I was having this problem with my BU-353 USB-puck GPS a little while ago.
The gpsd "cgps" program put it into SiRF binary mode without me asking,
and that creates (AFAICT) pseudo-NMEA strings when a client asks for raw
this was with gpsdrive built from SVN (Oct 2008?) and I believe gpsd
2.36-2~bpo40+1 for debian etch from backports.org.
The "solution" was to reset the GPS to output in NMEA not binary SiRF
protocol using scripts/gpsd_nmea.sh from the gpsdrive source code.
which is just:
echo "Switching the GPS connected to the local Gpsd into NMEA Mode ..."
(echo "N=0"; sleep 1 ) | telnet localhost 2947
no idea if it helps with the Garmin,
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