[time-nuts] World's most precise.... wall clock

[Magnus Danielson]

Hi,

On 2021-03-10 17:04, Bob kb8tq wrote:
> Hi
>
>> On Mar 10, 2021, at 9:39 AM, Charlie <charlie at drhabekost.com> wrote:
>>
>> Bob-
>>
>> As a rank amateur e astronomer, I am a  lurker. I am amazed at what I have
>> learned here. I know that there are differences between the meaning of
>> precision and accuracy, but please correct my understanding if I am
>> imprecise.
>>
>> I have a need for precise time, as all sorts of calculations are dependent
>> on precise geocentric position, and of course time to convert to other times
>> e.g. sidereal, utc, etc., as related to the motion control of a large
>> telescope.
>>
>> I have an old hp z3805a; seems to be really precise, agreeing with my
>> location (surveyed). Other gps's that I have seem to wander more.
> I suspect that is a function of how the 3805 presents the data. 
>
>> 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, and a really stable 10 Mhz
>> output ( I might add that I am a Ham, where 1 uhz  error is detrimental).
> Sub ns *jitter* is doing well at 1 second with GPS. Accuracy is different 
> than jitter. Since the GPS clock is not a direct expression of UTC from 
> BIH ( nothing is … sorry about that …) there is some back tracking to get
> *very* accurate time. 
>
>> Assuming I can afford an upgrade, would  getting a more precise PPS/time
>> unit then and feed that data into separate OCXO? Getting both seems out of
>> my league.
> If you have the $300 to $2000 for a multi band GNSS timing receiver, it will 
> indeed help a bit. How much will depend a lot on the state of the ionosphere 
> and the correction process. Troposphere also gets into things. I don’t know of
> any receiver that directly estimates Tropo delay. 

There is means to infer Tropo-delay with single receivers, but it is not
very accurate that I've seen. However, the usual way is to use nearby
receivers as reference. Eventually as the actual position is known,
tropo errors can be inferred more directly for a fixed receiver.

It is worth noting that you do not only want a good multi band GNSS
timing receiver, you also want a good phase-stable and multi-path
rejecting antenna to go with it, such as choke-ring or pin-wheel. Much
of carrier-phase properties is lost in a bad antenna.

Another aspect is that if you care about very accurate time, you need
the antenna, cable and receiver calibrated, as the delay through these
is not fully cancelled in the reception processing. In precise
positioning processing, the antenna should be of known type and properly
orienter such that the phase center calibration is compensated actively.

If you do not need a paper to show how good you are, you can do pretty
good guestimates to roughly your delays and compensate those. At some
point you end up wanting to do a real calibration anyway, if you try to
push the limit downwards.

>
>> Seems that  could the best of both worlds.
> Best would team the fancy receiver with a fancy standard. An OCXO is better
> than a TCXO. Most Rb's beats the OCXO long term. A Cs will beat them both
> if you run out long enough. You then get into things like GNSS disciplined 
> Passive Hydrogen Masers. Properly done they should perform quite well. A
> disciplined Active Maser would / could beat a Passive Maser …..
>
> The better the “flywheel” the better the result, at least for frequency / stability. 
> It will count off seconds quite nicely. Just how far off from “right” those seconds
> are is a bit unclear. ( = you still are not accurate)
>
> For accuracy you need a path back to BIH and the “official” definition of UTC.
> That’s true even with the brand new fresh from the factory disciplined Active
> Maser that you sold the house to buy ….. There are lots of nasty little delays
> that creep into the mix …. All of them need to be taken care of to below your
> target error level. If you are after < 10 ns accuracy, this could get pretty exciting. 

BIPM these days, not BIH. There is plenty of things and thorns. I've
touched on some.

I've seen some crazy stuff. Temperature stabilized concrete pillars,
temperature stabilized coax. All depends on how deep your pockets goes
and crazyness.

I have yet to go that crazy, but at times I wonder.

Cheers,
Magnus