[Success Story]
Satellite Radio Gets Serious
Sirius Satellite Radio delivers 100 channels of CD-quality music, sports, and news to auto and home receivers.
All electronic design engineers face design challenges, but few have to face the enormous undertaking that confronted the engineers who decided to build a next-generation satellite system to deliver high-quality digital audio to U.S. vehicles and homes.
It's not easy or inexpensive. But if you haven't already experienced it, the outcome is more than spectacular, far better than you would expect from radio (see "Test Report," p. 44). This is the story about how Sirius Satellite Radio (www.sirius.com) developed and launched its system.
WHAT IS IT? Sirius Radio is one of two satellite radio companies licensed by the FCC to deliver digital audio by satellite. This service, more formally known as Digital Audio Radio Service (DARS), was established in 1992. After many applications and reviews, the FCC whittled the number of applicants down and granted broadcast rights to Sirius and its competitor XM Satellite Radio (www.xmradio.com). These companies paid for this spectrum—Sirius $89 million and XM $92 million. The license authorizes broadcast rights in the microwave S-band at 2.3 GHz. After many years of system development, satellite launches, and receiver design, both companies launched their services in 2002.
The primary application for this service is constant coast-to-coast coverage of radio for cars. We have all experienced the problem of trying to listen to radio on a long trip. Both AM and FM stations fade in and out as we drive into and out of their coverage area. And signal reception is generally poor as well as variable. With the DARS systems, radio coverage throughout the 48 continental states is solid and continuous.
Sirius and XM each offer 100 channels of digital radio, mostly music channels but also news, sports, and other entertainment formats. Special car radios have been designed, and as of last year, were offered as an option by most major vehicle manufacturers. Sirius receivers are available as an option in Ford/Lincoln/Mercury vehicles as well as those of DaimlerChrysler, BMW, Jaguar, Porsche, Mercedes-Benz, Volvo, Mazda, and several others. XM radios are available primarily in GM vehicles. At $12.95 per month, the Sirius service offers the benefit of commercial-free music 24/7. After-market auto receivers and home receivers are also available at most popular consumer electronic stores, such as Best Buy, Circuit City, Good Guys, Sound Advice, Tweeters, and Crutchfield by mail order.
THE SPACE SEGMENT There are two basic ways to cover a given area with satellites. The traditional approach and the one used by most space communications systems is to put up a geostationary satellite over the desired area. Geostationary satellites are put into a circular orbit around the equator about 22,300 miles (yes, miles) from earth. In such an orbit, the satellite speed matches the rotation of the earth, so the satellite is always overhead to any observer or station on earth. Sirius' competitor XM Radio uses this system with two satellites providing full U.S. coverage. The equatorial geostationary orbit is unique and currently jammed with satellites side by side only a few degrees apart. As with the frequency spectrum, we're simply running out of space.
Sirius takes the other approach of using elliptical orbits. The company has three elliptical orbits over the U.S. They are geosynchronous, meaning that their rotational period is 24 hours just like a geostationary satellite. The satellite apogee (high point) is 29,200 miles over Canada and the perigee (low point) is 14,900 miles. The orbits function in a way where two satellites are over the U.S. at all times. The satellites are spaced eight hours from one another, and each satellite is over the U.S. for about 16 hours. All three transmit the same data.
The elliptical orbits offer the advantage of a very high angle of coverage. With a conventional geostationary satellite, the line-of-sight path runs at a very low angle of elevation above the equator (about 30°) to the south from the U.S. Because microwave transmissions are direct-line-of-sight, signals from geostationary satellites encounter many more obstacles like trees and buildings. With elliptical orbits, the satellites are more directly overhead (always above 60°) and thereby avoid most earth obstacles. Yet at such distances, the attenuation from satellite to earth is enormous. Typical signal strength in the U.S. is −102 dBi, meaning that a hot receiver is needed.
All communications satellites are space-based repeaters that receive an uplinked signal, which is translated to another frequency and retransmitted back to earth. In the Sirius system, the digitized music and talk is uplinked from studios in New York City and retransmitted back to earth.
Mike Ledford, VP of engineering for Sirius, explains the system. Sirius is assigned 12.5 MHz of spectrum from 2320 to 2332.5 MHz centered on 2.32625 GHz (2326.25 MHz). This spectrum is roughly divided into thirds. One third is assigned to transmitting satellite #1 (TDM1) centered at 2322.3 MHz, one third to the terrestrial repeater network (more on that later) centered on 2326.25 MHz, and one third to transmitting satellite #2 (TDM2) centered on 2330.2 MHz. This gives each satellite roughly 4-MHz bandwidth.
Reprints
