[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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