ardupilot VS RIOT

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

RIOT-OS is somewhat popular in academia, it tries to be very approachable but there are still plenty of low hanging fruits with all kinds of hardware and you can use it for software and thesis projects.

Just one example: https://github.com/RIOT-OS/RIOT

You could also take a look at RIOT-OS. http://riot-os.org

RIOT-OS has a lot of unit tests for all the boards they support. And it's written in C. https://github.com/RIOT-OS/RIOT/tree/master/tests

Another option is using RIOT-OS. But that unfortunately doesn't support PWM for the Weact. It does support PWM for the Bluepill and Blackpill STM32f103 and the Nucleo-f446 After quick search. \ https://riot-os.org

If you want to use C, I would suggest trying out RIOT-OS. It's a modern embedded operating system, written in C and focussed on IOT. It works on a lot of different boards. https://github.com/RIOT-OS/RIOT

Alternatively, you can also write your code in an RTOS. They often have scripts that make flashing and debugging easy. One that I personally like, and also supports Arduino, is RIOT-OS.