[ntp:questions] Accuracy of GPS device
unruh
unruh at wormhole.physics.ubc.ca
Thu Sep 1 23:47:11 UTC 2011
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 LVC:
>> > 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 -1.084
>> > 0.780
>> > +canon.inria.fr .GPSi. 1 u 399 1024 377 55.110 0.218
>> > 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 servers
> 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
infomation.
>
> > I have another machine (Linux, CentOS 5.6) that is a client to these
>> stratum
>> > 1 FreeBSD machines. Here's the relevant information:
>> >
>> > $ ntpq -pcrv 10.0.2.2
>> > remote refid st t when poll reach delay offset
>> > jitter
>>
>> >==============================================================================
>> > +10.0.2.10 .GPS. 1 u 211 256 377 0.159 -0.139
>> > 0.350
>> > *10.0.2.9 .GPS. 1 u 71 256 377 0.166 -0.136
>> > 0.468
>>
>> That is a huge offset for being on the same lan, and for being only
>> .15ms away.
>
>
> It's really strange... I am getting on another LAN connected to this one
> these values...
>
> $ 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 81.94.123.16 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.
>
> Here in Portugal our Time Dissemination Authority has two stratum 2 servers.
> One of them shows this:
>
> $ ntpq -p ntp02.oal.ul.pt
> remote refid st t when poll reach delay offset
> jitter
>==============================================================================
> *ntp01.oal.ul.pt .GPS. 1 u 113 128 377 5.125 -0.263
> 0.320
> ntp03.oal.ul.pt .INIT. 16 u - 1024 0 0.000 0.000
> 0.000
> -ntp04.oal.ul.pt 194.117.9.138 2 u 57 128 377 0.377 -0.056
> 0.080
> +ntp05.oal.ul.pt .GPS. 1 u 62 128 377 0.296 0.128
> 0.058
> +ntp06.oal.ul.pt .IRIG. 1 u 56 128 377 0.310 0.104
> 0.045
>
> 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.
>
>> tick# ntpdate -p8 -q tock
>> > server 10.0.2.9, stratum 1, offset -0.000004, delay 0.02577
>> > 1 Sep 10:23:45 ntpdate[3537]: adjust time server 10.0.2.9 offset
>> -0.000004
>> > sec
>>
>> That probably says more about the symmetry of the path than the offset
>> of the machine.
>
>
> OK. But this is a 24 port Gigabit switch from Cisco. I wouldn't expect
> asymmetry but it could be.
>
> > Are my GPS clocks OK? Does this happen due to the network latency? Are my
>> > stratum 2 servers OK?
>>
>> Your GPS seems to be consistant with each other ( that is all one can
>> say without another time source to compare them to). Your offsets are
>> large compared with the delay times ( was the machine recently heated
>> up due to working harder?)
>
>
> All these machines sit in a room temperature controlled at 20 ?C. 10.0.2.2
> is a backup server that just does some work every hour but nothing huge.
>
> 10.0.2.2 has been running for quite a while and it doesn't seem to get lower
> offsets. Could it be because it's running Linux? I've heard Linux is not as
> good as FreeBSD for time keeping.
All my machines are Linux machines. Linux is fine for timekeeping.
>
> Thanks!
>
> Regards,
> Miguel
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