High altitude pseudo satellites (HAPS) are uncrewed, solar-electric aircraft that cruise in the stratosphere for days or weeks, doing some of the work of a satellite from a far lower altitude. Their endurance depends on light, efficient solar cells.
The cell technology behind them, covered in the guide to flexible solar cells, explains why a thin, bendable solar skin matters more on these aircraft than on almost any rooftop.
What a high-altitude pseudo-satellite is
The International Telecommunication Union defines a high-altitude platform station as a radio station located on an object at an altitude of 20 to 50 kilometres, held at a nominal, fixed point relative to the Earth. The term “pseudo-satellite” describes the role: a HAPS can relay radio signals or carry cameras over a wide area, but it is an aircraft or airship rather than an orbiting spacecraft. It stays far closer to the ground than a geostationary satellite, which orbits at approximately 35,786 km.
Two vehicle families exist. Lighter-than-air airships use helium for buoyancy, while fixed-wing aircraft such as Airbus Zephyr rely on continuous solar energy to maintain altitude, according to the IEEE Technology Navigator. This page focuses on the solar-powered, fixed-wing aircraft, because their wing is also their power plant.
The stratosphere suits solar flight for practical reasons. Zephyr, for example, operates above 60,000 feet, above weather and conventional air traffic, so clouds rarely block the sun and airliners are not a conflict. Winds are not zero, though: stratospheric winds average 5 to 20 m/s at 20 km altitude and can spike in winter jet stream conditions, which is one reason station-keeping is a design problem.
How a solar aircraft stays aloft through the night
The principle is a daily energy cycle. During daylight the wing-mounted solar array powers the motors and the avionics, and any surplus charges batteries. After sunset the batteries carry the aircraft until the sun returns. Airbus describes Zephyr as a solar-electric aircraft with secondary batteries that recharge during the day for night flight, and it states that Zephyr remains the only fixed-wing HAPS to have demonstrated day and night endurance in the stratosphere.
The wing therefore has to do three jobs at once: generate lift, carry the solar cells and stay extremely light. A heavier aircraft needs more lift, which needs a bigger wing, which needs more cells and more battery. Every gram of cell, packaging and wiring feeds back into the energy budget, which is why designers care about power per unit of weight and not efficiency alone.
NASA’s Helios Prototype shows how large these arrays became. NASA records state that more than 65,000 solar cells were mounted on the upper surface of the wing, that the cells were about 19-percent efficient, and that the array could produce a maximum output of about 35 kW at high noon on a summer day. For night operation NASA planned an experimental fuel cell-based electrical system instead of batteries alone.
Milestones from Pathfinder to Zephyr
Solar-powered high-altitude flight developed in steps, first in altitude and later in duration. The altitude work came from NASA’s Pathfinder and Helios aircraft, and the endurance work came from Zephyr, which was developed by QinetiQ before Airbus took it over.
| Aircraft | Date | Result |
|---|---|---|
| Pathfinder Plus 3 | August 6, 1998 | 80,201 ft, then a world altitude record for propeller driven aircraft |
| Helios Prototype | August 13, 2001 | 96,863 ft, a world record for a non-rocket powered aircraft |
| Zephyr (AALTO flight) | 28 April 2025 | 67 days, 6 hours and 52 minutes of continuous stratospheric flight |
Altitudes come from the NASA technical memorandum on the Helios record flight, and the endurance figure comes from the AALTO announcement.
The Helios Prototype had a wingspan of 247 ft and reached its record altitude on August 13, 2001, according to NASA’s technical report. NASA later reported that the aircraft was lost: it crashed during a checkout flight on June 26, 2003, before a planned long-endurance mission.
Zephyr’s flights then moved the benchmark from altitude to endurance. A 2018 report says the earlier Zephyr prototype stayed in the sky for 14 days, and Zephyr S, which took off on July 11 from Arizona, stayed up for 25 days, 23 hours and 57 minutes. In 2022 the US Army’s Zephyr completed 64 straight days of flight after taking off on June 18, well past the 30-day goal described for that test. AALTO, the Airbus subsidiary that now runs the programme, then reported that Zephyr concluded 67 days, 6 hours and 52 minutes of continuous stratospheric flight on 28 April 2025, after launching from Kenya on 20 February 2025.
