[time-nuts] ublox NEO-M8T improved by insulated chamber?

Gary E. Miller gem at rellim.com
Tue Nov 7 23:39:01 EST 2017

Yo Tom!

On Tue, 7 Nov 2017 20:16:09 -0800
"Tom Van Baak" <tvb at LeapSecond.com> wrote:

> > Which is small compared to the published GPS time resolution
> > (IS_GPS_200H, page 54) of 90 ns.  
> Correct. GPS performs far better than the original spec. Like the
> Mars rovers...

Of course, but then you are on a wing and a prayer, not engineering.

> As a result we're now all used to ~10 ns level of performance out of
> GPS, even in a $5 receiver.

I'm sure we are not talking about the same thing here.  Your talking
about GPS time?  I'm talking about UTC as output from a GPS, after it
converted from GPS time.

I am NOT talking about the 'performance' of GPS. What is performance?
We talking about frequency stability, or position accuracy, or we
talking about absolute offset from USNO UTC time?  I'm talking about
the later. I'm talking about the spec about how close the GPS time is
to UTC time. Your GPS converts the GPS time to UTC time depending on an
ephemeris parameter that the GPS owners say is 90 ns (one sigma).

Sure, you may get better, but when you are looking at subtle error sources
that is surely one to look at.

How exactly do you measure offset of your GPS time output to absolute
UTC time?

> Closer to ~1 ns is possible when you dig
> into the bag-of-timing-tricks like zero-D mode, sawtooth correction,
> antenna calibration, multi-path mitigating antennae, dual-frequency
> receivers, external frequency references, post-processing,
> temperature stabilization of antenna, cables, receiver, etc.

Yes, of course, but NONE of that fixes the GPS to UTC offset problem.
It makes the GPS time much better, but does not solve the problem that
the GPS to UTC offset is only good to 90 ns (one sigma).  Is your GPS
getting a better offset correction somewhere else?  Otherwis it has NO
way to compute/calculate/devine that offset.

> So the
> industry big boys are getting sub-cm levels of positioning and sub-ns
> levels of timing. It's all pretty cool. Some time nuts are not far
> behind.

I have seen cm level precision myself.  But that is unrelated to the issue
I bring up.  The positioning depends on stable GPS time, and I agree GPS
time is much more stable than 90 ns.  I thought the subject was UTC offsets.

> Note also that relative timing, such as needed by a GPSDO frequency
> standard is always much better than absolute timing, such as needed
> by a UTC time standard. This is because many of the unknown offsets
> (antenna, cable, receiver RF and f/w) magically cancel when used as a
> GPSDO. This is why some GPSDO can get down to parts in 10^14th
> frequency stability over a day.

Yes, I 100% agree, and totally unrelated to my point.  Frequency stability
is only loosely correlated to absolute time accuracy.  Stable !=

> There's a slide I remember seeing that shows how GPS timing accuracy
> has improved since the early days. It's page 9 (attached) of:

I agree, GPS accuracy is great, but I am NOT talking about GPS timing,
I am talking about UTC timing accuracy.

I thought the problem was that the UTC time from the GPS was wandering
on a diurnal time frame.  The GPS can be perfect to one hundred 9s,
the GPS position can be perfect to 100 nines, but if the transmitted
GPS time to UTC time offset is said, by the US Military, to be only 90
ns (one sigma), then I'd listen to them when it matters.

Time for us all to actually read the standard and argue what that means.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588

	    Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin
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