In 1945 Arthur C Clarke suggested that satellites might carry telephone conversations.(There were one or two earlier references but it was an article by Clarke in Wireless World that really started people thinking.) His vision was of three geostationary communications satellites serving all the communications needs of the world.
Geostationary satellites orbit at 35,800 kilometres above the Earth's equator. At this height, the satellite takes 24 hours to go around the planet and therefore appear to hang still in the sky. Clarke, with clever mathematics, worked out not only that it was quite possible to launch these satellites but also that the whole of the Earth's surface could be covered with just three satellites. Within 20 years his vision started to come true with the launch of the first successful geostationary satellite, Syncom III in 1964, and today there are some 150 civilian geostationary communications satellites in operation.
But there are disadvantages to the geostationary orbit. To put a satellite into such a high orbit uses a lot of fuel, so launches are expensive. Because the satellite is so far from Earth it needs to have a powerful transmission system, which makes it heavy, and also requires powerful transmitting equipment on Earth. Finally, even though messages travel at the speed of light, with a round trip of 70,000 kilometres there is a noticeable and annoying delay in conversations.
So 50 years after Clarke's inspirational article, the accepted wisdom is being challenged by the vision of a new type of mobile telephone service. Using lower orbits, satellites will introduce many new ways of communicating, including direct links between pocket-sized phones.
The first company to attract public attention in this new field was Motorola, the world's largest mobile phone manufacturer, when it announced Iridium in 1990. Iridium originally consisted of 77 small satellites spinning around the Earth in seven necklaces at a height of around 800 kilometres. The satellites were designed to allow business travellers to carry a pocket telephone with them wherever they went and to make and receive calls at will.
The $4.5bn project has now been slightly simplified to a mere 66 satellites, the first three of which will be launched this November and the next three in January. All the satellites will be launched by mid-to-late 1998, when the system will go operational.
Iridium uses some pretty impressive tricks, not least satellite-to-satellite communications, something only previously tried in military communications satellites. This means if you were in rural China your call could be beamed up to a satellite passing over Tibet, passed along to another Iridium satellite hurtling over Iran to a third over Europe before finally being beamed down to Britain. Iridium's biggest challenge is to reach agreements with all the individual governments to allow it to operate in more than 200 sovereign countries.
Motorola and its partners are gambling that these difficulties can be ironed out and that there will be a big market among international business travellers.
The company plans to launch special phones that will work with the local mobile phone service and which will connect to the Iridium service only when necessary. No prices have been given for the phones themselves but calls are set to cost around $3 per minute.
Iridium faces two big competitors, Globalstar and ICO. Globalstar plans to use 48 satellites with eight spares to provide services similar to Iridium's, but also plans to provide links to rural communities in developing countries. It plans to do 50 per cent of its business with fixed village phone boxes. Call costs may range from $0.50 to $3.50 per minute, which will surely restrict use to the middle classes. The $2.2bn project will launch its first four satellites in July 1997 and plans to be operational by the middle of 1998 and fully operational by the end of 1998 or the middle of 1999.
However, Iridium's biggest challenger is probably ICO. ICO was set up by Inmarsat, a UN-affiliated organisation originally set up to provide satellite services for maritime users. But Inmarsat is becoming more commercial and ICO, in which Inmarsat now only has a minority stake, will be a private company.
ICO will use a higher intermediate orbit, at 10,330 kilometres, but by using bigger, more powerful satellites it will still need only the same signal strength from hand-held units as conventional mobile phones.
There are other operators trying to enter the market but it does look to be becoming more than a little crowded. "I would expect that the market may not be sufficient for all the systems that are currently planned," says Tim Farrer, consultant at the researchers Analysys.
This lower-orbit technology will not just be used for telephony traffic. Iridium offers a basic 2,400 bits per second (bps) fax and data ability. ICO offers 4,800bps and Globalstar will offer data services at 9,600bps. But one low Earth orbit company is already operational in a new data-only market. Orbcomm has so far launched the first of its 43kg (95lb) satellites and will complete its constellation of 36 satellites next year. The satellites operate as a store and forward messaging system. As the satellite passes overhead, your phone system will transmit the information up to it; when it goes over the destination, it will beam it down again.
But all these systems pale into technological insignificance compared with the adventure cooked up by Bill Gates of Microsoft and Craig McCaw, a successful US mobile phone entrepreneur.
Their project, called Teledesic, plans to launch 840 small satellites into orbit 680 kilometres above the Earth's surface. Dividing the world up into 53km by 53 km cells, the system will provide a massive data-carrying capacity. Each cell will be able to support more than 1,800 simultaneous voice conversations, or 18 broadband transmissions. Teledesic expects its project to cost $9bn, and it hopes to launch its first satellites in 1998. However, it remains largely a paper project, so there is no guarantee it will lift off.
Meanwhile Iridium, Globalstar, and ICO seem almost certain to complete their projects. Whether all three can find a market among the wandering mobile phone users remains to be seen.