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NTP and GPS
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Miguel Gonçalves
Apr 30, 2013, 5:08:14 PM4/30/13
to
Hello fellow NTP users!
I have a few topics I would like to discuss with you guys. I describe my
configuration first and then I wrote some questions. This is a lengthy
message but I believe that some topics are interesting for other users.
In my network I have 2 dedicated low powered i386 boxes with GPS receivers
from different brands (Garmin 18 LVC and Sure Electronics GPS Evaluation
Board).
At the moment both servers only have their GPS clock as sole reference.
I have a Cisco router that gets the time from these two local stratum 1
servers and from more 5 stratum 2 servers (close to me) on the Internet. I
know routers aren't good timekeepers but this is intended as a backup as
you'll see further down.
Every device in my network runs NTP and requests the time from the 2
stratum 1 servers and from the Cisco router.
Now the questions...
If one of the stratum 1 goes nuts the Cisco will mark it as a falseticker
and discard it's data, right? I believe the Cisco will then choose the
other stratum 1 to sync from. Right?
If the Internet connection is overloaded (not usual here) the Cisco will
choose predominately the stratum 1 servers. Right?
Is this circular topology (a client using a server that in turn references
the other two sources the client is using) a good idea?
I know a better approach would be to add another stratum 1 (having 3 in
total) and ignore completely the Internet servers. Can I really get away
with the above configuration? I am not looking for the ultimate precision
but I intend to use these two receivers I have to their fullest.
How good is NTP detecting a problem with a reference clock? Let's imagine
there's a problem with one particular satellite and makes the time jump a
few mili-seconds. With only one reference clock will NTP follow blindly or
ignore it? Adding another GPS would be pointless but a radio receiver would
be OK I guess. The problem is that I don't have any radio signals in my
country (I could get a GLONASS receiver). GPS receivers designed for timing
usually employ the TRAIM algorithm and can discard bad data. Does NTP do
something like this? I mean, if my NTP is synched and running fine I would
expect NTP to discard obviously bad data. OTOH, temperature can have a huge
impact on this and NTP doesn't know how hot the server is running.
In the end, I might buy a Meinberg NTP server for my network and have 3
sources and then I will be able to completely ignore the Internet servers
and validate my servers among each other.
Thanks for reading and I will appreciate your contributions.
Kind regards,
Miguel
I have a few topics I would like to discuss with you guys. I describe my
configuration first and then I wrote some questions. This is a lengthy
message but I believe that some topics are interesting for other users.
In my network I have 2 dedicated low powered i386 boxes with GPS receivers
from different brands (Garmin 18 LVC and Sure Electronics GPS Evaluation
Board).
At the moment both servers only have their GPS clock as sole reference.
I have a Cisco router that gets the time from these two local stratum 1
servers and from more 5 stratum 2 servers (close to me) on the Internet. I
know routers aren't good timekeepers but this is intended as a backup as
you'll see further down.
Every device in my network runs NTP and requests the time from the 2
stratum 1 servers and from the Cisco router.
Now the questions...
If one of the stratum 1 goes nuts the Cisco will mark it as a falseticker
and discard it's data, right? I believe the Cisco will then choose the
other stratum 1 to sync from. Right?
If the Internet connection is overloaded (not usual here) the Cisco will
choose predominately the stratum 1 servers. Right?
Is this circular topology (a client using a server that in turn references
the other two sources the client is using) a good idea?
I know a better approach would be to add another stratum 1 (having 3 in
total) and ignore completely the Internet servers. Can I really get away
with the above configuration? I am not looking for the ultimate precision
but I intend to use these two receivers I have to their fullest.
How good is NTP detecting a problem with a reference clock? Let's imagine
there's a problem with one particular satellite and makes the time jump a
few mili-seconds. With only one reference clock will NTP follow blindly or
ignore it? Adding another GPS would be pointless but a radio receiver would
be OK I guess. The problem is that I don't have any radio signals in my
country (I could get a GLONASS receiver). GPS receivers designed for timing
usually employ the TRAIM algorithm and can discard bad data. Does NTP do
something like this? I mean, if my NTP is synched and running fine I would
expect NTP to discard obviously bad data. OTOH, temperature can have a huge
impact on this and NTP doesn't know how hot the server is running.
