Hi, I am trying to use a Garmin GPS18x LVC receiver to synchronize my laptop 
clock using NTP.  I have found several documents with similar instructions as 
to how to make this work and so far I've had no luck.  I am able to receive the 
GPS NMEA string via a usb port:

~$ cat /dev/ttyUSB0
I�����b����r����br�12207.2689,W,2,09,0.8,10.6,M,-32.4,M,,*4E

$GPGSA,A,3,05,13,15,16,20,21,25,26,29,,,,1.2,0.8,0.9*3D

$GPGSV,3,1,11,05,21,045,15,13,22,086,27,15,30,129,29,16,19,317,25*7A

$GPGSV,3,2,11,20,35,220,19,21,53,298,26,25,20,188,17,26,34,292,20*7C

$GPGSV,3,3,11,29,71,104,37,02,01,080,00,10,09,217,00*42

$GPRMC,215057,A,3727.0074,N,12207.2690,W,000.0,020.0,100818,013.8,E,D*09

$GPGGA,215057,3727.0074,N,12207.2690,W,2,09,0.8,10.6,M,-32.4,M,,*40

$GPGSA,A,3,05,13,15,16,20,21,25,26,29,,,,1.2,0.8,0.9*3D

$GPGSV,3,1,11,05,21,045,15,13,22,086,27,15,30,129,29,16,19,317,25*7A

$GPGSV,3,2,11,20,35,220,19,21,53,298,26,25,20,188,17,26,34,292,20*7C

$GPGSV,3,3,11,29,71,104,37,02,01,080,00,10,09,217,00*42


If I monitor the GPS input via gpsmon I can see all the information is there, 
including the 1PPS pulse:

/dev/ttyUSB0 4800 8N1         NMEA0183>
┌──────────────────────────────────────────────────────────────────────────────┐
│Time: 2018-08-10T21:58:28.000Z Lat:  37 27' 00.215" N Lon: 122 07' 15.888" W  │
└───────────────────────────────── Cooked TPV ─────────────────────────────────┘
┌──────────────────────────────────────────────────────────────────────────────┐
│ GPRMC GPGGA GPGSA GPGSV PGRMT                                                │
└───────────────────────────────── Sentences ──────────────────────────────────┘
┌──────────────────┐┌────────────────────────────┐┌────────────────────────────┐
│Ch PRN  Az El S/N ││Time:      215828           ││Time:      215828           │
│ 0   5  45 18  16 ││Latitude:     3727.0036 N   ││Latitude:  3727.0036        │
│ 1  13  82 23  20 ││Longitude:   12207.2648 W   ││Longitude: 12207.2648       │
│ 2  15 126 32  34 ││Speed:     000.0            ││Altitude:  45.8             │
│ 3  16 315 21  19 ││Course:    194.5            ││Quality:   2   Sats: 08     │
│ 4  20 222 39  22 ││Status:    A       FAA: D   ││HDOP:      1.0              │
│ 5  21 302 55  23 ││MagVar:    013.8E           ││Geoid:     -32.4            │
│ 6  26 287 34  24 │└─────────── RMC ────────────┘└─────────── GGA ────────────┘
│ 7  29 112 68  16 │┌────────────────────────────┐┌────────────────────────────┐
│ 8  10 219 12   0 ││Mode: A3 Sats: 5 13 15 16 2 ││UTC:           RMS:         │
│ 9  25 188 18   0 ││DOP: H=1.0   V=0.9   P=1.3  ││MAJ:           MIN:         │
│10                ││TOFF:  0.611384951          ││ORI:           LAT:         │
│11                ││PPS:  0.000169339           ││LON:           ALT:         │
└────── GSV ───────┘└──────── GSA + PPS ─────────┘└─────────── GST ────────────┘
(70) $GPGSV,3,2,10,20,39,222,22,21,55,302,23,26,34,287,24,29,68,112,16*78


However, ntp does not recognize and reject the SHM peers


ntpq> pe
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 0.ubuntu.pool.n .POOL.          16 p    -   64    0    0.000    0.000   0.000
 1.ubuntu.pool.n .POOL.          16 p    -   64    0    0.000    0.000   0.000
 2.ubuntu.pool.n .POOL.          16 p    -   64    0    0.000    0.000   0.000
 3.ubuntu.pool.n .POOL.          16 p    -   64    0    0.000    0.000   0.000
 ntp.ubuntu.com  .POOL.          16 p    -   64    0    0.000    0.000   0.000
 SHM(0)          .NMEA.           0 l    -   16    0    0.000    0.000   0.000
 SHM(1)          .PPS.            0 l    -   16    0    0.000    0.000   0.000
#208.76.53.137   216.218.254.202  2 u   46   64   17   76.696    3.673   0.552
-12.167.151.2    198.148.79.210   3 u   51   64   17   78.516    2.003   0.281
+x.ns.gin.ntt.ne 249.224.99.213   2 u   48   64   17    4.576    0.418   0.464
-mdnworldwide.co 216.218.254.202  2 u   50   64   17   49.261    0.094   1.015
*darwin.kenyonra 127.67.113.92    2 u   45   64   17    5.362    0.354   0.494
-lv1.justaguy.be 127.67.113.92    2 u   47   64   17   23.820   -1.319   0.603
-66-188-102-122. 34.225.6.20      3 u   52   64   17   81.602   -3.496   4.033
+propjet.latt.ne 14.82.97.170     2 u   46   64   17    4.421   -1.750   1.819
-nu.binary.net   204.9.54.119     2 u   49   64   17   53.228   -6.678   0.723
-dns-e.wdc-us.ho 64.113.44.54     2 u   50   64   17   76.942    1.555   1.099
 golem.canonical 145.238.203.14   2 u   52   64   17  137.693    0.367  15.467
-195.21.152.161  195.66.241.2     2 u   49   64   17   10.487   -1.236   0.707
#64.52.22.96 (64 199.102.46.72    2 u   47   64   17   13.617    0.187   0.980
 pugot.canonical 17.253.34.125    2 u   53   64   17  139.481    0.624  35.522

