Satellite communication

Satellite communication is a relatively new alternative for long-distance communication. It features many advantages over conventional point-to-point radio. Instead of transmitting an analog signal directly from the vessel to a shore station, a digital signal is transmitted upward to a satellite. The satellite then relays the signal to another satellite or to a receiver elsewhere on the surface of the earth.

Satellite communication is private. When you use conventional SSB or VHF, everybody with a receiver in range can monitor your conversation. Signals transmitted for satellite communication, however, are highly directional, making them much more difficult for the casual eavesdropper to pick up. In the case of digital signals, data can be encrypted, making it extremely secure against even a determined spy. Satellite communication allows direct access to the global communication infrastructure (telephone and computer networks). SSB or VHF, on the other hand, both require an intermediary--such as a ship-to-shore operator--to make the appropriate connections ashore.

Satellite communication is not greatly affected by atmospheric or meteorological conditions. The signal does not have to bounce since it only needs to reach an overhead satellite, which is always within line-of-sight. Because the signal is being transmitted primarily upward, it passes through a relatively thin layer of the Earth’s atmosphere. SSB and VHF transmissions, however, must push their way through a great quantity of distortion-producing atmosphere as they travel across the surface of the Earth.

Geosynchronous satellites

Most communications satellites are in geosynchronous, or geostationary, orbits. This means that each satellite is at an altitude (22, 300 miles or so) such that its speed around the earth matches the earth’s rotation. Both the satellite and the surface of the Earth are rotating around the Earth’s axis, but since they are rotating at the same rate, the satellite appears to stay in one place over the Equator. This simplifies signal transmission since once an antenna on the ground is directed toward the satellite, it does not have to be readjusted. The dish antennas used to receive satellite television are directed toward geosynchronous satellites. A vessel as sea, however, does not stay in one place. Even if it did, though, it would tend to provide an insufficiently stable surface for precise antenna alignment.