Broadband links offer an appetising slice of the Internet access pie, and satellite systems are one way to provide it. But solutions that lie closer to the earth are also more than just pie in the sky, as Terry Edwards reports.

Suddenly, it seems, everybody wants to be in the race to transform the 'world wide wait' into a 'world wide comparatively quick and moderately easy to get at' facility. It's a given, of course, that high-speed Internet access requires broadband communications links, and with the global market for Internet backbones expected to be worth US$20 billion (E18.9 billion) by 2004, there's a great big appetising pie out there just waiting to be sliced - and the industry can smell it. Thus, among the projects either on the drawing board or on the way to becoming reality are broadband satellite systems and broadband terrestrial networks (LMDS and the like) - and the new 'subspace' flying base station systems that are high altitude but, by definition, certainly not orbital.

Any examination of the pros and cons of subspace systems must begin by asking the fundamental question: do we really need broadband communications systems at all? Posing such a query may seem heretical in this day and age - especially considering all the time, money and effort that has gone into their development - but the outlook for the broadband satellite market is, at best, clouded; and in parts, and for various companies, both gloomy and stormy - especially as new, less expensive but extremely competitive non-satellite options will shortly come on stream.

Provided that companies can convince the financial markets to back their schemes, it can make perfect business sense to spend tens of billions of dollars on infrastructure investments. However, the proof of the pudding comes with quantifiable and reliable returns on the investments (ROI) made. Companies have to justify their claim that they are spending to exploit real opportunity, and this is not only a matter of the rate of return on investment but also of its timing - how long after the system is up and running does payback start?

Some very big and very rich companies and industry consortia have already leveraged their assets and made financial agreements to fund massively expensive satellite projects. And, as Iridium is proving, the market can bring even the biggest and highest-tech project to its knees. That said, at the terrestrial level a tried and tested fibre-optic infrastructure is already in place and seems to be able to handle everything that the demands of broadband communications can throw at it.

However, fibre is, by definition, a fixed system and therefore inherently incapable of coping with the demands of mobile. The same is true of the growing network of terrestrial broadband millimetre-wave systems, dominated by local multi-point distribution services (LMDS). They too are best able to meet fixed rather than mobile requirements.

Given that state of affairs, for some time now it's been accepted that the solution to high-speed Internet access and other connection facilities for mobiles will be through broadband satellite. The satellite industry and its many partners, many of whom are betting their bottom line on the success of broadband satellite systems, have hogged the limelight for years, but there are alternatives that cost much less and might do an equally good - or, dare one whisper it, perhaps even a better - job.

The biggest and best-known broadband satellite projects are probably Astrolink, EuroSkyWay, SkyBridge and Teledesic. Each of these projects is well advanced, technologically viable, and properly financed, and all have the benefit of considerable political support from their nations of origin. These systems are designed to provide a combination of metropolitan and rural coverage and, if they work in the way they are supposed to, should cope well with mobile needs.

How the high fliers compare

However, if these companies get their marketing, their services and their tariffs right (as Iridium so conspicuously didn't), the sheer number of subscribers they could attract might, ironically, affect the quality of service they can provide. It is expected that the global market for broadband subscribers will exceed 300 million by the year 2004, and if that estimate is anywhere near the truth, the most complex communications architectures will be necessary just to keep the systems up and running.

At first glance, some of the proposed subspace projects seem, shall we say, eccentric. And, to be fair, the notion that airships and circling planes might carry transponders and provide broadband services can, and still does, cause either raised eyebrows or suppressed guffaws. However, these projects are genuine and serious; they have been costed out, are technically feasible and, if deployed, will no doubt work. Furthermore, worldwide regulatory approval for the use of stratospheric platforms was granted by the Federal Communications Commission of the US in July 1997 and by the International Telecommunications Union in November that same year - so these things are rather more than castles in the air.

Thus, although until now the subspace programmes have had a much lower visibility than the broadband satellite systems, they may well yet prove themselves to be at least as important - if not more important.

So what exactly do these subspace flying-base station systems consist of? Well, operating very much closer to the surface of the earth than the low-earth orbit satellites (LEOs), they are made up of either 'aerostats' (which are basically a kind of balloon) or special aircraft that carry transponder platforms for broadband communications services.

Dirigible and balloon-based comms systems have been used quite extensively by the military for a number of years, but the relevant technology had not been available in the civilian arena until recently, so balloons were not considered to be viable alternatives to satellite systems. Aircraft too, despite their easy availability, were precluded from serious consideration mainly because of doubts about their reliability while airborne and the cost of their maintenance while on the ground.

However, the latest developments in airframe and engine construction should permit slow-flying aircraft to hold position in tightly controlled and specially designated aerial zones for many hours at a stretch. That said, as these things would be constantly circling big cities, no regulator expecting to be re-appointed nor any legislator hoping to be re-elected is likely to sanction the use of unmanned robot vehicles. As for the manned vehicles, spare a thought for the pilots. While necessarily well paid they will surely have one of the most boring jobs on (or in this case, just off) the face of the earth.

The chart above demonstrates the main comparisons between broadband satellite and broadband subspace systems. Among the advantages common to all subspace systems are: comparatively low capital investments (subspace systems come in at about $100 million (EUR94.6 million) each), quick deployment, short ROI, and even shorter transmission delays (similar to terrestrial). On the downside, subspace systems will never be able to provide the combination of global urban and rural coverage, or metropolitan concentration via the spot beams, that satellite systems can. Most of the proposed non-orbiting subspace projects will look to the cities and big connurbations for their subscriber base; several organisations have plans to deploy geostationary subspace stratospheric balloons and dirigibles in the near future.

