ardupilot VS cortex-m3-rtos

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

I will add my own project here https://github.com/t4th/cortex-m3-rtos

I decided to have similar architecture in my own private project ( https://github.com/t4th/cortex-m3-rtos ), because the underlying concept is that, it is all just an array of structs and I used C++ to Abstract those arrays as different modules for cleaner architecture.

Here is project that I was experimenting with C++14 instead of doing it in plain C: https://github.com/t4th/cortex-m3-rtos/

Here is repo https://github.com/t4th/cortex-m3-rtos You have two branches with simple prove of concepts if you are curious about technical problems only.