Hi Brent,
I spent a little while skimming over the 254 PAGE! pdf of the manual
from the SparkFun website. The device is certainly well documented. I
couldn't find where they specify the timing relationship you specify.
That's very interesting if it's consistent. I'm currently using a
GlobalSat BU-353 USB only GPS, outputting only the GPGGA sentence only
at 57600 baud. I'm getting ~ +/- 15 ms peak offsets between the
computer clock and the GPS polling every 8 seconds with jitter averaging
around 3 ms.
Based on your note and some data in the Trimble manual, I've reset my
GPS to output only the GPZDA sentence. I looked at the link below to
find the mode command to set NTPD to recognize this sentence:
http://www.eecis.udel.edu/~mills/ntp/html/drivers/driver20.html
It turns out mode 8 does the trick, plus the other bits I add to get to
57600 baud, so it works out to mode 72. So, the GPS part of my ntp.conf
is thus:
server 127.127.20.5 prefer minpoll 3 maxpoll 3 mode 72
fudge 127.127.20.5 time2 0.3550 refid GPS1
So, I'm going to let that run a while and see if it's more stable than
the GPGGA setup.
Sincerely,
Ron
On 2/8/2012 6:37 PM, Brent Gordon wrote:
One cool thing about the Copernicus, beside the PPS, is that the
datasheet specifies the delay from the PPS to start for the ZDA
sentence. This means you can use the serial data directly without
having to use the PPS. On other GPS units I've looked at, the delay
from PPS to ZDA varies greatly (300 ms +/- 150 ms).
Brent
On 2/8/2012 3:23 PM, Ron Frazier (NTP) wrote:
Hello all,
While researching potential GPS modules to experiment with for PPS
output, I discovered that SparkFun has quite a lot of modules, some
of which have PPS and some of which don't. You can't necessarily
tell from the product page. You have to check the data sheet. You
can go to their website and search for gps. This link should also
work. Just thought I'd pass it along.
http://www.sparkfun.com/search/results?term=gps&what=products
Here is one unit which does have PPS but it's not obvious from the
product page. There are others as well.
12 Channel Copernicus II GPS Receiver - from Trimble -
http://www.sparkfun.com/products/10922
--
(PS - If you email me and don't get a quick response, don't be concerned.
I get about 300 emails per day from alternate energy mailing lists and
such. I don't always see new messages very quickly. If you need a
reply and have not heard from me in 1 - 2 weeks, send your message again.)
Ron Frazier
timekeepingdude AT c3energy.com
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