[ntp:questions] Re: May ntp be off by a second in case of GPSsource?

Eric ejensen at spamcop.net
Fri Dec 30 20:30:00 UTC 2005

On Fri, 30 Dec 2005 19:47:28 GMT, KomkaP at prolan.hu (Komka Péter) wrote
for the entire planet to see:
>> NTP doesn't require a local reference clock at all.
>I meant not "cannot do without a GPS" but "(in case of GPS) cannot do with 
>*any* type of GPS". (It does matter the behaviour of the GPS. To which PPS the 
>stamp belongs (if either).)
>(NMEA itself does not specify that?..)

Well, I'm not sure if I understand your question, but there are many
types of GPS receivers, and NTP can interface with more than one type,
but probably not all types.

Some GPS receivers seem to be adapted for timing functions, but there
are many that are not good for timing, only for location.  NTP doesn't
know or care where a reference clock gets its time from.  GPS, ACTS,
CDMA, rubidium, etc.  

This stuff is done in layers, and the layer below NTP is the clock
interface spec, usually determined by the clock manufacturer.  NTP
must be told which type of clock it is interfacing to, and there must
already be an NTP driver that is specific to that
manufacturer/interface spec (or you could write one).

Some clocks try to emulate an interface that was already defined by a
different manufacturer, so that the same NTP driver can be used.  Some
of those alternative clocks don't emulate the original reference clock
specification correctly, and lead to the trouble you seem to be
having.  Sometimes there are setup options within the clock to improve
its NTP compatibility.

I guess you answer is that NTP cannot interface with any type of GPS,
only the supported ones.

Which clock are you using, and what NTP driver are you using?

- Eric

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