Topics: Communications satellite, Satellite, Geosynchronous orbit Pages: 2 (550 words) Published: February 5, 2013
XM Radio uses two Boeing HS 702 satellites, appropriately named “Rock” and “Roll”, placed in parallel geostationary orbit, one at 85 degrees west longitude and the other at 115 degrees west longitude. Geostationary Earth orbit (GEO) is about 22,223 miles above Earth, and is the type of orbit most commonly used for communications satellites. XM Radio’s ground station transmits a signal to these two GEO satellites, which bounce the signals back down to radio receivers on the ground. The radio receivers are programmed to receive and unscramble the digital data signal, which includes the digital audio, the song title, the artist, and the genre of music. All of which are displayed on the radio (Bonsor, 2004). Since GEO satellites are above the equator, the terminals on the ground must have a good view of the sky to receive signals from them. This posed a challenge for XM, since obstacles on the ground, such as buildings or tall trees, can block listeners traveling in their cars from receiving the GEO satellites’ signals. Their solution was to supplement their system with a network of repeaters, ground transmitters consisting of antennas on buildings and other sites that receive satellite signals from an optimally placed antenna and retransmit them (Silverstein, 2004). These repeaters are primarily located in urban areas where the loss of the satellite signal most commonly occurs. Each XM receiver contains a proprietary chipset, consisting of two custom integrated circuits designed by STMicroelectronics and uses a small, car-phone-sized antenna to receive the XM signal (Bonsor, 2004). Each of these XM receivers is equipped to receive signals from both of the Boeing satellites as well as from a repeater simultaneously. Provided one of these sources is available, the radio will play with no interruptions. In the event a signal is briefly unavailable, the receivers also have buffers that store programming for several seconds to ensure continuing programming (Silverstein,...
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