Hi Paul!
Beware: long e-mail ahead!
On 6 December 2011 14:49, Paul Duncan <p...@noc.ac.uk> wrote:
> Hi Miguel,
>
> Thanks very much for getting back to me.
No problem. I've been helped before so I'll do my best to help you.
>
> Okay, so I have mostly copied yours and this is what I am using now:
>
> # Modified ntp.conf from Miguel Goncalves
>
> server 127.127.20.0 prefer mode 2 minpoll 4 maxpoll 4
> fudge 127.127.20.0 flag1 1 flag2 0 flag3 1 flag4 1 time2 0.195
>
> peer ntp.my-inbox.co.uk
>
>
> tos mindist 0.250
>
> statistics loopstats
> statsdir /var/log/ntp/
> filegen loopstats file loop type delay enable
>
> restrict 127.0.0.1
> restrict 139.166.0.0 mask 255.255.0.0
>
> restrict default nomodify notrap nopeer
>
> So, questions:
>
> 1) The peer line just needs to be another NTP server somewhere, right?
>
Peer creates a symmetric relationship with another NTP machine. We can get
the time from them and they can get the time from us. I am a time nut and I
have three different GPS receivers that peer with each other:
backup# ntpq -pn
remote refid st t when poll reach delay offset
jitter
==============================================================================
+192.168.111.1 .GPS. 1 u 16 16 377 0.255 -0.041
0.011
+192.168.111.2 .GPS. 1 u 11 16 377 0.258 -0.051
0.006
*192.168.111.3 .PPS. 1 u 13 16 377 0.269 -0.065
0.009
> 2) What does the line starting "tos" do?
>
It allows a difference of 250 ms between the PPS and the NMEA string.
Sometimes the NMEA output from the receiver has a bit of jitter and the PPS
is decoupled from the NMEA data. The default is 10 ms. It should be
increased in situations like this. It's not advisable to set it to a large
number: as low as possible is the best.
Either way, it does not seemed to work. Output from ntptime and ntpq below:
>
> oot@darkstar:/etc# ntpq -p
>
> remote refid st t when poll reach delay offset
> jitter
> ==============================**==============================**
> ==================
> *GPS_NMEA(0) .GPS. 0 l 15 16 377 0.000 -67.594
> 13.658
> root@darkstar:/etc# ntptime
Apparently the NMEA data is being received. I am not familiar with Linux
and PPS but I believe your kernel should have a PPS patch applied. Did you
do it?
ntp_gettime() returns code 0 (OK)
> time d288a989.f40bfde0 Tue, Dec 6 2011 14:47:37.953, (.953308550),
> maximum error 8175995 us, estimated error 13079 us, TAI offset 0
>
> ntp_adjtime() returns code 0 (OK)
> modes 0x0 (),
> offset -64.840 us, frequency -0.564 ppm, interval 1 s,
> maximum error 8175995 us, estimated error 13079 us,
> status 0x2001 (PLL,NANO),
> time constant 4, precision 0.001 us, tolerance 500 ppm,
> root@darkstar:/etc#
>
Definitely no PPS there. I have a machine that has GPS receiver and I
configured it differently:
server 127.127.20.0 mode 18 prefer minpoll 4 maxpoll 4 prefer
fudge 127.127.20.0 flag1 0 flag4 0 time2 0.404
server 127.127.22.0 minpoll 4 maxpoll 4
fudge 127.127.22.0 flag2 0 flag3 1
tos mindist 0.25
Let me explain: there's the NMEA driver (127.127.20.0) with PPS disabled
(flag1 0) and with a 404 ms fudge for the serial NMEA data. Additionally
there's the PPS from the GPS (127.127.22.0). Note the prefer on the first
driver.
You could check that your PPS is not working if you use something like
server 127.127.22.0 minpoll 4 maxpoll 4
fudge 127.127.22.0 flag2 0 flag3 1
server a.close.ntp.server prefer
tos mindist 0.25
The server you'll be using can be a stratum 2, 3 or even greater. NTP will
only use it to number seconds and find out about leap seconds and other
"stuff". You'll get great accuracy from the PPS you are getting from the
PPS. If you use this configuration don't forget to add the symlink in /dev
(/dev/pps0 should point to your serial/parallel port). If this does not
work you are not getting the PPS signal into your server.
So, I'm now thinking that the mistake must be in the way I have run the
> configure script when building the software. I think I just used
> --enable-NMEA. Should I have done something else?
>
--enable-NMEA is enough. My configuration uses kernel PPS and that should
be enabled in the kernel.
In FreeBSD it's quite easy: I just add a line OPTIONS PPS_SYNC to the
kernel configuration file, recompile and reboot. I believe the most recent
Linux kernels already have this patch but you should look for it. Did you
read http://time.qnan.org/?
> Thanks!
>
> Paul.
>
You're welcome!
Cheers,
Miguel
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