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Dr David Maddison describes high-altitude Aerial Platforms All air vehicles are limited by physics, some by imagination, all others are limited by fuel. Chris Kelleher, inventor of Zephyr. Special balloons and fixed-wing aircraft can act as long-endurance aerial platforms for observation and communication. Known as high-altitude platform stations/systems (HAPS), they fly above most planes but below satellites. Weather, scientific and military balloons are similar; they also ascend to great heights but do not necessarily stay there for a long time. Image Source – Airbus 2023 – https://mediacentre.airbus.com/mediacentre/media?mediaId=604534 A pplications for HAPS include communications, military or civilian surveillance, scientific observations and even amateur/hobbyist uses. They are especially useful when a satellite would take too long to organise or be too expensive, as these applications can often be effectively served by some other type of platform located within the Earth’s atmosphere. HAPS are sometimes called ‘pseudo-­ satellites’ or ‘atmospheric satellites’. Such platforms are uncrewed and fly at the highest possible aircraft altitudes or above. They are either special aeroplanes designed for high-altitude flying or lighter-than-air craft (a balloon or an airship) that rise due to a buoyant gas like hydrogen (H2) or helium (He). The lowest practical orbital altitude for a satellite is around 160km, while the highest any aircraft can fly is 26km (85,000ft) for the retired SR-71 ‘Blackbird’. The currently flying aircraft with the highest maximum altitude is the U2 surveillance plane at around 21km (70,000ft) sustained, although the CIA version of that same plane could cruise at 22.7km (74,600ft). By comparison, the Concorde could cruise at a mere 18.3km (60,000ft) above sea level. Current commercial jets typically fly at altitudes of 9-12.5km (30,000ft to 41,000ft) with a maximum service 14 Silicon Chip ceiling of up to 13.1km (43,000ft). That includes aircraft like the Airbus A380, Boeing 787-8 and 787-9. Military jets such as the F-35A in service with the RAAF have a service ceiling of 15.2km (50,000ft). This leaves a large gap from around 20km to 160km, unreachable by satellites due to too much atmospheric drag, and aircraft due to a lack of lift and oxygen to power their engines. The Fédération Aéronautique Internationale (FAI) considers space to start at the Kármán line, which is defined as an altitude of 100km. However, there is still too much air at that altitude for satellites to realistically orbit. The stratosphere is an area of the atmosphere that starts at around 7km at the poles, 10km at mid-latitudes and 20km at the equator and extends to an altitude of 50km (see Fig.2). This is the area in which HAPS platforms usually fly, most typically at around 20km (65,600ft). Part of the region from 20km to 160km can be accessed by ‘aerostats’ such as balloons or airships. The highest ever balloon flight was 53km (173,900ft), by the BU60-1 in 2002. Rockets can also access that region, but they usually don’t stay there for long! Why fly in or near the stratosphere? Fig.1: the US Air Force Project Manhigh gondola from 1955-58. Source: www.thisdayinaviation. com/2-june-1957/screen-shot2018-06-01-at-11-53-55/ In the stratosphere, there is little or no turbulence. The air is almost still, so it provides a stable platform for surveillance; it is ‘above the weather’. Winds are minimal at around 20km up. As the air is still, less structural mass is required to deal with turbulence and less engine power (for powered Australia's electronics magazine siliconchip.com.au vehicles) is required to overcome it. Because the air is so thin in the stratosphere, it is difficult to generate lift, so the aircraft has to be as light as possible. That usually means limited propulsive power, so they must fly slowly. But as the air gets thinner with altitude, the aircraft has to fly faster to maintain sufficient lift. Studies find the best balance for the lowest power consumption to maintain lift and altitude is around 20km (65,600ft). These light aircraft are naturally fragile, and the limited power means they take a long time to reach the target altitude. They must take off and land under calm conditions, which precludes launches in places like the UK, where it usually is too windy for such launches. Another consideration is that wherever a solar-powered HAPS aircraft is launched, there must be enough daylight hours for the solar panels to charge batteries for night-time operation, precluding launches at the poles in winter (for example). Lighter-than-air HAPS aircraft The first hydrogen balloon was made in France in 1783 by Jacques Charles and the Robert brothers. The first crewed free-flight in a lighter-­than-air aircraft