On 2/23/11 4:48 AM, Bob Camp wrote:
Hi
Hop rates below 1,000 per second are far more common in simple systems than
anything faster than that. At VHF, you are looking at a everybody being within
one hop of each other. That makes the idea of a GPS based code start fairly
reasonable.
Bob
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. (you could add a manpack sized GPS receiver to a manpack
sized radio) There were a lot of interesting schemes proposed for things
like clock syntonization so the hops/chips stay in phase. And, given
the usual application of SS links in the 70s and 80s (Skywalker sound
excepted) there was a lot of thought to the jammability and/or
jam-resistance of such schemes. Most SS systems have a generalized
vulnerability during acquisition, since you don't have any process gain
until you've acquired sync. (parallel sync schemes help a huge amount,
because you might be able to reject "false locks", while a serial sync
scheme might decide to lock on the wrong thing if it hits it first in
the search... sort of a global vs local minima thing)
These days, though, with ubiquitous GPS and very tiny, low power GPS
receivers, it would be very attractive. Getting 1 microsecond sync
would be trivial (after computing the nav solution).
And, if you want to be clever, if you've already implemented hardware
correlators and code space searchers, you could use GPS to get an
initial approximate phase for a fast Direct Sequence system, and then
use the same hardware to acquire the signal of interest.
By the way, the usual terms of art for frequency hoppers are "slow" and
"fast" hoppers, differentiated by whether the hop rate is slower or
faster than the symbol rate. For an example of exactly in the middle,
there were (are?) a raft of schemes for MFSK where the constellation was
much larger than needed for the number of bits per symbol (say you're
encoding 4 bits per symbol, and you have 2000 possible frequencies).
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