IN THE hope that Americans will pay to listen to radio programmes
uninterrupted by annoying advertisements, two companies are planning to spend
$1 billion each to do what no one previously thought possible: deliver
multi-channel radio by satellite to moving vehicles.
Sirius Satellite Radio of New York City and XM Satellite Radio of Washington
DC have each paid the US government $100 million for satellite radio
licences, and are preparing rival services. Motorists will be asked to pay
around $500 for a receiver and $10 a month for 100 channels of
commercial-free music, news and sport. But both firms face tough technical
hurdles.
To pick up a satellite signal, you normally need to point a dish aerial
precisely at the source. That鈥檚 impossible on a moving vehicle. The antennas for
the new systems, which look like small pods, are cut-down versions of military
antennas, which change their direction of focus electronically rather than by
moving physically.
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The big problem is that 鈥渃oncrete canyons鈥� in cities will block out the
satellite signal, so ground-based repeater stations will be needed to fill in
dead areas. The antennas will have to switch seamlessly to the repeaters when
the satellite signal is lost.
The two companies hope that a combination of high-power satellites,
computer-style error correction and memory chips that store鈥攐r
鈥渂uffer鈥濃�攖he sound will smooth over any gaps in reception. But until the
billion-dollar services start, no one knows if either of them will work.
Unfortunately for consumers, XM and Sirius have adopted incompatible
technologies. XM broadcasts music programmes at a fixed 64 kilobits per second,
while Sirius varies the bit rate continuously. So if one network fails, the
receivers won鈥檛 work for the rival service.
Sirius already has its three satellites in high elliptical orbits, ready for
the start of services later this year. The first XM satellite is due to be
launched from the Sea Launch ocean platform on 28 February, with a second
scheduled for the summer.
The Sirius satellites鈥� elliptical orbits mean that each can spend 16 hours of
the day over the US, then race round the rest of the world in eight hours. So
there will always be two satellites in sight of North America. The receiving
antenna follows one of the satellites across the sky, switching to the
alternative if transmission is blocked by a building or some other
obstruction.
XM鈥檚 two satellites will sit in geostationary orbits. They will be lower in
the sky than the Sirius craft, making it more likely that the signal will be
blocked by hills, trees or buildings.
While both firms believe their experiments will work, both admit to
鈥渦ncertainties鈥� over the technology.