Terry wrote:

> still no sign of gps synchronisation

As root or with sudo,

 killall gpsd

to stop the daemon, then have a look to see if the GPS is
sending data:

 stty -F /dev/ttyACM0 -echo
 cat /dev/ttyACM0

Should give you regular bursts of NMEA messages.  Look out
for the $GPGLL and $GPRMC which contain date and time
respectively.

Now try gpsd in foreground

  gpsd -bnN -D3 /dev/ttyACM0

which should print bunches of stuff as each burst of NMEA
packets is processed by gpsd.

Leave it running in foreground and open up another shell.
ntpd is probably already running, so in the other shell
run

 ntpq -p

After a short period of time you should have

      remote           refid      st t when poll reach   ...
======================================================   ...
*SHM(0)          .GPS.            0 l    1   16   377    ...

where the * in front of SHM shows that ntpd is getting the
time from gpsd via some shared memory variables.  The 'when'
value will be counting up.

In ntp.conf I'm using

 server 127.127.28.0 minpoll 4 maxpoll 4
 fudge 127.127.28.0 time1 0.051 refid GPS

The 0.051 is probably not correct, it was just a cut-and-paste
from somewhere else.   When you get it working you might want
to tweak the 0.051 for your actual comms latency.

--
Paul Nicholson
--