How GPS works (was Bus Information Signs)
 

In article , Tom
Anderson 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.
I believe it's actually CDMA rather than frequency-hopping. Although
the two are probably equivalent in some deep way.

You're right, and I mis-read.

Yes, this isn't complicated. There's a pseudo-random number generator
algorithm, for which each satellite has its own seed, and the receiver
knows the algorithm and the seeds. The generator and seed are used to
produce a 1023-bit code which is used to modulate the carrier in the
CDMA scheme.

It's actually even simpler than that. There are two 10-bit shift
registers which each generate a 1023 bit sequence. The second one has
the property that if you take any two intermediate bits and XOR them
together, you get the same 1023 bit sequence but shifted by some number
of bits. The output of the first register and two intermediate bits of
the second register are XORed together to form the 1023-bit code. Each
satellite uses a different pair of intermediate bits.

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.

That's certainly what wikipedia says: the 'navigation message' goes at
50 bits per second,

Right; thus 20 copies of the 1023-bit pattern per bit. The data bit is
XORed with the code to produce the transmitted signal.

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How GPS works (was Bus Information Signs)
 

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

How GPS works (was Bus Information Signs)
 

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)

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A blog about transport projects in London

How GPS works (was Bus Information Signs)
 

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

How GPS works (was Bus Information Signs)
 

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.

--
Clive D.W. Feather | Home:
Tel: +44 20 8495 6138 (work) | Web: http://www.davros.org
Fax: +44 870 051 9937 | Work:
Please reply to the Reply-To address, which is: