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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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3 Answers 3

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

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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I'm not sure but I know enough cryptographic theory to take a guess.

There is no two way communication going on outside of your telephone call.

This is just guess based on how I would do it but it is pretty straightforward. I may be wrong but this would seem to be the simplest way.

The satellite broadcasts channels digitally encrypted. Lets call it CHANNEL_1. Lets say it requires KEY_A to decrypt. You can not decrypt these channels without the secret key, KEY_STATION.

Your satellite radio receiver has a serial number and a secret key associated with it. Lets call it KEY_RECEIVER.

You call up the satellite company, you tell them your serial number and they look up KEY_RECEIVER associated with it.

(There may be some variation of the following but this would be the gist of it.) They also have a public radio channel that goes through all the receivers that are subscribed and tells them the secret key. Lets call this CHANNEL_2. So this channel goes one by one and telling, "Hey, radio serial number xxxx, Listen Up. Here is the key for the station KEY_STATION, but first I'm going to encrypt it with your specific key, KEY_RECEIVER, so you are the only one who can decrypt this key addressed specifically to your unit.

So KEY_STATION is wrapped inside KEY_RECEIVER. I guess it would take about 6 minutes to loop through all of the satellite radio receiver serial numbers and key information, which would explain the lag time.

So your radio receiver never actually talks to the satellite, it just listens. But this way they transmit the secret key to unlock the station just to you and the people who paid for it by only sending it in a way that only the people who paid for it can receive it.

They probably change the KEY_STATION pretty regularly and if you keep paying the bill they will keep broadcasting to your receiver the current KEY_STATION but if they stop paying they can take your receiver and its key out of the public channel.

Of course that is just a basic guess from my limited knowledge of cryptography but that is how I would do it.

In summary:

  1. Person Calls Satellite Company and tells them Radio Receiver Serial Number
  2. Satellite Company looks up KEY_RECEIVER from database
  3. Satellite wraps KEY_STATION in KEY_RECEIVER to make ENCRYPTED_BLOB
  4. Satellite sends out Attention Radio Receiver Serial Number here is ENCRYPTED_BLOB
  5. Radio Receiver takes ENCRYPTED_BLOB and uses KEY_RECEIVER to decode it to get the original KEY_STATION
  6. Radio Receiver takes KEY_STATION to tune into the desired station and decrypt that.
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This sounds like a heck of a lot of speculation and not really a physics answer either. –  Brandon Enright Jun 1 at 5:26

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