What are your NMEA connections? If you have rx+/rx- and tx+/tx- you have a RS422 bases electrical connection. Although you can connect the two it may be unreliable.
See this for more info
https://www.actisense.com/wp-content/upl...-4-1-1.pdf
See this for more info
https://www.actisense.com/wp-content/upl...-4-1-1.pdf