Satellite communications

Satellite communications include a wide range of technologies and applications that depend on high-altitude electronic relay(s), most often artificial satellites in Earth orbit. The article is not titled "communications satellite", since many applications involve multiple orbiting vehicles, and also a great deal of equipment on Earth. The definition is sufficiently broad to include communications that use communications relay devices operating within the atmosphere, including unmanned aerial vehicles, lighter-than-air devices carrying electronics, or manned aircraft with a primary communications relay function.

Operating frequency tradeoffs

UHF satellites

Satellites operating in the Ultra-High Frequency (UHF) offer the advantage of simple receivers and antennas, with plausible connectivity between the satellite and a handheld or other portable receiver. The disadvantage is that such communications rarely support high data rates, but that is perfectly acceptable in some applications, where short messages are meaningful. Civilian examples include Emergency Position Indicating Radio Beacons (EPIRB), which send short distress messages to the COSPAS-SARSAT constellations.

Military communications involving primarily text messaging or low-speed facsimile also work well with UHF, not needing complex antennas. Examples include the early United States Navy FLTSATCOM, followed by the current UHF Follow-On (UFO) system, which, in turn, will be succeeded by the Mobile User Objective System. For their particular operating environment, the military can afford additional signal processing that provides more bandwidth, but even they will operate in other frequency ranges when high data rates are needed.

Orbital tradeoffs

==Advanced features==