ntpq> as

ind assid status  conf reach auth condition  last_event cnt
===========================================================
  1 33742  8811   yes  none  none    reject    mobilize  1
  2 33743  8811   yes  none  none    reject    mobilize  1
  3 33744  8811   yes  none  none    reject    mobilize  1
  4 33745  8811   yes  none  none    reject    mobilize  1
  5 33746  8811   yes  none  none    reject    mobilize  1
  6 33747  802b   yes    no  none    reject clock_alarm  2
  7 33748  802b   yes    no  none    reject clock_alarm  2
  8 33749  1524    no   yes  none    backup   reachable  2
  9 33750  1324    no   yes  none   outlier   reachable  2
 10 33751  1424    no   yes  none candidate   reachable  2
 11 33752  1324    no   yes  none   outlier   reachable  2
 12 33753  163a    no   yes  none  sys.peer    sys_peer  3
 13 33754  133a    no   yes  none   outlier    sys_peer  3
 14 33755  1324    no   yes  none   outlier   reachable  2
 15 33756  1424    no   yes  none candidate   reachable  2
 16 33757  1324    no   yes  none   outlier   reachable  2
 17 33758  1324    no   yes  none   outlier   reachable  2
 18 33759  1024    no   yes  none    reject   reachable  2
 19 33760  1324    no   yes  none   outlier   reachable  2
 20 33761  1524    no   yes  none    backup   reachable  2
 21 33762  1024    no   yes  none    reject   reachable  2

This is what I have added to the ntp.conf file 

# /etc/ntp.conf, configuration for ntpd; see ntp.conf(5) for help

driftfile /var/lib/ntp/ntp.drift

# Enable this if you want statistics to be logged.
statsdir /var/log/ntpstats/

statistics loopstats peerstats clockstats
filegen loopstats file loopstats type day enable
filegen peerstats file peerstats type day enable
filegen clockstats file clockstats type day enable

# Specify one or more NTP servers.

# Use servers from the NTP Pool Project. Approved by Ubuntu Technical Board
# on 2011-02-08 (LP: #104525). See http://www.pool.ntp.org/join.html for
# more information.
pool 0.ubuntu.pool.ntp.org iburst
pool 1.ubuntu.pool.ntp.org iburst
pool 2.ubuntu.pool.ntp.org iburst
pool 3.ubuntu.pool.ntp.org iburst

# Use Ubuntu's ntp server as a fallback.
pool ntp.ubuntu.com

# Access control configuration; see /usr/share/doc/ntp-doc/html/accopt.html for
# details.  The web page 
<http://support.ntp.org/bin/view/Support/AccessRestrictions>
# might also be helpful.
#
# Note that "restrict" applies to both servers and clients, so a configuration
# that might be intended to block requests from certain clients could also end
# up blocking replies from your own upstream servers.

# By default, exchange time with everybody, but don't allow configuration.
restrict -4 default kod notrap nomodify nopeer noquery limited
restrict -6 default kod notrap nomodify nopeer noquery limited

# Local users may interrogate the ntp server more closely.
restrict 127.0.0.1
restrict ::1

# Needed for adding pool entries
restrict source notrap nomodify noquery

# Clients from this (example!) subnet have unlimited access, but only if
# cryptographically authenticated.
#restrict 192.168.123.0 mask 255.255.255.0 notrust

restrict 172.56.3.0 mask 255.255.255.0 nomodify


# If you want to provide time to your local subnet, change the next line.
# (Again, the address is an example only.)
#broadcast 192.168.123.255

# If you want to listen to time broadcasts on your local subnet, de-comment the
# next lines.  Please do this only if you trust everybody on the network!
disable auth
broadcastclient

#Changes recquired to use pps synchonisation as explained in documentation:
#http://www.ntp.org/ntpfaq/NTP-s-config-adv.htm#AEN3918

#server 127.127.8.1 mode 135 prefer    # Meinberg GPS167 with PPS
#fudge 127.127.8.1 time1 0.0042        # relative to PPS for my hardware

#server 127.127.22.1                   # ATOM(PPS)
#fudge 127.127.22.1 flag3 1            # enable PPS API

# GPS Serial data reference
server 127.127.28.0 minpoll 4
fudge 127.127.28.0 time1 0.183 refid NMEA

# GPS PPS reference
server 127.127.28.1 minpoll 4 prefer
fudge 127.127.28.1 refid PPS


I am at a loss as to what is happening and how to fix it.  Any suggestions are 
welcome
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