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

unruh unruh at wormhole.physics.ubc.ca
Tue Apr 13 16:54:25 UTC 2010

On 2010-04-13, Rob <nomail at example.com> wrote:
> unruh <unruh at wormhole.physics.ubc.ca> wrote:
>> On 2010-04-13, lhommedumatch <ludocluba at yahoo.com> wrote:
>>> On 13 avr, 00:48, unruh <un... at wormhole.physics.ubc.ca> wrote:
>>>> On 2010-04-12, lhommedumatch <ludocl... at yahoo.com> wrote:
>>>> > I'm working on a oceanographic vessel
>>>> > ntpgmtaceb is our reference clock that is synchronised with a gps at
>>>> > the beginning of a cruise.
>>>> Why not put a Garmin GPS 18 ?on board the ship and get gps time on the
>>>> whole cruise?
>>> That's our policy. I can't decide for that.
>> If time is important then those who can decide need to be informed. If
>> it is not important, why worry about 1ms. Esp as over the course of a
>> cruise the time is apt to be out by seconds, not ms. 
> In many applications, it is more important that all devices within
> some environment agree upon the same standard time, than that this
> standard time is within a small offset of true atomic time.
> This has been discussed before here, but you don't seem to get it.
> He may well want all his computers to be within a millisecond of the
> same time reference, but may not at all care what that reference is.
> There are many applications where this is a reasonable expectation.

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. 

Now if what he wants is to sync all of the computers on board the vessel
to a single time source, not matter how bad that time source is, then I
agree, that suggesting a gps is largely irrelevant ( except it can be
used to make his single source a good time source rather than a terrible
one). There are a number of procedures for synching a large number of
systems. But that was NOT what he asked about. You may well be a better
mind reader than I am, however. 

> Suggesting a Garmin GPS to be added to each and every configuration
> brought up by someone is really not adding much.  When it is not there,
> it is not there.  And the configuration may still be very useful.

Of course. That is why it is important for people to ask questions based
on what they need, rather than on what they think is the solution to
their need. 

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