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

[HÃ¥kan T Johansson]

Dear Keelan,

On Sat, 10 Jun 2023, Keelan Lightfoot via time-nuts wrote:

> 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

I think what is plotted is microseconds, not nanoseconds.  Since it 
looks like it wraps at 1000000.

If so, the blue curve is just a free-drifting clock?
(4 second total drift)

Best regards,
HÃ¥kan



>
> 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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>>
>>
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