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I just got XM radio again after a brief period without it. A customer service rep said that the satellite needs to beam my signal to me in a six minute time window. How exactly does this happen? How does the satellite know where to find me, and how exactly does it "talk" to my car? Why does it have a six minute time window?

I tagged this as radiometry, but I'm not quite sure what topic this is.

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2 Answers

It doesn't find you - XM uses three (IIRC) satellites in geostationary orbit as long as you are on the right continent you should get a signal.

I'm guessing that the six minutes is something to do with ensuring your receiver is authorized . Presumably it broadcasts some sort of key that unlocks your receiver when you pay - so you pay and within 6mins it will have sent the key to turn on your unit.

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Right but how does it "send the key" to my unit? –  John M Mar 2 '13 at 6:03
It's broadcasting a digital radio signal so it's very easy to send any other digital data as well. The satelite sends blocks of compressed audio (like Mp3) which is buffered in the unit so you can download minutes of playback in a few seconds. That's how it has so many channels –  Martin Beckett Mar 2 '13 at 6:14
What frequencies does it transmit on? –  Ataraxia Mar 2 '13 at 6:29
@ZettaSuro according to their site, S-Band around 2.4Ghz –  Martin Beckett Mar 2 '13 at 6:34
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Answer to " How does the satellite know where to find me, and how exactly does it "talk" to my car? "

Although GPS communications are not bidirectional, it's virtually possible to estabilish a bidirectional communication, i.e. make the satellite "send specific data on request". The satellite actually sends data to all people around you, but if data are encrypted by a password which only your device knows, it's just like if satellite was communicating only with your device , or like if you was asking to satellite specific data.

Something similar happens on some satellite TV decoders, which can be progammed from remote by the owner although he does not own any device capable of actually sending data directly to the decoder, which can only receive data from satellite antenna: user actually connects to TV broadcaster server using standard internet connection, inform it that he wants to start his decoder at specific time, and server tell satellite to start spreading around a "start recording order" to millions of decoders around... but only one will accept the order and use it to start recording.

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