was an untethered hot air balloon invented by the Montgolfier brothers and demonstrated in 1783. It was flown over Paris by Pilâtre de Rozier and the Marquis d’Arlandes. Manned high-altitude balloons were launched in the 1930s in pursuit of altitude records. For example, Explorer II ascended to 22,066m (72,395ft) in 1935. The US Air Force’s Project Manhigh (Fig.1) was undertaken in 195558 and achieved the following altitudes for manned balloons. Manhigh I: 29,500m (96,800ft); Manhigh II: 30,942 m (101,516ft); and Manhigh III: 29,900 m (98,100ft). In 1960, under the auspices of the US Air Force Project Excelsior, Joseph Kittinger skydived from a balloon at 31,300m (102,960ft), a record not beaten until Felix Baumgartner’s descent in 2012 from an estimated 39km (around 128,000ft). Project Moby Dick Project Moby Dick was a Cold War era project of the USA to fly espionage siliconchip.com.au balloons with cameras over the then Soviet Union. The Soviets protested when they found the remains of one in 1956. Project Skyhook Project Skyhook balloons were launched by the United States Navy Office of Naval Research from 1947 until the late 1950s, for atmospheric research at very high altitudes. The first such balloon carried a 29kg payload to 30km (100,000ft). About 1500 such balloons were launched. Some highlights of this project are as follows. In 1948, a three-balloon cluster was launched. In 1948 and 1953, Skyhook balloons measured radiation in the atmosphere between 27km (90,000ft) and 32km (105,000ft). In 1949, a manned launch took place. In 1954, two balloons with telescopes were launched to photograph a solar eclipse from a high altitude. In 1957, a 30cm telescope was launched to photograph the sun, providing the sharpest photographs of the sun taken to that date. Project Genetrix Project Genetrix, also known as WS-119L, was a US program of the 1950s to send surveillance balloons over China, Eastern Europe and the Soviet Union. They flew at 9-18km (30,000ft to 60,000ft). Soviet surveillance balloons The Soviets also had their own fleet of surveillance balloons they sent towards the West. Quoting from the website at siliconchip.au/link/abl1: ...in 1956, the OKB-424 design bureau — also known as the Dolgoprudny Automatics Design Bureau (DKBA) — was established, especially for the task of making new military aerostats... ...The first task of OKB-424 was to copy a US photo-reconnaissance balloon that had come down on Soviet territory. Over the next 60 years, DKBA produced around 20 types of free-­floating balloon envelopes, with volumes ranging from 11,500 cubic feet [326m3] to 21,190,000 cubic feet [600,000m3], each of which could carry various kinds of mission equipment. The largest of them was the Ukolka series of balloons from the 1960s, Australia's electronics magazine Fig.2: HAPS typically reside in the upper part of the troposphere or lower part of the stratosphere. Original source: https://w.wiki/6doG (author Kelvin Case, CC BY-SA 2.5) August 2023  15 Cannon Cosmic Ray Plates Parachute which had a capacity of 21,190,000 cubic feet and could lift a 660-pound [300kg] payload to an altitude of 147,600 feet [45km]. Project Mogul Radio Beacon Gondola Ballast Project Mogul was a US program conducted during 1947-1949 that launched balloons carrying microphones to listen for the noises of Soviet atomic blasts. When one of these balloons went down, the result was the “Roswell Incident”, which was claimed to be a UFO. Since Project Mogul was highly classified at the time, the object’s true nature was never disclosed. Project Strato-lab Fig.3: Operation Stratomouse, 1955. The cannon was to sever the payload at the end of the mission. The temperature and pressure inside the gondola were transmitted via a radio beacon, and ballast could be dropped or the payload separated by radio control. Source: https://academic.oup.com/milmed/ article/119/3/151/4933143 Fig.4: a Google Loon launch in New Zealand in 2013. Source: https://w. wiki/6dpb (CC BY 2.0). Project Strato-lab was developed from Project Skyhook (see above) and ran from 1954 to the early 1960s. They were manned balloons that contributed significantly to the space flight program by measuring radiation at altitude and testing pressure suits. The maximum altitude achieved was 34.7km (113,740ft). Operation Stratomouse (1955) In 1955, the US Air Force undertook a balloon flight program to determine if primary cosmic rays, which are strongly present at high altitudes, were hazardous to humans. Mice were chosen as the experimental test subjects, along with tissue cultures and cosmic ray measuring equipment. Helium balloons of 56,600m3 (Fig.3) were constructed by a company called Winzen Research (https://w. wiki/6dpT), a pioneer in high-altitude scientific balloons. The balloons were made from polyethylene and, uninflated on the ground, were 76m long. Fully inflated in the stratosphere, they had a diameter of 53m. An altitude of 40km (131,500ft) was reached on one of the flights, with flight durations of up to 26 hours. For a fascinating full account of this project, see https://academic.oup.com/ milmed/article/119/3/151/4933143 Google Project Loon Project Loon (https://x.company/ projects/loon/) was a project of Google’s parent company, Alphabet, to use HAPS balloons (Fig.4) at an altitude of 18-25km (59,000-82,000ft) to provide internet access in remote areas. Manoeuvring was to be achieved by altitude control to move the balloons into layers with different wind directions. One test balloon achieved a flight duration of 312 days in 2020. The balloons used were Raven Aerostar Super Pressure Balloons (see below) composed of polyethylene about 0.076mm thick. They were around 15m across and 12m tall. They also carried an electronics box weighing 10kg plus a 100W solar panel. The project was terminated in January 2021 due to a lack of profitability. Aerostar Aerostar (https://aerostar.com/­ products/balloons-airships) is a US manufacturer of high-altitude, long-duration stratospheric balloons (Fig.5), some of which are steerable, such as the Thunderhead model. The Thunderhead exploits different wind directions at different altitudes to provide directional control. Aerostar was previously associated with Google and their Project Loon, now discontinued, despite making significant technical advances. Sceye Fig.5: an Aerostar super pressure balloon at launch with the payload in the foreground. Note the solar panels. Source: https://aerostar.com/products/ balloons-airships/super-pressure-balloons 16 Silicon Chip Australia's electronics magazine Sceye (www.sceye.com) is a Swiss company (also with offices in Roswell, New Mexico, USA) developing an airship (Fig.7) for applications such as broadband delivery, atmospheric monitoring, agricultural monitoring and security surveillance (eg, border protection). It uses a hull fabric of unspecified composition that is said to be five times stronger and 1500 times more gastight, UV-resistant and ozone resistant than existing materials. Its advanced lithium sulfur batteries have an energy siliconchip.com.au Helium or hydrogen for balloons? Fig.6: the Czech Stratosyst Skyrider can stay aloft for weeks or more. Source: www.stratosyst.com Helium is extremely expensive for balloons, and the supply is very limited. Hydrogen is cheap and of unlimited supply, but flammable. Hydrogen is not considered suitable for human flight in balloons or airships ever since the Hindenburg disaster. Still, it can be used in uncrewed balloons, provided proper safety precautions are taken during filling. Hydrogen is typically used in weather balloons, including those launched by Australia’s Bureau of Meteorology (BoM). Nevertheless, many organisations still prefer to use helium, even for unscrewed balloons. Our recent report on the Australian International Airshow in Avalon Airport (May 2023; siliconchip.au/ Article/15773) included mention of the Sierra Nevada Corporation (www. sncorp.com) developing an LTA-HAPS (lighter-­than-air higher altitude platform station) for long-term ISR (Intelligence, Surveillance, Reconnaissance) for military missions. It is designed to fly for up to 60 days at 23km (75,000ft) with a 50kg payload. To do this, they partnered with balloon maker World View Enterprises (https://worldview.space/), as described in the news article found at: siliconchip.au/link/abl2 World View Enterprises use their balloons for remote sensing with what they call a Stratollite (Fig.8), a portmanteau of stratosphere and satellite. A Stratollite flies at 15-23km (49,00075,000ft) and can be launched as a constellation. These tandem balloons have an upper balloon containing helium or much cheaper hydrogen, and beneath that, a ballast balloon for altitude control. Winds often blow in different directions at different altitudes, so by varying its altitude, it is possible to have a limited ability to control the position. Technically, this type of balloon is known as a ‘variable altitude air ballast balloon system’ (VAABBS). The lifting balloon is known as a zero-­pressure Fig.7: a Sceye airship climbing. It can reach 20km altitude and is expected to go into commercial operations in around one year. Source: www.sceye. com Fig.8: the Stratollite can alter its course by varying its buoyancy and thus altitude. Source: World View. density greater than 400Wh/kg. An ultra-thin laminated solar cell ‘cape’ covers much of the airship’s surface that is 50-85% lighter than conventional solar cells. One such airship was launched in New Mexico in June 2022; it took two hours to ascend into the stratosphere and then maintained its position for 24 hours. Commercial operations from 20km (65,600ft) up are expected in about 9-15 months. Stratosyst Stratosyst (www.stratosyst.com) is a startup company from the Czech Republic developing the Skyrider HAPS. It is expected to take a payload of 12kg, have a power supply that can deliver up to 5kW, fly at an altitude of 20km and have a mission duration of weeks to months (see Fig.6). World View Enterprises (2012 – present) siliconchip.com.au Australia's electronics magazine balloon (ZPB), while the ballast balloon (one or two) are super-pressure balloons (SPBs). Beneath the ballast balloon is a ‘ladder’ that contains solar panels to charge the batteries. Beneath that is the gondola or “Stratocraft”. The ZPB is made of UV-resistant polyethylene with a volume of about 23,000m3. The SPB beneath it has a pumpkin shape and operates at a higher pressure than the surrounding atmosphere. Its pressure is varied to alter buoyancy and thus altitude by a compressor in the Stratocraft. The concept of using both a lifting balloon and a ballast balloon (one or more) or tandem balloon originates in the “Sky Anchor” system developed by Texas A&M University in 1976. The orientation of the ladder can be changed to ensure the solar panels have maximum exposure to the sun; the orientation of the Stratocraft can also be altered to suit requirements. The Stratocraft can carry a payload of 50kg. Continuous power of 250W and instantaneous power of up to 1000W are available. At the end of a flight, the Stratocraft separates from the structure and August 2023  17 Figs.9 & 10: the human-powered AeroVironment Gossamer Penguin (left) and 2kW AeroVironment Solar Challenger (above). They and their predecessors pioneered techniques of lightweight construction, low drag and solar power that were later used in HAPS aircraft. Source: www.nasa.gov/centers/ armstrong/news/FactSheets/FS-054-DFRC.html descends via a steerable parachute, guided to a designated landing location. Stratollites have sensor packages that can photograph the surface with a 5cm/pixel resolution (5cm GSD) vs 25cm GSD for a commercial satellite. Plus, they can take infrared imagery, radar imagery and RF signals can be received and processed. Word View has an online portal for customers to examine the data that has been collected. World View is also developing nearspace tourism and has produced a pressurised gondola (with restroom!) with the intention of launching at various locations around the world, including Australia. Passengers will be taken to 30km (100,000ft) and flights are expected to take place from 2024 – see https:// worldview.space/space-tourism/ on human-powered aircraft turned out to be valuable research put towards building HAPS aircraft. An athlete such as a Tour de France contender can sustain a continuous power output of a few hundred watts for several hours, so that is how much power is available for sustained human-powered flight. A HAPS aircraft such as the Zephyr (see below) uses around 100-200W of power to cruise, so it’s arguably more efficient than early human-power aircraft. HAPS aircraft have the advantage that they don’t have to carry the weight or volume of a human, although the battery, motor and other electronics might come close to that. Some milestones were: ● 1974: NASA Sunrise II was the first radio-controlled solar-powered aircraft. ● 1977: the first human-powered flight in the AeroVironment GossaHeavier-than-air mer Condor. Its empty weight was HAPS aircraft 31.75kg. Similar ultralight construcHAPS aircraft must be lightweight, tion techniques were later used on have very low drag and fly with lit- HAPS aircraft. tle power. Those parameters are ● 1979: the AeroVironment Gosall also requirements for human-­ samer Albatross became the first powered flight; hence, the past work human-powered aircraft to cross the English Channel with an empty weight of 32kg. ● 1980: the AeroVironment Gossamer Penguin (Fig.9) was the first solar-powered aircraft capable of carrying a human with an empty weight of 30.8kg. ● 1981: the AeroVironment Solar Challenger (Fig.10) was the first solar-powered aircraft to cross the English Channel with an empty weight of 90kg. NASA ERAST program The NASA Environmental Research Aircraft and Sensor Technology (ERAST) program that ended in 2003 produced UAVs that could perform long-duration science missions at 18km (60,000ft) and above. Among other vehicles, it resulted in the solar or fuel-cell-powered Pathfinder, Centurion and Helios aircraft (see Fig.11). NASA Pathfinder (first flight 1995) The NASA Pathfinder by AeroVironment (see Fig.12) was the first aircraft built under NASA’s ERAST Program to develop long-duration, high-altitude aircraft for science missions. Fig.12: the solar-powered NASA Pathfinder over Hawaii Fig.13: the NASA Centurion first flew in 1998. Source: www. on the 28th of August, 1997. Source: