[time-nuts] Carrier phase tracking
Didier Juges
didier at cox.net
Mon Feb 19 17:19:28 UTC 2007
Layman explanation, be nice to me please...
The notion of carrier with a spread spectrum system is theoretical.
There is no "carrier" signal being sent continuously and modulation
sidebands that contain the information, as you would with an AM signal.
This is more like an FM signal, where the carrier is not always present
in the transmitted signal, depending on the modulation index, or like
single sideband, where the carrier is purposely removed. If you looked
at the GPS signal with a spectrum analyzer, it would look just like
noise. You need the right correlator to see something.
The "carrier" information is reconstituted in the receiver by software
algorithms, which essentially remove the modulation to compute what the
carrier should be like.
Didier KO4BB
PS: it used to be early cheap GPS receivers could only decode one
satellite at a time, so they had to train on the signal from one
satellite, then decode it, then switch to the next and so on in
sequence. This delayed the availability of a fix by a lot, and tracking
while moving, well, sucked. Then parallel receivers appeared, where the
signal processor was powerful enough to decode 4, then 6, then 8 then
all 12 signals at the same time (in parallel). You may remember when
parallel receivers became popular, all advertisements would prominently
display that feature. Now, you take it for granted. There are at most 12
visible satellites at the same time, so there is no need for more than
12 channels (at least for a "single frequency" L1 receiver)
Peter Vince wrote:
> I sort of understand the idea of correlation in order to receive the
> signals from several satellites all on the same frequency, but I
> wonder if someone has a simple explanation of how the carrier phase
> can be tracked when there are several carriers all at once? Does it
> rely on the different Doppler-shifts making them distinguishable?
>
> Thanks,
>
> Peter
>
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