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See Inside February 2007

Song Beams

Satellite radio can send the same 100 channels of music, talk and sports to you at any street corner in the nation. Yet someone next to you may receive a different set of channels, and a third person who does not subscribe cannot pick up a thing. How can the service blanket the country yet be so discriminating?

Three companies provide the world's satellite radio: XM Satellite Radio and Sirius Satellite Radio for the U.S.; and WorldSpace for Africa, Asia and Europe. XM uses two geostationary satellites and about 800 low-power ground repeaters scattered around cities where tall structures can block the satellites' line of sight. Sirius has three birds in highly inclined, elliptical orbits and about 100 high-power repeaters, each bathing a metropolitan area. Both architectures provide equally reliable service, says Dan Goebel, a senior research scientist at the Jet Propulsion Laboratory in Pasadena, Calif., who previously designed amplifiers for ground repeaters.

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