[ntp:questions] FreeBSD and SureGPS

fRANz andrea.francesconi at gmail.com
Tue May 1 21:11:07 UTC 2012

Hi guys,
I played with a Sure GPS board and a Soekris box (net4801) with
FreeBSD 8.2 to use as an internal time source.
More info about the scenario:

* FreeBSD 8.2 with PPS enabled kernel;
* SureGPS (http://www.sureelectronics.net/goods.php?id=99) connected
by RS-232 connector on COM2 (TTL-level positive-going signal on TP9
soldered directly to pin 1 of the RS-232 connector, hack described in
David Taylor NTP site);
* ACPI and FIFO disabled;
* ntpd 4.2.6p2 from FreeBSD packages;

ntp# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
xPPS(1)          .PPS.            0 l   11   16  377    0.000    5.321   2.636
oGPS_NMEA(1)     .GPS.            0 l   10   16  377    0.000    5.285   2.651
xkraken2.bilink.  2 u  396 1024  377   63.208  -23.734  26.911

ntp# ntptime
ntp_gettime() returns code 0 (OK)
  time d34acb6a.99ffa380  Tue, May  1 2012 22:51:54.601, (.601557152),
  maximum error 6598 us, estimated error 23 us, TAI offset 0
ntp_adjtime() returns code 0 (OK)
  modes 0x0 (),
  offset 5003.852 us, frequency 254.143 ppm, interval 256 s,
  maximum error 6598 us, estimated error 23 us,
  time constant 4, precision 0.001 us, tolerance 496 ppm,
  pps frequency 254.143 ppm, stability 52.972 ppm, jitter 46.538 us,
  intervals 187, jitter exceeded 73, stability exceeded 4, errors 2.

ntp# ntpdc -c kerninfo
pll offset:           0.00495389 s
pll frequency:        254.143 ppm
maximum error:        0.007119 s
estimated error:      4.4e-05 s
status:               2107  pll ppsfreq ppstime ppssignal nano
pll time constant:    4
precision:            1e-09 s
frequency tolerance:  496 ppm
pps frequency:        254.143 ppm
pps stability:        52.972 ppm
pps jitter:           2.6096e-05 s
calibration interval: 256 s
calibration cycles:   187
jitter exceeded:      73
stability exceeded:   4
calibration errors:   2

ntp# cat /etc/ntp.conf | grep -v ^#
server mode 18 minpoll 4 prefer
fudge flag3 1
server mode 18 minpoll 4
fudge flag1 1 flag2 0 flag3 1

server it.pool.ntp.org iburst minpoll 10

driftfile /var/db/ntp.drift

statsdir /var/log/ntp/
filegen clockstats file clockstats type day enable
filegen peerstats file peerstats type day enable
filegen loopstats file loopstats type day enable
statistics clockstats peerstats loopstats
logfile /var/log/ntp/messages

Some questions about my implementation:
jitter looks like ok for you? ntp.conf configuration is ok or can be better?
what about GPS_NMEA source (PPS source is referred to this one)
instead PPS considered falseticker? Should I use both? Is it
why server added (it.pool.ntp.org) is always considered as
falseticker? Could be a remote (pool) server problem?

Thank you for any help.

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