[time-nuts] ublox NEO-M8T improved by insulated chamber?
attila at kinali.ch
Sun Nov 12 10:57:31 EST 2017
On Wed, 8 Nov 2017 14:24:08 -0600
"Chris Caudle" <chris at chriscaudle.org> wrote:
> On Wed, November 8, 2017 12:55 pm, Gary E. Miller wrote:
> > I knew about the errata, I left out that detail to see when someone
> > actually read my citation.
> OK, I don't want to quibble about versions, and whether the "latest
> version" is 200H or 200H including errata, but I think we both agree that
> the currently operating performance would be described by the rollup
> document, i.e. the offset information for GPS time to UTC time should be
> within 20ns 1 sigma, as opposed to the older 90 ns 1 sigma description.
The GPS standard only specifies a _target_ value, what they want to achieve.
The actual value has been consistenly better than what is described in the
standard for decades. Why standard not updated, you ask? Because in a
standard you want to be conservative. People rely on that value and if
anything happens and for some reason that value gets much worse than usual,
you still want tto be within specs.
> > Do you now see how measured GPS time/location can be very precise, but
> > UTC from a GPS less so? Have you read the entire 3.3.4?
> Yes, and I do not really understand the "1 sigma" description. Is the
> error really random? I'm not sure how the term 1 sigma applies to error
> distributions other than a Gaussian distribution, so what "20 ns at 1
> sigma" really means moment to moment for the time value I get from a GPS
> receiver is not completely clear to me. At a simplistic level I would
> interpret that 66% of the PPS ticks are within 20ns of the "true" UTC
> tick, 33+% could be farther away than 20ns from "true" second tick.
This is what it means. The distribution is not completely gaussian.
There are some periodic (12h, 24h, and various others) contents that
look more sinusoidal than random, but if you look at the raw distribution,
without extracting any of the periodic components, it looks quite gaussian.
> The general interest in GPS based time transfer covers a wide range of
> uses, so whether you actually care about absolute offset from UTC or not
> needs to be made more explicit in discussions about vaguely defined
If you are really doing time transfer using GPS and you care about
performance. Then you are using at least common-in-view mode with some
heavy post-processing. This brings you close to a 5ns one-sigma uncertainty.
If you calibrate your GPS receivers frequently, you can get down to 200ps.
(over a few 100km baseline).
If you need better than that, you go to Timetech ( http://www.timetech.de/ )
and ask them to loan you one of the TWSTFT systems.
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering. -- The Doctor
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