Probably the best known of these is Sky Station International, of Washington DC, which will have its aerostats aloft and in service early next year.

Not one to undersell itself, the company has it that, taken together, its vision and technology make up one of the most significant innovations since the invention of the communications satellite.

The ultimate plan is that, by 2005, the Sky Station 'Stratospheric Telecommunication Service', will comprise a flotilla of 250 helium-filled balloons hovering 21km above the world's biggest cities. At 21km, the balloons will be at just over twice the height that today's commercial jet airliners fly.

If things go to plan, and there seems little reason why they shouldn't, in just a few years these balloons will be a familiar sight to airline passengers and will appear as a constellation of bright new stars to viewers on the ground.

Elsewhere, the Japanese Ministry of Posts and Telecommunications has advanced plans for an airship-based broadband access network; and Italy's Turin Polytechnic is working with the Italian Space Agency (ISA) to develop and build a long-endurance aerostat that will be powered by fuel cells for night operation and solar energy by day. The total investment for the Turin/ISA program is expected to amount to only about $3 million (EUR2.8 million) and the operating cost might be as low as $345 (EUR326) per hour.

At that sort of level, the investment needed is at least an order of magnitude lower than that required for LMDS or other broadband terrestrial wireless networks, and about two orders of magnitude below the investment levels associated with most broadband satellite projects.

Meanwhile, in Israel, Tel Aviv University is developing a network called RotoStar. An unmanned rotating-wing aircraft rather than a balloon or an airship, this system would actually hover at the same altitude as both Sky Station and the proposed system from Turin/ISA. However, unlike these designs it would be capable of remaining on station for between four days and an astonishing six months.

At present, the main contender in the high-flying aircraft-based broadband-access stakes is HALO - which stands for High Altitude Long Operation.

The HALO system consists of a whole fleet of high-flying aircraft, each of which carries broadband transponders.

Proteus, the specialised aircraft designed for Angel Technologies of St Louis, Missouri in the US by the Wyman-Gordon company, sports a large underbelly antenna and, flying 16km above the city centres, will be capable of delivering very high-speed communications services. The HALO-Proteus aircraft is designed to augment both terrestrial microwave towers and orbiting satellites, and, it is claimed, will fly fixed-pattern stratospheric routes to provide metropolitan wireless services, at much lower cost and with more flexibility and better quality of service than either current satellite or fibre-based services. HALO will offer a variety of fixed and mobile wireless services, including voice, data and video.

And the HALO system is not just pie in the sky. In August last year Angel Technologies conducted a flight test using ground and airborne electronics developed by Massachusetts-based Raytheon Company. The result was a record-breaking 52Mbit/s wireless link between the ground and a Proteus testbed aircraft in flight. Under this teaming agreement, Raytheon has responsibility as the prime electronics systems integrator for both the airborne and ground segments of the HALO network. The plan for now is that 100 Proteus aircraft will be built.

In commercial service, the Proteus aircraft will circle for periods up to 16 hours at a time at an altitude of 16km. From this height the aircraft can provide coverage footprints, or 'cones of commerce' as the company prefers to term them, with diameters of up to 120km. Twenty-four hours a day, seven days a week, 365 days a year broadband communications availability will be provided. The aircraft will communicate between HALO gateways, high-capacity business premises and domestic subscriber premises and also provide connections to remoter metropolitan centres.

Angel Technologies says that, for individual subscribers, a system of this type should, in the very early years of the new century, readily provide 1.5Mbit/s Internet connection at a price tag of around $40 (EUR38) per month. In the case of dedicated commercial customers connect speeds of up to 54Mbit/s are promised and the total network capacity exceeds 16Gbit/s.

Fittingly it will be Los Angeles, the 'City of Angels', that will be the first place to have access to an Angel Technologies network. Thirty further US metropolitan areas are on the list for service. Thereafter, it is expected that major cities in many other parts of the world could soon deploy such wireless networks.

Harping On About HALO's Advantages

Compared with terrestrial networks, systems such as HALO offer the following distinct advantages: rapid network deployment ubiquitous coverage from day one line-of-sight communications to 99.99% of city rooftops no towers and no local zoning restrictions network flexibility plus quick and easy upgrading. Compared with satellite systems, HALO also has the following advantages: it is between 20 and 2,000 times closer to users it provides much greater and much cheaper availability electrical power is on board the aircraft substantial capacity can be allocated directly over densely populated regions the network is extremely flexible and can be readily upgraded financing is on the basis of one market at a time. With high-flying, long-duration manned aircraft systems the main challenges are likely to be human rather than technological; the future of such systems probably lies with unmanned, remotely piloted, automatically navigated, aircraft. Also, given the present state of the market, it is not too far-fetched to speculate that, in the medium to long term, a substantial proportion of broadband high-speed Internet access will be facilitated not by satellite systems alone but by a combination of subspace technologies. Some systems may even be hybrids of broadband satellite and subspace. And in all cases - even if, against all apparent odds, satellite systems do gain the ascendancy - integration with existing and planned terrestrial networks will be vital to the long-term viability and success of HALOs. This means efficient handovers between broadband platforms and third-generation mobile terrestrial networks, as well as backward compatibility with second generation networks, will be essential.