In the end, I might buy a Meinberg NTP server for my network and have 3
sources and then I will be able to completely ignore the Internet servers
and validate my servers among each other.
Thanks for reading and I will appreciate your contributions.
Kind regards,
Miguel
unruh
May 1, 2013, 2:00:55 AM5/1/13
to
On 2013-04-30, Miguel Gon?alves <miguel.barbo...@gmail.com> wrote:
> Hello fellow NTP users!
>
> I have a few topics I would like to discuss with you guys. I describe my
> configuration first and then I wrote some questions. This is a lengthy
> message but I believe that some topics are interesting for other users.
>
> In my network I have 2 dedicated low powered i386 boxes with GPS receivers
> from different brands (Garmin 18 LVC and Sure Electronics GPS Evaluation
> Board).
>
> At the moment both servers only have their GPS clock as sole reference.
>
> I have a Cisco router that gets the time from these two local stratum 1
> servers and from more 5 stratum 2 servers (close to me) on the Internet. I
> know routers aren't good timekeepers but this is intended as a backup as
> you'll see further down.
>
> Every device in my network runs NTP and requests the time from the 2
> stratum 1 servers and from the Cisco router.
>
> Now the questions...
>
> If one of the stratum 1 goes nuts the Cisco will mark it as a falseticker
> and discard it's data, right? I believe the Cisco will then choose the
> other stratum 1 to sync from. Right?
Nope. It has no idea which the correct clock is. If one disappears
> Hello fellow NTP users!
>
> I have a few topics I would like to discuss with you guys. I describe my
> configuration first and then I wrote some questions. This is a lengthy
> message but I believe that some topics are interesting for other users.
>
> In my network I have 2 dedicated low powered i386 boxes with GPS receivers
> from different brands (Garmin 18 LVC and Sure Electronics GPS Evaluation
> Board).
>
> At the moment both servers only have their GPS clock as sole reference.
>
> I have a Cisco router that gets the time from these two local stratum 1
> servers and from more 5 stratum 2 servers (close to me) on the Internet. I
> know routers aren't good timekeepers but this is intended as a backup as
> you'll see further down.
>
> Every device in my network runs NTP and requests the time from the 2
> stratum 1 servers and from the Cisco router.
>
> Now the questions...
>
> If one of the stratum 1 goes nuts the Cisco will mark it as a falseticker
> and discard it's data, right? I believe the Cisco will then choose the
> other stratum 1 to sync from. Right?
entirely, then it will use the other, but if one wanders off into the
wild blue yonder, then ntp has no idea which one it is. That is why you
need three servers, so two can outvote the bad one. One of them could
even be a network server (pool server) which will not be as accurate,
but will at least tell which of the two has gone crazy.
>
> If the Internet connection is overloaded (not usual here) the Cisco will
> choose predominately the stratum 1 servers. Right?
>
> Is this circular topology (a client using a server that in turn references
> the other two sources the client is using) a good idea?
>
> I know a better approach would be to add another stratum 1 (having 3 in
> total) and ignore completely the Internet servers. Can I really get away
> with the above configuration? I am not looking for the ultimate precision
> but I intend to use these two receivers I have to their fullest.
>
> How good is NTP detecting a problem with a reference clock? Let's imagine
> there's a problem with one particular satellite and makes the time jump a
> few mili-seconds. With only one reference clock will NTP follow blindly or
monitored.
> ignore it? Adding another GPS would be pointless but a radio receiver would
> be OK I guess. The problem is that I don't have any radio signals in my
> country (I could get a GLONASS receiver). GPS receivers designed for timing
> usually employ the TRAIM algorithm and can discard bad data. Does NTP do
> something like this? I mean, if my NTP is synched and running fine I would
> expect NTP to discard obviously bad data. OTOH, temperature can have a huge
> impact on this and NTP doesn't know how hot the server is running.
world and you will have far worse problems than worrying about the time
on your machines.
On the other hand, your receivers going down are much more likely (My
two GPS18 have both died after three years. My two sure's one died
because the antenna was crap and died. I have to find a place to get a
new, cheap antenna.
ntpd will more or less take care (slowly) if the servers get hot. It
cannot do anything about about the server dying of course.
>
> In the end, I might buy a Meinberg NTP server for my network and have 3
> sources and then I will be able to completely ignore the Internet servers
> and validate my servers among each other.
