The Sirius signal is separated into three carriers, one each for the two satellites, and the third for the terrestrial repeater network where available. Sirius receivers decode all three 4 MHz carrier signals at once to achieve
signal diversity. This is in contrast to XM which uses six carriers and decodes three 2 MHz carriers to economize on receiver power consumption and complexity at the cost of channel-changing speed. There is an intentional four-second delay between the two satellite carrier signals. This enables the receiver to maintain a large buffer of the audio stream, which, along with
forward error correction, helps keep the audio playing in the event that the signal is temporarily lost, such as when driving under an overpass or otherwise losing
line-of-sight of any of the satellites or ground repeater stations. A third, separate signal is uplinked to the AMC-6
Ku-band satellite and received by
satellite dishes for the ground repeater network. This third signal is broadcast on a third segment of the signal.
Signal architecture and early prototypes The technology for Sirius Satellite Radio receivers as well as some of the uplink equipment, and the studio encoder, originated at Bell Labs in the late 1990s and subsequent years. The studio encoder was a result of Bell Labs' efforts in statistical multiplexing of perceptual audio coded signals, a cousin of the MP3 standards. The waveform design for the terrestrial and satellite signals, as well as the early prototype receivers, were implemented in an
FPGA logic and tested in the field to verify the performance of the receivers. This work was contracted by Sirius to
Lucent Technologies, at the time a spinoff of AT&T. Early prototypes were followed by a number of generations of
ASIC custom designed chipsets, supplied at first by Agere Systems and later supplied by Agere Systems and their competitor STMicroelectronics. Three signals from three different sources (satellite, satellite, and terrestrial) are therefore combined in the receiver as radio signals, (not as audio signals). The three signals need to be combined constructively (avoiding situations where bad signals pollute good signals) in the receiver before being decoded. Heavy error correction is applied to the signals. All three signals contain the same audio content on all the channels that the receiver can receive, with the exception of one audio program waveform being transmitted ahead of the other two by approximately four seconds. With this time skew, the signals, once realigned, need to see an 8-second obstruction of overpass fade in order to lose audio content. This increases the robustness of the signal delivery in most driving conditions. In order to recover meaningful signal and error-free audio from a signal impaired by interference and fading, the receiver uses concatenated Reed-Solomon block coding and Forward Error Correction encoding and decoding (codec). This technique was proven in the early days of satellite modems in the late 1970s. Linkabit, then run by Irwin Jacobs prior to his involvement with Qualcomm, offered such a codec for rack mounting in satellite earth stations. The Sirius signal uses more robust error correction on control channels than on the audio content, trading off error correction and bandwidth differently for separate categories of bits in the signal waveform. The terrestrial carrier is an OFDM QPSK signal, and cousin to WiMax and LTE, with the particular feature that more than one transmitter operates on the same frequency, forming a single frequency network. A number of transmitters can be placed around a city to create coverage that is less subject to fading than if a single transmitter were used. The satellite signal is QPSK. Both satellite and terrestrial signals have hierarchical modulation superposed on the original signal, a measure created to add bandwidth at a small expense in the satellite link budget for decoding the core audio content. This architecture has worked remarkably well in avoiding drop out of audio signal when driving under highway overpasses, and when scintillating (very deep and frequent losses in signal strength caused by radio fading from trees) conditions exist. Since Sirius and XM separately entered the market with incompatible waveforms on the satellite, one would logically conclude that the merged company will eventually evolve the signal format again to take advantage of their size, but this is a speculative statement. The use of a satellite and terrestrial combined service has been adopted by the
DVB-SH standard, and companies such as ICO communications who cooperated with Alcatel-Lucent on system design and field trials. ONDAS, a Madrid-based company, also adopted this pioneering system architecture.
