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. 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 that can fly 64 days; few are as advanced. 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.
Definitions
Studies
In 1983, Lockheed produced A Preliminary Study of solar powered aircraft and Associated Power Trains for the NASA, as long endurance flight could be compared to suborbital spacecraft.
The NASA ERAST Program (Environmental Research Aircraft and Sensor Technology) was started in September 1994 to study high-altitude UAVs, and was terminated in 2003.
In 2002, Preliminary reliability design of a solar-powered high-altitude very long endurance unmanned air vehicle was published. The European Union CAPECON project aimed to develop HALE vehicles, while the Polish Academy of Sciences proposed its PW-114 concept that would fly at 20 km (66,000 ft) for 40 hours.
Design
Applications
Atmospheric satellites could be used for weather monitoring, as a radio relay, for oceanography or earth imaging like an orbital satellite for a fraction of the cost.
Airplanes
Reconnaissance aircraft like the late 1950s Lockheed U-2 could fly above 70,000 ft (21,000 m) and the 1964 SR-71 above 80,000 ft (24,000 m).
Operational
Prototypes
HAL CATS Infinity
CATS Infinity being developed by HAL, NAL and NewSpace Research. Its scaled down model underwent first flight in 2022. In February 2024, a test flight of the scaled down prototype weighing 23 kg was tested with a wingspan of 12 m at an altitude of 3 km was carried out on Chitradurga Aeronautical Test Range with a duration of eight and a half hours. Officials stated that the development is now expected for completion in 2027. In the next test, expected to happen in March 2024, the duration shall be increased to 24 hours.
Airships
Unmanned Stratospheric airships are designed to operate at very high 60,000 to 75,000 feet (18.3 to 22.9 km) altitudes during weeks, months or years.
The first stratospheric powered airship flight took place in 1969, reaching 70,000 feet (21 km) for 2 hours with a 5 pounds (2.3 kilograms) payload.