ublox gps LEA-6T received a sentencce with an invalid checksum. sentence was

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hello, I am using a Ublox lea 6T GPS for robot navigation. to connect the GPS with raspberry pi I've used TTL USB. after nmea_navsat_driver package installation and running:

rosrun nmea_navsat_driver nmea_serial_driver _port:=/dev/ttyUSB0 _baud:=9600

I got a warning msg:

Received a sentence with an invalid checksum. The sentence was:'\x\xc\xefkl

and when I run: rostopic list

/fix
/heading
/rosout
/time_reference
/vel

when I want to display topics I got no data