Understanding Satellite Radio

If you're on the road a lot, you know how frustrating it is to have radio stations fade out as you drive out of their broadcast areas. Satellite radio offers the much better alternative of fade-free nationwide coverage -- you're never out of range. Plus, you get over a hundred channels of entertainment, with a good mix of music and talk, all with high-quality digital sound. It's a great deal if you love music, but hate the technological and format restrictions of traditional terrestrial radio.

How Satellite Radio Works

In the United States, there are two competing satellite radio services: SIRIUS Satellite Radio and XM Radio. The two companies announced a merger on February 19, 2007, but the merger will not be completed until late 2007 or early 2008. Until then, both companies use similar -- although ultimately incompatible -- technologies. In essence, the two companies bounce digital signals off orbiting satellites; the signals are then received and decoded by compatible satellite receivers back on Earth. Both SIRIUS and XM signals are transmitted in the 2.3GHz S-band, reserved by the FCC in 1992 for digital audio radio service (DARS) transmissions.

XM uses two Boeing HS 702 satellites, dubbed "Rock" and "Roll." The two satellites are in geostationary orbit, in fixed positions approximately 22,000 miles above the equator. (XM has a third satellite already built and awaiting emergency launch, just in case something happens to the two orbiting satellites.)

XM's satellite signals are supplemented by a series of ground-based repeater antennas in major metropolitan areas. This helps to reduce the loss of the satellite signal that sometimes comes when you drive between tall downtown buildings.

Instead of geosynchronous satellites, SIRIUS uses three Space Systems/Loral satellites in an inclined elliptical constellation. This configuration puts at least one satellite over the U.S. at all times; each satellite spends about 16 hours a day over the continental U.S. SIRIUS' satellites are also higher in the sky than XM's satellites, thus alleviating the reception problem in downtown "canyons" -- although the company still utilizes some ground-based repeater antennas.

Michael Miller is a writer and commentator on technology and digital lifestyle topics.