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A high-altitude platform station (HAPS, which can also mean high-altitude pseudo-satellite or high-altitude platform systems) or atmospheric satellite is a long endurance, high altitude aircraft able to offer observation or communication services similarly to artificial satellites. Mostly unmanned aerial vehicles (UAVs), they remain aloft through atmospheric lift, either aerodynamic like airplanes, or aerostatic like airships or balloons. High-altitude long endurance (HALE) military drones can fly above 60,000 ft (18,000 m) over 32 hours, while civil HAPS are radio stations at an altitude of 20 to 50 km above waypoints, for weeks.
High-altitude, long endurance flight is studied since at least 1983, and demonstrator programs followed since 1994. Hydrogen and solar power are proposed as alternatives to conventional engines. Above commercial air transport and wind turbulence, at high altitudes, drag as well as lift are reduced. HAPS could be used for weather monitoring, as a radio relay, for oceanography or earth imaging, for border security, maritime patrol and anti-piracy operations, disaster response, or agricultural observation.
While reconnaissance aircraft could reach high altitudes since the 1950s, their endurance is limited. Very few HALE aircraft are operational like the Northrop Grumman RQ-4 Global Hawk. There are many solar powered, lightweight prototypes like the NASA Pathfinder/Helios, or the Airbus Zephyr having fly 64 days, while many are not as advanced. Conventional aviation fuels are used in prototypes since 1970 and can reach an endurance of 60 hours like the Boeing Condor. Hydrogen aircraft can even fly longer, a week or longer, like the AeroVironment Global Observer
Stratospheric airships are often presented as a competition, but if a few prototypes were built, none is operational yet. Flying in the stratosphere, the most well known ballon high-endurance project is Google Loon, using helium-filled high-altitude balloons.