To overcome the high costs of conventional satellites, researchers are looking at solar-powered craft hovering at high altitude to deliver cheaper telecoms.
With so many protests against mobile phone masts and warnings that space is filling up with satellites and other junk, it would be easy to believe there was nowhere left to put equipment to transmit the radio signals we rely on for our communications infrastructure.
However, a project being carried out by researchers at the University of York, in concert with organisations across Europe, aims to exploit a layer of the atmosphere currently unused. The idea is simple enough. Rather than use a rocket to launch an expensive satellite into space, why not float it at high altitude in the Earth's atmosphere?
The £3.9m European Commission-funded Capanina project aims to develop High Altitude Platforms (Haps) - solar-powered aircraft and airships - which can be kept practically stationary at a height of about 20km. Such aircraft can be launched and maintained at a fraction of the cost and time it takes to lay cables underground or launch conventional satellites.
The aircraft carry transponders for services such as mobile phones, broadband internet, digital television and radio, and military and civilian surveillance. They maintain their position using solar-powered electric motors.
David Grace, of the University of York's communications research group, said, "The main benefit is that high-capacity communications can be achieved with less communications infrastructure and can be quickly deployed."
One example of where the technology could be used is to assist in disaster relief, or for emergencies. Haps could also be used to provide a communications infrastructure where it currently does not exist, especially over large areas in countries such as India or Brazil, or in rural areas.
The project aims to deliver network bandwidth of 120mbps within a Hap's coverage area, which has a 60km diameter. Vehicles travelling at up to 300km per hour on the ground would be able to access the communications infrastructure when travelling within the coverage area.
The build and launch costs of Haps are favourable compared to satellites. Whereas the cost of a satellite launch can run into millions of pounds, Hap launch costs are minimal.
Grace said that excluding payloads, solar-powered aeroplane-type Haps are likely to cost about £3.5m and airship-based Haps will cost in the region of £17.6m. "Satellites typically cost upwards of £88.4m," he said.
An aircraft Hap requires an airfield and a runway to launch, but an airship Hap requires just a hangar or a ship.
In terms of ongoing maintenance, the Hap itself requires maintenance only when it lands.
The on-station altitude for a Hap is between 17km and 20km, where it needs a power source to maintain its position within a 6km radius. The aircraft continually flies into the wind and uses electric motors to avoid drifting and so maintains a static position.
In the 17km to 20km altitude range, wind speeds average 120km an hour, much slower than the jetstream, which blows at the height airliners travel. Because of a temperature inversion, the climate is not damaging to the Hap and its communications equipment.
At launch, the solar-powered aircraft would be flown up to the operating altitude during the day, using several electric motors. Once on station, the number of motors used would be reduced to two or three to maintain its position; energy stored in fuel cells would power the motors overnight.
Energy storage is one of the obstacles the Hap project still needs to overcome. Grace said further development work was needed to improve the efficiency of the fuel cells and the design of the communications payload.
He also said further work was required to finalise the design and technology. "We are still at an early stage - the field only began in the early 1990s. If you think of the money spent on terrestrial communications, the Hap industry is very small and undeveloped, and probably equal to the satellite industry in the 1960s."
The Capanina project began in November 2003 and will run for three years. It follows on from work done in the Helinet project (2000 to early 2003), which illustrated the potential of broadband delivery from high-altitude platforms and came up with an outline system design.
The University of York's communications research group and physical layer research group in the electronics department are involved in the European Commission-funded £3.9m Capanina project.
Capanina will develop a test-bed using low-altitude tethered craft, stratospheric balloons and an outline design for a system to deliver 120mbps to on-board wireless Lan on vehicles such as trains travelling at up to 300km an hour.
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