On 3/21/2012 2:49 AM, David J Taylor wrote:
"Ron Frazier (NTP)" <timekeepingntpl...@c3energy.com> wrote in message
news:4f692255.2020...@c3energy.com...
On 3/20/2012 11:21 AM, David J Taylor wrote:
[]
You /will/ see variation in the serial output from the Sure device,
as you will in many NMEA devices. For the Sure device, one
measurement is here:
http://www.leapsecond.com/pages/MG1613S/
under the heading "NMEA Latency". The graph here is 100
milliseconds full scale.
http://www.leapsecond.com/pages/MG1613S/nmea-jitter-1.gif
That's funny, there's this line in the text.
"In a 15 minute run (900 seconds) the mean latency was 350.2 ms with
a standard deviation (jitter) of 10.7 ms. "
Then there's the graph, which seems to show a variance in NMEA start
time of 75 ms or so. The two seem to contradict each other.
How so? Are you taking the peak-to-peak figures from the graph and
comparing it to the standard deviation? SD isn't a peak-to-peak value.
You already mentioned the Garmin previously, and the Sure now, and I
have reports of similar NMEA drifting behavior from other SIRF units.
So, it appears that most, if not all GPS's exhibit a variance in the
timing of NMEA data of 50 to 120 ms or so. That would definitely put
a limit on what you could do with NMEA only data.
Yes, in typical GPS receivers the NMEA data is only accurate to a
second - it says where the unit was at the UTC second preceding the
data. I don't believe that jitter in NMEA output time is limited to
one particular chipset.
(Is jitter RMS, SD, peak-to-peak?).
I noticed that Dave Hart later posted this reply to your question. I'll
reference that below.
NTP's jitter is root mean squares of offsets from the clock filter
register (last 8 responses, more or less).
Cheers,
David
Perhaps I've been misusing some of the terms. There appear to be 5
items we could discuss / graph.
A) The moment to moment offset of my PC's clock from the GPS clock when
I sample it. In my case, this is NMEA data. This is where I've been
reporting + / - 10 ms accuracy. I, perhaps wrongly, thought this peak
to peak range, or maybe half of it, was the jitter.
B) The longer term (a few days) variance of the NMEA data from UTC. You
could call it variance, wobble, drift, I don't know if there is an
official name for it.
C) What your ntp plotter plots as the red line on the jitter tab with a
loopstats file. Not sure how that's derived.
D) What your ntp plotter plots as the black line on the jitter tab with
a loopstats file, a weighted moving average, presumably of the red line.
E) What the Meinburg Time Server reports for each peer as jitter. This
is probably the same data as NTPQ - p.
Probably, C and E are the only ones that match Dave Hart's definition
quoted above.
For my personal purposes, an RMS value doesn't do me as much good. I am
mainly concerned with A and B. My main concerns are, how far my PCs'
clocks are off from each other, and how far they're off from UTC. So, I
could say the following to characterize my GPS system.
1) My PC clock is almost always within + / - 10 ms of GPS time due to
short term offsets from the GPS NMEA data.
2) My GPS time is almost always within + / - 70 ms of UTC time due to
longer term variance in the timing of the NMEA data.
and consequently
3) My PC clock is almost always within + / - 80 ms (adding the two) of
UTC time due to short term and longer term changes in the NMEA data.
I have no idea what the proper names are to apply to these parameters.
Sincerely,
Ron
--
(PS - If you email me and don't get a quick response, don't be concerned.
I get about 300 emails per day from alternate energy mailing lists and
such. I don't always see new messages very quickly. If you need a
reply and have not heard from me in 1 - 2 weeks, send your message again.)
Ron Frazier
timekeepingdude AT c3energy.com
_______________________________________________
questions mailing list
questions@lists.ntp.org
http://lists.ntp.org/listinfo/questions
