I also have a Garmin GPS76 handheld unit on board. The last time that
the errror occurred with the Garmin 172c driving the autopilot, I
turned on the handheld unit and it did not have a problem acquiring
the GPS signal. The GPS76 would accurately show my position and
course at the same time as the Garmin 172c was having a problem. This
would indicate that there is no problem with the satellite signal.

In the Garmin 172c, when the error starts to occur, the following two
NMEA sentences are transmitted:

$GPRMC,235959,V,,,,,,,,,,N
$GPRMB,V,,,,,,,,,,,,,N

The first field of the $GPRMC sentence contains the time and the
second field is a 'V'. The 'V' indicates a navigational receiver
warning and none of the following fields exist except for the 'N' at
the end. This 'N' means that the data is not valid.

In the $GPRMB sentence, the same indication occurs.

If you are using a Mac computer, download A HREF="http://
www.gpsnavx.com/downloads/GPSUtility.dmg"GPSUtility/A to display
the NMEA information.