Stratellites (the term comes from combining the words stratosphere and satellite) are high-altitude, unmanned airships designed to operate continuously for weeks, months, or even years at a time. Their main purpose is to hover over one geographical location and function as a relay and/or transmission source for radio signals, including as broadband, mobile phone, and digital television transmissions. The idea is currently being developed by the company Sanswire and others.
A stratellite would hover about 20 kilometers up, above the jet stream and most weather, and could service an area about 320 kilometers in diameter, or about 480,000 square kilometers. Lag times would be many times less than that for orbiting satellites carrying the same type of signals, but would cover a larger area than traditional transmission towers.
The frame and components would be made of very lightweight but strong modern composites, and the topside would be covered over with high-efficiency solar cells. These cells would feed power into onboard batteries, which in turn would run the onboard systems and the electric propeller engines it would use for station keeping. Though designed to land at frequent intervals for maintenance, a well-designed stratellite could stay in the air for many months or even years without tending.
A system of overlapping stratellites is thought to be cheaper to create and maintain than a similar system of transmission towers, as fewer stratellites would be needed to cover any particular region. Stratellites would also suffer less total wear as they fly above most damaging weather, and so despite their more advanced design may end up needing less total maintenance and repair over their operational lifetimes.
Stratellites can also be used for long-term surveillance of a particular area, for military, research, or other purposes. Unlike spy and surveillance satellites, their coverage of an area would be constant, whereas the satellites will be able to gather data only during certain portions of their orbit.
Stratellites were seen in the novel Spin by Robert Charles Wilson as well as other scifi sources.