David Woolley
May 1, 2013, 3:11:18 AM5/1/13
to
Miguel Gonçalves wrote:
>
> If one of the stratum 1 goes nuts the Cisco will mark it as a falseticker
> and discard it's data, right? I believe the Cisco will then choose the
> other stratum 1 to sync from. Right?
Noting the lack of sufficient votes for a falseticker, already
>
> If one of the stratum 1 goes nuts the Cisco will mark it as a falseticker
> and discard it's data, right? I believe the Cisco will then choose the
> other stratum 1 to sync from. Right?
mentioned. ntpd will normally synchronise to both stratum one servers,
and, with a lower weighting, to any higher stratum servers, although it
will only report the error statistics and source identity of one of them.
>
> If the Internet connection is overloaded (not usual here) the Cisco will
> choose predominately the stratum 1 servers. Right?
>
> Is this circular topology (a client using a server that in turn references
> the other two sources the client is using) a good idea?
>
> I know a better approach would be to add another stratum 1 (having 3 in
> total) and ignore completely the Internet servers. Can I really get away
> with the above configuration? I am not looking for the ultimate precision
> but I intend to use these two receivers I have to their fullest.
>
> How good is NTP detecting a problem with a reference clock? Let's imagine
> there's a problem with one particular satellite and makes the time jump a
in pre-surveyed mode, a time receiver could use just one.
> few mili-seconds. With only one reference clock will NTP follow blindly or
> ignore it? Adding another GPS would be pointless but a radio receiver would
1 millisecond error would still be within the allowable error bounds,
even without the increases to those error bounds to allow for time since
last measurement and network propagation delays, so it is very unlikely
that a clock that differed by only or two milliseconds.
In any case, if the radio clock reports a fault, the stratum one server
will continue to serve time, but with an increasing root dispersion. It
will get ignored when that root dispersion gets too high.
Miguel Gonçalves
May 3, 2013, 6:47:39 AM5/3/13
to
Hi!
Many thanks for your suggestions!
In the meantime I've just bought a Trimble Acutime Gold and used a
RS422-to-RS232 converter and apparently it is working fine on a FreeBSD 8.3
machine (NTP 4.2.7p367).
I am getting a bit of jitter on the serial port. On FreeBSD 7 there was the
possibility of disabling the UART's FIFO but I can't seem to find how to do
it on FreeBSD 8.
Any help would be greatly appreciated.
Here is some more relevant information about the machine and set up:
--- /etc/ntp.conf ---
server a.b.c.d iburst
server k.l.m.n iburst
server 127.127.29.0 mode 3
fudge 127.127.29.0 minpoll 4 maxpoll 4
fudge 127.127.29.0 flag2 0
statsdir /var/log/ntp/
filegen peerstats file peers type day link enable
filegen loopstats file loop type day link enable
filegen clockstats file clock type day link enable
--- /etc/ntp.conf ---
acutime# ntpq -p
remote refid st t when poll reach delay offset
jitter
==============================================================================
*GPS_PALISADE(0) .GPS. 0 l 2 16 377 0.000 -0.248
0.034
+a.b.c.d w.x.y.z 2 u 51 64 377 10.941 -0.520
0.425
+k.l.m.n w.x.y.z 2 u 51 64 377 9.201 -1.984
0.410
--- /var/log/ntp/loop ---
56415 38463.327 -0.000358705 144.772 0.000047732 0.005580 4
56415 38479.326 -0.000290738 144.768 0.000050705 0.005449 4
56415 38495.326 -0.000327819 144.763 0.000049209 0.005396 4
56415 38511.327 -0.000327237 144.758 0.000046050 0.005347 4
56415 38527.327 -0.000257539 144.754 0.000049626 0.005191 4
56415 38543.327 -0.000308850 144.749 0.000049840 0.005134 4
56415 38559.326 -0.000247656 144.745 0.000051397 0.004984 4
56415 38575.327 -0.000247452 144.742 0.000048096 0.004850 4
56415 38591.327 -0.000281144 144.737 0.000046540 0.004784 4
56415 38607.327 -0.000318896 144.732 0.000045534 0.004794 4
56415 38623.327 -0.000256571 144.729 0.000047956 0.004694 4
56415 38639.326 -0.000294313 144.724 0.000046801 0.004669 4
56415 38655.327 -0.000294382 144.720 0.000043799 0.004646 4
56415 38671.327 -0.000228657 144.716 0.000047101 0.004518 4
56415 38687.327 -0.000268336 144.712 0.000046239 0.004467 4
56415 38703.327 -0.000301464 144.707 0.000044810 0.004485 4
56415 38719.326 -0.000236024 144.704 0.000047878 0.004384 4
56415 38735.327 -0.000244381 144.700 0.000044883 0.004307 4
56415 38751.327 -0.000279280 144.696 0.000043759 0.004301 4
56415 38767.327 -0.000237060 144.692 0.000043570 0.004221 4
--- /var/log/ntp/loop ---
What can I do to improve this?
