The future looked bleak for high altitude platform systems (HAPS) as an option for non-terrestrial network (NTN) communication after several high-profile failures such as Google’s Project Loon and Facebook’s Aquila.
But these were blamed more on flaws related to the specific project than HAPS and interest simmered to be revived under subsequent projects that now show much greater promise. The situation could be compared with mobile broadcast, which is now at last in the air after a series of failures over a decade or more.
The coming of age for HAPS may well have occurred last month in July 2022 when Airbus launched the first significant commercial business in the field around its existing HAPS technology platform, called Zephyr. In the absence of clear standards and regulations for global deployment of HAPS at this stage Airbus is aiming to stimulate industrialization of the field through this new subsidiary of its Defense and Space division.
The move followed successful test flights of the solar-powered Zephyr, establishing itself as a leading contending platform among the current generation, alongside several others. The company’s plan is to offer telecommunications services itself via the platform, aiming just like those earlier ventures to help bridge the digital divide, as well as serve travelers at sea and air.
Operating in the stratosphere at a maximum height of about 50 kilometers (31 miles), HAPS is the closest to earth of the NTN options, the next being LEO starting at 250km and extending up to 2500km. It offers some advantages over LEO, such as shorter transmission distances and far slower movement relative to ground, such that Doppler shift of radio frequency is far less. But there are also hurdles to overcome, including sustaining the vehicles in flight indefinitely since some power is required, and addressing the issue of regulatory no-man’s land, given that the stratosphere lies between airspace governed by nations just below, and outer space subject to different rules above.
HAPS airborne vehicles also require command and control in a way that satellites do not. Although the stratosphere is above the troposphere where weather occurs, it is still subject to variations in the wind, and occasional quite extreme conditions as when sudden stratospheric warming (SSW) occurs, as it did late in 2020. Then, while the top of the stratosphere does not vary much around the globe, there is a big difference in the height of boundary with the troposphere below, ranging by a factor of three – from 19km over the equator to just 6km over the poles. This means that a HAPS vehicle flying at say 14km, the height adopted for some of the trials, would actually be in the troposphere in lower latitudes and subject to the greater vagaries of atmospheric weather including rain, hail and lightning, as well as wind.
Still, these are not unsurmountable hurdles, and the bigger challenge is almost certainly the matter of where airspace ends, and outer space begins. That point presently lies somewhere within the stratosphere, certainly above the tropopause even in low latitudes, but operators need clarity in order to deploy vehicles in or above the airspaces of numerous jurisdictions.
A single HAPS can cover an area of about 140km in diameter, which is equal to hundreds of cells on the ground, but equally less than an LEO satellite, and far below a GEO satellite capable of covering a whole continent. It would take about 60 HAPS vehicles to cover a country the size of the UK, and over 100,000 for the whole globe, although in practice there would not be blanket coverage. Instead, global coverage would involve a combination of ground-based cells, HAPS and satellites, with some overlap.
Nonetheless, a HAPS service would have to negotiate the distinct differences between air and space legal regimes around the world until international agreement is reached. This is likely to impede roll out of HAPS services in the short to medium term.
The HAPS Alliance, a collaborative forum of companies and research institutions in the field, including Airbus, has played a critical role keeping HAPS alive during the period between those earlier failures and the current time of renewed optimism. It has been involved addressing technological challenges, standards, and the regulatory issues.
There are two main classes of HAPS carrier, with different pros and cons. They can be aerostatic vehicles lighter than the surrounding air, exploiting buoyancy like helium balloons lower down, to stay airborne. The alternative is a powered aerodynamic fixed wing aircraft or drone heavier than air, designed in principle to remain flying in the stratosphere throughout its working life. The first category in turn can be divided into aerostatic balloons, and airships, which may have more supporting structure and greater payload capacity.
The aircraft or drone category requires more significant power and so can also be distinguished by their energy source, with some being solar powered and others carrying hydrogen fuel cells.
Apart from the power, one of the main technological challenges is in designing an antenna for the vehicle that delivers enough signal power while being energy-efficient and sufficiently light. Two firms based in Cambridge, UK, have addressed this in a collaboration leading to a proof-of-concept vehicle first demonstrated early in 2022. This vehicle carried an antenna designed by Cambridge Consultants, part of Capgemini, in collaboration with Stratospheric Platforms Limited (SPL), measuring and weighing 120 kilograms, to become what the companies claimed was the world’s largest commercial such airborne system.
This involved various tests connecting a 5G base station with the airborne antenna and retail mobile devices including a smartphone, leading to demonstration of 4K video stream delivery to a handset at an average latency of 1ms above the actual terrestrial network speed.
That latency is far better than can be achieved when a satellite network is involved. The area covered by the 5G signal was 450 square kilometers, during three-way video calls between the land-based test site, a mobile device operated from a boat and a control site 950km distant.
The current objective is for SPL to launch the first commercial service in Germany during 2024.
In most cases HAPS works by connecting the airborne vehicles to ground base stations via a gateway. With this configuration HAPS can operate over a range of frequencies across the three 5G bands, including 700 MHz and 900 MHz in the lower band, 1.2-1.7 GHz, 1.9, 2.5, and 3.4-3.5 GHz in the mid band, then 26, 38 GHz and 70-80 GHz in the millimeter wave ranges. There has also been a demonstration over unlicensed spectrum at 5.8 GHz. Multiple bands have been used during tests to support feeder links to transmit data from smartphones to terrestrial Internet lines successfully alongside the HAPS links, as well as for general redundancy.
One benefit of HAPS shared by other NTN options is the ability to reach users beyond the range of terrestrial links in the vertical as well as horizontal plane, primarily aircraft, especially as they fly over more remote areas.
The growing conviction that HAPS has a key role to play in the full mix of 5G platforms has more recently attracted some more big names into the field, or at least reawakened their interest.
One is Japan’s Softbank, which in September 2021 acquired around 200 patents from Alphabet’s Loon, bringing its total number of patents relating to HAPS to 500. Softbank had developed a relationship with Loon through its own subsidiary HAPSMobile, as well as with California-based AeroVironment. This led to SoftBank investing $125m in Loon, with the option of topping this up. Loon’s development of high-altitude network connectivity including an LTE video call from a solar-powered, fixed-wing unmanned aerial vehicle flying in the stratosphere late in 2020 had been achieved with help from HAPSMobile.
Softbank therefore had several fingers in the HAPS pie anyway and saw Loon as a commercial rather than technological failure. Loon might have been able to close the digital gap but not the economic one, failing in the words of its CEO Alastair Westgarth being unable “to get the costs low enough to build a long-term, sustainable business”.
That is still the challenge for the options on the table today, although developments such as that smaller antenna from Cambridge Consultants and SPL should help square that economic equation.