[time-nuts] U-blox teaser

Dana Whitlow k8yumdoober at gmail.com
Sat Feb 27 16:18:30 UTC 2021 

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.

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 <kb8tq at n1k.org> wrote:

> Hi
>
>
>
> > On Feb 27, 2021, at 9:41 AM, Dana Whitlow <k8yumdoober at gmail.com> 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 <kb8tq at n1k.org> 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 <jra at febo.com> 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 <jra at febo.com>
> >> 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 <jra at febo.com>
> >> 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
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