Why the solar cell matters so much at 60,000 feet
A trade-press account of Alta Devices’ 2019 product launch lists what solar technology faces at this altitude. At 60,000ft, ultraviolet radiation from the sun is more intense than on the earth’s surface. Lift matters, so the cells must be extremely light-weight, and in-flight mechanical stresses mean the solar material must be flexible and durable. The announcement says flight at this altitude demands solar technology that is different from the type used in terrestrial installations or in space, which is why HAPS arrays are a specialist category.
Zephyr is small by aircraft standards. Airbus said it has a wingspan of 25 meters, entirely covered with solar panels, and a total weight of 75 kg. On an aircraft this light, a cell that saves weight or area changes what it can carry.
Gallium arsenide (GaAs) cells became the focus of one company in particular. Alta Devices, a Sunnyvale, California maker of thin GaAs cells and a Hanergy company, announced a product aimed at this market in 25 April 2019. The product used single-junction GaAs cells with a record efficiency of 29.1%. Earlier, in a 28 April 2016 announcement, the company had claimed that on a typical high-altitude long-endurance aircraft its material needs less than half of the surface area and weighs a quarter as much as competing thin-film technologies for the same power. That is a manufacturer’s claim, not an independent measurement. The same announcement reported a one-sun efficiency of 31.6%, certified by the US Department of Energy’s National Renewable Energy Laboratory.
The company’s lighter-weight cells also reached smaller aircraft. In 2013, its cells enabled a prototype solar-powered version of AeroVironment’s small Puma AE to stay aloft for over nine hours. Small aircraft of that kind are the subject of the page on solar powered drones, which sit at the opposite end of the size range from stratospheric platforms.
What these aircraft are used for
The most discussed uses are communications and observation. Airbus says a Zephyr with a connectivity payload can act as a tower in the sky, with an approximate coverage reach of up to 7,500 square kilometres, and that paired with its Strat-Observer service it can provide high-resolution imagery and near-real-time video from the stratosphere. Alta Devices’ 2019 announcement listed intelligence, surveillance and reconnaissance, search and rescue, border patrol and mapping alongside communications.
Commercial service is still being developed. AALTO’s statement named Japan as the market for initial targeted entry-into-service in 2026. Its record flight crossed the Intertropical Convergence Zone and passed through 7 different flight information regions on the way to Australia, which shows that airspace rules, and not only engineering, shape how these aircraft will operate.
Weight-saving cell designs studied for aircraft also interest vehicle makers, and the page on the solar car roof looks at that very different use of a thin solar layer.
Limits and open questions
Records do not equal a finished service. The IEEE Technology Navigator notes that researchers describing HAPS networks argue six months or more of platform endurance is required for commercial viability. Each long Zephyr flight also ended: after the Army flight Airbus said Zephyr experienced circumstances that ended its flight, and the record flight terminated over a designated aviation sanctuary area in the Indian Ocean.
Three engineering limits stay in view. Winter jet stream conditions can spike wind speeds, which makes holding position harder. The energy budget is tightest in the dark and in low-sun seasons, because the batteries must carry the full load of the night. And the stratosphere is an extreme ultraviolet, ozone and thermal environment, so packaging, not just the cell, decides how long an array lasts.
None of this makes HAPS a replacement for satellites or for ground networks. They fill a niche between the two, and the open question is whether their reliability and cost can reach the level that long-term operators need.
Frequently asked questions
What does HAPS stand for?
HAPS stands for high-altitude platform station, and the same acronym is used for high-altitude pseudo-satellite. The ITU definition places the station at 20 to 50 kilometres altitude.
How long can a solar high-altitude aircraft stay in the air?
The longest reported flight is Zephyr’s 67 days, 6 hours and 52 minutes, ending on 28 April 2025. It followed a 64-day flight in 2022.
How does a solar aircraft fly at night?
Batteries charged by the wing’s solar cells during the day power the motors after dark. Airbus describes Zephyr as having secondary batteries that recharge during the day for night flight.
Why do these aircraft fly in the stratosphere?
The stratosphere sits above weather and conventional air traffic. Zephyr flies above 60,000 feet, where sunlight is not blocked by cloud.
Which solar cells suit these aircraft?
Cells must be very light, flexible and durable, and able to withstand intense ultraviolet radiation at 60,000ft. Gallium arsenide cells were the approach Alta Devices offered for this market.
