CatPilot VS ardupilot

Has similar code generation approaches but written in C and looks better generalized

CatPilot gives model based design tools for creating specific configuration faster.

CatPilot has convenient code generation tools, DSL integration layer for fast functions and behaviour creation

They are widely used in narrow circles :)

Two of (arguably the best) open source RC aircraft flight controllers (ArduPilot and PX4) are using extended Kalman filters in their state estimators (essentially sensor fusion that provides attitude/position estimate):

https://github.com/ArduPilot/ardupilot/tree/master/libraries...

https://github.com/PX4/PX4-Autopilot/blob/main/src/modules/e...

I'm not that familiar with cleanflight/betaflight/inav scene to know what the FPV racer flight controllers use.

2. https://ardupilot.org/

Trimble also has some nice stuff, I think the BD940 is interesting. I do not know if it's recommended or not, but Ardupilot has a driver for the Septentrio GPS (https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_GPS/AP_GPS_SBF.cpp), as well for Trimble GPS (https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_GPS/AP_GPS_GSOF.cpp). So people have used these brands for drones.

For programmable open source autopilot: Arduino > Ardupilot (https://ardupilot.org).

You might also want to look at something like Ardupilot

I'd like to emphasize the reference to Ardupilot bc there is a lot of material already developed there: a lot about navigation and guidance from UAVs, and the low-level stuff. Check this out