[time-nuts] Interesting paper: Don't GPSD' your Rb...

[ws at Yahoo]

Poul

I agree, 1PPS from many GPS engines, if sawtooth correction is not used, can 
add all sorts of errors like hanging bridges.
Some can sound really bad, like non zero bias over short or very long time 
periods and 12 ns of early or late wonder.
It's interesting to talk about these problems, But from a practical point 
you do not need to fix them, because all these errors are small compared to 
what the errors are from the received GPS signal including it's short term 
wonder and long term near 12 and 24 hour cycles due to satellite orbits 
cycles.

If you want something without the1 PPS problems, can use an optimized tuned 
Tbolt.

ws

*****************
From: "Poul-Henning Kamp"

>>I don't understand that.  What am I missing?
>
> You are missing that the average of the 1PPS pulse only can be trusted
> to be zero over a timescale of many hours.
>
> This is an error-source distinct from GPS reception, caused by the
> picking a preexisting flank nearest to the epoch, with no attempt
> to keep the average of the resulting error zero.
>
> Imagine that the GPS receivers clock happens to run on perfect
> frequency for a while:  That means that the flank used to generate
> the PPS will have a fixed location relative to the epoch, for instance
> always 12 nanoseconds early or late.
>
> I belive that some GPS receivers have deliberately de-tuned Xtals
> for this very reason, but unfortunately that is only a partial
> fix, as the problem is a modulus-issue, so not only is perfect
> frequency bad, but "perfect +/- n Hz" is equally bad.
>
> The hanging bridges Tom has plots of on leapsecond.com, arises when
> the frequency of the GPS xtal changes.
>
> At one point I tried putting a 1W resister close to the xtal and
> feed it with a very slow sine-wave to see if "jittering" it would
> get me an average of zero of shorter timespans.  My experiment
> was inconclusive, but the idea is not unsound.
>
> Poul-Henning Kamp
>