Hi
> On Feb 27, 2021, at 11:18 AM, Dana Whitlow <[email protected]> wrote: > > Thanks, Bob. > > It seems to me that, depending on the positions of sats visible to one's GPS > antenna and the spatial distribution of free electron density in the > ionosphere, > the ionospheric contribution to position errors could sometimes largely > cancel. > But that observation may (or may not) reflect strongly on one's ability to > get > accurate absolute time from GPS on "average" days. > > During my Arecibo Observatory days we used NIST's TMAS service to keep > our H-maser-based station clock synced with UTC. Our user community > (mainly VLBI and pulsar timing people) seemed pretty satisfied with +/- > 100ns > accuracy, so I tried to do better by keeping things well within +/- 50 ns > during > my reign. IIRC, NIST was claiming that TMAS could produce results mostly > within about +/- 20 ns. > > To be honest I'm baffled by how time transfer much better than that could > be achieved in practice. One way (mentioned about a month back on the list) is a two way transfer via satellite. The “delay is equal in both directions” assumption is pretty good in this case. Once you have that as a baseline, you can measure the performance of other approaches. One of *many* starting points to rumble down this rabbit hole: https://www.nist.gov/pml/time-and-frequency-division/time-services/common-view-gps-time-transfer <https://www.nist.gov/pml/time-and-frequency-division/time-services/common-view-gps-time-transfer> I would suggest starting with David Allan’s paper (referenced in the link above) as a pretty good starting point. Bob > > Regarding Q3, yes I'm aware that *some* GPS receivers do the estimation of > ionospheric delay. What I was asking was: Do any of the relatively > inexpensive > receivers to which we time-nuts have access do this? Here I'm speaking of > those being sold for no more than a few hundred USD. > > Dana > > > > On Sat, Feb 27, 2021 at 9:08 AM Bob kb8tq <[email protected]> wrote: > >> Hi >> >> >> >>> On Feb 27, 2021, at 9:41 AM, Dana Whitlow <[email protected]> wrote: >>> >>> I've long understood that ionospheric delays and variations thereof lead >> to >>> *position* >>> uncertainties in GPS navigation receivers, to the tune of perhaps 10m >>> (2DRMS IIRC)., >>> and that these are said to constitute the single largest GPS error >> source. >>> >>> Q1: Would this not imply timing errors of comparable magnitude (10's of >>> nsec) >>> for a single band GPS? >> >> Once all the signals “hit” the antenna, the delays are mostly common mode. >> Instead of showing up as a position error, they show up as an error in the >> time estimate. Since time is one of the things you estimate in the >> solution >> (along with X,Y, and Z) it get’s it’s own independent solution. >>> >>> Q2: Why have I not seen this issue raised in connection with the present >>> discussion >>> about achievable absolute timing accuracy? >> >> GPS time transfer is often done to the sub-ns level. There are a number of >> papers on this. >> >>> >>> Q3: Do any of the "modern" timing GPS receivers available to civilians do >>> dual-band >>> reception in a way that includes estimation of (and correction for) >>> said delays and >>> their variations? I know that Garmin, for one, is now selling L1/L5 >>> handheld GPS >>> receivers (GPSMAP66sr and GPSMAP65s), but I've seen no indication >>> that these >>> units make any attempt at doing such corrections. >> >> Yes, some receivers do an estimate of ionospheric delay based on the >> variation of that delay with frequency. This does not help with >> tropospheric >> delay or all of the various “common mode” issues we have been talking >> about. >> It is also unclear how the “unknown” timing variation between the bands >> due to the antenna impacts these solutions….. >> >> Bob >> >>> >>> Dana >>> >>> >>> >>> On Sat, Feb 27, 2021 at 7:43 AM Bob kb8tq <[email protected]> wrote: >>> >>>> Hi >>>> >>>> The same 20 or so ns delay in a saw would also apply to the >>>> saw (or tight filter) in some timing antennas. It also would apply >>>> to the saw(s) in some modules. Even if the tolerance is “only” >>>> a couple ns on each of them, you *could* have 3 or more in the >>>> chain. >>>> >>>> Lots of numbers to crunch to get to 5 ns “absolute”. One could go >>>> grab a GPS simulator and start poking. First step would be to find >>>> a simulator that is spec’d for a < 5 ns tolerance on the PPS into >>>> GPS out. I do believe that rules out the eBay marvels that some >>>> of us have lying around ….. >>>> >>>> Simpler answer would be a quick “clock trip” with your car full >>>> of 5071’s …… hour drive over to NIST and then back home. >>>> That sounds