On Mon, 28 Apr 2008, Clive D. W. Feather wrote:

In article , Tom Anderson
writes

Rather, there's one frequency, producing a stream of bits which are the sum
of the signals from all the satellites. CDMA lets you filter that stream
and recover a single satellite's signal. Basically, the CDMA decoder takes
the raw, summed signal, plus one of the 1023-bit codes, and gives you back
the satellite signal that was modulated with that code.

Right - the mathematics to do this turns out to be very simple (well, to
programmers it will).

The code input and satellite signal need to be in sync,

Or you record one cycle of the data and run all 1023*31 possibilities through
the algorithm.

Doh! Yes, that would be rather quicker.

Although it's actually a bit more complicated than this, since the signals
from the satellites won't be in sync at the CDMA modulation level (the
time of flight of the signal being 60 ms, variable by tens of ms, and the
modulation period being 1 us), so the way the signals add isn't just a
straightforward addition of bits, it's a situation called asynchronous
CDMA. But still, you take a roughly similar approach, i think. Anyway, the
electronics bods have worked it all out!

tom

--
catch my hand and come with me - close your eyes and dream

On 29 Apr, 15:16, Tom Anderson wrote:
Anyway, the electronics bods have worked it all out!

Well mine doesn't seem to do anything clever (it's a few generations
old Garmin). It shows which channels its tuned into and fresh out of
the box it'll cycle betweeen 1-12, 13-24, 25-36*, until it finds at
least one satellite, then it starts whittling.

U

(* not sure if it goes up to 36, and not sure what the channels
correspond to exactly)

--
http://londonconnections.blogspot.com/
A blog about transport projects in London

On Tue, 29 Apr 2008, Mr Thant wrote:

On 29 Apr, 15:16, Tom Anderson wrote:
Anyway, the electronics bods have worked it all out!

Well mine doesn't seem to do anything clever (it's a few generations old
Garmin). It shows which channels its tuned into and fresh out of the box
it'll cycle betweeen 1-12, 13-24, 25-36*, until it finds at least one
satellite, then it starts whittling.

Okay, interesting. I'd guess things are quicker now.

(* not sure if it goes up to 36, and not sure what the channels
correspond to exactly)

It'll be the number of the satellite. There are 31 at the moment, and
could be more - i don't know if 36 is a design limit.

tom

--
Just add a little flange and phase in

In article , Tom
Anderson writes
(* not sure if it goes up to 36, and not sure what the channels
correspond to exactly)

It'll be the number of the satellite. There are 31 at the moment, and
could be more - i don't know if 36 is a design limit.

I believe the "xor two outputs to shift the sequence" system has 36
possible shifts, of which 31 are used for satellites and the others for
special purposes.

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