Ron Frazier (NTP) wrote:
On 3/20/2012 2:25 AM, David J Taylor wrote:
"unruh" <un...@invalid.ca> wrote in message
news:JDU9r.22132$_c5.11...@newsfe09.iad...
[]
Of course the question still is why in the world did the system go nuts
when it was on Local. That itself should not have happened.
If some software had told the system clock to run fast, it simply
stays running fast, even on Local.
Ron is using a single GPS device, over USB, without the backup of a
few Internet servers to stop such a thing happening, and the GPS has
already shown itself to be problematical. NTP would normally have
simply rejected the errant GPS data and not cause the PC clock to run
wild, but without the Internet servers as backup, what is NTP to do?
I don't think it has a choice other than to believe the GPS, even if
it's incorrect or faulty.
Ron, perhaps in the future you could adopt a similar configuration to
one I've mentioned before - add some Internet servers with a long
polling interval as a second opinion for NTP:
_________________________________________
server <ref clock stuff>
server 0.us.pool.ntp.org minpoll 10 iburst
server 1.us.pool.ntp.org minpoll 10 iburst
server 0.uk.pool.ntp.org minpoll 10 iburst
server 1.uk.pool.ntp.org minpoll 10 iburst
_________________________________________
using servers [network] local to yourself, of course.
Cheers,
David
Hi David T,
Eventually, I do plan to have the server preferences as follows:
Time server machine:
GPS
Internet as backup
Hypothetically speaking, what if I don't want it to distribute time if
it's working in internet mode?
Easy, configure it that way.
Non time server machines
GPS (if attached)
Local time server (if available)
Internet as backup
However, I only plan to do that after thoroughly testing the GPS by
itself for a week or two to see if it's stable. I originally had the
internet servers on with this unit. It completely surprised me by
having this tendency to drift apparently and have periodic heart
attacks. Unfortunately, this odd behavior may exist in all SIRF III and
possibly other SIRF units. It was only by turning off the internet
servers that I was able to get some clean graphs of exactly what the GPS
was doing. When I had the internet servers enabled, once the GPS
starting acting odd, which it shouldn't do at all, NTPD would clock hop
NMEA gives me around +/- 10ms mean, 20-30ms rms and
40-80ms maximum
PPS gives me around 0.000ms mean, 0.004ms rms and
0.015-0.035ms maximum
Attempting to compare time vs NMEA offset doesn't get
you anywhere.
There are more productive ways of spending your time
such as sorting out your wireless network or having a
wired link to one pc to confirm if the delays you're
seeing are due to the wireless network or your provider.
David
to the internet. Normally, that would be OK. However, as discussed
previously, even my errant GPS is more accurate over the short term than
the internet for me. With the internet conection, I get + / - 50 ms
variations in time over a span of an our. With the GPS, I get + / - 60
ms variations over several days, with a few wild corrections during its
heart attacks. Those are two bad choices, but I think I'd still rather
run on the GPS. The only way I can prevent clock hopping is by
noselecting the internet servers. Even if I end up with internet
servers turned on, which I expect to, I think it's much better to know
about these GPS anomalies before putting it into long term service.
Anybody considering using a SIRF III or maybe even any SIRF unit for
timekeeping should be warned by my experience, test the unit, and make
sure it's up to the task. These problems could even affect SIRF units
with PPS outputs, although I don't know. I'll probably decommission
this unit from timekeeping duty and relegate it to navigation duty,
although I'm not sure how trustworthy it is for that when it's throwing
a temper tantrum.
I've already committed to getting better (hopefully) equipment.
(Shipping from Hong Kong or where ever seems to take a LONG time when
you're waiting on something.) Hopefully, the Sure board will be much
more stable and reliable. I'm planning to do the same extensive testing
on the Sure for a week or two. I'll start out just plugging the Sure
into my serial - USB converter using the same com port as the Globalsat
unit was running on. I want to see how it does with NMEA only data for
a while. I'm hoping NOT to see substantial drifting from UTC and NOT to
see any heart attacks every few days. I expect lots of jitter, since a
number of variable length sentences are being output. Then, I plan to
turn off all but GPGGA and test some more, and maybe tinker with the
baud rate. Then, if I can solder the board without killing it, I'll
engage PPS through the serial - USB port and test that for a while.
Then, I'll try it with PPS and real serial on my other computer, the
only one with a serial port.
Hopefully, when I'm done, I'll have a qualified unit running stably and
accurately for the whole network to use. I've acquired a case and some
hardware to mount the device similar to yours. Once I learned that it
was only 3" x 3", that made me nervous as far as soldering and all, but
we'll see what happens.
By the way, do you think I should update to Dave H's latest binaries?
I'm at 4.2.7p259 on Windows. Almost all these discussions have been
about Windows. Linux is a whole other ballgame. The NTPD there from
the repositories is about 2 years old, and I'm reluctant to go outside
the repositories because of the numerous problems it creates. One very
serious Linux user on a local message board said even he doesn't compile
his own programs because of possible problems. I tried it once and all
sorts of scripts and file locations that Ubuntu expects got broken.
Sincerely,
Ron
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