Editing XM Satellite Radio (section)

== Technology ==
XM provided digital programming directly from two high-powered satellites in [[geostationary orbit]] above the equator: XM Rhythm at 85° west [[longitude]] and XM Blues at 115° west longitude<ref>{{cite web|url=http://www.boeing.com/defense-space/space/bss/factsheets/702/xm3-4/xm3-4.html |title=Nationwide Digital-Quality Radio Programming XM-3 and XM-4 |access-date=October 19, 2010 |url-status=dead |archive-url=https://web.archive.org/web/20101204112821/http://www.boeing.com/defense-space/space/bss/factsheets/702/xm3-4/xm3-4.html |archive-date=December 4, 2010 }}</ref> in addition to a network of ground-based repeaters. The combination of two satellites and a ground-based repeater network was designed to provide gap-free coverage anywhere within the contiguous U.S., the southern tip of Alaska, and in the southern part of Canada. The signal could also be received in the Caribbean Islands and most of Mexico (reports have stated that areas north of [[Acapulco]] were able to receive a steady signal<ref name="Acapulco">{{cite web |url=http://www.hawaiithreads.com/showpost.php?p=16992&postcount=28 |archive-url=https://web.archive.org/web/20070928054702/http://www.hawaiithreads.com/showpost.php?p=16992&postcount=28 |url-status=usurped |archive-date=September 28, 2007 |title=XM radio in Hawaii |work=HawaiiThreads.com |date=December 22, 2004 |access-date=December 28, 2006}}</ref>); however, XM is not yet licensed for reception by paid subscribers living in these areas.

The original satellites, XM-1 ("Rock") and XM-2 ("Roll") suffered from a generic design fault on the [[Boeing 702]] series satellite (fogging of the solar panels), which meant that their lifetimes were shortened to approximately six years instead of the design goal of 15 years.<ref name="SpaceAndTech">{{cite web|url=http://www.spaceandtech.com/digest/flash2001/flash2001-082.shtml |title=Boeing 702 Satellites Solar Arrays Possibly Defective |work=SpaceandTech.com |date=September 29, 2001 |access-date=December 25, 2007 |url-status=usurped |archive-url=https://web.archive.org/web/20071226225020/http://www.spaceandtech.com/digest/flash2001/flash2001-082.shtml |archive-date=December 26, 2007}}</ref><ref name="ADA430372">{{cite journal |title=Charging of Mirror Surfaces in Space |first=Shu T |last=Lai |journal=Journal of Geophysical Research |volume=110 |issue=A1 |year=2005 |doi=10.1029/2002JA009447 |pages=A01204 |bibcode=2005JGRA..110.1204L|doi-access=free }}</ref> To compensate for this flaw, XM-3 ("Rhythm") was launched ahead of its planned schedule on February 28, 2005, and moved into XM-1's previous location of 85° WL. XM-1 was then moved to be co-located with XM-2 at 115° WL, where each satellite operated only one transponder (thus broadcasting half the bandwidth each) to conserve energy and cut the power consumption in half while XM-4 ("Blues") was readied for launch. Subsequently, XM launched ground-spare XM-4 ("Blues") ahead of schedule on October 30, 2006, into the 115° WL location to complete the satellite replacement program. On December 15, 2006, XM-1 was finally powered down and then drifted back to its original location at 85° WL, where it remained as a backup to XM-3. XM-2 was similarly powered down and remained as a backup to XM-4. This makes the current active satellites as XM-3 "Rhythm" and XM-4 "Blues" with two in-orbit spares.<ref name="XMbluesPR">{{cite press release|url=http://xmradio.mediaroom.com/index.php?s=press_releases&item=1404|title=XM Completes New State-of-the-Art Satellite System With 'Rhythm' and 'Blues'|publisher=XM Radio via PRNewswire|date=December 18, 2006|access-date=December 28, 2006|url-status=dead|archive-url=https://web.archive.org/web/20070224222631/http://xmradio.mediaroom.com/index.php?s=press_releases&item=1404|archive-date=February 24, 2007}}</ref><ref name="FCC00368">{{cite web |url=http://hraunfoss.fcc.gov/edocs_public/attachmatch/DOC-265875A1.pdf |title=Satellite Space Applications Accepted for Filing |work=FCC.gov |date=June 9, 2006 |access-date=December 28, 2006}}</ref>
[[File:Xm radio headquarters2.jpg|thumb|right|225px|XM Satellite Radio headquarters in Washington, D.C., near the [[NoMa – Gallaudet University (WMATA station)|NoMa – Gallaudet University]] Metro station.]]

On June 7, 2005, [[Space Systems/Loral]] announced that it had been awarded a contract for XM-5.<ref name="LoralPR">{{cite press release |url=http://ssloral.com/html/pressreleases/pr20050608.html |title=Space Systems/Loral to Build Next Generation Satellite for XM Satellite Radio |publisher=Space Systems/Loral |date=June 7, 2005 |access-date=December 28, 2006 |url-status=dead |archive-url=https://archive.today/20130202232715/http://ssloral.com/html/pressreleases/pr20050608.html |archive-date=February 2, 2013 }}</ref><ref name="SkyrocketXM5">{{cite news |url=http://space.skyrocket.de/doc_sdat/xm-5.htm |title=XM 5 |work=Skyrocket.de |date=June 2005 |access-date=December 28, 2006}}</ref> XM-5 was to feature two large, unfurlable antennas. Sirius' Radiosat 5, also to be built by Loral, was slated to have a similar single large antenna.

