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