[time-nuts] U-blox teaser

Sat Feb 27 14:41:06 UTC 2021

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?

Q2: Why have I not seen this issue raised in connection with the present
discussion
      about achievable absolute timing accuracy?

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.

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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