www.dfrc.nasa.gov/ nasa.gov/centers/armstrong/news/FactSheets/FS-056-DFRC. Gallery/Photo/Pathfinder/HTML/EC97-44287-2.html html 18 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.11: the planform evolution of the NASA solar-powered aircraft designed under the Environmental Research Aircraft and Sensor Technology (ERAST) program. Original source: https://w.wiki/6doD In 1995, it set an official record altitude for solar-powered aircraft of 15.4km (50,567ft). It also set an unofficial altitude record of 21.8km (71,500ft) and had a ground speed of 24-40km/h. In 1998, it was modified into the Pathfinder Plus (more on that later). It had a wingspan of 29.5m. It weighed 252kg and could carry a payload of 45kg. Endurance was 14-15 hours with 2-5 hours on battery power. NASA Pathfinder Plus (first flight 1998) The AeroVironment Pathfinder Plus is a modification of the Pathfinder that climbed to 24.4km (80,206t) altitude in 1998. It was the second NASA ERAST aircraft with a wingspan of 36.3m, a Fig.14: the NASA Helios in its HP01 high-altitude configuration. Source: www.nasa.gov/pdf/64317main_helios. pdf siliconchip.com.au weight of 315kg and a payload of up to 67.5kg. The solar array produced up to 31kW at high noon in summer. It had 14 electric motors of 1.5kW each and flew at around 27-34km/h. Like the original Pathfinder, its endurance was 14-15 hours with 2-5 hours on battery power alone. In 2002, it was involved in atmospheric satellite tests from 20km (65,600ft), transmitting HDTV and 3G signals. Only 1W of transmission power was required. NASA Centurion (first flight 1998) The NASA AeroVironment Centurion (Fig.13) first flew in 1998 and Fig.15: this dramatic image demonstrates the fragility of this type of aircraft as it disintegrates and falls into the Pacific Ocean. Source: www. nasa.gov/pdf/64317main_helios.pdf Australia's electronics magazine was the third ERAST aircraft. It was designed to fly to 30.5km (100,000ft), although no official altitude attainment was recorded. It had a wingspan of 63m, weighed 529kg and could carry a payload of 45-270kg. Its endurance was, once again, 14-15 hours with 2-5 hours powered by its lithium battery alone. NASA Helios (flights in 1999 – 2003) The NASA Helios (Fig.14) was the fourth aircraft of the ERAST program and a modification of the Centurion. A 12m wing section was added to the Centurion for a new wingspan of 75.3m. In 2001, it achieved a world record for sustained horizontal flight by a winged aircraft of 29.524km (96,863ft). It had two possible configurations. HP01 was optimised for altitude and used solar cells and a battery to power 14 motors. HP03 was optimised for endurance and used solar cells, a battery and a fuel cell to power 10 motors. The HP01 weighed 600kg empty weight and could carry a payload of 329kg. It was lost in a dramatic accident in 2003 – see Fig.15. You can read the accident investigation report at www. nasa.gov/pdf/64317main_helios.pdf Solar Impulse 1 (2009) Solar Impulse 1 by André Borschberg August 2023  19 Fig.16: an artist’s impression of the Titan Aerospace Solara 50. Source: https://w.wiki/6doE and Bertrand Piccard of Switzerland first flew in 2009. As the name suggests, it was solar powered and used LiPo batteries so it could continue flying at night. In 2010, they took it for a manned flight over a complete night/ day cycle (26 hours). Solar Impulse 2 (2015 – 2016) Solar Impulse 2, also by André Borschberg and Bertrand Piccard, completed a manned circumnavigation of the world in 2015-16, although it involved 16 stops (17 stages). Titan Aerospace Solara 50 concept (2015) Titan Aerospace was a US company that existed from 2013-14 before being acquired by Google. Google planned to use the Solara 50 (Fig.16) and subsequent models as atmospheric satellites to deliver services such as internet, real-time Earth images, voice, navigation and mapping. The aircraft was expected to fly at around 20km (66,600ft) and spend five years continuously in the air. It had a 50m wingspan, was 15m long and could carry a payload of 32kg. The ground area to be serviced was expected to be 17,800km2. Fig.17: an illustration of one aircraft from the DAP concept. It has a wingspan of 39m, a wing area of 64m2 and a gross weight of 192kg. Source: www.nasa.gov/ sites/default/files/thumbnails/image/niac_engblom_phii.png Unfortunately, the aircraft crashed due to structural failure early on its maiden flight in 2015, and Google shut down the company in 2016. Dual-Aircraft Platform (DAP) concept (2015) This is a very unusual idea from Embry-Riddle Aeronautical University. It involves two powered aircraft tethered together that take off from a runway and ascend to around 60,000ft using both solar and battery power. The lead aircraft is called SAIL, while the towed aircraft is called BOARD (Fig.17). At about 60,000ft (18.3km), the two aircraft separate by around 1km and utilise the difference in wind speeds at the two locations in an analogous manner to kite surfing (Fig.18). The SAIL aircraft provides lift for both aircraft, while the BOARD aircraft provides directional control, like a keel. The propeller can be used as a wind turbine to recharge batteries under certain conditions, and missions of up to several months or years are thought possible. However, it has yet to be flight tested. For more information, see the video titled “DAP Configuration” at https://youtu.be/fidiDPaLWWw Facebook Aquila (2016 – 2018) Facebook Aquila, developed by Ascenta in the UK, was a HAPS intended to provide Internet access in remote areas (see Fig.19). It was designed as a flying wing about the size of a Boeing 737, with a wingspan of 43m, but weighed just 399kg. It was to fly at 27km (90,000ft) during the day, dropping to 18km (60,000ft) at night. The planned endurance was three months, to provide internet access to an 80km radius below the flight path. The project was cancelled in 2018. Airbus Zephyr (2001 – present) Zephyr is a solar electric HAPS platform that uses solar during the day Fig.20 (left): the Airbus Zephyr. Source: https://mediacentre.airbus.com/mediacentre/media?mediaId=604534 Fig.21 (right): the Zephyr 8/S in flight, presumably soon after launch, during 2021 tests in the United USA to demonstrate wireless broadband service delivery. It undertook 18 daytime flights. Source: https://mediacentre.airbus.com/ mediacentre/media?mediaId=557935 20 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.18: the dual-aircraft platform (DAP) aerodynamic concept, analogous to kite-boarding. Source: www.nasa.gov/sites/default/files/thumbnails/image/ engblom_sail_board.jpg Fig.19: the Facebook Aquila drone in flight. Source: Meta – http:// siliconchip.au/link/ablk and batteries at night – see Fig.20. The Zephyr was initially conceived and designed by Chris Kelleher for QuinetiQ around 2001 (QuinetiQ is an offshoot of the UK Ministry of Defence). In 2013, the project was sold to Airbus Defence and Space and is now under the Airbus business unit AALTO HAPS (www.aaltohaps.com). Zephyr went through a series of models, and in 2004, the Zephyr 4 was demonstrated in Australia. In 2010, the Zephyr 7 achieved a flight duration record of 14 days, 22 minutes and eight seconds, which was longer than any other unrefuelled aircraft flight at that time. It flew as high as 21.5km (71,000ft). In 2018, a Zephyr 8/S broke another record with a flight of 25 days, 23 hours replacing 250 terrestrial mobile phone towers and providing coverage over a 7500km2 area. Other possible applications include military reconnaissance, communications relay and environmental monitoring. Zephyr is optimised for operations at around 20km, the ideal balance between power required for propulsion and altitude. At this altitude, the line-of-sight (LOS) is about 500km, so a radio reception area of 1000km diameter could be established (eg, using UHF). Zephyr can travel up to 1852km (1000nmi) if it is not kept on station in one area. It takes about eight hours to ascend to its usual altitude of 20km and, due to its highly efficient aerodynamics, it takes about 24 hours to descend from that altitude. and 57 minutes. In 2022, a Zephyr 8/S was lost due to a mechanical failure after flying for 64 days. The Zephyr 8/S (Fig.21) has a wingspan of 25m, weighs 62-65kg, has a service ceiling of 23km (76,000ft), a rated endurance of 624 hours (26 days) and can carry a 5kg payload. Another variant, the Zephyr T, has a wingspan of 32m, weighs 145kg and can carry a payload of 20kg. The Zephyr can be used for various HAPS applications. One example is surveillance with the Airbus OPAZ Earth observation payload. OPAZ has an electro-optical (EO) sensor that provides an 18cm resolution and an infrared sensor for night and day operations. It can also be used as a ‘mobile phone tower in the sky’ (Fig.22), Fig.22: the ‘mobile phone tower in the sky’ concept for Zephyr. Coverage is expected over an area of 7500km2, equivalent to 250 ground towers. D2D is short for direct-to-device. Original source: www.aaltohaps.com/mobile-connectivity siliconchip.com.au Australia's electronics magazine August 2023  21 Fig.23: an artist’s concept of the Stratospheric Technologies aircraft, to be launched from a balloon and then use a plasma engine to stay aloft. Source: https://stratospherictechnologies.com/technology Fig.24: the Hawk30 (in 2020, renamed Sunglider), a product of HAPSMobile. Source: NASA / Carla Thomas During ascent and descent, it is vulnerable to bad weather because of its light structure, so the weather must be carefully monitored (this applies to all HAPS). duration of several months. It can provide a phone service area 200km in diameter for use by