Drone for Electrical Engineering research

I’m planning to start a research project focused on developing a trajectory tracking control algorithm for a small, low-cost indoor drone.

The main requirement is to use a platform that allows me to access and modify its internal control code, so I can implement and test custom control strategies.

I initially considered using the Parrot AR.Drone 2.0, since it seems to have been a popular choice for this type of research, but because it has been discontinued, I’m not sure whether it is still a good option.

So I’d like to ask:

Are there any current alternatives with similar characteristics that would be suitable for this kind of project?

Ideally, I’m looking for something that is:

• small and safe for indoor use
• relatively affordable
• suitable for control and trajectory tracking research
• and compatible with custom firmware or low-level control modification