With my recent (November 2011) purchase of the MT3329 GPS receiver from DIY Drones, it may have the Nav Speed Threshold set so that the positional information is not updated if the unit is not moving. In other online discussions (for example, SparkFun), it appears that this threshold can be set or turned off with a $PMTK command (397). But this command is not listed in the PMT commands referred to on the DYI site. Will the unit I got from you accept this command? Is it ON by default?
Check http://diydrones.com/forum/topics/gps-position-hold-in-quadcopter. It has a string to set it to 0.2 m/s. I used it and it worked.
Yes, it's on by default and set to 1 meter per second.
You can find the command reference here:
$PMTK397,1.0*3C <= 1 meter per second
$PMTK397,0*23 <= 0 meters per second or off
Note that this is for standard NMEA mode. When switching to DIYD binary mode the filter seems to be set to zero or close to it.
John is correct, and the unit will reset to its defaults when power is removed. The command will have to be sent to the unit after each power up. Wait a half second or so to ensure the GPS is booted and in the mood to receive commands.