On 3/14/2012 11:31 AM, David Lord wrote:
Ron Frazier (NTP) wrote:
On 3/14/2012 7:14 AM, David Lord wrote:
Ron Frazier (NTP) wrote:
....
Based on advice from David Taylor, I just changed my
configuration. Previously, I had only the GPS selectable and the
internet servers noselected for testing purposes to try to
determine where a slow drifting behavior is coming from. Now, I
have changed that so the GPS is noselected and the New York NIST
server is the preferred and only selectable peer. I am monitoring
the GPS and other internet servers for comparison. So, it will be
a couple of days before this configuration accumulates some stats
files.
Even though I have been tinkering with this GPS for a couple of
months, I have never seen anything like the 50 second jump I
started this thread about. That problem may not be reproducible
for some time, if at all.
Faulty GPS, incorrect configuration, anybodies guess?
Internet servers give me an rms offset about 610us, whilst GPS
with PPS gives about 4us. There are day to day variations and
some rare 'events' when GPS or an internet server goes bad.
You must LIVE on the internet backbone and have blazing fast routers
to get performance like that. My typical performance from internet
servers is offsets of + / - 60 ms.
Hi Ron
I don't live on the internet backbone nor do I have blazingly
fast anything. I have a slow 2Mbit/s ADSL connection.
You must have a very broken internet connection to get offsets
that high. A few times I've been routed by satelite or some
wireless connection and I also sometimes use mobile broadband
but still don't see offsets as large as 60ms.
GPS without PPS, Globalsat BR304, wasn't worth using as ntp
source due to large variations in offset from the NMEA sentences
that were tried with RMC being best giving 50% of offsets under
10ms but maximum offsets being near 100ms.
With my BU-353, which is similar, by setting the baud rate to 57,600,
programming for ONLY GPGGA sentence, and polling every 8 seconds, I
can keep my offsets from the GPS's estimate of true time to usually
around + / - 5 ms with spikes to 10 ms and almost never higher. If I
use ONLY the GPZDA sentence, which is essentially fixed length and
reports only time, I can get that down to + / - 3 ms most of the time
with spikes to 6 ms. However, David Taylor pointed out that the
GPZDA sentence doesn't have any validity check field, and the GPGGA
sentence does. We verified that the refclock.c code does check for
this. So, I'm back to using GPGGA even with a bit more jitter. That
way, if the GPS fails, ntpd is more likely to react gracefully.
What are you using as a reference to verify the offsets you
get are offsets from UTC?
Hi David L,
Allow me to clarify. As we've discussed, using the internet for time
keeping from here is awful. The only relatively precise source I have
for UTC time is my GPS. The main point I was trying to make is that I
can get NMEA data, even without PPS, and even over USB, from my GPS with
a jitter of around 3 ms. So, the stats I was quoting are loopstats of +
/ - 10 ms of the offset of the GPS time relative to my PC's clock
polling it every 8 seconds and with it as the only selectable clock for
testing purposes. So, if GPS time = UTC time, then my reported offsets
are + / - 10 ms to UTC. However, this file
http://dl.dropbox.com/u/9879631/drifting01-peerstats.20120312.jpg
that I posted earlier shows the PC locked to GPS time, which is the dark
jaggy line around zero. The other band of colored lines are the
internet servers that I'm monitoring for comparison. The graph seems to
indicate that either all the internet servers are slowly wandering, or
my GPS is slowly wandering (which would have the same appearance), or
neither is wandering and the internet offsets are being reported wrong
by whatever drives the peerstats file which I graphed. The most likely
candidate seems that the NMEA start time from my GPS is wandering around
+ / - 60 ms over about 4 days, although I'm not 100 % convinced it's not
a reporting error.
If the start time of the NMEA sentences is varying + / - 60 ms over
about 4 days, and since my PC is locked to GPS time, then my PC's time
will also vary about + / - 60 ms over about 4 days away from UTC. So,
all I really know for sure is that I'm locked to within + / - 10 ms of
the time that my GPS is reporting via NMEA data.
The problem with using my BU-353 in this way is that the start times
for the NMEA sentence seem to wander over about 60 ms in either
direction over a period of about 4 days. So, over a few days, my
computer's time will drift away from true UTC time by that amount,
and back again. However, over the short term, my pc's clock is much
more stable than if I use internet servers as my primary source.
Can you give your ntp.conf that result in that level of
offset from internet servers?
You can look at these if you like. Posting these via my dropbox is the
only way I know that's compatible with the mailing list. If you'd like
them another way, just let me know. I'm not worried about getting
stellar performance from the internet time servers. I just want to get
a good performing GPS server running, and then use the internet servers
only as a backup in case of GPS failure. Note that these files
represent a computer that is polling the internet only, and which has no
GPS as of yet.
http://dl.dropbox.com/u/9879631/TAZ%20loopstats%202012-03-07%20to%202012-03-14.jpg
http://dl.dropbox.com/u/9879631/ntp.conf-TAZ
http://dl.dropbox.com/u/9879631/loopstats.20120313-TAZ
http://dl.dropbox.com/u/9879631/loopstats.20120314-TAZ
Sincerely,
Ron
The ntp distribution might still have some advice and
tools for selection of suitable sources. I just select
from the list of uk public servers at ntp.org and try
each one to get rtt and their sources then select the
ones that are closest with different sources. If you
don't want to do that you can specify your local ntp
pool. You should select at least four sources. I have
about eight different sources split between
ntp0.lordynet.org and ntp1.lordynet.org.
David
--
(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
