So I've been trying unsuccessfully to interface a secondary GPS NMEA stream to my quad with Pixhawk installed. Finally I just connected my terminal program directly to Pixhawk and started sending a single GGA message.
Here is a sample message I've been sending:
$GPGGA,120000,2753.1354100,N,8232.2899630,W,1,08,0.9,31.1,M,-26.5,M,,*78 (CR and CL at the end)
Now depending on the port used (GPS or Serial 4/5) I get different results and both are incorrect:
When GPS port is used - GPS status oscillates every ~5 seconds between No GPS - No Fix - 3D Fix and then again in no particular order. It can go from No GPS to No Fix several times and then again to 3D Fix
When Serial4/5 port is used - GPS status takes about 5 seconds to change from No GPS to 3D Fix, however it stays in 3D Fix forever after that, even if you stop sending the message. 8| After restarting Mission Planner and connecting to Pixhawk again it will display No GPS
I have been sending that sentence at a rate of about 2-3 Hz. If it is sent with frequency less than 1Hz the status never changes at all.