Electronic Design

Don't Touch That Dial

Imagine leaving San Diego, Calif., driving to Portland, Maine, and listening to your favorite radio station all the way there. Well, that time is almost here. The satellite digital audio radio service (SDARS) came about in 1997 when the FCC auctioned off a portion of the spectrum.

Both Sirius Satellite Radio Inc. and XM Satellite Radio, Washington, D.C., were awarded licenses to build subscription-based, digital systems in the spectrum between 2320 and 2345 MHz for the purpose of national broadcasting by satellite. Originally, the plan was to use geostationary satellites at longitudes of approximately 80° and 110° west. These two satellites were intended to cover the contiguous United States. In the northern regions like Maine, however, this would have meant a fairly low angle of elevation—typically 25° to 35°.

But most mobile receivers can't count on any particular line of sight. While moving, they would suffer blockages from small buildings, trees, and even trucks. So, the plan was changed to what are known as inclined, elliptical, geosynchronous orbits. With this arrangement, there will always be at least two orbits in view of North America at all times, but at a higher angle of elevation for the northern section of the country.

Three satellites will be implemented, evenly spaced over the 24-hour day. Each satellite will spend eight hours in an idle state below the equator and 16 hours above the equator transmitting. Every satellite is 47,102 km above the Earth at apogee.

Sirius plans to broadcast up to 100 channels, with 50 of them devoted to commercial-free music. Listeners will pay a monthly subscription fee of $9.95 for the service—less than the cost of a compact disc. Service is expected to begin this year. More information can be found at either www.siriusradio.com or www.xmradio.com.

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