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 after nine years of development.

Definitions

High-altitude long endurance (HALE) High-altitude, long-endurance (HALE) aircraft are non-weaponized military drones capable of flying at over 32 hours, like the USAF RQ-4 Global Hawk or its variants used for ISR. This is above and longer than Medium-Altitude, Long-Endurance (MALE) aircraft flying between during 24 hours, more vulnerable to anti-aircraft defense, like the USAF ISR/strike MQ-9 Reaper or its variants.

High-altitude platform station (HAPS) defined by the International Telecommunication Union (ITU) as "a station on an object at an altitude of 20 to 50 km and at a specified, nominal, fixed point relative to the Earth" in its ITU Radio Regulations (RR). HAPS can also be the abbreviation for high-altitude pseudo-satellite.

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. In 1984 was published the Design of Long Endurance Unmanned Airplanes Incorporating Solar and fuel cell propulsion report. In 1989, the Design and experimental results for a high-altitude, long-endurance airfoil report proposed applications as a radio relay, for weather monitoring or cruise missile targeting.

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 July 1996, the USAF Strikestar 2025 report forecast HALE UAVs maintaining air occupation with 24 hours flights. The Defense Airborne Reconnaissance Office made demonstrations of long-endurance UAV craft. In September 1996, Israel Aircraft Industries detailed the design of a HALE UAV.

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 for 40 hours. Luminati Aerospace proposed its Substrata solar-powered aircraft that would fly in formation like migratory geese to reduce the power required for the trailing aircraft by 79%, allowing smaller airframes to remain aloft indefinitely up to a latitude of 50°.

Design

Power Power is required for continuous operation, limiting endurance by the need for refueling. Persistent solar-powered aircraft need to store daylight energy for the night, in electric batteries, or in fuel cells.

Altitude selection Drag is reduced in the tropopause thin air, well above the high winds and air traffic of the high troposphere between . Maintaining a position facing variable winds is a challenge. Relatively mild wind and turbulence above the jet stream is found in most locations in the stratosphere between , although this is variable with the latitude and season. Altitudes above are also above commercial air transport. Flying in the tropopause at is above clouds and turbulence with winds below , and above FAA-regulated Class A airspace ending at .

Comparison to satellites A lower altitude covers more effectively a small region, implies a lower telecommunications link budget (a 34 dB advantage over a LEO, 66 dB over GEO), a lower power consumption, and a smaller round-trip delay. Satellites are more expensive, take longer to deploy, and cannot be reasonably accessed for maintenance. A satellite in the vacuum of space orbits due to its high speed generating a centrifugal force matching the gravity. Changing a satellite orbit requires expending its extremely limited fuel supply.

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. Other uses include border security, maritime patrol and anti-piracy operations, disaster response, or agricultural observation. They could bring internet connectivity to the 5 billion people lacking it, either with 11,000 airplane UAVs or with balloons like Google's Project Loon.

Radiocommunication services Commercial air transport cruises at approximately –. HAPS can deliver radio connectivity to users, as an altitude above enable line-of-sight propagation of at least . HAPS could deliver bandwidth and capacity similar to a broadband wireless access network, like WiMAX, over a coverage area similar to that of a satellite. Military communications can be improved in remote areas like in Afghanistan, where mountainous terrain interferes with communications signals. World Mobile is testing a platform flying for 6 days at around , which can serve up to 500,000 users in an area of 15,000 km² (161,458,656,000 ft²). Within that area, connectivity can be delivered directly to devices across 3G, 4G, 5G, and future 6G standards.

Surveillance and intelligence The Northrop Grumman RQ-4 Global Hawk UAV is used by the US Air Force for surveillance and security. It carries a radar, optical, and infrared imagers; and is able to transmit its data in realtime.

Real-time monitoring An area could be monitored for flood detection, seismic monitoring, remote sensing and disaster management.

Weather and environmental monitoring For environment and weather monitoring, high-altitude balloons can deploy scientific equipment to measure environmental changes or to keep track of weather. In partnership with The National Oceanic and Atmospheric Administration (NOAA), NASA has started using Global Hawk UAVs to study Earth's atmosphere.

