Trying to stream NMEA GPS data into GPS port of my Pixhawk won't work

Hello, I am trying to fly without using GPS so I generated specific NMEA lines and I constantly stream them into my Pixhawk GPS port. When I stream the very same data into a Raspberry Pi and run gpsd library (gpsmon), it detects it as gps and gives me a valid lat long data but when I stream it into the gps port in Pixhawk, I see no data on QGroundControl, what may be causing this? Thanks.