Xm was the first and has over 150 channels and four Geostationary (they hold a pretty still orbit, low lattitude angle, over North America) Satellites, two of the originals are the back-ups, seeing as they have slightly clouded solar collectors. They were supposed to last 15 years but as you can see they are not going to cut it. Xm has about 800 ground 'repeaters' of the low (50 to 100) Watt power range to fill in between tall buildings and such in the cities.
You may see them on corner rooftops of tall buildings. XM has allocated some bits of data transmission for 'On Star' due to an agreement with GM and others. Xm uses the upper half of the alloted band of 2332.5 to 2345 MHZ for its transmission. The repeaters use the same frequency to supply complete coverage to the 'individual' recievers, which buffer the transmitted data by 4 seconds, so that when you drive under an overpass you cannot hear the pause.
A unique 26th floor of a building in Manhattan, has a floor that was poured on top of a bladder inflated to isolate the street noises and vibrations from the transmitter that is the 'uplink' to the Satellite, so that the noise doesn't waste valuable bits of data. Now wasn't that intelligent? I believe Sirius 'uplink' data transmission comes out of NJ. When you pay for the programing you desire, the system sends the info to the 'Birds', they in turn send the code that blocks the rest from being recieved by your reciever. Your reciever has an internal code which the 'Birds' recognise as friends so you can listen to whatever your heart desires. Now aren't those smart 'Birds'?
Sirius uses the lower half of the frequency bandwidth at 2320 to 2332.5 MHZ. They have three satellites up, a fourth stored for emergency (in California) and a fifth to be used for the latest coverage that is not compatible with the early version individual recievers. The original 3 are in a lazy '8 semi-stationary' orbit that meanders down to South America and back. South America has not yet payed for the licensing. (but that doesn't mean they don't use it). The Sirius system ensures two Satellites are over the USA at any given time with 16 hours overlapping coverage by each. The fifth satelite is to be truly geostationary. Sirius has about 100 High power repeaters in the 400 to 2000 watt category, placed high on mountain tops and tall transmitting towers. The same 4 second rule is used by them as well.
XM and Sirius are prevented from a merger by the Justice dept. They will soon co-operate to stem billions of dollars in losses from competing with each other. XM has over 7 million customers while Sirius has over 6 million but is gaining customer base at a faster rate. New 'Data compression algorythms' are going to increase the channels available as time allows.
Europe, India, Africa, and Asia are covered by "world Space" Which is now totally a part of XM. WS uses two Geostationary Satellites on the Equator over the Euro-asian Continents. WS has a unique 'Flashlight' type of beam which is very selective about which country gets to listen to the broadcast. the technology was part of the reason that XM used their cash revenues to buy back the 20 percent of outstanding shares that were held by WS.
The most interesting feature of these 30 or so latest comunication Satellites all around the world, is the fact that they use a futuristic space age system, called "Ion Propulsion" to help them maintain their position in Geostationary orbit. "Ion Propulsion" drives the 'Star Ships' in science fiction books for as long as I can remember, Star Ship 'Enterprise' being no exception. the "Ion Drive" operates for about an hour or more each day to 'nudge' the satellites back to the perfect position they need to maintain for great data transmission.
This latest bit of 'How things work' comes thanks to the magazine rack's, current 'Scientific American' complemented by my own link to 'Wikipedia'. You might want to check out the pros to get the total 'skinny'.
Now you know enough to sound intelligent at the bar with the 'Buds' or at the local beauty shop or even the barber shop. Now aren't you glad you stopped and visited my blog? Stop in again some time. I just love the attention.