[time-nuts] Improving on basic L1 timing

[Michael Wouters]

Hello Angus

If you have 3 rubidiums of similar stability + 3 counters, you could
do a 3-cornered hat.

Otherwise, GPS common view to a better clock may be an option. If you
are reasonably close to a national standards lab, you might be able to
use their time-transfer files to compare your rubidiums with their
time scale - not everyone makes them publically available though.
Otherwise, if there is an IGS station near you, you could use the
station RINEX files and IGS clock solutions which are freely
available. Many IGS stations have a H-maser as the local clock. But it
may be just as good to simply use the comparison with GPS time
inherent in the time-transfer file.

The advantage of generating a time-transfer file is the possibility of
then improving upon the various corrections broadcast by GPS,
effectively repeating what the GPS receiver does to generate its
realization of GPS time but with better data.

With post-processing, the short to medium term (less than 1 day)
performance  can be improved a bit as you are suggesting when you
referred to "atmospheric issues". Improved ionospheric models are
available  or if there is an IGS station nearby, for example, the
measured ionosphere could be used. Other improvements can be had with
good antenna coordinates and using final orbits in the processing.

What can you use for your time-transfer receiver ? Some low-cost
single-frequency receivers are suitable eg the Trimble Resolution T.
The essential requirement is the availability of  raw code
measurements - with these you can generate CGGTTS time-transfer files
and/or RINEX observation files.

At least part of the software infrastructure to do this exists: the
OpenTTP project (www.openttp.org) has software for CGGTTS and RINEX
file generation for a few older,single frequency receivers, with
support for some other,current receivers under active development.
There is other software around, but it is orientated towards dual
frequency receivers and carrier phase processing.

Although it would be relatively straightforward to hack in use of
improved ionosphere, using final orbits is a bit harder since these
are not parameterized the same way as the broadcast orbits. Maybe
someone on time-nuts has software to do the conversion (and this would
have to be hacked into the OpenTTP software, rather than the final
time comparison).

The sort of performance you get on a zero baseline is a TDEV of a few
ns - you can extrapolate frequency stability from that.
On a 1000 km baseline, you can compare two Cs to better than 1 part in
10^13 @ 1 day.

All of the above is software-oriented, whereas you seem to be looking
for a hardware solution, but that's what I know best.

Cheers
Michael

On Tue, Jun 14, 2016 at 6:16 PM, Angus <not.again at btinternet.com> wrote:
> Hi,
>
> I'm planning to test some rubidiums again, but since Santa never did
> get me that hydrogen maser I asked for, I'm still stuck with ordinary
> gps timing receivers as a separate medium to long term reference. The
> atmospheric issues are probably the main ones I would like to get rid
> of, although the more errors removed the better.
>
> It does not have to be done in real time, but even an single test run
> would last weeks, so there could be a lot of data to tie together.
>
> It would really need to be something that actually exists rather than
> just an idea of how it might be done, since I really just don't have
> time for any more major projects anytime soon. I've found from
> experience that too much time spent making the test gear etc means
> that I don't get the time to actually use it!
>
> I'm also looking for something that's not too expensive - like up to
> hundreds rather than thousands of pounds.
> A good cesium or 2+ frequency gps with relevant options might be fine,
> but also rather out of my price range.
>
> BTW, I do plan on uploading the end results, in case anyone is
> interested.
>
>
> If anyone knows of some way to do this, (or even has something
> appropriate they want to sell) I'd appreciate hearing about it, on or
> off-list.
>
> Thanks,
> Angus.
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