It’s a bird, it’s a blimp …

The higher a radio antenna, the longer its range. That’s why the tops of the highest buildings in any city bristle with a porcupinelike thatch of antennae.

But no matter how high the building, range is limited by obstructions such as hills, other buildings and the curvature of the earth.

Two aerospace companies believe they’ve found a way to build the ultimate antenna tower, one that will eliminate obstruction problems that plague terrestrial systems. Both hope to use high-altitude aircraft or airships to provide wide geographic coverage for cellular, wide-band data and high-definition television at a fraction of the cost of satellites and without the thousands of towers required to operate a cellular telephone system.

Communications satellites eliminate the obstruction problem by positioning antennae and transmitters 22,300 miles above the earth in a geostationary orbit. But satellites are expensive and difficult, if not impossible, to repair.

The two high-altitude communications platform companies — Advanced Technologies Group (ATG) in Bedford, England, and SkyTower Inc., a subsidiary of AeroVironment Inc. in Monrovia, Calif. — both plan to use advanced, high-tech aircraft they call “stratospheric satellites.” The aircraft would operate at an altitude of 12 miles and provide multimegabit wireless data service and cellular telephone service over an area ranging from 30 to 300 miles. Wider areas could be covered by launching more aircraft.

The two companies have pursued dramatically different designs. ATG will base its “tower in the sky” on a lighter-than-air platform whose design heritage goes back to World War I. But Gordon Taylor, marketing director at ATG, which is headquartered in a dirigible hangar, emphasizes that there’s little comparison between the company’s StratSat (for stratospheric satellite) high-tech airship and the dirigibles and blimps of yore.

The StratSat uses helium for lift — not the flammable hydrogen that destroyed the Hindenburg zeppelin at Lakehurst, N.J., in 1937 — and solar energy to power its electric motors and communications gear. It also has an auxiliary diesel engine. Since the StratSat operates at such a high altitude, it avoids the high winds and storms that led to the crash of a U.S. Navy dirigible in 1925. On-board, dual Global Positioning System (GPS) units will help keep the StratSats “on-station” above specific service areas, Taylor adds.

ATG has years of experience building blimps, including Fuji’s advertising blimp, and it pioneered such technologies as “fly-by-wire” fibre-optic control systems years before airplane manufacturers did, Taylor says. This experience is being applied to the design of the StratSat airship, which is about 650 ft. long (the length of three Boeing 747s).

The StratSat is designed to stay on-station for five years but can easily be brought down for repairs or upgrades to the communications payload — something almost impossible to do with a communications satellite.

ATG plans to lease the airship, or sell its services, to telecommunications companies such as cellular or wireless mobile data operators. ATG believes it can make a profit selling multimegabit data service at US$5 a user per month, with the operators adding charges to their customers on top of that.

ATG plans to launch its first commercial StratSat within two to three years, and within five years, the company plans to have 150 to 225 in operation worldwide. ATG estimates it would take only 19 StratSats to cover the entire U.K. for third-generation (3G) high-speed data service, saving operators the costs of building and maintaining 10,000 cell towers.

SkyTower has based its stratospheric satellite on an innovative flying wing design with 14 solar-powered electric motors. The gossamer-wing craft uses composite materials including carbon fibre, epoxy Kevlar and plastic foam.

SkyTower tested its concept in July, beaming 3G and high-speed data signals from a 7-inch dish mounted on a NASA-funded aircraft 12 miles above the Hawaiian island of Kauai. Stuart Hindle, vice president of strategy and business development at SkyTower, says the tests proved the viability of using a stratospheric aircraft as a communications platform operating in a tight, circular orbit above a service area.

He says SkyTower expects to begin commercial service within two years, using production versions of the NASA test aircraft as the tower in the sky for advanced communications systems.

Weston Henderek, an analyst at ARS Inc. in La Jolla, Calif., describes the high-altitude communications platforms as a “pretty far-out concept.”

But, he adds, they do have a value proposition in comparison with broadband satellite services such as those of Bellevue, Wash.-based Teledesic LLC. High costs keep delaying the launch date for those systems, which could provide the high-altitude platforms with a window of opportunity, Henderek says.