On 2010-02-09, David Lord <[email protected]> wrote: > BK wrote: >> Details: >> >> First I installed NTP4.2.4p8 >> Then I updated the binaries with your updates labeled 4.2.7p8 >> Then I installed the serial PPS driver 20091228 >> >> Although for NTP purposes, you would like only one output string, I >> have to output two NMEA strings because there is another device >> looking at the serial stream also. I am outputting GGA and RMC >> messages. According to the GPS manufacturer (I am using a Garmin >> GPS15H) the PPS signal is applied just before outputting the NMEA >> sentences that would be for that time period. I have the PPS signal >> set to 80ms width. One oddity about my configuration is that the NTP >> server will not be up 24x7. The machine will be booted, and I would >> like the ntpd to discipline the local clock to a reasonable (+-10ms) >> accuracy within 10 minutes. I have another machine that I will then >> synchronize to the computer with the GPS. > > 10 minutes might be difficult from a cold start. How long from > bootup to ntpd starting? How far out will clock be after bootup?
The big problem is the rate difference-- it takes a long time to correct ( less than halving of the error per hour). > I've been using "ntpd -q" before starting ntpd. That takes > around 5 minutes before time is set to usually well within 10ms. > After that ntpd is started and it's another few minutes before > it's serving time from nmea and another few minutes before it's > using pps to condition the clock. > >> >> Looking at the documentation it appears you can specify which NMEA >> string to use so perhaps this is better >> >> server 127.127.1.0 >> #the below line is just to remind me how cruddy the base clock might >> get >> fudge 127.127.1.0 stratum 5 > > I don't believe I've ever seen stratum 5 from internet > but I had fudge 127.127.1.0 stratum 10 for the systems with > better free running clock and stratum 12 otherwise. > >> #tell NMEA reflclock to use RMC messages >> server 127.127.20.1 minpoll 4 mode 1 > > I had poor results (higher jitter) with nmea and > minpoll 4 both with/without serial DCD connected and now > have minpoll 6. > >> # tell NMEA refclock to use PPS >> fudge 127.127.20.1 refid gps stratum 1 flag1 1 > > What is point of refid and stratum here? As already > explained upthread you will not see the .gps. if you set > stratum 1, and you certainly shouldn't set any stratum > that may cause problems to clients should your system > deliver incorrect time. > >> It really doesn't matter to me if I install the serialPPS driver or >> not. From your discussion it appears using the serialPPS driver is >> more accurate than the NMEA driver? Does that make it a better choice >> or is that just making it more confusing? > > Use whichever works best on your system, or whichever is > simpler (just nmea with pps on DCD). > > Last year I had following avg over couple of days for each: > > NetBSD 5.0.0_PPS, GPS18X-LVC > > nmea RMC type 20 pps on DCD offset=0.07 > > nmea RMC type 20 + PPS type 22 pps on DCD offset=0.002 > > But this year with gps positioned to have better view, both > methods tried are looking similar with offset < 0.01ms > > > David _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