practical for most of us :) :) >>>> >>>> Bob >>>> >>>>> On Feb 26, 2021, at 9:29 PM, John Ackermann N8UR <[email protected]> wrote: >>>>> >>>>> A while ago I tried doing a decidedly non-anechoic measurement with a >>>> VNA exciter going to a 1500 MHz ground plane and the receiver connected >> to >>>> the antenna (with a known delay cable) and I got a similar result, but >>>> there was enough noise that I didn't think I could nail it down to >> within >>>> 10 ns. >>>>> >>>>> I've also measured GPS antenna splitters and they tend to have 20-ish >> ns >>>> delays, mainly due to the SAW filters. I did surgery on an HP splitter >> to >>>> remove the filters so it could be used for L1 and L2 and that dropped >> the >>>> delay down to only 1 or 2 ns. >>>>> >>>>> So there's definitely lots of stuff to calibrate if you want to get >>>> accurate time transfer. >>>>> >>>>> John >>>>> ---- >>>>> >>>>> On 2/26/21 8:02 PM, Michael Wouters wrote: >>>>>> Typical L1 antenna delays range from 20 to 70 ns. >>>>>> I know of only one antenna for which a delay is given by the vendor >> and >>>> the >>>>>> technique used was just to measure the electronic delay ie by >> injecting >>>> a >>>>>> signal into the circuit. To do it properly, you need a setup in a >>>> microwave >>>>>> anechoic chamber with transmitting antenna etc. The practical >> difference >>>>>> may be small though, 1 or 2 ns ( sample of one antenna!). >>>>>> Cheers >>>>>> Michael >>>>>> On Sat, 27 Feb 2021 at 11:42 am, John Ackermann N8UR <[email protected]> >>>> wrote: >>>>>>> They're claiming "even better than" 5 ns for relative time, which >> given >>>>>>> the 4 ns jitter seems at least sort-of reasonable. But until someone >>>>>>> shows me otherwise, I'm still thinking that getting better than 25 ns >>>>>>> absolute accuracy is a pretty good day's work. >>>>>>> >>>>>>> John >>>>>>> ---- >>>>>>> >>>>>>> On 2/26/21 5:26 PM, Bob kb8tq wrote: >>>>>>>> Hi >>>>>>>> >>>>>>>> I can’t think of many antennas (multi band or single band) that >> claim >>>> to >>>>>>> know their >>>>>>>> delay to < 5 ns. Simply having a *differential* delay spec of < 5 ns >>>> is >>>>>>> quite good. >>>>>>>> Same thing with delay ripple, you see specs out to around 15 ns on a >>>> lot >>>>>>> of antennas. >>>>>>>> None of this is getting you to the actual total delay of the >> antenna. >>>>>>> It’s a pretty good >>>>>>>> bet that number is a bit larger than either of these. >>>>>>>> >>>>>>>> Some of the ripple probably comes out in the standard modeling. I’m >>>> not >>>>>>> sure that >>>>>>>> the differential delay is taken out that way. Total delay, not taken >>>> out >>>>>>> in any obvious >>>>>>>> fashion ( at least that I can see). If the F9 has a built in antenna >>>>>>> database, that’s not >>>>>>>> mentioned in the doc’s. Any benefit from the corrections would have >> to >>>>>>> be part of >>>>>>>> post processing. >>>>>>>> >>>>>>>> No, that’s not the same as talking about the F9 it’s self doing X >> ns, >>>>>>> but it would be part >>>>>>>> of any practical system trying to get close to 5 ns absolute >> accuracy. >>>>>>>> >>>>>>>> 5 ns *relative* accuracy between two F9’s? I probably could buy that >>>> if >>>>>>> the appropriate >>>>>>>> one sigma / on a clear day / with the wind in the right direction >> sort >>>>>>> of qualifiers are >>>>>>>> attached. >>>>>>>> >>>>>>>> Bob >>>>>>>> >>>>>>>>> On Feb 26, 2021, at 4:27 PM, John Ackermann N8UR <[email protected]> >>>> wrote: >>>>>>>>> >>>>>>>>> It's interesting that they talk about the F9 receivers offering 5 >> ns >>>>>>> absolute time accuracy. Does anyone know of tests confirming that, >> and >>>>>>> what sort of care was required in the setup to get there? >>>>>>>>> >>>>>>>>> John >>>>>>>>> ---- >>>>>>>>> >>>>>>>>> On 2/26/21 9:34 AM, Robert LaJeunesse wrote: >>>>>>>>>> FWIW. No detailed content, and a rather quick read. "Five key >> trends >>>>>>> in GPS". >>>>>>>>>> https://www.u-blox.com/en/blogs/insights/five-key-trends-gps >>>>>>>>>> _______________________________________________ >>>>>>>>>> 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. >>>>>>>> >>>>>>>> >>>>>>>> _______________________________________________ >>>>>>>> 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. >>>>>>> >>>>>> _______________________________________________ >>>>>> 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. >>>> >>>> >>>> _______________________________________________ >>>> 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. >> >> >> _______________________________________________ >> 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. _______________________________________________ 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.