Thanks in advance for any help!
Kind regards,
Miguel
> ______________________________**_________________
> questions mailing list
> ques...@lists.ntp.org
> http://lists.ntp.org/listinfo/**questions<http://lists.ntp.org/listinfo/questions>
>
Many thanks for your suggestions!
In the meantime I've just bought a Trimble Acutime Gold and used a
RS422-to-RS232 converter and apparently it is working fine on a FreeBSD 8.3
machine (NTP 4.2.7p367).
I am getting a bit of jitter on the serial port. On FreeBSD 7 there was the
possibility of disabling the UART's FIFO but I can't seem to find how to do
it on FreeBSD 8.
Any help would be greatly appreciated.
Here is some more relevant information about the machine and set up:
--- /etc/ntp.conf ---
server a.b.c.d iburst
server k.l.m.n iburst
server 127.127.29.0 mode 3
fudge 127.127.29.0 minpoll 4 maxpoll 4
fudge 127.127.29.0 flag2 0
statsdir /var/log/ntp/
filegen peerstats file peers type day link enable
filegen loopstats file loop type day link enable
filegen clockstats file clock type day link enable
--- /etc/ntp.conf ---
acutime# ntpq -p
remote refid st t when poll reach delay offset
jitter
==============================================================================
*GPS_PALISADE(0) .GPS. 0 l 2 16 377 0.000 -0.248
0.034
+a.b.c.d w.x.y.z 2 u 51 64 377 10.941 -0.520
0.425
+k.l.m.n w.x.y.z 2 u 51 64 377 9.201 -1.984
0.410
--- /var/log/ntp/loop ---
56415 38463.327 -0.000358705 144.772 0.000047732 0.005580 4
56415 38479.326 -0.000290738 144.768 0.000050705 0.005449 4
56415 38495.326 -0.000327819 144.763 0.000049209 0.005396 4
56415 38511.327 -0.000327237 144.758 0.000046050 0.005347 4
56415 38527.327 -0.000257539 144.754 0.000049626 0.005191 4
56415 38543.327 -0.000308850 144.749 0.000049840 0.005134 4
56415 38559.326 -0.000247656 144.745 0.000051397 0.004984 4
56415 38575.327 -0.000247452 144.742 0.000048096 0.004850 4
56415 38591.327 -0.000281144 144.737 0.000046540 0.004784 4
56415 38607.327 -0.000318896 144.732 0.000045534 0.004794 4
56415 38623.327 -0.000256571 144.729 0.000047956 0.004694 4
56415 38639.326 -0.000294313 144.724 0.000046801 0.004669 4
56415 38655.327 -0.000294382 144.720 0.000043799 0.004646 4
56415 38671.327 -0.000228657 144.716 0.000047101 0.004518 4
56415 38687.327 -0.000268336 144.712 0.000046239 0.004467 4
56415 38703.327 -0.000301464 144.707 0.000044810 0.004485 4
56415 38719.326 -0.000236024 144.704 0.000047878 0.004384 4
56415 38735.327 -0.000244381 144.700 0.000044883 0.004307 4
56415 38751.327 -0.000279280 144.696 0.000043759 0.004301 4
56415 38767.327 -0.000237060 144.692 0.000043570 0.004221 4
--- /var/log/ntp/loop ---
What can I do to improve this?
Thanks in advance for any help!
Kind regards,
Miguel
> questions mailing list
> ques...@lists.ntp.org
> http://lists.ntp.org/listinfo/**questions<http://lists.ntp.org/listinfo/questions>
>
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