The plan is familiar: park an antenna high in the stratosphere and then relay signals to and from devices below. Such an airborne transceiver could blanket urban areas with wireless coverage more cheaply than satellite-based alternatives while avoiding the need to build forests of mast-mounted base stations on the ground. In the past, various remote-controlled airplanes, balloons and blimps have been proposed to keep antennas aloft for months on end, but few have ever made it into the air, and none have operated commercially.

Previous failures, however, do not daunt the latest contender for the prize. This spring engineers at Sanswire Networks, an Atlanta-based Wi-Fi provider, plan to test a prototype of a high-tech airship that they claim could supply mobile communications service to major metropolitan areas for as long as 18 months at a stretch. "It's like a big pontoon boat in the sky," says company chairman Michael K. Molen. If the concept proves successful, fleets of whale-shaped "Stratellites"--short for stratospheric satellites--will fly 20 kilometers up to where the air is so thin that solar-powered electric motors can keep the $10-million-plus ships in geostationary "orbits."

Beyond standard financial and political difficulties, these projects face formidable technical difficulties, observes aerospace engineer James D. DeLaurier of the University of Toronto. A veteran of a late-1980s effort to fly a microwave beampowered drone, DeLaurier explains that the region above "20 kilometers is no atmospheric Shangri-la, as was once thought." Despite the rarefied air in the stratosphere, wind pressures are still significant, he notes, adding that the "area is full of ozone, ultraviolet radiation and even atomic oxygen, all of which degrade materials over time." Helium management is another issue: the enclosing envelope bag, which has to be thin to reduce weight, "looks like Highway 95" to helium molecules, which zip right through it, DeLaurier says. "So you have to have a way to replenish lost lifting gas."

To overcome such obstacles, Sanswire has hooked up with Vernon Koenig, an expert in lighter-than-air technology who designed an airship for an earlier, underfinanced effort. Called Sanswire 1, the one-third-scale prototype will rely on a lightweight skin of body-armor-grade polyethylene fibers and incorporate thousands of square meters of photovoltaic film on the upper exterior. Once builders install the engines, high-altitude propellers and a triple-redundant computer guidance system, flight-testing will be conducted above a California U.S. Air Force base.

A full-size Stratellite would be nearly as large as a football field. It would be able to loft a 1.5-ton payload using mostly helium and would employ nitrogen to maintain the shape of its outer skin as the craft rises and falls. Some 37,000 cubic meters of helium inside will expand nearly 17-fold by the time the ship reaches cruising altitude. The commercial airship will most likely include a technology to manufacture lifting gas, perhaps hydrogen, to replace lost helium.

Molen says the aerodynamic lifting body-type shape means the airship will be able to "fly" under power, maneuvering its way up rather than simply floating higher. Presumably this approach will enable the ship to navigate past the jet stream and maintain its position.

As with the earlier, similar efforts, it all sounds promising. But the question remains whether the idea will really fly.