[time-nuts] Re: Use of dual frequency GPS for NTP server and other purposes

[Todd Smith]

As has been mentioned on the list recently, Jackson Labs builds a M12M
multi-GNSS receiver that is a drop-in replacement for the Furuno
receivers that are in the Syncservers that you are running right now.

If I am not mistaken, Galileo uses close enough frequency to GPS that you
can reuse the existing GPS antenna and receive both systems with a
multi-GNSS receiver like the Jackson Labs units.

Todd

On Tue, Oct 4, 2022 at 9:29 PM Bob kb8tq via time-nuts <
time-nuts at lists.febo.com> wrote:

> Hi
>
> > On Oct 4, 2022, at 8:33 PM, Tim Lister <listertim at gmail.com> wrote:
> >
> > On Tue, Oct 4, 2022 at 4:57 PM Bob kb8tq <kb8tq at n1k.org> wrote:
> >>
> >> Hi
> >>
> >> The main issue with using the output of a GPS receiver directly is
> >> dropouts. They can be caused by a variety of things. When the GPS
> >> “goes away” you loose all timing information.  Do these various things
> >> ( jamming, birds, lightning, …..) impact you? If they do, depending on
> >> just how the loss of timing impacts you, you do or don’t have a problem.
> >>
> >> Into an NTP server, the impact will still be there. How great an impact
> >> it is depends a *lot* on what the next level source going into the
> server
> >> is. The commercial NTP gizmos tend to have things like OCXO’s or Rb
> >> standards in them. That gives them a bit better performance than a
> >> typical DIY computer based NTP server.
> >
> > Yes they would definitely impact us. I neglected to mention that we
> > have the rubidium oscillator option as backup in these units to
> > provide holdover. Mostly to provide insurance/holdover for the case
> > that weather takes out the antenna to give time to ship a replacement
> > out from HQ in CA to whichever site was affected. Given that one of
> > the time servers provides the IRIG-B signal for the 400 Hz control
> > loop for the drives for a ~20 ton 2-meter diameter telescope, I'm not
> > keen on a homebrew Raspberry Pi solution however much I like them for
> > my own time-nut home use...
>
> So indeed, having timing drop out is not a good thing ….
>
> >
> >>
> >> If you *do* go with a dual freq GPS, you can do a very good job of
> working
> >> out the location of your antenna. With some effort that can improve the
> >> net timing accuracy. If post processing timing correction is ok in this
> >> case, that also is a lot easier / more accurate with a full set of dual
> freq
> >> GPS data to correct against.
> >
> > A "real time" (latency ~<30 minutes) measure of the PWV would be
> > helpful for scheduling/optimization of observations but there's also
> > likely to be value in an improved value for use during later analysis
> > at timescale >12 hours. Haven't done enough reading around of the
> > literature on determining PWV from GPS to get a sense of where the
> > error budget and improvements lie. I'm assuming something like a 48
> > hour initial survey and having that post processed by e.g. the NRCan
> > PPP service after the final GPS orbits are available would likely be a
> > benefit but unclear how having a proper geodetic phase center vs a
> > $200 bullet L1/L2/L5 antenna matters. Similarly it's not clear to me
> > how much improvement in the water vapor determination comes from the
> > increasingly precise (but increasingly delayed) GPS orbit products -
> > it may be e.g. the Ultra Rapids are Good Enough.
>
> For about $100 ( delivered ) you can get a “pretty good” Chinese multi
> band antenna. It’s not as good as fancy $2,000 antenna, but it’s way
> better than a hockey puck. With that and a week’s data into NRCan
> (it’s free ….. ) you can be pretty sure of your location to < 10 CM. That’s
> good enough to “not matter” for timing.
>
> >>
> >> So, when it works, you might be down into the < 10 ns range. When that
> >> bird sits on that antenna …. microseconds …. takes a nap … milliseconds
> ….
> >
> > We're not an optical observatory so not in the pulsars & masers
> > category. Driving demand on accuracy for us is normally very close,
> > fast moving Near Earth Objects which can move at several hundred
> > arcseconds/minute. If we don't want the timing uncertainty to degrade
> > the position accuracy by more than the star catalog does, then we need
> > timing precision in the ~1-10ms range. Determining diameters from
> > occultations of stars by Kuiper Belt Objects, which have a transverse
> > velocity of ~15 km/s, leads to similar requirements.
>
> The advantages of all this stuff ( single band vs multi band ) are in the
> < 100 ns
> range. If you do a good job on the single frequency stuff ( sawtooth
> correction ….)
> the improvement is < 20 ns.  There is a footnote down somewhere that
> mentions
> some sort of “99.9% of the time” stuff. There are unusual solar events
> that can
> push things well above the 20 ns range.
>
> Yes, there are other factors. There is another chunk for GPS <-> UTC.
> There also
> is a chunk for antenna / cable delays and the like. This stuff impacts
> both approaches.
> Normally that all should be < 100 ns. The delay part should be stable to <
> 20 ns in
> a typical setup.
>
> >>
> >> Lots of tradeoffs.
> >
> > As always, tempered often by a lack of funds.
>
> Somehow money always matters :)
>
> Bob
>
> >
> > Tim
> >
> >>
> >> Bob
> >>
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