[ntp:questions] Re: GPS Time source offset

Bjorn Gabrielsson bg at lysator.liu.se
Sat Oct 22 07:14:28 UTC 2005

"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:

> DJ wrote:
> >I then took a $200 etrex hand-held GPS, set it to NMEA mode, plugged in

Should work.

> >1. Why the offset

The NMEA string is not sent exactly on time. (It takes time for the
GPS processor to compute the solution, format and send the NMEA string.

This means that you have to 'calibrate' away a hopefully stable
offset. Various GPS receivers has more or less jitter on this delay. 

> >2. How to adjust for it - so I can make a cheap linux/Etrex GPS stratum
> >1 server

Your approach should be working.

> A timing receiver will typically have a Pulse Per Second output and
> the leading edge of the pulse will mark the "top" of the second.  

Shure, but for ntp use, calibrating the offset down to the jitter
level, usually give well below 1ms jitter. This is no worse than the
time given by the 60/77.5kHz timing signal receivers.

> The
> text message sent through the RS232 port tells you which second the
> last pulse marked.   The navigation receiver sends a text message with
> the latitude, longitude, height, time, the numbers of the satellites
> in view, etc, etc.  The message is sent whenever a very busy CPU can
> get around to it.  The time is an afterthought!
> I think you probably have the wrong tool for the job.

For keeping track of a cesium, shure! But as a source of time for a
ntp server, it might be of use. Knowing its limitations...


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