Listers,

   I was using NTPD with the following configuration for the GPS (Garmin 
18xLVC):

### /etc/ntp.conf
driftfile /var/lib/ntp/drift

server clock.fmt.he.net noselect
server ntp-1.vt.edu noselect
server clock.nyc.he.net noselect
server time-b.nist.gov noselect

# Garmin GPS 18 LVC
server  127.127.20.0    mode 0 minpoll 4 maxpoll 4 prefer
fudge   127.127.20.0    time2 0.600 flag1 1 flag2 0 flag3 1 refid PPS

It would report offsets of about 0.001 with low jitter as well.

Trying to use the same unit with chrony + gpsd and this configuration:

# Internet sources
server clock.fmt.he.net noselect
server ntp-1.vt.edu noselect
server clock.nyc.he.net noselect
server time-b.nist.gov noselect

refclock SHM 0 offset -0.6 delay 0.5 refid NEMA noselect
refclock SHM 1 offset -0.6 precision 1e-9 lock NEMA refid PPS

# Record the rate at which the system clock gains/losses time.
driftfile /var/lib/chrony/drift



Yields wildly varying offsets :(

MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
#? NEMA                          0   4   377     9   +675ms[ +675ms] +/-  253ms
#* PPS                           0   4   377     8  +7747ns[+8253ns] +/- 5644ns
^? clock.fmt.he.net              1   6   377    55   -600ms[ -600ms] +/-   35ms
^? ntp-1.ipv6.vt.edu             2   6   377    53   -601ms[ -601ms] +/-   35ms
^? clock.nyc.he.net              1   6   377    54   -600ms[ -600ms] +/- 7765us
^? time-b.nist.gov               1   6   377    55   -600ms[ -600ms] +/- 6549us

MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
#? NEMA                          0   4   377    10   +669ms[ +669ms] +/-  253ms
#* PPS                           0   4   377     8  +2954ns[+2721ns] +/- 5613ns
^? clock.fmt.he.net              1   6   377    38   -600ms[ -600ms] +/-   35ms
^? ntp-1.ipv6.vt.edu             2   6   377    36   -601ms[ -601ms] +/-   36ms
^? clock.nyc.he.net              1   6   377   101   -600ms[ -600ms] +/- 7765us
^? time-b.nist.gov               1   6   377    37   -600ms[ -600ms] +/- 6610us

Chronyc tracking

Reference ID    : 50505300 (PPS)
Stratum         : 1
Ref time (UTC)  : Thu Sep 07 22:48:47 2017
System time     : 0.000001081 seconds fast of NTP time
Last offset     : +0.000000692 seconds
RMS offset      : 0.000000814 seconds
Frequency       : 22.380 ppm fast
Residual freq   : +0.000 ppm
Skew            : 0.010 ppm
Root delay      : 0.000000001 seconds
Root dispersion : 0.000015589 seconds
Update interval : 16.0 seconds
Leap status     : Normal


GPSMon Screenshot:

https://img.bsd-unix.net/screenshots/user1/2850Ab83e.png

ntpshmmon output:

#      Name   Seen@                Clock                Real               L 
Prec
sample NTP0 1504824422.059456899 1504824422.059016714 1504824422.000000000 0  -1
sample NTP1 1504824422.400228108 1504824422.399984786 1504824423.000000000 0 -20
sample NTP0 1504824422.559968988 1504824422.059016714 1504824422.000000000 0  -1
sample NTP1 1504824422.900433425 1504824422.399984786 1504824423.000000000 0 -20
sample NTP0 1504824424.045028574 1504824424.044241505 1504824424.000000000 0  -1
sample NTP1 1504824424.400442519 1504824424.399978781 1504824425.000000000 0 -20
sample NTP0 1504824424.545348968 1504824424.044241505 1504824424.000000000 0  -1
sample NTP1 1504824424.900928345 1504824424.399978781 1504824425.000000000 0 -20
sample NTP0 1504824426.041318136 1504824426.041166576 1504824426.000000000 0  -1
sample NTP1 1504824426.400096238 1504824426.399988587 1504824427.000000000 0 -20
sample NTP0 1504824426.541773498 1504824426.041166576 1504824426.000000000 0  -1
sample NTP1 1504824426.900437395 1504824426.399988587 1504824427.000000000 0 -20
sample NTP0 1504824428.051476959 1504824428.050505815 1504824428.000000000 0  -1


Any help would be appreciated!

-- 
             
Bryan G. Seitz

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