[ntp:questions] Re: GPS Time source offset

Richard B. Gilbert rgilbert88 at comcast.net
Fri Oct 21 19:38:19 UTC 2005


DJ wrote:

>I have a 4000 GPB ($6000 ?) GPS time source - it works beautifully.
>I then took a $200 etrex hand-held GPS, set it to NMEA mode, plugged in
>into a serial port on a Linux machine, I then did
>ln -fs /dev/ttyS0 /dev/gps0 and configured ntp.conf
>
>server 127.127.1.0
>fudge 127.127.1.0 stratum 10
>
>server 127.127.20.0
>
>I started ntpd and then my linux platform appears as a stratum 1
>server. However, it seems to have an approx 300mS offset from the 4000
>GBP GPS time source. I tried
>
>fudge 127.127.20.0 time1 0.3
>
>or
>
>fudge 127.127.20.0 time1 -0.3
>
>or even
>
>fudge 127.127.20.0 time1 500.0
>
>(+ restarting ntpd each time)
>
>But it seemed to make no difference to the time served from linux
>Any idea
>
>1. Why the offset
>2. How to adjust for it - so I can make a cheap linux/Etrex GPS stratum
>1 server
>
>
>DJ
>
>  
>
I'm not familiar with the Etrex but GPS receivers can be optimized for 
navigation (where am I?) or for timing (what time is it?)    All GPS 
receivers solve for latitude, longitude, height and time and the 
solution, if executed properly, yields a time within microseconds or 
even within nanoseconds of the correct time.   The difference between 
navigation and timing occurs in the presentation of the data.

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.  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.




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