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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