[time-nuts] Re: GPS receiver with reference clock input

[Keelan Lightfoot]

Bob,

I did that experiment a while back. Running a static survey in my back yard
with the receiver exposed to the sun over the course of the day, the
internal clock offset was quite a mess, with 4 milliseconds of drift over 9
hours:

http://beefchicken.com/dump/clockoffsetbad.png

Once I acquired the receiver with the reference input, I used a fairly
recently calibrated HP 5316A with the OCXO option as a clock source, and
kept the receiver indoors in my underground lair where the temperature is
fairly constant. The results were much better, with -3 ns of drift over a
16 hour survey.

http://beefchicken.com/dump/clockoffsetgood.png

The results are interesting, but in terms of the static survey solution,
(and to be painfully reductive), the terrible clock seems to have had
little impact on the final survey, with both surveys having identical error
ellipses. And the internally clocked survey had the odds stacked against
it; it was a shorter survey, and there were a number of trees blocking a
clear view of the sky.

What are the other applications? These receivers were specifically marketed
as CORS receivers, I imagine having a reference station running for years
with a clock with milliseconds of drift per day might have some negative
consequences...

- Keelan

On Fri, Jun 9, 2023 at 5:07 PM Bob Camp <kb8tq at n1k.org> wrote:

> Hi
>
> Simple answer:
>
> Feed the 10 MHz out of your Rb (or whatever). Send the data off to NRCan
> (or wherever). Get
> back a plot that lets you know how many parts in 10^-15 your standard is
> off frequency.
>
> There are other applications. The more general answer is: to remove local
> clock issues from
> the solution.
>
> Bob
>
> > On Jun 9, 2023, at 7:37 PM, Keelan Lightfoot via time-nuts <
> time-nuts at lists.febo.com> wrote:
> >
> > In my pile of Trimble GPS receivers, I have a small handful of 4000SSi
> CORS
> > receivers with a BNC clock reference input, and in the receiver firmware
> I
> > can switch the receiver over to the external 10 MHz clock. It's all
> > documented in the manual, but what I don't understand is... why?
> >
> > I'm trying to reconcile my mental model of the GPS receiver, and in my
> > model, any critical timing in the receiver is all relative to the
> received
> > GPS signals, what would I gain from feeding the 10 MHz from a GPSDO other
> > than the more stable OCXO in the GPSDO? I imagine it might improve jitter
> > on the 1PPS signal...
> >
> > Thanks,
> >
> > Keelan
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>