[time-nuts] I've been thinking about a GPS receiver experiment

Dana Whitlow k8yumdoober at gmail.com
Tue Oct 24 23:09:04 UTC 2017

Hello Skip,

I have a theory, but it will be interesting to see what others say.
Assuming that the
1 PPS error to which you refer is the so-called "sawtooth" error, I've come
to suspect
that the rate at which the individual PPS pulses walk across the sawtooth
is related to,
and likely proportional to, the error of the internal (or in this case, the
external) clock
oscillator.  If I'm right about its being proportional, then it seems to me
that having
the GPS's clock oscillator right on would freeze the PPS error at some
fixed value,
not necessarily zero.  If true, you'd experience a constant bias error to
the timing of
the PPS pulses.

Now you would seem to be in the perfect position to refute or verify my
provided you have the means to vary an external clock's frequency in a
way, by watching how the PPS error behavior changes as a function of the

If you manage to try the experiment, I'd greatly appreciate hearing the

Dana    K8YUM

On Tue, Oct 24, 2017 at 5:17 PM, Skip Withrow <skip.withrow at gmail.com>

> Hello time-nuts,
> I've been thinking about a GPS receiver experiment and just wondering
> if there are any opinions or prior experience that might save me a lot
> of time.
> What I have been thinking about doing is taking a GPS receiver
> (Novatel OEM4-G2) that has provisions for an external clock (5 or 10
> MHz) and driving it with a rubidium oscillator (that has 1pps
> disciplining, (such as the X72 v5.05 or SRS PRS-10).  The GPS even has
> settings for OCXO/rubidium/cesium dynamics.
> Then, (and here is the unknown part) what if the GPS receiver 1pps is
> used to discipline the rubidium?  This basically forms a feedback
> loop, so could either hurt or help - depending.  Supposedly the better
> oscillator would give a better GPS solution.  And the better solution
> (1pps) should provide a better oscillator frequency.
> We know that GPS receivers using asynchronous clocks have 1pps errors
> and hanging bridges (OEM4 is spec'd at 20ns rms), If the oscillator is
> on 10MHz and disciplined will the 1pps error be reduced such as the
> Thunderbolt?
> Comments appreciated.
> Regards,
> Skip Withrow
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