[time-nuts] U-blox teaser

Dana Whitlow k8yumdoober at gmail.com
Sun Feb 28 01:07:02 UTC 2021 

Thanks for the link, Bob.  I got to know both Victor Zhang and Mike
Lombardi during my
stay at Arecibo, but to my regret have never met either in person.

Dana


On Sat, Feb 27, 2021 at 6:01 PM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
>
>
> > On Feb 27, 2021, at 11:18 AM, Dana Whitlow <k8yumdoober at gmail.com>
> 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 <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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