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

[Magnus Danielson]

Hi,

On 2021-03-11 00:38, Bob kb8tq wrote:
> 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 …..

Well, there is a high correlation with near-by stations on the tropo.
Post-processing pushes the errors way down.

It used to be that you had local sensory package to provide input for
tropo estimation. That faded out because it was not providing much
improved result than nearby observations.

Also, tropo has a different function for how it adds up in the
pseudo-range measurements to that of other sources, such as ionsphere.
So, separation can be done once the ionsphere shift is gone, which
double-frequency allows. Higher-order parts of ionsphere can be observed
if you like, and that gives a finer detail about the wavefront angle
into the tropo, if you want to play that game. Yeah, the signal does not
go line-of-sight through any of those, that's an approximation in
itself. There are people scratching their heads and refining it over time.

Kind of pitty that tropo has the same sign on code and carrier and seems
relative insensitive to frequency. Ah well, at least we can cancel the
ionspheric shift.

Cheers,
Magnus