How does Satellite
Radio work?

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   How Satellite Radio works?

Satellite Radio

 How does it work?

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 Satellite Radio Receivers
 Satellite Radio beams the signal up to a satellite, and
  the limitations of ground-based transmitters

   Satellite Radio - Basic Concept

Satellite radio is such a remarkably simple concept that one might wonder why it took until 2001 for the first space-based audio service to make its debut in the United States.

satellite radio At least itís simple on the surface: Take a music, news or talk station, beam the signal up to a satellite, and overcome the limitations of ground-based transmitters whose signals generally drop off as distance increases. Then make sure the programming is more appealing than traditional radio stations and cut down on the number of commercials in exchange for a monthly subscription fee.

But as it turns out, satellite radio is a whole lot more complex than it seems on paper Ė and it took cutting-edge technology to make the systems operated by Sirius Satellite Radio and XM Satellite Radio work.

XM and Sirius are not the first companies to enter the satellite radio industry: Worldspace Corp., a firm based in Washington, has provided satellite radio in Asia, Africa and Europe since 1998. But Worldspace was intended primarily for use in
One of XM's two seven-meter uplink satellite dishes located at the company's headquarters.
fixed locations, while the systems used by XM and Sirius are optimized to reach U.S. listeners on the go.

satellite radio
Sirius uplinks its signal from New Jersey, with backup
  dishes in Manhattan (above).

It took a number of years to develop the XM and Sirius systems.

Engineers had to figure out how to squeeze dozens of individual channels into a relatively small amount of bandwidth and come up with reliable methods of beaming signals from thousands of miles in space to roving antennas smaller than tennis balls.

They also had to develop inexpensive circuitry, or chipsets, to enable receivers to decode the satellite signals, which are encrypted to prevent reception by non-subscribers. Both firms are working on newer versions of their chipsets that will be smaller and use less power.

Sirius and XM each took somewhat different approaches, although the end result, from a lay personís perspective, is the same: 100 channels of music, news, sports and other fare available virtually anywhere in the continental United States. The companies are trying to distinguish themselves with programming and attitude.

XMís system uses two very powerful satellites floating in space directly above the equator. The spacecraft are in geostationary orbit -- they appear from the ground to remain in fixed perches, because they move around the Earth at the same speed the planet is rotating.

Geostationary satellites are commonly used for all sorts of space-based communications because they enable use of inexpensive, fixed antennas. Satellite TV and Internet systems are two examples of consumer-oriented technologies that use this type of satellite.


Repeat that, please

Since geostationary spacecraft are above the equator, terminals on the ground must have a decent view of the southern sky to receive signals from them. This posed a challenge for XM, since listeners in cars often pass by obstacles, such as buildings, foliage or hills, which can block geostationary satellite signals.

XMís solution is a network of repeaters Ė antennas on buildings and other sites that receive satellite signals from an optimally placed antenna and retransmit them. The repeaters are located primarily in built-up areas, where loss of the satellite signal is most likely to occur.

Each XM receiver is equipped to receive signals from both of the companyís Boeing 702 satellites and a repeater simultaneously. As long as one of the sources is available, the radio will play without interruption. In addition, the receivers have buffers that store programming for several seconds, allowing operation to continue even if no signal is available momentarily.

Sirius uses a trio of Loral FS1300 satellites in unique elliptical orbits in an effort to avoid the problems posed by geostationary satellites.

The orbits, shaped like figure eights, allow the satellites to appear higher in the sky than XMís, cutting down on the potential for a listener to be out of range of a satellite signal -- and allowing Sirius to have a much smaller number of repeaters.

Siriusí repeater network also avoids the need for specialized antennas that can track the companyís non-geostationary satellites as they move about the sky, Sirius feeds its repeaters using capacity on a geostationary satellite leased from a traditional satellite operator. Listeners canít tell that the signals they receive via the repeaters do not travel over Siriusí fleet of satellites.

The Sirius satellites each spend about 16 hours over the United States, then whip around the other side of the Earth and return eight hours later for another stint hovering over Siriusí listening area, according to Ted Hessler, the companyís vice president of space segment and enterprise operations.

Two Sirius spacecraft cover the United States at any given time, Hessler said.

In the studio

XM and Sirius both operate digital broadcast centers that combine dozens of individual recording studios with huge amounts of storage to hold hundreds of thousands of compact discs worth of music in digital format.

satellite radio1. Sirius and XM both produce live and taped programming, ranging broadly from Alanis Morissette, right, to sports and news.

Programmers just point and click at the material they want to play, and it airs directly from the storage system at the appointed time. During transmission, the system also adds a short description of the music or other material for display on a small receiver screen. That is one unique advantage to satellite radio -- you can find out the artist and song title as each piece of music plays.

The 22 terabytes of storage capacity at XMís facilities in Washington can hold about 250,000 CDs, said Anthony J. Masiello, XMís senior vice president of operations.

Terry Smith, senior vice president and chief technology officer of Sirius, said his companyís studios in mid-town Manhattan have about seven terabytes of storage. While that is less than XM has, Smith says itís plenty.

"Our library is constantly being refreshed as new content comes in," Smith said.

Both companies also maintain large collections of CDs to augment their digital libraries. They also retransmit programming that originates elsewhere, such as news, sports and comedy channels, and maintain studios where artists perform live.

Another, less visible key to satellite radio is digital compression, a technique to use radio spectrum as efficiently as possible. Both satellite radio broadcasters use sophisticated algorithms to squeeze as much material as they can into the available bandwidth without causing audio quality to degrade.

XM and Sirius are each allocated 12.5 megahertz of radio spectrum by the U.S. Federal Communications Commission.

Satellite Radio is looking far beyond the dashboard.

Satellite radio has turned the corner. First positioned as a mobile entertainment service for long-distance truckers and road warriors, pay radio is now heading home, to the park, and anywhere else people want to listen to music. Thanks to the recent success of Delphi's SKYFi, a portable amplifier/speaker shell with a slot for an XM Satellite Radio receiver module, XM--and its competitor, Sirius--are looking far beyond the dashboard.

The Delphi SKYFi--at a street price of $99 for the boombox with amplifiers and speakers, and $99 for the receiver module

satellite radio
The programming is beamed to satellites from dishes operated by satellite radio operator.

satellite radio3. The satellites broadcast the signal back to Earth, where it's picked up directly by receiver units. The signal is also received and rebroadcast by repeater stations in metropolitan areas. XM uses two geostationary satellites (right) that remain constantly above the United States. Sirius uses three satellites, two of which are always over the country.

satellite radio car 4. A receiver buffers the broadcast for a few seconds, so if it loses the satellite signal it can use one from a repeater station, helping insure a continuous broadcast. Overpasses and tall building are particular problems.

itself--breaks the $200 price barrier. That appears to have caught the attention of cost-conscious consumers who also have to pony up $9.99 per month for XM Radio service. Sony's home unit cost $300 to $350 when it was introduced in 2001. In 2002, Sony stopped producing the home unit when it decided not to continue the line.

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