The wireless universe is filled with innovation, and sometimes these innovations come off, at first glance, as being just a little wacky. One of these is airships stationed miles above the Earth, working like floating cell towers, that offer an intriguing direction for broadband wireless communications.

First, a little background. Communications satellites have been around since the early 1960s, with the Echo 1A and Telstar 1 satellites. The beauty of a satellite used for wireless communications is that the higher the orbit, the more of the Earth below can be "seen," in a radio sense, by the satellite.

Many satellites orbit the Earth at about 23,300 miles, where they move at roughly the rate that the Earth revolves about its axis. This means the satellite appears to hang motionless in the sky, and thus can be accessed simply by pointing a satellite dish antenna at that point.

A satellite at that altitude can see about a third of the Earth's surface and is thus great for broadcasting applications such as satellite TV and radio.

Satellites are especially good in broadcasting applications because the same information is being delivered to all receivers. If, however, we want to send typical Internet traffic, different for each user, through a satellite, capacity would quickly become a constraint and costs would soar. In addition, there is the problem of the round-trip delay of about half a second due to the speed of light, making real-time communications (such as for voice) at least a little painful.

Low-earth bankruptcies
These two challenges led to the idea of placing satellites in lower orbits. Capacity improves, and the delay associated with geosynchronous orbits is eliminated.

However, more satellites are required, because they don't have as big a view of the Earth, and it's necessary to hand off signals between satellites as they orbit past. Telephony-oriented satellite systems such as those of Iridium and Globalstar, and a number of planned data systems involve lots of satellites (which are not cheap) and booster rockets (which often cost even more).

And there's significant risk involved in launch, operations and even the business side. As a result, both Iridium and Globalstar have been through bankruptcies. What's needed, then, is a cheaper, low-altitude solution.

So, why not apply the idea of satellites to vehicles that operate within the atmosphere? The general concept is called "tall towers," because these platforms look like very tall (60,000 to 70,000 feet!) microwave relay towers. The idea is simple: Fly an aircraft or unmanned airship up to that altitude and equip it with radio relay hardware.

It will look like a big "cell in the sky," except that there's no need for it to hand off signals to another cell. It doesn't have to - the platform can see a very large metropolitan area below, and the aircraft or airship is relatively stationary.

One problem with this approach is that frequency reuse - where we can use the same frequency separated by a few miles on the ground because of the limited range of propagation of that signal over the surface of the Earth - isn't feasible. This can't be done with a tall tower, but we can apply a fairly large swath of frequencies to each platform.

Angel's Halo... or a blimp?
One early attempt (now inactive) at a tall tower was by Angel Technologies, which planned to use a HALO (high altitude, long operation) aircraft called Proteus T, designed by legendary aerospace engineer Burt Rutan.

Rutan recently gained fame as the designer of SpaceShip One, the first privately built manned spacecraft and the winner of the Ansari X Prize. Since hiring pilots is expensive, an unmanned airship approach might look more attractive.

The concept is still somewhat unproved, but it involves placing a blimp at the altitudes noted above, where winds are light and the amount of energy required to keep the airship on-station is fairly minimal.

An early effort using unmanned blimps was made by Skystation International, but that project literally never got off the ground. Two new companies, however, are forging ahead with the airship idea. One of these is SkySpectrum, the commercial arm of SkySentry, which develops airships for military applications.

The most visible venture in this area at the moment, however, is Sanswire Networks, which is interested in building a nationwide network of airships with coverage of up to 300,000 square miles each. They are quite far along, and you can see photos of their first "stratellite," Sanswire One, here.

But is it safe?
The biggest question I get about the airship concept is safety. What happens if something goes wrong and one of these monsters lands on someone's house? And, of course, will any company get the funds necessary to make the tall-tower vision a reality?

Since we can never have too much wireless capacity, I hope so. My guess for now is that the airship approach will be used to augment, rather than compete with, terrestrial cellular and broadband services. But then, it's early yet.

Craig J. Mathias is a principal at Farpoint Group, an advisory firm specialising in wireless networking and mobile computing. This article appeared in Computerworld. He can be reached at [email protected]