High-altitude platform station

A high-altitude platform station (HAPS, which can also mean high-altitude pseudo-satellite or high-altitude platform systems), also known as 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 has been studied since at least 1983, and demonstrator programs since 1994. Hydrogen and solar power have been 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 have been capable of reaching high altitudes since the 1950s, their endurance is limited. One of the few operational HALE aircraft is the Northrop Grumman RQ-4 Global Hawk. There are many solar powered, lightweight prototypes like the NASA Pathfinder/Helios, or the Airbus Zephyr that can fly for 64 days; few are as advanced as these. Conventional aviation fuels have been used in prototypes since 1970 and can fly for 60 hours like the Boeing Condor. Hydrogen aircraft can fly even longer, a week or longer, like the AeroVironment Global Observer.

Stratospheric airships are often presented as a competing technology. However few prototypes have been built and none are operational. Among balloons specifically, the most well known high-endurance project was Google Loon, using helium-filled high-altitude balloons to reach the stratosphere. Loon was ended in 2021.