Solar-powered drones like the PHASA-35 have spent years stuck on the same problem: sunlight by day, batteries by night, and short winter days that can mean days of flying on stored charge alone. British defence firm BAE Systems thinks the fix is to stop treating the aircraft like a battery user and start treating it like a phone on a charger. Under a £15.7 million contract with the UK’s Advanced Research + Invention Agency (ARIA), its solar stratospheric drone will be fitted with a system that beams electricity up to it from a transmitter on the ground.
That drone is the PHASA-35, a High Altitude Pseudo Satellite (HAPS) designed to loiter at roughly 66,000 feet (20,000 metres) for months, serving as a permanent communications relay and surveillance platform. At 150 kg all-up with a 35-metre wingspan, it weighs about as much as a motorbike yet carries a 15 kg payload of sensors and comms equipment.
Beamed power is not new in concept. A space-based solar power concept has been circulating for decades, and the same physics is what lets satellites harvest energy from a ground station. What is new here is the choice of aircraft as the receiver. Because a plane holds a steady position above one spot on the ground, unlike a satellite that whips across the sky, the geometry for beaming is dramatically easier.
What PHASA-35 Already Does
Before the beaming project, PHASA-35 was already pushing the limits of what a battery-supported platform can do. It first flew in June 2023 from Spaceport America in New Mexico, reaching above 66,000 feet and staying up for 24 hours. A 2024 trial pushed past the same ceiling again, and the aircraft has since been ground-tested for 72 hours in a simulated stratospheric environment. It cruises at about 49 knots (90 km/h), just slow enough to hold station rather than travel.
Why the Night Is the Problem
The math behind the difficulty is simple. Even near the equator, a stratosphere circling aircraft eventually drains its batteries on the dark side of the planet. Higher up, the problem compounds. At 66,000 feet the air is thin, so a solar array spread across a 35-metre wing has to work hard for every watt, and the batteries sized to cover the night are a significant share of the aircraft’s total weight. A dead battery at 20 kilometres does not mean a safe landing, it means an unpowered glide home from the stratosphere.
That is why the beaming project targets winter specifically, rather than treating power as a nice-to-have. Northern Europe gets very little usable sunlight for long stretches, and the UK funding body is deliberately testing whether a stratospheric aircraft can be kept aloft through a British winter. Bob Davidson, CEO of BAE’s Prismatic subsidiary, framed the project as a way to make an already proven platform “even more capable.”
How the Beam Works
Ground stations transmit microwave energy in a tight, controlled beam toward a dedicated receiver built into the aircraft’s wing. That receiver is a rectenna: a lightweight antenna paired with a rectifier that turns the incoming radio waves back into usable electricity, feeding the motors and topping up the batteries. The project runs over about three and a half years, with the flight demonstration due at the end of it.
It is worth being precise about scale. This is not a scheme to fly a Boeing 737 off a transmitter. ARIA’s programme is deliberately narrow: the demonstration target is net delivery of 300 watts to a payload at altitude for seven days while maintaining line of sight with a fixed ground location. From there, the agency wants a credible route to 3 kW. The £15.7 million PHASA-35 contract covers the flight demo platform, not the transmitter itself, which is being developed separately by Harwell-based Space Solar, working with microwave specialist MuWave on the ground unit.
Why Aircraft Beat Satellites for This
There is a deeper reason power beaming is appearing on drones before it appears on satellites. A stratospheric aircraft holds one position, so a single ground station can track it indefinitely. Satellites are in constant motion, which is why conventional satellite beaming has stayed largely experimental.
For the same reason, the cost story is compelling. A 150 kg aircraft that can fly for months, be recovered, repaired, and relaunched, offers something a satellite launched on a rocket cannot: an aircraft that returns to the ground. That is the argument BAE is making for HAPS generally: replacing an expensive orbit with an aircraft that can be serviced in a hangar.
What It Means for Satellite Internet
For readers tracking the broader space-based solar power and microwave beaming field, the significance is that the receiver technology now has a real flight test scheduled against a hard number. The math is unsurprising: PHASA-35 needs on the order of 300 watts continuously for the payload but also needs power for its own motors and battery top-up, so the actual transmitted power is considerably larger. The payload target of 300 W and the 3 kW aspiration are different numbers, and the gap between them is the entire remaining engineering problem.
Who Else Is Doing This
BAE is not alone. Airbus’s AALTO ran the Zephyr platform for a 67 days continuous flight in 2025, a record for a fixed-wing HAPS, though a later prototype was lost after ditching into the Indian Ocean. AALTO is targeting 200 days. Prismatic is aiming at around 180 days. Both companies are treating power beaming as a shortcut past the battery problem rather than waiting for better cells.
A separate UK project run by Space Solar under the ARIA programme, called Project HAWK, is developing the same 5.8 GHz link at a smaller scale with a conformal rectenna and MuWave’s ground transmitter, and will fly on PHASA-35 too.
It is also fair to say that this is a technology programme rather than a service rollout. ARIA committed £70 million across 18 teams over three and a half years, with a narrow proof point rather than a near-term national broadband plan. Whether a 300-watt, seven-day link matters to anyone depends entirely on what the payload actually is.
Why It Matters
The honest headline is that a solar drone will be able to plug itself into the ground, like a mobile phone on a charger, but the scale of that charger is a transmitter the size of a building and a wing-mounted receiver that has not flown yet. The £15.7 million contract is for development over three and a half years, not a capability, and the £70 million ARIA programme behind it has a deliberately modest target of 300 watts for a week. With PHASA-35 having already demonstrated 24-hour stratospheric flight, though, the platform is proven even though the power link is not.