smartphones and IoT devices. It is envisaged to use it in areas with no existing coverage, such as islands or remote areas, for natural disaster relief or to provide communications links for drones. The Sunglider is a development of the NASA Pathfinder and NASA Helios. In 2020, a demonstration flight lasted for 20 hours and reached 19.1km (62,500ft). Its operational altitude is intended to be 20km. Stratospheric Technologies (2016 – present) Stratospheric Technologies (web: https://stratospherictechnologies. com/overview) is developing a HAPS (Fig.23) that is launched by balloon and then released at an altitude of around 30km (98,000ft). After that, it is powered by plasma engines that derive their power from solar panels and ascends to 35km (115,000ft). We don’t have specific details of the plasma engines, but Fig.25 shows how atmosphere-breathing electric propulsion works. Electric power ionises atmospheric gases and then accelerates them to generate thrust. At night, the platform gradually glides down to around 20km; when it becomes light again, the panels can again produce power for the engines so it can ascend. The plasma propulsion system is unaffected by low air density and is said to be the first plasma propulsion system that operates in the atmosphere. When the platform needs to return to Earth for maintenance, it glides to a landing area. The company says that potential use cases for the platform include telecommunications, weather forecasting, imaging and surveillance, including civil and military applications. It is not in commercial use at the moment. For more details, see the video titled “Stratospheric Technologies” at https://youtu.be/4D1TAV_aocc Hawk30/Sunglider (2018 – present) The Hawk30, renamed Sunglider in 2020, is a product of the Japanese company HAPSMobile (website: www. hapsmobile.com/en/) – see Fig.24. It has a wingspan of 78m, a cruise speed of 110km/h and is designed for a flight Kraus Hamdani Aerospace Kraus Hamdani Aerospace (also called KHA; https://krausaerospace. com/) has developed the K1000ULE Rev-P 4.8m wingspan drone, demonstrating a powered flight duration of 26 hours so far. However, this drone is designed to fly like a glider or bird and extend its mission time using thermals. Fig.25: how atmosphere-breathing electric propulsion works. Original source: https://w.wiki/6doF (CC BY-SA 4.0). 22 Silicon Chip Australia's electronics magazine siliconchip.com.au This aircraft is not strictly a HAPS as it is only intended to fly to 6.1km (20,000ft) to perform various observation and communication functions. Balloons of uncertain origin and purpose Mysterious balloons over the United States were in the news recently. One was ultimately shot down by US Air Force planes. Fig.26 shows one of these balloons from a US Department of Defense U-2 high-altitude reconnaissance aircraft. While parts of the balloon were recovered, at the time of publishing, the origin and purpose of these balloons are not known for certain. The Chinese government said it was a civilian weather balloon that was blown off course. BAE Systems PHASA-35 (2020 – present) Promoted as having the “wingspan of a 737 and weight of a motorcycle”, and being able to carry a 15kg payload, the BAE Systems PHASA-35 (www. baesystems.com/en/product/phasa-35) is described as a high-altitude long-endurance (HALE) unmanned aerial system (UAS). It is solar-­powered and can operate over an area of interest for several months – see Fig.27. It provides a persistent, stable platform for monitoring, surveillance, communications and security for military and civil applications. It can also be used in disaster situations, for agricultural monitoring, environment monitoring, Earth observations and border monitoring. It can potentially deliver 5G communications in a disaster or remote area. Its wingspan is 35m; it weighs 150kg and flies at 20km (65,600ft). It can also be used as part of a constellation of identical aircraft. For more information, see the video titled “PHASA-35 - Persistent High Altitude Solar Aircraft” at https:// youtu.be/Z7NE-rcDtGs What to do if you find a downed weather balloon If you find a weather balloon, it should be considered dangerous if it is still inflated, even partially, as it likely contains flammable hydrogen. The advice from the BoM is to call the Fire Brigade. Secondly, it will have a radiosonde. The BoM says these can be disposed of in regular household garbage or recycling; you do not need to return them. We suggest a better use. You can use and reprogram the radiosondes for amateur radio purposes, including tracking any balloons you may launch (subject to appropriate laws). That balloon has likely been for a journey into HAPS territory; they typically achieve 16-35km altitude, according to the BoM. You can view the video by Australia’s Peter Parker, VK3YE, titled “A mystery package from a mystery sender” at https://youtu.be/_-cwbIiinkA Also check out “Repurposing Vaisala RS41 radiosondes for amateur radio high-altitude balloon tracking” by 0xfeed at siliconchip.au/link/ablg Fig.26: “A U.S. Air Force pilot looked down at the suspected Chinese surveillance balloon as it hovered over the Central Continental United States February 3, 2023” – from US DoD. Source: www.dvidshub.net/ image/7644960/u-2-pilot-over-central-continental-united-states Fig.27: BAE Systems’ PHASA-35. Source: www.baesystems.com/en/product/ phasa-35 Tethered drones While not strictly speaking HAPS, tethered drones such as quadcopters and multi-rotor drones can provide persistent aerial observation, surveillance, reconnaissance and communications at altitudes up to a few hundred meters. Tethering a drone involves connecting a power and data cable from a ground station to a drone. siliconchip.com.au Fig.28: launching a tethered Teledyne FLIR Skyranger drone. Source: www.flir.fr/news/pressreleases/flir-acquirestethered-drone-assetsand-technology-fromaria-insights/ Australia's electronics magazine August 2023  23 The flight duration is then limited only by the power available and how long the drone can last before requiring a motor overhaul etc. Existing drones, including consumer types, can be converted to tethered operation. However, dedicated tethered drones are available, designed for particular commercial or military applications (see Fig.28). Tethering can be done from a stationary position, a moving vehicle such as a ship at sea, a land vehicle or even a person carrying the ground station in their hands or, more likely, in a backpack. One potential use for a tethered drone is for aerial filming in areas subject to commercial airspace restrictions, where untethered drones can’t be used; for example, near airports. One example is the LIFELINE tethering system (www.lifeline-drone. com) that works with consumer DJI drones like the Phantom 4 Series V1 & V2, Mavic Pro, Mavic 2 Pro, Zoom & Enterprise, Inspire 1, and Inspire 2. UAVOS Inc. We can’t tell you too much about the UAVOS product as our Malwarebytes software warns us not to visit their website at the time of writing. However, you can safely view the video titled “HAPS (High Altitude Pseudo Satellite) by UAVOS” at https://youtu. be/1YsloiRVEzs PICO balloons PICO balloons are a form of amateur HAPS that anyone with an amateur radio license can participate in. We first mentioned these balloons in the February 2015 issue (“Reach for the Sky”, siliconchip.au/Series/281). They are basically standard helium-­ filled Mylar party balloons that you can buy at any party supply shop carrying a tiny transmitter, solar panel and GPS module. The payload can weigh 13g or less but relays the balloon’s position using weak signal protocols such as JT9, JT65 and WSPR at 10-25mW. These balloons can stay aloft for Wind direction varies with altitude The wind speed and direction usually change with altitude, not only in the stratosphere but at any height – see Fig.29. By altering the altitude of a balloon, it is possible to achieve some directional control. Smart software and information from weather resources can help a lighter-than-air HAPS platform stay on station. many weeks and even circumnavigate the Earth several times, see: • https://picospace.net/ • www.picoballoons.net Balloon tracking website You can track amateur balloons at https://amateur.sondehub.org/ If you want to track scientific and weather balloons such as from the BoM, check out: • siliconchip.au/link/able • siliconchip.au/link/ablf Links and videos ● “B-Line to Space: The Scientific Balloon Story”: https://youtu.be/ sPQ-tMoAHkY ● “China’s Balloon: One Question NO ONE Is Asking!”: https://youtu.be/ eeAFCclFXUY ● More about the Chinese balloon over the USA: https://stratocat.com. ar/2023-03-e.htm ● L.E. Epley (1990) “A system architecture for long duration free floating flight for military applications”: www. osti.gov/biblio/6525013 ● “Stratosphere: The Uncharted Territory in Networks | Halim Yanikomeroglu | IEEE YP Ottawa | 14Mar2023”: https://youtu.be/ XyGGQoCt5M0 ● A website about Stratospheric balloons: https://stratocat.com.ar/ indexe.html ● Information about high-altitude balloons: farleyflightaerospacellc. SC space/FFA.html Distance to the horizon by altitude To indicate the desirability of using HAPS, this table shows the distance to the horizon as a function of altitude. Theoretically, a radio beam could reach the horizon from a HAPS at the indicated altitude. Altitude Distance to the horizon 1km 113km 5km 252km 10km 357km 15km 438km 20km 505km 25km 565km 30km 619km Fig.29: how a balloon can control its direction of travel by varying its altitude. 24 Silicon Chip Australia's electronics magazine 50km 800km 160km 1438km siliconchip.com.au