XM Radio is a satellite radio service. Satellite radio is a technology that for the most part has been around for many years. For instance, many television studios have been using satellites to beam TV signals from far away locations to viewers for decades, however satellite radio has been in operation in America since 2001.
Satellite radio is pretty easy to describe. The programs for satellite radio include music, talk shows, live sporting events and sport analysis shows. These shows are usually sent from one central location, where they are sent from the ground to satellites rotating in space that then broadcast the signal to those listeners with the hardware such as special antennas and receivers to pick up the signal. Here is some more information
Satellite Radio Starts from the Broadcaster
It is important to note that XM satellite radio initially starts from the studio, where the programming is put together. Programming such as music, talk shows, sports analysis shows, etc are created and then transmitted usually from a main source to two satellites over head. (It is also possible that live events such as news or sporting events are sent individually directly from the location where they are happening to the two satellites overhead). Usually programming is digitized and then compressed to fit lots of data into a small bandwidth of radio frequency which XM satellite is allowed to operate on.
XM Radio has two satellites that are in geostationary orbit located above the equator. Geostationary orbit means that the satellites are moving at the same speed as the earth turns, making it able to be in the same position as the earth at all times. This way, XM satellite radio can always be pointing to their satellites and feeding them programming and the satellites can easily broadcast their signal to the millions of listeners that have antennas and receivers pointed towards the geostationary satellite.
A special antenna is necessary to receive XM satellite radio. Usually these antennas are very small in size, smaller than a normal tennis ball and can be placed on top of car roofs, in an office or at home. They are not that expensive and cost usually less than $50. These antennas make it possible to catch the specific digital radio signal that is broadcasted and send it to the special XM Satellite Receiver.
Because satellites that are in geostationary orbit, their antennas must have a clear line of sight to the southern sky to pick up signals clearly. Obviously not everyone will have a clear line of sight (especially if you live in an urban location) so a technique used to increase the likelihood of anyone using this technology to have good reception are repeaters. A repeater is a large antenna located in key positions around the country, where a satellite signal can be received and then repeated or rebroadcasted to antennas facing any direction, south, north, east, west and in between.
Satellite receivers enable listeners to receive the digital audio broadcasts being relayed off of the satellites in geostationary orbit. These satellite receivers come in many different styles and some have different functions. They all include a chipset that can decode the encrypted signal sent from the satellite. The satellite sends its programming signal encrypted so that people that do not have a receiver or do not pay the monthly subscription fee can not have access to it.
Receivers can come in the form of auto car stereos, add on receivers that hook up to your already installed car stereo, boom boxes, home audio receivers with satellite receiver capability and portable devices that allow you to pick up the satellite signal no matter where you go. These portable devices have their own energy source and are similar to size and shape as iPod's. These receivers usually cost from about $30 to over $300.