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.

At least its 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 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 and Africa since 1998. But Worldspace was intended primarily for use in fixed locations, while the systems used by XM and Sirius are optimized to reach U.S. listeners on the go.

From the ground up

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 persons 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.

XMs 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.