The Final Countdown for Satellite Radio
Jul 1, 2001 12:00 PM, By Chriss Scherer, editor
(Left) A Proton rocket lifts one of the Sirius satellites into orbit. (Right) XM's Roll takes off from the Sea Launch platform.
(Left) The XM (left) and Sirius system control centers monitor the satellite parameters.
Figure 3. Sirius and XM use their S-band frequency allocations in very different ways. (Click the image for a larger view.)
Radio currently has three different forms: terrestrial, Internet and the newest innovation, satellite radio, which delivers multiple audio channels directly from the sky to the listener. Apart from the delivery method, the most unique difference between satellite radio and the other forms is the subscription-based business model. Subscription services are not a new concept. In fact, many cable TV systems already offer audio entertainment channels as an option, but these channels are typically jukeboxes without any announcers � or personality.
The two satellite radio service providers, Sirius Satellite Radio and XM Satellite Radio, have assembled talented programming staffs to format many of the 100 audio channels each will offer. The providers plan to offer many niche programming choices, but will draw from a much larger population, so the niche audience will also be greater.
Satellite radio basics
Both Sirius and XM have satellite systems in the sky ready to go. Because satellite reception requires a line-of-sight signal path, both providers are building terrestrial repeater networks to supplement coverage in metropolitan areas. The exact number of repeaters varies for each location. In some major markets, the terrestrial network is complete and ready for use. Some terrestrial networks will not be completed until after the service officially launches.
Let's look at the audio chain and move through the system. Sirius has built a studio complex in New York City (see the Facility Showcase in the November 2000 issue of BE Radio), and XM has built its studio complex in Washington, DC (a Facility Showcase on XM will be featured in the October 2001 issue.) Both facilities use the latest technologies to route audio and data through their facilities. In addition, both providers have agreements with program source providers, so some content will be created by an outside source and then transported to these network facility control centers. XM has also built a studio facility in the Country Music Hall of Fame in Nashville.
From these extensive studio facilities (where digital audio file storage is measured in terabytes and approaches petabytes), the audio and data are uplinked via X-band to the orbiting satellites. The satellites then transmit the signal back to Earth on S-band. Both providers have footprints that cover the continental United States. Agreements are in place to prevent the signals from extending into Mexico and Canada.
Repeaters are needed in areas where satellite reception will be hindered by buildings and other obstructions in the line-of-sight path. While both services will launch nationally, only some of the terrestrial networks are completed. As additional subscribers are obtained, the repeater networks outside the largest cities will be completed.
The main difference between the systems are the satellite orbital paths. XM has two satellites in a geostationary orbit at 85� and 115�. This places them 22,223 miles above the equator south of the Alabama/Georgia and the California/Arizona borders.
Figure 1. The two providers are a study in satellite technology. XM uses two satellites in geostationary orbits (blue path), while Sirius uses three satellites in highly eliptical orbits (red path).
Sirius has three satellites placed in highly elliptical orbits. At any given time, at least one of the three satellites is visible to receivers. Each satellite is over the United States for about 16 hours each day at an angle of 60 to 90 degrees. Figure 1 shows a comparison of the orbital paths for each service. Figure 2 shows the effective flight path of the Sirius satellites after combing the theoretical flight paths with the Earth's orbit.
The terrestrial repeater network works with the satellite signals to provide a more robust transmission system. A listener can receive up to three signals at any time: two satellites and a repeater. A listener's radio need only receive one of the three signals to function. The terrestrial repeater network adds another level of redundancy to the system.
Figure 2. The effective Sirius satellite coverage factoring orbital paths and Earth's rotation. (Click the image for a larger view.)
Signal reception of the two systems are similar in their basic theories but differ in execution. Because of the geostationary orbits, it is easier to start with the method that XM uses. The signal is divided into two ensembles, each containing half the data being transmitted. These signals are sent to both satellites (named Rock and Roll) where they are received, downconverted to S-band, and then transmitted back to Earth. While XM has a total licensed spectrum of 12.5MHz, notice that each satellite only uses a portion of this space, as shown in Figure 3. The listener receives the data from one or both satellites. The terrestrial repeaters also receive the satellite signal and then retransmit it on the appropriate frequencies. In addition, each satellite and repeater adds a finite amount of time delay (up to a few seconds) to the signal, which provides greater robustness. The overall system data throughput is 3.28Mb/s.
This method provides three kinds of diversity. Spatial diversity is provided by the physical differences in the location of the satellites and terrestrial repeaters. Frequency diversity is provided by the placement of signals across the spectrum allocation. Finally, time diversity is provided by the small amounts of delay added to the signal from each source.
Sirius divides its X-band spectrum into two portions, each containing the entire data package. The two two portions are then sent to the two visible satellites.
Each packet of data has a unique identifier so that a receiver can properly reassemble the datastream regardless of the source. As the receiver moves, the signal strength from each source will vary. The receiver will constantly search for the data it needs to reconstruct the signal.
Many existing broadcasters have expressed concern over Sirius and XM providing local content through the terrestrial network. While every repeater site does have a direct connection back to the providers' operation center, this link is only for telemetry and control. There is not enough bandwidth to provide additional audio services. Besides, with more than 1,000 repeater sites across the country, the cost of connecting each of these sites would be quite high. Also, the X-band uplink does not have the capacity to carry the additional localized programming.
The Sirius method transmits two different signals to the two satellites visible at that time. Both uplink signals contain the complete programming data. The satellite then converts and retransmits the signal back to Earth. Figure 3 shows the S-band spectrum division. Since there are three satellites and two signals, the frequencies rotate between them as each satellite comes into view. One satellite uplink feed is delayed by four seconds to introduce time diversity.
Instead of receiving the S-band transmission and rebroadcasting it on the terrestrial repeaters like XM, a separate VSAT link is used to send data to the terrestrial network. Data is uplinked on 14GHz and then downlinked on 12GHz to the terrestrial repeaters. Because the Sirius satellites appear higher in the sky and are not over the equator, fewer terrestrial repeaters should be required to fill in the built-up areas.
Both providers are using the Lucent PAC (Perceptual Audio Coder) algorithm to encode the audio. While the first generation of receivers will only be able to receive the service of one provider, Sirius and XM have an agreement to work together on receivers that can receive both services.
Not just audio
While satellite radio is an audio service, it has one feature that will draw obvious listener attention. In addition to digital audio, Program-associated data (PAD) is transmitted to the receiver. FM stations have been able to transmit RBDS data for several years with limited acceptance. Because the new services require a new receiver, the data capacity can be included from the beginning.
The PAD content is stored on a server and recalled as needed. Most on-air audio playback systems from outside the United States can provide this type of data storage and manipulation, mainly because RBDS (or RDS as it is called in Europe) has seen a much greater acceptance. Some domestic systems can also store and manipulate this data as well. Sirius uses a Prophet Systems system, and XM uses a Dalet system for audio storage and playback. In addition to PAD, Non-program-associated data (NPAD) can also be stored and displayed as needed.
For the listener, a subscription-based radio service will be something new. The anticipated monthly cost will be $12.95 for Sirius and $9.95 for XM. Wall Street analysts predict that by the end of 2001 Sirius will have 10,000 subscribers and XM 50,000, with the numbers reaching about 2 million subscribers each by 2003 and 4 million by 2004.
Satellite radio will provide another entertainment and information source to potential radio listeners. The additional data and display capability will add a new spin to the long-established medium. An evolutionary step in radio is about to take place.