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?
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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