Hi, I'm trying to use the nmeanavsatdriver stack with a TOPCON GPS unit. However, the nmeaserialdriver starts and issues the following warnings:

   $ rosrun nmea_navsat_driver nmea_serial_driver _port:=/dev/ttyUSB4 _baud:=115200 
    [WARN] [WallTime: 1410482892.574839] Received a sentence with an invalid checksum.                 Sentence was: 
    [WARN] [WallTime: 1410482893.088193] Received a sentence with an invalid checksum.                 Sentence was: $GPGSA,M,3,16,32,20,11,22,01,,,,,,,2.71,1.81,2.01*04

but then goes quiet and doesn't publish any data across the fix and velocity topics. Via a serial monitor I can confirm that the NMEA sentences are being published correctly with the correct checksums. The above two messages seem to be just startup errors.

However I can get the system to work with nmeatopicserialreader combined with the nmeatopic_driver on the same settings. However in this configuration it doesn't issue velocity information (from the NMEA RMC message).

Any suggestions on how to get the nmeaserialdriver to work?