On 2017-01-24 14:47, juergen perlinger wrote: >> I'm using this GPS module with a MTK3339 chipset. Bought from here: >> https://www.adafruit.com/product/790 >> And I'm using ntpd with have PPS support compiled-in. >> Juergen, thanks for pointing out the documentation. I've understood more >> about NMEA receivers now. >> I've tried Brian's and Juergen's server and fudge configuration, using >> time2 to align with the NMEA data-stream and now getting better results. >> I don't have a scope, so how do you suggest I determine the correct values >> to use for time2 ? > It's a bit of a chicken-egg problem. An easy way would be to have some > decent time servers (you seem to have that) and mark you cklock to > calibrate as 'noselect'. Take a note of the offset every hour or every > other hour, and after some time (1/2 day or so -- the longer, the > better) you have enough samples to get a decent correction for your > initial estimate. (don't forget to remove the noselect statement after > setting the final fudge value ;) > There are also some more hints about fudge calibration in the ntp docs, > but that needs some digging. I guess there are more elaborate ways, but > this worked for me. > Note that the serial timing (time2) is only critical if there is no PPS > signal. Fine tuning of the PPS delay (time1) needs more equipment, I'm > afraid. I never went into that -- for my purposes 50usec is definitely > close enough. (That's a guesstimate, of course. Most of it will be > caused by interrupt latency.)
Garmin gives the values 18.2ns for their cable delay per metre and 122.2ns for antenna plus receiver delay for their 18x-LVC, to which you need to add the port delay for the serial hardware interrupt, plus interrupt delay for the system hardware and software, which probably swamps the rest. You could calibrate the port and system delay with a series of loopback interrupt timing tests on a system supporting an accurate hardware counter like x64 constant/invariant TSC. There's nothing like that available for Adafruit/GlobalTop MKT-3339. The discussion below may be helpful to the OP: http://lists.ntp.org/pipermail/questions/2013-September/036165.html as may: http://open.konspyre.org/blog/2012/10/18/raspberry-flavored-time-a-ntp-server-on-your-pi-tethered-to-a-gps-unit/ -- Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