Receiver technology The receiver is designed to mitigate and retain signal quality in hostile signal conditions and the relatively weak signal levels from distant satellites. Because the satellites are not all geostationary they appear and disappear over the horizon. Terrestrial signals are present only in major cities to augment the satellite signals. Approximately five chipset versions were built by Agere and approximately 4 versions were built by
STMicroelectronics after the initial prototypes, although all of the early receivers included an Agere chipset known as Northstar. This platform enjoyed the highest volume of chipsets to date, representing the bulk of total production from 2002 to 2010. As of 2010, most of the chipsets are produced by STMicroelectronics. At the heart of a Sirius receiver is a custom application-specific integrated circuit (
ASIC) chip called the
Baseband Integrated Circuit currently the STA240, which is produced by STMicroelectronics. The chip contains embedded
ARM7TDMI and ARM946E-S microprocessors synthesized from
IP cores. Every baseband has a unique Electronic Serial Number (or Sirius ID). Another major section of a Sirius receiver is the tuner. The tuner is also a custom
ASIC, the STA210. The tuner connects to the antenna, and receives the incoming satellite and terrestrial signals at 2.315 GHz and downconverts them to
intermediate frequency signals at around 75 MHz. The strength of the signals is approximately −50dBm in clear-sky conditions. The IF signals are fed to the STA240, which are digitized, demodulated, error-corrected, de-interleaved, and decrypted using specialized circuits on the chip. The baseband processor utilizes a 16MB
SDRAM memory to buffer four seconds of one of the satellite signals in order to bring it into time coincidence with the other for
Maximal-ratio combining. On newer receivers with a "pause" feature, a dual-port PSRAM is employed to store up to 60 minutes of the selected channel. The baseband processor outputs digital audio over a Serial Peripheral Interface, which is fed to a D/A converter to produce the analog audio signal. The front-end of a Sirius receiver is called the
head unit, required to display descriptive text (such as the category, channel, artist, and song name) and provide controls to the user. This is implemented by the third-party designers of Sirius-ready receivers, using a microprocessor of their choice. Sirius offers car radios and home entertainment systems, as well as car and home kits for portable use. The Sirius receiver includes 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. Sirius broadcasts using 12.5 MHz of the
S band between 2320 and 2332.5 MHz. Audio channels are digitally compressed using a proprietary variant of
Lucent's
Perceptual Audio Coder compression algorithm and encrypted with a proprietary
conditional access system. Sirius has announced that they intend to implement
hierarchical modulation technology to economize on bandwidth up to 25%. Each receiver must be connected to an external antenna, which is included with the receiver. Antenna placement is crucial to receiving a clear signal. In some locations users have experienced difficulty receiving the Sirius programming because the signal is not consistently strong. For the best reception, antennas should be placed such that they have an unobstructed view of the sky (preferably on rooftops without overhanging
eaves or trees). If this is not an option, the antenna should be placed on an exterior wall. When placing on an exterior wall, the antenna should be mounted to a wall which faces the southern continental United States in order to minimize the likelihood of the building itself blocking the signal.
