[ntp:questions] NTP.conf using Dave Hart code.

unruh unruh at wormhole.physics.ubc.ca
Tue Feb 9 23:00:10 UTC 2010


On 2010-02-09, David Lord <snews at lordynet.org> 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




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