Strange ntpmon offset

[Massimiliano Fago]

Dear All,

I have a strange behavior with gpsd and ntpmon on my raspberry system.

I have this version:

gpsd: 3.22 (revision 3.22)

ntpmon ntpsec-1.2.0+ 2021-04-20T14:25:51Z (git rev 1feb8f90d)

my gps is u-blox and I have an external antenna.
MAX-M8Q-0

my ntp.conf file is:

refclock shm unit 1 minpoll 4 maxpoll 4 refid PPS flag4 1 prefer

# GPS Serial data reference (NTP0)
#server 127.127.28.0 minpoll 4 maxpoll 4
#fudge 127.127.28.0 time1 0.9999 refid GPS
refclock shm unit 0 time1 -0.0900 refid GPS flag4 1 noselect

GPS signal is very good:

lqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqklqqqqqqqqqqqqqqqqqqSeen 21/Used 11k
x Time:        2021-11-29T19:28:16.000Z (18)xxGNSS   PRN  Elev   Azim   SNR Usex
x Latitude:         40.47071280 N           xxGP  2    2  44.0   76.0  38.0  Y x
x Longitude:        17.24413700 E           xxGP  6    6  14.0   38.0  20.0  Y x
x Alt (HAE, MSL):     72.762,     35.224 m  xxGP 12   12  57.0   63.0  31.0  Y x
x Speed:             0.05 km/h              xxGP 24   24  29.0  156.0  22.0  Y x
x Track (true, var):   204.3,   4.0     deg xxGP 25   25  68.0  336.0  42.0  Y x
x Climb:             0.00 m/min             xxGP 29   29  58.0  233.0  40.0  Y x
x Status:         3D DGPS FIX (16 secs)     xxGP 31   31  20.0  315.0  33.0  Y x
x Long Err  (XDOP, EPX):  0.55, +/-  2.0 m  xxGP 32   32  25.0  267.0  38.0  Y x
x Lat Err   (YDOP, EPY):  0.61, +/-  2.3 m  xxGL  9   73  28.0  114.0  31.0  Y x
x Alt Err   (VDOP, EPV):  1.30, +/-  2.5 m  xxGL 10   74  69.0   62.0  20.0  Y x
x 2D Err    (HDOP, CEP):  0.82, +/-  1.8 m  xxGL 21   85  39.0  193.0  25.0  Y x
x 3D Err    (PDOP, SEP):  1.53, +/- 1899 m  xxGP 11   11  37.0   73.0   0.0 uN x
x Time Err  (TDOP):       0.86              xxGP 20   20   2.0  103.0   0.0  N x
x Geo Err   (GDOP):       1.76              xxSB120   33  32.0  225.0   0.0  N x
x Speed Err (EPS):       +/-  0.8 km/h      xxSB123   36  41.0  159.0  33.0  N x
x Track Err (EPD):        n/a               xxSB136   49  42.0  199.0   0.0  N x
x Time offset:           -0.853875848 s     xxGL  1   65   5.0  278.0  25.0  N x
x Grid Square:            JN80ol92          xxGL  2   66   6.0  333.0   0.0  N x
mqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqjmMore...qqqqqqqqqqqqqqqqqqqqqqqqqqj

My problem is that I have strange value on offset data. It some time
is good like 0.045 for example and some time is more than 1 second:

     remote           refid      st t when poll reach   delay   offset   jitter
*SHM(1)          .PPS.            0 l    4   16  377   0.0000 1000.052 654.6609
 SHM(0)          .GPS.            0 l   28   64   37   0.0000 765.2188   0.5516
ntpd ntpsec-1.2.0+ 2021-04-20T14:25:51Z        Updated: 2021-11-29T20:26:39 (8)
 lstint avgint rstr r m v  count    score   drop rport remote address
      0   2.34    0 . 6 2    136    0.755      0 44339 localhost

Please give me some suggestion.

Massimiliano