[ntp:questions] Accuracy of GPS device
unruh at wormhole.physics.ubc.ca
Fri Sep 2 06:56:28 UTC 2011
On 2011-09-02, Miguel Gon?alves <mail at miguelgoncalves.com> wrote:
> Comments bellow...
> On 2 September 2011 00:47, unruh <unruh at wormhole.physics.ubc.ca> wrote:
>> On 2011-09-01, Miguel Gon?alves <mail at miguelgoncalves.com> wrote:
>> > Hi!
>> > Thanks for your reply. My comments bellow.
>> > On 1 September 2011 18:24, unruh <unruh at wormhole.physics.ubc.ca> wrote:
>> >> On 2011-09-01, Miguel Gon?alves <mail at miguelgoncalves.com> wrote:
>> >> > Hi all!
>> >> >
>> >> > I have two internal FreeBSD with GPS receivers attached (Garmin 18
>> >> > 10.0.2.10 / Sure Evaluation Board:10.0.2.9). Both machines are on the
>> >> same
>> >> > LAN segment (VLAN).
>> >> >
>> >> > For redundancy, I've configured a Cisco switch as a stratum 2 server.
>> >> Here's
>> >> > the relevant information:
>> >> >
>> >> > $ ntpq -pcrv 10.0.2.254
>> >> > remote refid st t when poll reach delay offset
>> >> > jitter
>> >> > +ntp0.as34288.ne .PPS. 1 u 814 1024 377 72.750
>> >> > 0.780
>> >> > +canon.inria.fr .GPSi. 1 u 399 1024 377 55.110
>> >> > 0.400
>> >> What are those machines? You have names rather than IP addresses.
>> >> Are they your pps machines?
>> > No. This is a stratum 2 server and it gets the time from stratum 1
>> > thus the names and not IP addresses.
>> What I am asking is what the mapping is between these names and the
>> numbers you have later. I assume that some of those names are the same
>> machine as the IP addresses you list below but We do not have that
> Which numbers?
> 10.0.2.254 is a stratum 2 server that gets time information from outside
> servers (I've included the information). canon.inria.fr is a server in
> France that has a GPS clock receiver AFAIK.
> 10.0.2.254 is just a Cisco switch that I set up internally for comparison to
> the the stratum 1 servers (10.0.2.9 and 10.0.2.10).
> 10.0.2.2 is another server (Linux, CentOS) that gets time from 10.0.2.9,
> 10.0.2.10 and 10.0.2.254. I've written all this before. I thought I was
> clear enough.
It is completely unclear to me what your question is. Your 10.0.2.254 is
an outside switch.
>> > $ ntpq -p 10.0.99.99
>> > remote refid st t when poll reach delay offset
>> > jitter
>> > *10.0.2.10 .GPS. 1 u 21 256 377 0.173 0.196
>> > 0.008
>> > +10.0.2.9 .GPS. 1 u 93 256 377 0.175 0.191
>> > 0.014
>> > +10.0.2.254 184.108.40.206 2 u 149 256 377 0.583 -6.884
>> > 0.152
This tells me that your two GPS receivers are consistant with each
other, but I have no idea why the offset is larger than the delay, and
why the offsets are so large. On a lan, the offsets should be a factor
of 20 or so less than what you are getting here.
That the external router is 7 ms out just tells me that it is really
poorly synced with the outside world.
>> > This is a FreeBSD embedded PBX machine running Asterisk. The machine is
>> > mostly idle. What kind of offsets should I get with local machines?
>> in the 10s of usec range max. Certainly less than the delay.
> tens of usec is good... Anyone here which can explain why NTP isn't getting
How could we?
Maybe you are running a virtual BSD machine, and thus the clocks are
wonkey. Maybe you have lousy hardware. Who knows.
>> > Assuming ntp04, ntp05 and ntp06 are on the same LAN I see offsets higher
>> > than 100 us. Is it possible to decrease these numbers?
>> Sure. all my systems have offsets in the 10us range-- on the same lan
>> as my time server.
>> Mind you I do use chrony, not ntpd but even ntpd should be in a few 10s
>> of usec.
> Can ntpd really get there? I'll try to query some good public servers to see
> what others are getting...
Sure it can. It can get better than 30us. But why you are not getting it
is impossible for us to say.
>> All my machines are Linux machines. Linux is fine for timekeeping.
> OK. No criticism intended. I'm just a BSD guy.
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