[time-nuts] Newbie question about GPS

[Attila Kinali]

On Mon, 2 Mar 2020 09:13:24 -0500
Bob kb8tq <kb8tq at n1k.org> wrote:

> > Hi All,I hope some one can help me to find the answer.1. Let say some one 
> > can extract the L1 carrier frequency 15xx MHz and convert it down to let 
> > say 1MHz and put it to phase comparator and made the PLL to generated the 
> > LO to convert and extract the carrier. Is this possible ? Have some has do 
> > it ? Can share any thoughts ?
> 
> The carrier is significantly doppler shifted due to the way the satellites 
> orbit the earth. Without correcting for that, the carrier makes a poor
> reference. This is only the first of *many* corrections applied…..

To put a few numbers in here:

Doppler shift a stationary GPS receiver sees is about +/-6kHz
or about 4ppm. On top of that we have ionospheric delay variation
that is in the order of several 10s of ns. Tropospheric delay in
several ns. We are not yet talking about the relatively high
noise in the signal. Even after decorrelation we are only around
20-50dB SNR (low SNR -> high jitter; an XO has something in the order
of 80-120dB SNR an OCXO is around 140-160dB). Or things like multi-path.
All things which are fairly detrimental to getting a good frequency
standard out of the GPS carrier.

So you _need_ the full system solution to get to have a reasonably
stable output. And even then, GPS is very noisy. A cheap OCXO is more
stable than GPS up to several 10s of seconds. A good OCXO will be more
stable than GPS up to several 100s to a few 1000s of seconds.
If you go further, a cheap Rb vapor cell standard like the FE-5680A
is more stable than GPS up to around 10ks. A good Rb standard like
the PRS10 or HP5065 is more stable than GPS for up to 100ks (about a day)
If you go for what is curently up in research for Rb standards,
we are probably talking about 2-5 days for the cross over
(look at the papers published by Mileti/Affolderbach's group
at University of Neuchâtel and Micalizio/Calosso/Gozzelino
at INRIM in Torino).

			Attila Kinali

-- 
In science if you know what you are doing you should not be doing it.
In engineering if you do not know what you are doing you should not be doing it.
        -- Richard W. Hamming, The Art of Doing Science and Engineering