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.