[time-nuts] clocks for amateur radio and astronomy (was: World's most precise.... wall clock)

[Tim Lister]

On Wed, Mar 10, 2021 at 2:21 PM Bruce Griffiths
<bruce.griffiths at xtra.co.nz> wrote:
>
> Unless something akin to VLBI or pulsar timing is involved millisecond accuracy will usually suffice for amateur astronomy.

I think some of the most demanding requirements for optical astronomy
by amateur/semi-pros are set by observing Near Earth Object (NEO)
asteroids or observing outer solar system objects occultations (when
these objects pass in front of a star). NEOs can have motion rates up
to several hundred arcseconds per minute at close approach and you
don't want to degrade the measured position by any more than about
0.2". Similarly outer solar system objects such as Kuiper Belt Objects
will have a velocity across the line of sight during the occultation
of around 10-15 km/s and size in the few hundred meters to few hundred
km range. Both of these work out to wanting time accuracies around the
few millisecond regime.

Cheers,
Tim

>
> Bruce
> > On 11 March 2021 at 06:31 Bob kb8tq <kb8tq at n1k.org> wrote:
> >
> >
> > Hi
> >
> > > On Mar 10, 2021, at 12:17 PM, Attila Kinali <attila at kinali.ch> wrote:
> > >
> > > On Wed, 10 Mar 2021 06:39:46 -0800
> > > "Charlie" <charlie at drhabekost.com> wrote:
> > >
> > >> My question is thus: It seems that procuring a more precise PPS/time output
> > >> unit is quite a bit more costly than what I have; even more costly is a unit
> > >> that has both more precise PPS/time output,
> > >
> > > The question is more whether it is good enough. How accurate
> > > do you need your time? How precise does it need to be?
> > > If the answer to both questions is smaller than 100ns,
> >
> > I suspect that’s a typo. “larger than 100 ns” ( 1 us > 100 ns ….)
> > would seem to be the correct way to look at it.
> >
> > > then you
> > > are good with anything you can get your hands on, even the cheapest
> > > GPS receiver. If you need better than that, you need to look
> > > more closely and calculate each contributor to uncertainty,
> > > both random and systematic and see where you can and want to
> > > improve (or compromise).
> > >
> > >
> > >> and a really stable 10 Mhz
> > >> output ( I might add that I am a Ham, where 1 uhz  error is detrimental).
> > >
> > > At what time scales you need that 1µHz stability? Is it for
> > > a few seconds? Or over a few hours? How accurate does it have
> > > to be?
> > > And are you sure it is really 1µHz @ 10MHz? Because that's
> > > a stability better than 1e-13. Which isn't something most people
> > > just have in their lab. You need a an exceptionally good OCXO to
> > > reach down there (an OCXO that costs you as much as a car)
> > > and even those reach it only over a very limited Ï„ between
> > > about 1s and maybe 100s. Beyond a Ï„ of 1000s you have the option
> > > of using a HP5065, possibly with Corby's Super-HP6065 modifications.
> > > Even GPS takes until somwhere like 100ks to reach down to 1e-13.
> >
> > Even crazier if you are talking about your transmit frequency at VHF :) :).
> > Welcome to why Hz, uHz, etc normally are replaced with ppm, ppb, and
> > the like … That’s been the case in every place I’v ever worked on this stuff.
> >
> > Bob
> >
> > >
> > > If you need better than that, either at shorter or longer Ï„,
> > > then we are clearly deep in atomic clock territory. Either
> > > hydrogen maser. caesium beam standard or cold atomic clock.
> > >
> > >                     Attila Kinali
> > > --
> > > The driving force behind research is the question: "Why?"
> > > There are things we don't understand and things we always
> > > wonder about. And that's why we do research.
> > >             -- Kobayashi Makoto
> > >
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