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

[Bob kb8tq]

Hi

> On Mar 10, 2021, at 4:57 PM, Magnus Danielson <magnus at rubidium.se> wrote:
> 
> 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.

If you have a “Tropo Observatory” that gives you anything close to 
24 / 7 / 365 data you are *very* lucky. Indeed there are people doing
Tropo by looking at GPS and saying “what’s left is Tropo” ….. If
that’s the approach your local observatory is using …. hmmmm …..

Bob

> 
> 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
> 
> 
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