Under LINUX there is the gpsd service which provides SHM services.

In ntp.conf include lines:-

# ... and use gpsd daemon for time sync
server 127.127.28.0 minpoll 4 maxpoll 4
fudge 127.127.28.0 time1 -0.245 refid GPS stratum 15
server 127.127.28.1 minpoll 4 maxpoll 4 prefer
fudge 127.127.28.1 refid GPS1

gpsd then provides the following (from ntpq):-

remote      refid     st   t   when   poll   reach   delay   offset   jitter
=====      ===     ===   =   ====   ===   =====   ====   =====   ======
LOCAL(0)  .LOCL.     13   l   21   64   377   0   0   0.002
=-SHM(0)  .GPS.     15   l   9   16   377   0   36.775   12.287
*SHM(1)      .GPS1.       0   l   6   16   377   0   0.032   0.003

Note:

SHM(0) is the time from NEMA string (to establish second) this can deviate a lot, recommend you add another server (Not shown).

SHM(1) is the PPS from GPS

Related Topics: SHMRefclockUsers