[time-nuts] carrier phase tracking GPS receiver

Magnus Danielson cfmd at bredband.net
Mon Feb 19 15:08:53 EST 2007

From: Didier Juges <didier at cox.net>
Subject: Re: [time-nuts] carrier phase tracking GPS receiver
Date: Mon, 19 Feb 2007 06:37:35 -0600
Message-ID: <45D99A0F.1020103 at cox.net>

> Magnus Danielson wrote:
> >> However if all you have is a 10811 it will limit the short term 
> >> satbility of such a GPSDO.
> >> The phase carrier measurements will have a lower noise floor than the 
> >> 10811 for tau > 10s or so.
> >> A better oscillator (FTS1200, FTS100 Oscilloquartz OSA8607 etc.) will 
> >> have a significantly lower noise floor.
> >>     
> >
> > Indeed. Carrier phase will cut in early as the quieter source than the PPS
> > method.
> >   
> That by itself seems like a significant advantage, being able to set the 
> loop much faster and therefore being phase locked sooner after power up, 
> for portable applications.

You can acheive much greater speedup by a combined frequency/phase approach.
You will get a very accurate frequency error estimate, so you will very
quickly be close enought to go into phase lock. At least if your clock isn't
too noisy. So, the lock-in time should not be the major concern, but rather
the behaviour of the full setup when running. Also, another classic trick is
to vary the bandwidth, so you have a much wider bandwidth in the beginning and
then step down towards your target bandwidth as some suitable conditions have
been met prior to the step.

> It seems that in this case, the acquisition time of the receiver will be 
> the most significant delay.

Your OCXO needs to heat up anyway. You can usually acheive a good GPS lock in
that time. A full-fledge correction would probably require data collected over
some time anyway, so don't fool yourself here.

> I guess as a result, it will become more important to have an algorithm 
> that effectively filters out outliers. Are there any such things as 
> hanging bridges with carrier phase receivers? (I hate asking that 
> question...)

Well, the resolution of carrier phase measures is below the noise level, so
whatever errors there would be will be noised out. Also, it is not static as
the satellite orbit and speed of change will practically ensure that there is
no hanging bridge, or at least not for very long. So, effectively no.

I was kind of expecting that question, it was just a matter of time before it
came up. :)


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