Pointers to info on how XM and Sirius systems work, specifically as
> regards authorization?
> When a satellite authorizes a given radio, does it do it by sending
> that radio (and, as a side effect, all other radios) some kind of
> digital key that combines with some unique built-in matching key in
> that particular radio to enable it to decode the broadcast content?
Yep. They're continuously sending out an authorization stream.
If so, presumably if you stop paying your bill it can also deauthorize
> your radio by sending something to wipe out the code?
No. They don't send out a deauthorization stream. Read on.
But suppose you put your radio in a shielded box, stop paying, wait a
> few months until they give up trying to deauthorize you, then bring it
> back out -- are you still authorized? Or does every authorized radio
> have to get reauthorized at some periodic interval?
Bingo. You have to get reauthorized at some periodic interval.
I did exactly what you describe: I de-installed my SkyFi for a period of months, and deactivated my account. When I plugged it back in some months later, it worked fine for several days. Then suddenly it stopped.
It needs to be reauthorized periodically. When it sees an entire authorization stream come and go but it's not on the list, it's deauthorized.
Bottom line: Do individual radios actually talk back to the satellites
> at any point? Or do authorization and other command signals flow only
> from the satellites to the radio?
It's one way. The radios don't talk back to anyone. It's really quite simple.