On 11 Feb 2007 08:34:03 -0800, wrote:

I have a Garmin GPSmap 172c GPS installed and connected to a Raymarine
ST4000 autopilot.

When the GPS is operating properly, it outputs a sequence of NMEA
navigational data every 2 seconds. This NMEA data is used by the
autopilot to steer a course to a waypoint. However, there is a problem
with the Garmin software in that every day at 23:59:59 GMT, it will no
longer transmit the NMEA information reliably. Sometimes the NMEA data
stops entirely and other times it becomes quite erratic and is
transmitted at 8 second, 20 second or some other random interval. When
this happens, the autopilot displays a 'No Data' warning message and
then steers the boat on the last valid course that it received. This
behaviour continues until 00:08:34 GMT at which time the GPS reverts
to the normal 2 second transmission interval.

Do you have WAAS corrections enabled? If so, you might try turning
WAAS off to see if that affects the problem.

Here, the WAAS satellites are very low on the horizon, and when they
hide behind a hill, my GPSMap/Sounder 168 spends so much time and
effort trying to get the WAAS correction data that it ignores, and
therefore "buggers up" the NMEA data output. For me, this problem
depends on my location, and it is hard to explain how time-of-day
could affect WAAS reception, but it may be possible.



--
Peter Bennett, VE7CEI Vancouver BC, Canada
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca