[time-nuts] Carrier phase tracking
pvince at theiet.org
Mon Feb 19 17:07:18 EST 2007
Thank you Didier - it sounds horrendous, and I'm glad I am not trying
to design the equipment to do it :-)
>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.
>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?
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