[time-nuts] PN sequence generation using GPS
hmurray at megapathdsl.net
Fri Feb 25 06:15:25 UTC 2011
jimlux at earthlink.net said:
> yes.. back in the day, people had proposed various "time of day" sync
> strategies, but they all seemed require more complexity and mass than it
> was worth. ...
I'm embarrassed that I didn't see this sooner...
If you use something like time-of-day to synchronize the PN generators, that
throws out one of the main reasons for using spread spectrum. It will work
fine if only a few people use it, but as soon as everybody uses the same
setup, they will all land on top of eachother because they will be using the
same sequence of frequencies at the same time.
So rather than (logically) resetting the PN generator at midnight (aka
loading it with some constant like all 1s), we need to load it with a random
seed known to both parties.
The military version would probably distribute that seed with the crypto keys.
If two people are arranging a contact, they could use the low order bits of
the contact time as the random seed. If your PN generator has N bits of
internal state, you need N bits of randomness. If you roll the dice to pick
a minute within a quarter hour and a second, you get 10 bits. If you need
more, you will have to do something like specify fractions of a second.
These are my opinions, not necessarily my employer's. I hate spam.
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