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.

I also have 2 Icom radios (M502 and M802), both of which are DSC
capable and have a Lat/Lon display. When they no longer receive valid
NMEA position information, the Lat/Lon display flashes, alternating
between the last valid position received and a series of question
marks. This is another indication that the GPS error is occurring.

I've been in contact with Garmin about this problem and sent them NMEA
log files that showed the error occurring. (Software is available that
will display the NMEA data in real time as it is being transmitted.)

After reviewing these log files, Garmin's Technical Support department
acknowledged that it is a problem in their software. The issue was
forwarded to their Software Engineering department in November of 2006
and I was told that a fix would be provided in 2 weeks. When the 2
weeks passed, I was informed that it would be another month and when
that passed, Garmin said that they didn't know when a fix would be
available. I'm still waiting for them to fix the problem.

When the GPS stops transmitting the NMEA navigational information, the
autopilot maintains the last heading that it received. If that heading
happens to be for example, a 30 degree course correction, then the
vessel will be steered off course by 30 degrees until it starts
receiving valid NMEA data again. I think that this represents a
significant hazard and so I cannot rely on the GPS to maintain an
accurate course.

I have notified the U.S. Coast Guard and submitted a 'Consumer Safety
Defect Report' to them. I don't know if they will pursue a recall or
just some kind of advisory but this issue needs to be resolved one way
or another.

The Garmin GPSmap 172c is running the latest firmware version which is
2.60, dated May 9, 2006. On the Garmin download site, the firmware for
the GPSmap 172, GPSmap 178 and GPSmap 178c also have that same date
for their latest updates. I'm trying to find out if this error also
affects these units.

Is anyone else using any of these Garmin GPS units and if you are, do
you get intermittent navigational errors with them?