[time-nuts] Carrier phase tracking

Matt Ettus boyscout at gmail.com
Mon Feb 19 18:01:22 EST 2007


On 2/19/07, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> >Does it
> >rely on the different Doppler-shifts making them distinguishable?
>
> It would have to, if they were not on different frequencies, they
> would sum to a single sinewave and you couldn't tell them apart
> (unless you have a direction sensitive antenna).

I don't wish to offend, but that is totally wrong.  Even if every
satellite were on the exact same frequency, with the exact same
doppler, and the exact same direction, you would still have no problem
separating the carriers of the multiple satellites.  The reason is
that the satellites do not transmit carrier.  Here is how:

   Everybody here understands how the PN codes are tracked by
despreading.  The despreading operation is multiplication, and when
you multiply the PN code by the original signal you get a carrier,
only from that one satellite.  Despread with a different PN code, and
you get a different carrier.

Thus, while the frequencies may all be the same, they are in separate
signals, one out of each correlator.

Matt



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