On 3/20/2012 5:19 PM, David Lord wrote:
Ron Frazier (NTP) wrote:
Hi David L,
See below.
On 3/20/2012 1:00 PM, David Lord wrote:
Ron Frazier (NTP) wrote:
....
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.
I'm not sure how to do that within the confines of ntp.conf. David
T. suggested I could run a Perl script every minute to shut down NTP
if the GPS fails. But, I'd rather keep NTP running and just not
distribute time on the LAN when my time server is polling the
internet. Which brings up a question. If my time server on my LAN
is attached to the GPS, that GPS is considered stratum zero and my
time server on the LAN appears to be a stratum 1 device to other
computers, right? Then, what if the time server stops using the GPS
and begins using internet stratum 2 servers as it's time source?
Does my LAN time server now present itself as a stratum 3 device to
the other PC's on the LAN? If so, they might automatically stop
using it and poll the internet stratum 2 servers themselves. That
would be fine.
"ntpd" is not designed to be restarted often, either by
scripts or by reboot to another operating system. If that
is your requirement use ntpdate to get you perhaps within
a second.
Ntp.conf provides options to limit responses from ntpd.
That begs the question, was your ntpd answering requests
when your server was 50 seconds out?
I don't currently have a server set up. I'm only testing on the one
computer at the moment.
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
The first thing a timekeeping newbie like me hears when asking about
accurate PC timekeeping is "go hang a GPS on it". So, back when I
started experimenting in January, and without the benefit of 2 months
of banging my head on this wall and all these discussions, I go to
Amazon, find a cheap GPS with good reviews, and buy it. I hook it
up, configure NTP, and start getting + / - 10 ms of offsets of the PC
versus GPS time. Since I think GPS time = UTC time, for most
practical purposes, I'm happy. For my particular purposes, + / - 10
ms is fine. I plan to
Your perception of GPS time was understandably wrong.
pursue PPS mostly for learning reasons, but I don't have to have
microsecond level accuracy. I just want my PC's clocks to be right
to less than 500 ms but would really prefer less than 10 ms. I also
want to be doing better than the + / - 50 ms I'm getting from the
internet at the moment. Another goal of having the GPS was possibly
to poll the internet less often. Now, only after 2 months of head
banging and discussions, do I find that the NMEA data is wandering.
My NMEA time data wanders.
It is not broken, the NMEA spec is for the time data to be
accurate to within one second.
If you require millisecond offset from GPS you need to use
PPS which will give timing within a few microseconds.
Otherwise if you still want to use NMEA rather than internet
servers you need to configure ntpd to accept the larger
variations in offset without dumping the source as
false-ticker.
David
So, since I could live with NMEA only if it didn't wander, even
though it only give + / - 10 ms accuracy, the question on my mind is:
Does only this GPS wander? Evidence is no. Others wander. David T
mentioned a wandering Garmin and someone on another list mentioned
another SIRF unit that does it?
Do all SIRF GPS's wander? Very possible, but evidence that I have is
inconclusive.
Do all NMEA outputs on all GPS's everywhere wander? I don't know
about that.
I think it's important to have this discussion, and I think it's
important that it goes on these public lists. If it turns out to be
true that almost no GPS with NMEA only output, or with PPS but that's
not used, will ever provide more than 100 ms peak - peak accuracy,
then that's important to know. Now, if that 100 ms performance is
consistent, then it may be adequate for some applications. It would
even be adequate for mine, if it weren't for the darned heart attacks
this particular unit seems to have. However, 100 ms doesn't get me
anything that I can't get with internet, so why bother? It could
still be used in places where internet connectivity is not available
or is intermittent.
If, on the other hand, there are some GPS's with NMEA only output
that can consistently provide + / - 20 ms accuracy or so, then I
think that's important info too.
Keep in mind, many computers don't have serial and never will, other
than serial - USB converters.
It's good that we get the information out there so other newbies
researching the issue can zone in on what's possible to do.
Speaking only regarding Windows systems for a moment:
So, let's say a person needs + / - 100 ms performance from UTC. He
can use the internet, even with wireless, and multiple routers, like
I have. Or he could use even a USB only GPS if there is a reason.
Say he needs + / - 20 ms performance. Internet may be an option
depending on the circumstances. GPS might be a better option
depending on the GPS.
Say he needs + / - 1 ms performance. Internet is probably not an
option, at least in my experience and my location. USB only GPS with
PPS through the serial - USB converter might work. Serial GPS with
PPS would work.
Say he needs + / - 20 us performance. Excluding the realm of
rubidium standards and such, serial GPS with PPS is probably the only
option.
So, the more this information is threshed out and clarified, the more
I can evaluate what options I have for my minimal needs, and the
better I can point someone in the right direction if I'm asked about
it, either professionally or personally. Also, the more other
newbies starting to explore such things will be able to make the
right decision with less headaches. If there were a USB GPS with PPS
support through DCD messges was already available, one such as myself
could jump right to the + / - 1 ms realm, and forget the rest unless
he need to go to + / - 20 us. That's what the people on the
thumbgps-devel group (Eric Raymond et al) are working on.
Sincerely,
Ron
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
--
(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
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