In American and Canadian metropolitan areas, XM and its Canadian licensee "Canadian Satellite Radio" (CSR) and operating as [[Sirius XM Canada]], owned and operated a network of approximately 900 terrestrial [[repeater]] stations, meant to compensate for satellite signal blockage by buildings, tunnels, and bridges. In the United States, XM owned and operated approximately 800 repeater sites, covering 60 markets; in Canada, CSR was installing approximately 80 to 100 repeaters that were planned to be owned and operated by CSR in the 16 largest Canadian cities. The actual number of repeater sites varies as the signal is regularly tested and monitored for optimal performance. The actual number of sites in the United States has dropped from the original 1,000 installed when the service first launched in 2001. The repeaters transmit in the same frequency band as the satellites. A typical city contains 20 or more terrestrial stations. Typically, the receiver owner is unaware when a terrestrial station is being used, unless he or she checks antenna information from the receiver being used. Due to a FCC filing in October 2006, the latest list of XM's US terrestrial repeater network was made available to the public. <!-- This data is available now in both [http://www.whatsonmyxm.com/gmaps/repeaters.php Google Base] (searchable by zip code or city/state) and [http://www.whatsonmyxm.com/gmaps/xmrepeaters.kml Google Earth] formats. -->

The XM signal used 12.5&nbsp;MHz of the [[S band]]: 2332.5 to 2345.0&nbsp;MHz. XM provided 128 kilobits per second of its bandwidth to [[OnStar]] Corporation for use with XM-enabled [[General Motors Corporation|GM]] vehicles, regardless of whether their owners are XM subscribers. American Honda also retained the right to some of the company's bandwidth to transmit messages to [[Acura]] vehicles via a service known as [[AcuraLink]].

XM NavTraffic, an optional service, transmitted coded traffic information directly to vehicle navigation systems using [[Traffic Message Channel|TMC]] technology.

Audio channels on XM were digitally compressed using the [[HE-AAC|CT-aacPlus (HE-AAC)]] codec from [[Coding Technologies]] for most channels. That said, some channels rely on the [[Advanced Multi-Band Excitation|AMBE]] codec from Digital Voice Systems (e.g., for voice channels), including all of the Traffic and Weather channels.

The XM radio signal was broadcast on 6 separate radio carriers within the 12.5&nbsp;MHz allocation. The entire content of the radio service, including both data and audio content, was represented by only two carriers. The other 4 carriers carried duplicates of the same content to achieve redundancy through [[Diversity combining|signal diversity]]. The data on each carrier is encoded using time-delayed and error-correction schemes to enhance availability. Effectively, the total radio spectrum used for content is a little over 4&nbsp;MHz.<ref>{{cite web|url=http://www.patentstorm.us/patents/7075946-description.html |title=Method and apparatus for audio output combining - US Patent 7075946 |access-date=April 3, 2007 |url-status=dead |archive-url=https://web.archive.org/web/20070926220151/http://www.patentstorm.us/patents/7075946-description.html |archive-date=September 26, 2007}} This XM patent describes the carrier allocation scheme</ref>

Each two-carrier group broadcast 100 8-kilobit-per-second streams in approximately 4&nbsp;MHz of radio spectrum. These streams were combined using a patented process to form a variable number of channels using a variety of bitrates. Bandwidth is separated into segments of 4-kilobit-per-second virtual "streams" which are combined to form audio and data "channels" of varying bitrates from 4 to 64 kilobits-per-second.<ref>{{cite web|url=http://www.patentstorm.us/patents/7075946-description.html |title=Method and apparatus for audio output combining - US Patent 7075946 |access-date=April 3, 2007 |url-status=dead |archive-url=https://web.archive.org/web/20070926220151/http://www.patentstorm.us/patents/7075946-description.html |archive-date=September 26, 2007}} XM Patent Method and apparatus for audio output combining</ref>

XM preprocessed audio content using Neural Audio processors that are optimized for the [[aacPlus]] codec, including [[spectral band replication]] (SBR). Audio was stored digitally in Dalet audio library systems using an industry-standard [[MPEG-1 Layer II]] at 384&nbsp;kbit/s, sometimes known as [[MUSICAM]]. The audio is further processed by the Neural Audio processors on the way to broadcast.

In the past, the [[Sirius XM Pops]] channel, which aired classical music, was broadcast in [[Surround sound|5.1 surround sound]]. The technology, titled [[XM HD Surround]], is the result of a partnership between XM and [[Neural Audio Corporation]] which provides content with six discrete channels of digital audio. The former [[XM Live]] channel also broadcast in this format for certain concerts and studio performances. XM manufacturing partners such as [[Denon]], [[Onkyo]], [[Pioneer Electronics (USA) Inc.]], and [[Yamaha (manufacturer)|Yamaha]] introduced home audio systems capable of playing XM HD Surround. It is not known if the XM HD Surround technology is used on the service as of 2015.