[ntp:questions] garmin 18x and linux

unruh unruh at wormhole.physics.ubc.ca
Sat Sep 3 14:35:37 UTC 2011

On 2011-09-03, David Woolley <david at ex.djwhome.demon.invalid> wrote:
> unruh wrote:
>> ?? Where does this come from? While it is true that there is the 128ms
>> rule ( which I believe can be adjusted) but where is this coming from?
>> And besides, Garmin GPS nmea can control to better than 10ms.
> Basically the 128ms, but one doesn't want to anywhere close to that 
> 128ms.  Note I said a few tens, which could be say 40-50.

I believe that the 128ms is user adjustable. 

> I was replying to someone that said they only wanted time to 1 second 
> and I was really pointing out that there can be time sources that would 
> satisfy that requirement, but would be unsuitable for use with NTP, so 
> by asking on an NTP newsgroup, they are effectively asking for times 
> that are significantly better than 128ms.

Which the nmea sentence from a gps certainly should be. 
I also believe it takes a few 128ms before ntp will step. It would be
grossly irresponsible if it stepped each time it got a 128ms (just
imagine the havoc when a single return packet was delayed by 1 second,
which can happen). Thus that time source would have to have a persistant
jitter of greater than 128ms. Ie, while you concern may be right it is
also irrelevant to the discussion here. 

Can one use a Garmin 18x nmea and ntp to control the clock to better
than 1 second? Yes.

Also note that this is an ntp newsgroup, NOT an ntpd newsgroup. Ie,
there exist many implimentations of ntp which are not ntpd and this is
also the newsgroup for them. Many of them do NOT have the 128ms rule,
which has always struck me a totally bizarre anyway ("We have a 500PPM
max rule, except when we run the system at infinite slow rate.")

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