[time-nuts] Sub 5e-13 ADEV GPSDO

Ole Petter Ronningen opronningen at gmail.com
Tue May 15 06:22:17 EDT 2018

Hi, Bert

Yes, the ionosphere is most definately taken out of the equation using this
method, as is a few other effects such as solid earth tides. I am working
on a writeup here http://www.efos3.com/GPSDO/GPSDO.html

Still work in progress, but it has come along some way - you're welcome to
a "preview".

The actual receiver does not matter very much, the method is not specific
to the Novatel. I am also running a Trimble NetRS which works just fine for
monitoring my maser, and I am sure many other would work, I know some
timing labs used a variant of the Ashtec Z-12; the only think that matters
is that 1) it will take an external reference frequency, and 2) you can get
the raw GPS observations out of it. If RTKLib can parse the obervation
messages directly, you will save yourself some hassle. (It can parse many
different formats, although I have only tested Novatel and Trimble RT17,
both work fine.) The PPP processing takes care of the rest. I also have a
writeup on how to use PPP for frequency monitoring here if you are
interested: http://www.efos3.com/ppp.html

I could be interested in borrowing a Furuno if you are willing, it could be
interesting to compare it to my maser - would give some pretty good
measurements of the PPS accuracy

On Tue, May 15, 2018 at 12:07 PM ew via time-nuts <time-nuts at febo.com>

> Thank you Ole
> Juerg and I spend most our time on frequency,control and measuring. We
> also see 2 E-12 frequency using GPS and blame the Ionosphere. Which brings
> up the Question does the way you use the OEMV-3 in your experiment use L1
> L2 compensation and where can I buy one? Looked for hours last night.
> Second have you looked at the Furuno GT-87 we bought the latest version
> which claims 1.7 nsec saw tooth . Buerklin in Germany sells them, we bought
> some and I expect boards tomorrow. If you want some boards I mail them to
> you.
> Bert
> In a message dated 5/14/2018 4:16:03 PM Eastern Standard Time,
> opronningen at gmail.com writes:
>  Hi
> Perhaps this is of interest to some. I finally got around to doing an
> experiment I've been mulling over for some time.
> In short, I use a Novatel OEMV-3 GPS receiver, clocked by an Oscilloquartz
> 8600 BVA, process the observations with RTKLib and real time corrections
> from igs - and use the resulting clock solutions as inputs to a software
> PLL that sets the EFC on the BVA via a Keysight 33510B function generator
> set to DC. A bit of a contraption, but it works surprisingly well (or
> perhaps I should say it surprisingly works!)
> I measured the GPSDO using a TimePod and my maser.
> There is *much* room for improvement, and probably too little data, but I
> thought the preliminary results worthy of a post: The ADEV is <5e-13 on the
> "hump". It passes through to e-14 territory around 6-7000 seconds, at least
> in this dataset. The TDEV flattens out around 2e-10 at 2000 seconds or so
> (a little wiggle on the end, but I choose to ignore that and blame it on
> not enough data).
> If the masks in TimeLab are to be trusted, it is very close to a 5071A high
> performance option in this region (400 to 10000 seconds), and of course
> much better at lower taus.
> The frequency accuracy stays within 2-3e-12, and the phase rarely wanders
> off more than 1 ns.
> I have pretty much everything to learn about tuning GPSDO's, but I think it
> shows that the approach is viable. I ofcourse also need to collect a *lot*
> more data, get a proper DAC in place and so forth, but still.
> To get some sort of baseline, I also processed in real time using the same
> correction stream observations from a Trimble NetRS clocked by the hydrogen
> maser (sadly not at the same time) - the resulting curves should pretty
> accurately show the limits of the approach - at least using the corrections
> I was using (CLK93; there are many to chose from, and frankly I don't know
> which would be best for something like this. )
> The .tim-files are here: http://www.efos3.com/downloads/GPSDO_14.05.18.zip
> As a final thought - this setup can be made "self documenting": log the
> observations to RINEX-files, and solve with PPP and IGS Rapid or other
> corrections. The resulting phase record should be good to at least low e-15
> from what I understand (and observe).
> Ole
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