[ntp:questions] Accuracy of GPS device

unruh unruh at wormhole.physics.ubc.ca
Fri Sep 2 16:25:36 UTC 2011


On 2011-09-02, Miguel Gon?alves <mail at miguelgoncalves.com> wrote:
>
> Very good point really. This is probably why the FreeBSD machine performs
> better than the Linux one. The FreeBSD is embedded and doesn't produce a lot
> of heat while the Linux one is a Dell regular server. Perhaps removing the
> lid on the Linux server will improve things.

Most systems have temperature sensors on the motherboard or on the cpu.
Keep track of the temperature to see if it is temp problems. 


>
>
>>> 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.
>>>
>>
>> It probably means there is jitter in the time from the servers.  Offset
>> doesn't measure the error in the internal time, it measures the estimated
>> instantaneous error in the measurements of that time.  A large error in the
>> measurement will produce a proportionate, but smaller error in the actual
>> time.
>
>
> OK. So I should look at ntptime? Or ntp -c kerninfo? I tried

You can keep track of the offset over time. That will give you a
measure. It is archived in the peerstats file-- the measured offset
against each peer at every measurement. 


>
> $ ntpq -p 10.0.2.2; ntpdc -c kerninfo 10.0.2.2
>      remote           refid      st t when poll reach   delay   offset
>  jitter
>==============================================================================
> +10.0.2.10       .GPS.            1 u  734 1024  377    0.182   -0.149
> 0.034
> *10.0.2.9        .GPS.            1 u  235 1024  377    0.163   -0.055
> 0.029
> pll offset:           -8.5e-05 s
> pll frequency:        44.545 ppm
> maximum error:        0.151092 s
> estimated error:      3.8e-05 s
> status:               0001
> pll time constant:    6
> precision:            1e-06 s
> frequency tolerance:  512 ppm
>
> So, despite 149 and 55 us I should consider the 38 us estimated error?
>
> For the FreeBSD PBX server
>
> asterisk# ntpq -pn ; ntpdc -c kerninfo
>      remote           refid      st t when poll reach   delay   offset
>  jitter
>==============================================================================
> +10.0.2.10       .GPS.            1 u  843 1024  377    0.202    0.033
> 0.005
> *10.0.2.9        .GPS.            1 u  991 1024  377    0.186    0.029
> 0.017
> pll offset:           2.4324e-05 s
> pll frequency:        151.490 ppm
> maximum error:        1.03863 s
> estimated error:      2.1e-05 s
> status:               2001  pll nano
> pll time constant:    10
> precision:            1e-09 s
> frequency tolerance:  496 ppm
>
> BTW, what is the difference between 2001 pll nano and 0001 in the status?
>
> Thanks a lot David! You were very helpful!




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