Reference page: http://www.catb.org/gpsd/gpsd-time-service-howto.html

cp /etc/ntp.conf /etc/ntp.conf.origin

<pre>
pool us.pool.ntp.org iburst

driftfile /var/lib/ntp/ntp.drift
logfile /var/log/ntp.log

restrict default kod nomodify notrap nopeer noquery
restrict -6 default kod nomodify notrap nopeer noquery
restrict 127.0.0.1 mask 255.255.255.0
restrict -6 ::1

# GPS Serial data reference
server 127.127.28.0
fudge 127.127.28.0 flag1 1 time1 0.9999 refid GPS

# GPS PPS reference
server 127.127.28.1 prefer
fudge 127.127.28.1 flag1 1 refid PPS
</pre>

ntpq -p

<pre>
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*SHM(0)          .GPS.            0 l    6   64    7    0.000   -2.353   1.951
 SHM(1)          .PPS.            0 l    -   64    0    0.000    0.000   0.000
</pre>