Satellite technology Sirius' satellites are named Radiosat because there is already a fleet of satellites named
Sirius, launched by Sweden's NSAB (Nordiska Satellitaktiebolaget, or Nordic Satellite AB, and known today as
SES Sirius) and used for general telecommunications and
satellite television throughout
Sweden and the rest of
Scandinavia. The current primary uplink facility for Sirius, which was formerly used as the uplink site for
Western Union's
Westar fleet of communication satellites from the early 1970s to the late 1980s, is located in
Glenwood, New Jersey. The original facility was located on the roof of the building housing the Sirius studios in
Rockefeller Center in
New York City but has since been decommissioned. Sirius' spacecraft Radiosat 1 through Radiosat 4 were manufactured by
Space Systems/Loral. The first three of the series were orbited in 2000 by
Proton K Block-DM3 launch vehicles, with the final three-satellite constellation completed on November 30, 2000. Radiosat 4, built as a ground spare for the now-decommissioned elliptical mission, was transferred to the Smithsonian Institution's
National Air and Space Museum in October 2012. It is on display at the
Steven F. Udvar-Hazy Center. The satellites were built on the
Space Systems/Loral 1300 platform. Before the elliptical-orbit satellites were decommissioned, all three satellites broadcast directly to the consumer's receiver, but due to the highly elliptical orbit only two of them broadcast at any given time. Today the satellites are located in the southern sky in the United States. Satellites Radiosat 1 through Radiosat 3, now decommissioned, flew in
geosynchronous (not
geostationary)
Tundra orbits. Like the geostationary orbit, the tundra orbit has a period of 23 hours, 56 minutes (one
sidereal day). Unlike the geostationary orbit, the tundra orbit is elliptical, not circular, and is inclined with respect to the
equator rather than orbiting directly over it. The eccentric orbit ensures that each satellite spends about 16 hours of each day high over the
continental United States. At least one satellite is always visible, with another often visible as well. The orbit's high inclination places
apogee just west of
Hudson Bay in
Canada, providing a much higher elevation angle for most of the country than is possible from a geostationary orbit. This was intended to reduce blockage from tall buildings in urban areas, allowing a much smaller terrestrial repeater network than does sister network XM, which uses geostationary orbits. This system has since been decommissioned in favor of newer geostationary satellites located at 96.0° and 116.15° that support both the Sirius and XM platforms. On June 8, 2006, Space Systems/Loral announced that it was awarded a contract for the fifth Sirius spacecraft. The new spacecraft features a nine-meter unfurlable reflector. The first four Sirius spacecraft used more traditional parabolic reflectors. The new satellite has been designed for geostationary orbit, unlike the other satellites in the constellation; the different orbit has the stated purpose of allowing for more consistent reception for fixed location users (many subscribers have reported having to regularly reposition their antennas for optimal reception). Radiosat 5 (FM-5) is in a
geostationary orbit at 96.0° West. It was launched June 30, 2009, and announced to be in service as of September 9, 2009. On October 14, 2010, the XM-5 satellite was launched aboard an
International Launch Services (ILS)
Proton vehicle. It was placed into a geostationary orbit at 85.2° West to serve the eastern half of the United States. It is named XM-5 because it serves as an in-orbit spare that can replace both the Sirius Radiosat satellites and the XM satellites. The satellite was manufactured by Space Systems/Loral and was fully operational on December 3, 2010. On February 29, 2008, the launch service provider International Launch Services (ILS) announced a contract which includes a launch of the SIRIUS FM-6 satellite on a Proton
Briz M launch vehicle. The launch planned for March 6, 2012, was canceled due to concerns with a design defect in the solar panel deployment. The Radiosat 6 (FM-6) satellite was launched on October 25, 2013, and was put in a geostationary orbit at 116.15° West which services the western half of the United States.
Satellites •
Sirius FM-1 (Radiosat 1) – launched June 30, 2000; decommissioned by Sirius XM in 2016 •
Sirius FM-2 (Radiosat 2) – launched September 5, 2000; decommissioned by Sirius XM in 2016 •
Sirius FM-3 (Radiosat 3) – launched November 30, 2000; decommissioned by Sirius XM in 2016 •
Sirius FM-4 (Radiosat 4) – ground spare, never launched; later donated to the
National Air and Space Museum in October 2012 •
Sirius FM-5 (Radiosat 5) – launched June 30, 2009 •
Sirius FM-6 (Radiosat 6) – launched October 25, 2013
Data services In addition to the audio programming, the Sirius broadcast stream also carries a
Data Services channel that is utilized by capable receivers and graphical display hardware. Some of the data services offered are traffic speed and flow, marine weather, and fuel prices to name just a few. Examples of capable hardware are the
Raymarine SR100 Satellite Weather receiver and the Alpine NVE-N872A Satellite Traffic Ready navigation system. == Receivers ==