While land-based cell towers bring Internet access to most of the world, in many developing nations, the service is 2G or 3G and it is not particularly reliable. And in areas affected by natural disasters, cell towers may be damaged or overwhelmed with traffic, leaving victims and rescue workers without connectivity. The HAPS Alliance, a consortium of companies in the HAPS (high altitude platform station) industry, has begun a collaborative effort to bring 5G Internet service to remote areas via autonomous, solar-powered aircraft. The alliance, which began with a partnership between HAPS Mobile and Google spinoff Loon, quickly expanded to include dozens of aerospace and telecommunication businesses, as well as universities, research labs and semiconductor manufacturers.
The group is building a fleet of “floating cell towers” that operate in the stratosphere, several kilometers above jet aircraft flight paths. In addition to providing telecommunication services, the array will also contribute to high-resolution earth observation, weather prediction and atmospheric modeling. HAPS can also enable the deployment of Internet of Things (IoT) devices to remote regions and disaster areas.
The Stratosphere
Commercial aircraft typically fly in the troposphere (lower atmosphere) or the low end of the stratosphere—up to 11 km, or 38,000 feet—in order to avoid the solar radiation, low pressure and frigid temperatures at higher altitudes. Satellites, whether in low-Earth orbit (LEO) or geostationary orbit, are very expensive to launch, making it difficult to update their equipment as new standards surface. Moreover, their distance from the Earth’s surface results in higher latency (time lag) and increased power requirements for mobile devices.
Recent developments in materials, solar power, batteries and artificial intelligence (AI) have made it possible to build cost-effective unmanned aircraft that can operate for months at a time with little to no human intervention. The members of the HAPS Alliance believe there’s a significant, untapped market for telecommunication services in the middle stratosphere (20-25 km), bridging the service gap between land-based towers and space-based satellites. The group is now deploying two types of aircraft outfitted with mobile communications equipment capable of delivering cell service, using all current standards up to and including 5G.
Loon Aircraft
Considering that Loon is one of the HAPS Alliance’s founding companies, it’s no surprise that the Loon balloon is one of the vehicles it’s using. Engineering.com covered the Loon when the first balloon launched as a pilot project; since then, “the little balloon that could” has matured, logging over a million flight hours and traversing 40 million kilometers.
In 2020, Loon completed a record-setting 10-month flight. The first leg involved testing its telecommunications ability over Peru for three months, after which it traveled around the globe. It then spent the next 10 months experimenting with its navigation systems in the South Pacific, eventually landing in Baja, Mexico.
Because the Loon is propelled only by air currents, the craft navigates by changing altitudes using a solar-powered fan to pump lift gas into and out of the balloon. By tapping into global weather forecasts, communicating with other Loons nearby, and learning from its past behaviors, the Loon can determine how high to fly in order to catch a ride on favorable winds. To accomplish this lofty goal, Loon engineers spent years designing navigation and control software.
Over time, they realized that every flight presented unique conditions and numerous variables that the algorithm hadn’t accounted for. Crunching all that data required AI, so the Loon team developed a smarter navigation system with help from another branch of the Google family tree: Google AI. Essentially, the AI tweaked the algorithm each time it ran, gradually improving its precision over time.
Simulations demonstrated that the newly developed machine learning (ML) algorithm (“Sleepwalk”) tracked a ground-based homing device (the “Romer Box”) at least as well as Loon’s previous system (“StationSeeker”). When the ML system eventually surpassed its predecessor in the simulator, the engineers decided to launch it for real, also sending up a StationSeeker as a control. Both Loons performed well in routine operations. When given more difficult tasks, like tracking a moving object on the ground (see the graph below) or maintaining a nearly stationary position over a spot, the ML system outperformed the man-made algorithm. The differences were even more pronounced as atmospheric conditions became more challenging, with the AI faring much better than the control.
Engineers noted that the ML navigator behaved more like a well-trained human pilot, making subtle changes to correct its course. The older system frequently overshot its target, forcing it to make abrupt changes in course and causing it to use more energy—a huge factor on an aircraft powered by solar panels and batteries.
Sunglider Aircraft
HAPS Mobile took a different approach, opting for an autonomous fixed-wing aircraft that runs on solar panels and batteries. The Sunglider’s 78 m (256 ft) wingspan is covered in solar panels, which power 10 propellers and charge its lithium-ion batteries during the day. At night, the craft runs on battery power. Flying at speeds up to 110 kph (68 mph), the Sunglider can remain airborne for months at a time.
In September 2020, HAPS Mobile launched the Sunglider (also called the HAWK30) to an altitude of 19 km (62,000 ft) to run a battery of tests related to propulsion, power management, navigation and communication. The plane’s structural integrity was verified during the most turbulent parts of the flight: ascending and descending through the jet stream. HAPS Mobile engineers were pleased when testers in Tokyo, New Mexico and Silicon Valley were able to hold smartphone-based videoconferences over a five-hour stretch, with all systems behaving as intended. Here’s a video describing some of the Sunglider’s engineering:
Payload
Both the Loon and the Sunglider carry an array of telecommunication equipment jointly developed by Loon and HAPS Mobile. The airborne cell stations are outfitted with multiple antennas capable of transmitting high-speed (1 Gbps) data to mobile devices up to 700 km away. Interference is avoided through the same protocols that prevent conflicts between overlapping cell towers. Loon recently tested the payloads on a group of 20 balloons, coordinating all the aircraft while keeping solid communication with ground-based mobile devices spanning a diameter of 4,000 km.
HAPS employs a dedicated frequency band (70-80 GHz) for its feeder link, which connects it to the Internet and landline phone services. The feeder can come from a land-based antenna or a satellite link, depending on the location of the aircraft.
Since the Sunglider could be traveling up to 110 kps, the antennas can be quickly rotated to maintain point-to-point connections. The entire payload, including the antenna array, is housed in an aerodynamic shell to reduce drag and to protect the antennas from forces associated with high velocities.
If the HAPS Alliance is successful, the group’s efforts could be a boon to rescue workers, disaster victims, scientists performing remote measurements, and people in developing nations. On the surface, HAPS seems like a niche market destined to siphon business from satellite and land-based cell services, but when it comes to communication, new applications tend to appear after the technology is in place. Three hundred thousand users are currently connected to the HAPS network, and dozens of companies are betting that the number will soar. Since the flight and communication abilities have been verified, the launch and recovery costs are low, and the payload is easily upgradable, it seems like a low-risk, high-reward gamble.