[ntp:questions] Sure GPS - Very High Jitter and Offset

Ken Link klink at numberzero.org
Mon Aug 15 17:46:04 UTC 2011

Got that sorted, I had to disable Apparmor instead of SELinux.

Now, though, I can't get PPS input to work. It appears the site that
hosts most, if not all, the information regarding LinuxPPS is down, so
I've been stumbling around various internet searches trying to find
all the information that I can. It appears that PPS support was added
to recent versions of the Linux kernel, so I upgraded my system to a
pre-compiled 2.6.38 version of the kernel, distributed via standard
Ubuntu apt channels. I'm trying to avoid compiling my own kernel if at
all possible.

I created a new udev rule to set the serial port to the correct UART
setting and add low_latency. It also runs "ldattach pps /dev/ttyS0"
and creates the appropriate symlinks for gps0 and gpspps0. However,
when I "cat /sys/class/pps/pps0/assert", it returns "0.000000000#0"
instead of a timestamp of when the PPS line was last asserted. The
hardware hasn't changed since I was running it on Windows where PPS
was working correctly. At this point I'm not sure if it's with the
kernel itself or just a configuration setting somewhere in the OS.

Does anyone know of any Linux CLI tools that display (in some way or
another) the serial control signals received on a serial port? It
would help tremendously if I had a program similar to the one on
Windows that showed when signals were asserted or de-asserted. This
would at least let me know that the PPS signal is being seen. The only
thing I see in the syslogs is the PPS driver being loaded on bootup,
but nothing once-per-second.


On Sat, Aug 13, 2011 at 8:12 PM, Ken Link <klink at numberzero.org> wrote:
> Following up on my previous posts...
> I compiled NTP 4.2.6p3 on my Ubuntu server (with the NMEA refclock
> enabled, debugging enabled and linuxcaps enabled) but I'm getting
> Permission Denied when it tries to access /dev/gps0 (symlink to
> /dev/ttyS0). I saw from searching that someone fixed it by disabling
> SELinux, but I don't have it installed.
> Command line: /usr/sbin/ntpd -p /var/run/ntpd.pid -g -u ntp:ntp -d
> ..."refclock_open /dev/gps0: Permission denied"...
> # ls -al /dev/ttyS0
> crw-rw---- 1 ntp ntp 4, 64 2011-08-13 19:20 /dev/ttyS0
> # ls -al /dev/gps0
> lrwxrwxrwx 1 root root 5 2011-08-13 19:06 /dev/gps0 -> ttyS0
> Any ideas?

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