The temperature was chilly - at least to two southerners - but even the coldest day in Rocky Mountain National Park is sweltering compared to the frigid -450° void of outer space. At 9,000 feet, we were well above sea level, but low compared to the orbiting satellites that transmit the signals for the XM and Sirius radio services from as high as 29,000 miles above terra firma. The top speeds of our Audi and BMW vehicles were fast by earthly standards, but positively sluggish compared to the 9,000-mile-per-hour velocity of the space vehicles in question. Our world and theirs could hardly be more different, yet we were comfortably linked by heavenly music floating down from above.

Music of the Spheres

For the most part, radio is a terrestrial business. Although AM and FM signals reach us through thin air (and radiate outward into space), radio-station transmitters use earthbound towers to broadcast their signals. Since that technology, along with government regulation, limits the range of reception, these stations can only provide local service. And since terrestrial broadcasting is analog, the signal will waver as you drive, and numerous noises will intrude. (Digital terrestrial radio is in the works, though, and could debut as early as next year.)

The XM and Sirius radio services are seeking to change the broadcasting business by using satellites to receive signals and then transmit them back to earth so they can be picked up throughout the lower 48 United States. This provides a virtually national radio service, since the same stations can be received in the entire coverage area. And since the transmissions are digital, they're immune to most of the reception problems that plague analog radio.

To receive the 100 channels from either service, you'll need a Sirius or XM car receiver and a subscription that costs $12.95 or $9.95 a month, respectively. XM provides 71 music channels and 29 news/talk/sports channels, and it's being supported by General Motors, Honda, Suzuki, Isuzu, and others. Sirius provides 60 music channels and 40 news/talk/sports channels, and it's partnered with Ford, DaimlerChrysler, BMW, Mazda, and others. Add-on receivers are available from a variety of mobile-electronics manufacturers, with home units to become available in the next few years.

XM has been available nationally since November 12, 2001. Sirius began its rollout earlier this year in Fort Worth and Houston, Texas; Jackson, Mississippi; and Denver, Colorado. It has since expanded into most of the Midwest and plans to offer service nationally by July 1. While the current receivers are designed to pull in either XM or Sirius exclusively, head units that can receive both are expected to be available in about five years.

Head to Head

Since analog terrestrial radio has served us well for over 80 years, do we really need digital satellite radio? Is it worth the premium cost over free radio? If it is, which is better - Sirius or XM?

To explore those questions, we requested test vehicles from the respective services and hit the road. Sirius supplied a BMW X5 3.0i with a Kenwood KTC-SR901 tuner, while XM set us up with an Audi S4 with a Pioneer GEX-FM903XM receiver. The BMW was outfitted with a high-end aftermarket all-Kenwood system that included a 7-inch touchscreen monitor for the AM/FM/Sirius receiver, a DVD-based navigation system, and 1,000 watts of amplification. The Audi's sound system was more modest. The XM receiver was patched into the factory installed Bose system using a BlitzSafe interface. (BlitzSafe creates a digital connection, as opposed to the FM-modulated connections that are commonly used to hook up an external component like a CD changer or portable CD player.) Bottom line: both systems were equipped to provide good sound quality.

We wanted answers to three basic questions:
1) How reliable is the signal reception?
2) How good is the sound quality (irrespective of reception interference)?
3) How good is the programming?

You wouldn't normally ask that last question when you're buying a radio. But, unlike AM and FM radio, Sirius and XM are "closed" systems, meaning that you can only listen to what their DJs provide. Sure, you can change channels, but you can't escape what the company is offering (unless you tune back to old-fashioned AM or FM broadcasts).

The Mile-High City

Our approach to the comparison was simple: Two vehicles. Two days. One of us in each vehicle, switching cars after one day. Both of us driving the same route at the same time, staying in touch by walkie-talkie. Each of us listening carefully for reception robustness, sound quality, and content. The first criterion is objective. Like so much digital technology, these are "all or nothing" systems. When they fail, they fall silent. The second criterion is semi-subjective. All listeners have different ideas about sound quality. The last criterion, of course, is completely subjective. One person's Beethoven is another person's Poison.
We wanted our road test to include both urban and rural areas. Even though the signals from the Sirius and XM satellites cover the entire continental U.S. except Alaska, things like buildings, mountains, tunnels, and bridges can cause gaps in the coverage that can be filled by local terrestrial transmitters that repeat the satellite signal. The receiver automatically switches to this "gap filler" frequency when it loses its lock on the satellite.

XM has already established its network of repeaters, but Sirius was only just completing its gap-filler infrastructure when we took our test drive. Its network in Denver, however, was up and working fine, which is why we chose that area.

The Denver area proved to be perfect for testing the services. The city's buildings were tall enough and the streets narrow enough to test the repeaters. And the twisting roads through the narrow canyons of the nearby Rocky Mountains, where there are no repeaters, stressed the receivers and their antennas to their limits.

We started our trip in the city's concrete and steel canyons. The Colorado Rockies were at home, so a trip downtown to Coors Field seemed in order. XM and Sirius use different technologies to deliver signals to their repeaters, but both were very reliable, shrugging off the skyscrapers along 17th Street without a hitch. Even an overpass walkway posed no problems. After several hours of in-town driving, each system had experienced exactly one momentary drop-out - in the same spot along a downtown street. We were impressed by the robustness of both signals. Of course, it would be a mistake to overgeneralize these results, since each system's urban reliability is largely a function of its repeaters - the results in your city may be different. After stopping to grab a hotdog at the ballpark, we turned toward the mountains.