On 02/07/2012 03:12 AM, Hal Murray wrote:
[email protected] said:
Not quite right. If you lock up the clock, you do not lock to the birds,
but to GPS time or UTC as received over GPS. The observed time of the birds
would be a bad solution since you can't see a particular bird continously
unless you is in geosync orbit.
It's even more complicated than that.
Suppose you decide to lock onto a bird that you can see. You have to correct
for Doppler so you can't use a simple PLL.
I considered writing that, but realizing that just not sweeing the same
bird continuously 24x7 should be a great hint that you have done
something wrong. The doppler is the second killer and then even if you
compensat that, each bird has their own frequency error and drift, which
is compensated by the nav message, but then you no long follow the bird,
but GPS time.
You can however use a simple PLL, the PLL in the tracking loop isn't
particularly advanced or special. It's not even aided except for initial
track-in. It's when you no longer wishes to track the doppler shifts
that you need to handle it, but you can compensate that out if you know
your position and even produce an analogue signal predicing and
cancelling the effect, in which case only the residue between predicted
and actual doppler will remain visible.
Cheers,
Magnus
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