[ntp:questions] how to have offset < 1ms

unruh unruh at wormhole.physics.ubc.ca
Tue Apr 13 18:32:29 UTC 2010

On 2010-04-13, Rob <nomail at example.com> wrote:
> unruh <unruh at wormhole.physics.ubc.ca> wrote:
>> That may be what he wants, but it is not what he asked for. He stated
>> that he was using an external source as his ntp server, and wanted his
>> machine to be within a ms of it. That is a job admirably suited to a GPS
>> receiver. He then stated that this was on an oceanographic vessel which
>> was set at the beginning of a cruise. A gps receiver again would be
>> ideally suited to giving the time throught the cruise. Unless the cruise
>> is underwater, the gps signals would be available at all times during
>> the cruise. 
> It seems that you simply WANT to read that he needs a GPS.

No, I simply think that having a GPS source is by far the simplest
solution to the problem he seems to have. 
He wants ms accuracy, GPS will give him usec accuracy. At present his
network connection is flakey-- changing from <1ms delay to 30ms delay,
and almost certainly asymmetric. It is impossible to get ms accuracy
under those conditions. There are thus a numbr of solutions. One is to
install a gps. This is almost certainly the simplest. The next is to
figure out why his network connection is so flakey. If as has been
implied it is daemon.com who is providing attrocious adsl service, then
there is little he can do to fix this, other than perhaps get another
provider. If there is some other reason, it is not obvious. 

Of all of these possible solutions, installing a gps clock is by far the
easiest and most accurate. 

> It was only stated that he wanted to sync his system to a reference
> that is named ntpgmtaceb.  Apparently a system on the LAN, when
> reading the ntpq output he posted.  There never was any requirement
> to get sync to something else than that ntpgmtaceb.  The sync needed
> to be within 1ms, but unfortunately the path to system to sync to has
> a delay of 29ms.  This is a bit strange for a LAN.  What is also strange
> is that this delay sometimes is a lot lower.

Yes. So he can spend a lot of time tracking this down, and quite
possibly discovering it is the network provider that is at fault.
> Now this is the info we work from.  The addition of GPS as a requirement
> or solution is entirely by you.

Of course. That is the thing about solutions. You provide extra
information or hardware that is needed to impliment the solution. Now if
you have a better one, I am sure he would welcome it. But I have not
seen it. 

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