Rocket launch More than 90% of atmospheric matter is below the high-altitude platform, reducing atmospheric drag for starting rockets: "As a rough estimate, a rocket that reaches an altitude of when launched from the ground will reach if launched at an altitude of from a balloon." Mass drivers have been proposed for launching to orbit.

Airplanes

Reconnaissance aircraft like the late 1950s Lockheed U-2 could fly above and the 1964 SR-71 above . The twin-turbofan powered Myasishchev M-55 reached an altitude of 21,360 m (70,080 ft) in 1993, a variant of the M-17 first flown in 1982, which reached 21,830 m (71,620 ft) in 1990.

Operational

Grob G 520 Egrett The manned Grob G 520 first flew on 24 June 1987 and was certified in 1991. Powered by a Honeywell TPE331 turboprop, it is wide, reached 16,329 m (53,574 ft), and can stay airborne for 13 hours.

Northrop Grumman RQ-4 Global Hawk The Northrop Grumman RQ-4 Global Hawk first flew on 28 February 1998 and was put into USAF service in 2001. The 131 ft (40 m) wide, 48 ft (14.5 m) long RQ-4 is powered by a single Rolls-Royce F137 turbofan, weighs up to 32,250 lb (14.6 t) at takeoff, and carries a 3,000 lb (1,360 kg) payload up to 60,000 ft (18,300 m) over more than 34 hours. It can be used as a radio relay and can carry electro-optical, infrared, synthetic aperture radar (SAR), and high and low band SIGINT sensors. A total 42 of them have been in service with the United States Air Force. It is the basis for the US Navy's MQ-4C Triton.

Prototypes

Solar powered

AeroVironment/NASA Pathfinder The HALSOL prototype, a 185 kg (410 lb), 30 m (98.4 ft) wide flying wing propelled by eight electric motors, first flew in June 1983. It joined the NASA ERAST Program in late 1993 as the Pathfinder, and with solar cells covering the entire wing added later, it reached on September 11, 1995 and then in 1997. The Pathfinder Plus had four sections of the Pathfinder wing out of five attached to a longer center section, increasing span to , it flew in 1998 and reached on August 6 of that year.

AeroVironment/NASA Centurion/Helios Prototype Flying in late 1998, the Centurion had a redesigned high-altitude airfoil and span increased to , 14 motors, four underwing pods to carry batteries, systems and landing gear. It was modified into the Helios Prototype, with a sixth wing section for a span, and a fifth landing gear and systems pod. It first flew in late 1999, solar panels were added in 2000 and it reached on August 13, 2001. A production aircraft would fly for up to six months. It broke up in flight in 2003.

Airbus Zephyr The Zephyr were originally designed by QinetiQ, a commercial offshoot of the UK Ministry of Defence. The UAVs are powered by solar cells, recharging batteries in daylight to stay aloft at night. The earliest model flew in December 2005. In March 2013, the project was sold to Airbus Defence and Space. The latest Zephyr 8/S model weighs , has a wingspan of , and reached .

Solar Impulse The first Solar Impulse manned demonstrator made its first flight on 3 December 2009, and flew an entire diurnal solar cycle in a July 2010 26-hour flight. The 71.9 m (236 ft) wide, 2.3 tonnes (5,100 lb) Solar Impulse 2 first flew on 2 June 2014, it could reach 12,000 m (39,000 ft) and its longest flight was from Nagoya, Japan to Kalaeloa, Hawaii over 117 h 52 min on 28 June 2015.

Titan Aerospace Solara Founded in 2012 in New Mexico, Titan Aerospace was developing large solar-powered, high-altitude atmospheric satellites similar to the AeroVironment Global Observer or QinetiQ Zephyr. Their wing, over wide, would be covered with solar cells to provide energy for day flight, stored in electric batteries for use at night. Costing less than $2 million, they could carry a payload for up to five years, limited by battery deterioration. In 2013, Titan was flying two fifth-scale test models and aimed to flight test a full-sized prototype by 2014. In March 2014, Facebook was interested in the company, led at the time by Eclipse Aviation founder Vern Raburn, for $60 million. Google bought Titan Aerospace in April 2014, managed to fly a prototype in May 2015 but it crashed within minutes and Titan Aerospace was shut down by early 2017.

KARI EAV