(Left) A Proton rocket lifts one of the
Sirius satellites into orbit. (Right) XM's Roll takes off from the Sea
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
(Left) The XM (left) and Sirius system
control centers monitor the satellite parameters.
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
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
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.
Figure 3. Sirius and XM use their
S-band frequency allocations in very different ways. (Click the image
for a larger view.)
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
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
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
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
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
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.