[ntp:questions] Accuracy of GPS device
mail at miguelgoncalves.com
Fri Sep 2 00:13:05 UTC 2011
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
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
> > $ 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
> > +10.0.2.254 126.96.36.199 2 u 149 256 377 0.583 -6.884
> > 0.152
> > 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
> > 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...
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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