regions. (You can thank Clarke's shortsightedness for the low cost of your satellite TV service; it didn't occur to Clarke to patent the idea, and since it had been published openly, nobody else could patent it either!)