Would suggest setting SERIAL3_BAUD=34 as the default GPS uart speed is set to this on ublox chipset. Ardupilot will switch it to 115200 and communicate at that speed after initialization. It works fine on UART3 for me using latest ChibiOS firmware on the same board as you.
You can also try using GPS passthru mode to see if you can connect ublox app to the GPS to check configuration. Ctrl-F for the Temp screen has an option for the passthru mode.
This is the port which corresponds with Serial3_ parameters:
According to the documentation here the default firmware expects the GPS connected to UART6. So instead of compiling a custom firmware (actually I tried but could not get past the compiling errors… ) i just moved the GPS to UART6 and used Mission Planner to flash the default firmware (3.9.0 stable). Mission planner immediately showed “No Fix” indicating that is was receiving GPS data and waiting for it to acquire the satellites.
@pauljatherton my understanding is that RX6/TX6 represent SERIAL6 not SERIAL3 (the documentation on this boards are just terrible isn’t it?)
In the middle of my first build. Trying to also use the BN-220 on serial 3, but i can’t see the parameter to disable i2c. The documentation for the omnibus f4 says to " by setting BRD_ALT_CONFIG to 1, the external I2C connection is disabled". Searching in the “full parameters list” in mission planner gives no results for this variable. Not sure what to do.
That update fixed it. Thanks for the help. But I’m still having GPS issues. I have triple checked the wiring on the GPS. Is it plug and play or do i have to configure the GPS in mission planner? (To my knowledge i have disabled I2C, and it looks like I have when i look into the compass section of config).
EDIT: Reflashing the omnibus for a second time with the 4.0.6 ardupilot did the trick.