Mobile Satellite Systems
In the early 1990s, many companies began to dream of mobile voice and data networks that would span the entire earth. Cellphones had already been inspired by the flip-top communicators of Star Trek,so the thought of being able to beam a message up to space was a marketer's dream. All would use low or middle orbits, eliminating the latency of VSATs and the problems of coverage at high latitudes. The first planned systems are summarized in Table 12.2.
With so many competing technologies on Earth—at the time, it was far from clear that GSM would emerge the winner, and CDMA was still in its experimental stages—satellite phones also had the potential to create a proper global standard. Why buy different phones for New York, Los Angeles, and Paris when one could work in all three, as well as in the Sahara Desert and at the South Pole?
By the time these networks were ready for launch, many had already been overtaken by terrestrial mobile systems. Cellphones became cheaper and lighter, while satellite phones were very bulky and cost thousands of dollars, with call charges to match. An early advertising slogan for one system was "If you can see the sky...," which inadvertently emphasized what it couldn't do—namely, work inside. That system, along with others, later filed for bankruptcy.
It might seem that mobile satellite services have no future, but investors are undeterred. Though business travelers might not need satellite phones, most people on the planet have still not made a phone call, so there is a compelling social case. There is also a large untapped market even in the richer countries. America is big and more than 50 percent of the United States is not yet covered by cellphone networks. The collapse of early systems is put down to marketing and their failure to anticipate the demand for high-speed data brought about by the Internet.
| System | Data Speed | Orbit | Number | Frequency Region (GHz) |
|---|---|---|---|---|
| Iridium | 2.4 kbps | 780 km (LEO) | 66 | 1.6 (L-Band) |
| ICO | 64 kbps | 10,390 km (MEO) | 10 | 2.0 (IMT-2000) |
| Globalstar | 9.6 kbps | 1,410 km (LEO) | 48 | 2.5 (S-Band) |
| Teledesic | 2 Mbps | 1,350 km (LEO) | 288 | 20 (Ka-Band) |
| Skybridge | 2 Mbps | 1,469 km (LEO) | 80 | 10 (Ku-Band) |
| Orbcomm | 2.4 kbps | 820 km (LEO) | 36 | 0.6 (L-Band) |
Mobile Satellite Telephony
Three schemes originally promised worldwide satellite telephony. They are sometimes referred to collectively as big LEOs, somewhat inaccurately because one used higher MEO orbits. Two of them went bankrupt in 1999, the main reasons being the huge expense and complexity of the networks coupled with very low data rates.
Iridium was the most spectacular failure, finally shutting down in March 2000. Many users who had paid thousands of dollars for handsets were cut off, including peacekeepers in Kosovo and an Iridium-sponsored expedition to prove that the system worked at the South Pole. Its 89 satellites, each the size and weight of a car, were scheduled to be "deorbited"—fired into the Earth's atmosphere at a shallow angle to ensure that they burn up safely.
Rival ICO was luckier. It filed for Chapter 11 protection before it had even launched its satellites, enabling something to be salvaged. With new investment that enabled its satellites to be refitted for higher data rates, it emerged in July 2000 as New ICO,promising broadband services. It is also the only satellite system to use the 2 GHz spectrum proposed by the ITU for IMT-2000 services, so may eventually interoperate with standards such as UMTS and cdma2000.
The most successful so far has been Globalstar, which is an intrinsically simpler system. As shown in Figure 12.5, it routes all calls via ground stations, while Iridium and ICO used ISL (Inter Satellite Links). This means that Globalstar doesn't work everywhere, because it needs a ground station to be in range of the satellite, but the savings in cost and simplicity seem to be worth the reduced coverage. Each station has a range of several hundred miles, so a station in Paris can cover the Irish Sea and one in Miami, the Caribbean. When every station is operational, the system should cover nearly everywhere except the Antarctic and the middle of the oceans.
Figure 12.5. Intersatellite Links, pioneered by Iridium
Mobile Satellite Telemetry
Though not as exciting as the big LEO projects, so-called little LEO constellations have proved more successful. They use as little as two satellites to offer narrowband data services, such as telemetry, short messages, and non-urgent paging. They do not offer voice or interactive data, keeping costs down. Most offer only partial coverage and can have latency of several minutes as a transmitter waits for a satellite to appear overhead.
The most successful so far is Orbcomm, which has launched 26 satellites and plans ten more to achieve greater coverage and lower latency. Analysts say that it has proved that there is a sustainable business case for little LEO schemes. The costs are low and the potential market is quite high. This has encouraged many other companies to follow it, the most ambitious being Leo One, planning 48 satellites. Around 20 other systems plan to use fewer satellites to focus on specific parts of the earth, mostly in the U.S.
Non-Space Networks
Satellites aren't the only way to put a wireless system in the sky. A company called Sky Station International plans to avoid the risk of rockets altogether by going back to a far older technology: balloons. Instead of orbiting Earth, its "satellites"will be suspended from huge, unmanned blimps floating on the edge of space, above the weather and all but the highest-flying jets.
A stratospheric balloon should be able to hold a steady position, with virtually no latency and a much stronger signal that any satellite. Being open to the air puts the equipment more at risk from the elements than satellites preserved in a vacuum, though it should be easier to repair if a fault does occur. The biggest drawback is that thousands would be needed to cover the whole world, which is why Sky Station is initially concentrating only on urban areas.
HALO (High Altitude Long Operation) uses a similar concept, with planes instead of balloons. This doesn't even make a pretense of global coverage, but focuses exclusively on cities, with initial rollout planned in California. The planes fly around all day in a tight circle, landing to refuel at night when the demand for capacity is lower. Burning jet fuel over Los Angeles's crowded skies doesn't help the city's smog problem, but supporters say it is no less expensive or polluting than launching rockets into space.