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

[Tim Lister]

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...

>
> 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.
>
> 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.
>
> Lots of tradeoffs.

As always, tempered often by a lack of funds.

Tim

>
> Bob
>