Hi > On Mar 10, 2021, at 4:57 PM, Magnus Danielson <[email protected]> wrote: > > Hi, > > On 2021-03-10 17:04, Bob kb8tq wrote: >> Hi >> >>> On Mar 10, 2021, at 9:39 AM, Charlie <[email protected]> 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 > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
