In article
, Mr
Thant writes
I think how it works is the receiver has to jump between frequencies
on the same pattern as the satellite it wants to listen to.

True, I believe.

The
receiver therefore needs to know which pattern each satellite is
using,

But this will be pre-programmed in some way.

and it also needs a separate tuner for each satellite. Because
most have far fewer tuners than there are satellites (usually 12, vs
30ish) they need to know in advance which satellites are overhead.

So this means a receiver needs to already know current approximate
location and the current time, and the orbit and frequency pattern
information about the satellites. This is why it takes ages for a
brand new or freshly reset GPS unit to get any sort of lock (up to an
hour).

That's not my experience, nor do I see why it needs to be like that.

You listen in on one of the frequencies (call it A) and look for a
regular "blip" (the frequency hopping sequence repeats every
millisecond). If 8 satellites are visible you should get 8 such blips at
various points in the cycle. You choose one of them and pick another
frequency (B) and start listening in an
ABBBBB...BBABBBBB...BBABBBBB...BBABBBB pattern until you get two blips
per millisecond. Hopefully only one or two satellites are using a
pattern which matches that and you can quickly find which. If the
ABBBBB... pattern gives you more than two blips, you've been unlucky and
found two satellites hitting A at exactly the same moment, but again you
pick one of the B hits and go for a third frequency.

For that matter, once you've locked in on an A signal at regular
intervals, you can simply try each of the 31 sequences and see which
ones work.

[Since there are less frequencies than there are steps in the sequence,
it's a bit more complicated than this. But that's the principle.]

I believe most of the delay in syncing up with satellites is reading all
the data from the signal, which is transmitted at a relatively slow
rate.

--
Clive D.W. Feather | Home:
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