We all have our favorite radio stations that we preset into our car radios, flipping between them as we drive to and from work, on errands and around town. But when you travel too far away from the source station, the signal breaks up and fades into static. Most radio signals can only travel about 30 or 40 miles from their source. On long trips that find you passing through different cities, you might have to change radio stations every hour or so as the signals fade in and out. And it's not much fun scanning through static trying to find something -- anything -- to listen to.

Photo courtesy XM Satellite Radio Satellite radio broadcasters promise crystal-clear music transmitted from thousands of miles into space.

Now, imagine a radio station that can broadcast its signal from more than 22,000 miles (35,000 km) away and then come through on your car radio with complete clarity. You could driv e from Tacoma, Washington, to Washington, D.C., without ever having to change the radio station! Not only would you never hear static interfering with your favorite tunes, but the music would be interrupted by few or no commercials.

XM Satellite Radio and Sirius Satellite Radio have both launched such a service. Satellite radio, also called digital radio, offers uninterrupted, near CD-quality music beamed to your radio from space.

Car manufacturers have installed satellite radio receivers in some 2001 and 2002 models, and electronics companies have launched several models of portable satellite radio receivers. In this article, you'll learn what separates satellite radio from conventional radio and what you need to pick up satellite radio signals.

Satellite radio is an idea nearly 10 years in the making. In 1992, the U.S. Federal Communications Commission (FCC) allocated a spectrum in the "S" band (2.3 GHz) for nationwide broadcasting of satellite-based Digital Audio Radio Service (DARS). Only four companies applied for a license to broadcast over that band. The FCC gave licenses to two of these companies in 1997. CD Radio (now Sirius Satellite Radio) and American Mobile Radio (now XM Satellite Radio) paid more than $80 million each to use space in the S-band for digital satellite transmission.

At this time, there are three space-based radio broadcasters in various stages of development:

• Sirius Satellite Radio is now operational in the United States, with its official launch on July 1, 2002.
• XM Satellite Radio launched commercial service in limited areas of the United States on September 25, 2001. (They were originally going to launch service September 12, but postponed the event because of the terrorist attacks on the United States.)
• WorldSpace is already broadcasting in Africa and Asia, and will begin broadcasting in South America sometime soon.

• Satellites
• Ground repeaters
• Radio receivers

XM Radio uses two Boeing HS 702 satellites, appropriately dubbed "Rock" and "Roll," placed in parallel geostationary orbit, one at 85 degrees west longitude and the other at 115 degrees west longitude. Geostationary Earth orbit (GEO) is about 22,223 miles (35,764 km) above Earth, and is the type of orbit most commonly used for communications satellites . The first XM satellite, "Rock," was launched on March 18, 2001, with "Roll" following on May 8. XM Radio has a third HS-702 satellite on the ground ready to be launched in case one of the two orbiting satellites fails.

Photo courtesy XM Satellite Radio This graphic illustrates how the XM Radio system works.

XM Radio's ground station transmits a signal to its two GEO satellites, which bounce the signals back down to radio receivers on the ground. The radio receivers are programmed to receive and unscramble the digital data signal, which contains up to 100 channels of digital audio. In addition to the encoded sound, the signal contains additional information about the broadcast. The song title, artist and genre of music are all displayed on the radio. In urban areas, where buildings can block out the satellite signal, XM's broadcasting system is supplemented by ground transmitters.

Photo courtesy XM Satellite Radio An XM Satellite Radio receiver

Each receiver contains a proprietary chipset. XM began delivering chipsets to its XM radio manufacturing partners in October 2000. The chipset consists of two custom integrated circuits designed by STMicroelectronics. XM has partnered with Pioneer, Alpine, Clarion, Delphi Delco, Sony and Motorola to manufacture XM car radios. Each satellite radio receiver uses a small, car-phone-sized antenna to receive the XM signal. General Motors has invested about $100 million in XM, and Honda has also signed an agreement to use XM radios in its cars. GM began installing XM satellite radio receivers in selected models in early 2001.

For $9.99 per month, subscribers can receive the XM signal. For that price, listeners get up to 100 channels of music, talk and news. Many of the channels have no commercials, with none of the channels having more than seven minutes of ads per hour. XM's content providers include USA Today, BBC, CNN/Sports Illustrated and The Weather Channel. The service bolsters that lineup with its own music channels.

Unlike XM, Sirius does not use GEO satellites . Instead, its three SS/L-1300 satellites form an inclined elliptical satellite constellation. Sirius says the elliptical path of its satellite constellation ensures that each satellite spends about 16 hours a day over the continental United States, with at least one satellite over the country at all times. Sirius completed its three-satellite constellation on November 30, 2000. A fourth satellite will remain on the ground, ready to be launched if any of the three active satellites encounter transmission problems.

The Sirius system is similar to that of XM. Programs are beamed to one of the three Sirius satellites, which then transmits the signal to the ground, where your radio receiver picks up one of the channels within the signal. Signals are also be beamed to ground repeaters for listeners in urban areas where the satellite signal can be interrupted.

While XM offers both car and portable radios, Sirius is concentrating on the car radio market. The Sirius receiver includes two parts -- the antenna module and the receiver module. The antenna module picks up signals from the ground repeaters or the satellite, amplifies the signal and filters out any interference. The signal is then passed on to the receiver module. Inside the receiver module is a chipset consisting of eight chips. The chipset converts the signals from 2.3 gigahertz (GHz) to a lower intermediate frequency. Sirius also offers an adapter that allows conventional car radios to receive satellite signals.

So far, WorldSpace has been the leader in the satellite radio industry. It put two of its three satellites , AfriStar and AsiaStar, in geostationary orbit before either of the other two companies launched one. AfriStar and AsiaStar were launched in October 1998 and March 2000, respectively. AmeriStar, which will offer service to South America and parts of Mexico, is not yet scheduled for launch. Each satellite transmits three signal beams, carrying more than 40 channels of programming, to three overlapping coverage areas of about 5.4 million square miles (14 million square km) each. Each of the WorldSpace satellites' three beams can deliver over 50 channels of crystal clear audio and multimedia programming via the 1,467- to 1,492-megahertz (MHz) segment of the L-Band spectrum , which is allocated for digital audio broadcasting.

The United States is not currently part of WorldSpace's coverage area. The company has invested in XM Radio and has an agreement with XM to share any technological developments. WorldSpace is going beyond one nation and eyeing world domination of the radio market. That might be overstating the company's intent a bit, but WorldSpace does plan to reach the corners of our world that most radio stations cannot. There are millions of people living in WorldSpace's projected listening area who cannot pick up a signal from a conventional radio station. WorldSpace says it has a potential audience of about 4.6 billion listeners spanning five continents.

Photo courtesy WorldSpace WorldSpace will be able to broadcast to the majority of the world's population when its AmeriStar satellite is launched.

WorldSpace broadcasters uplink their signal to one of the three satellites through a centralized hub site or an individual feeder link station located within the global uplink beam. The satellite then transmits the signal in one, two or all three beams on each satellite. Receivers on the ground then pick up the signal and provide CD-quality sound through a detachable antenna.

Photo courtesy WorldSpace Two of the WorldSpace satellite radio receivers

WorldSpace satellite receivers are capable of receiving data at a rate of 128 kilobits per second (Kbps). The receivers use the proprietary StarMan chipset, manufactured by STMicroelectronics, to receive digital signals from the satellites.