How do I achieve auto flight use pixhawk2.4.8 without GPS?

I want my plane to autonomously fly whitout GPS and avoid obstacles based on the terrain. And I also hope it can automatically recognize letters like “ABC”, then fly to where the letters are.

What technologies do I need to learn? I serched in internet and I think maybe slam,openmv can help me. But I have not learned too much about this so I’m very confused. Can you tell me more related technologies I need to learn?

I would appreciate it if you could also recommend me some related teaching videos.

This is quite a large topic you are going into. I would suggest starting to learn some basic computer vision, which will give you the basics for the rest that you will need. A place to start can be OpenCV’s tutorials: Tutorial
This might also help you with the part of automatically recognize letters. Might need some background info, which the youtube Cyrill Stachniss often has some good one. An example is this

After this I would as you said yourself look into SLAM and maybe also look into optical flow, which can be used for the navigation. Optical flow might be the most straightforward point to start with for these two. But both of these are quite advanced topics if you have no experience with them, so it will take quite some time, but they are also very interesting topics. Unfortunately I don’t have any good links to tutorials for this.

Thank you very much for your suggestion!

I have also seen some related technologies in the past few days. My current idea is to use the Raspberry Pi as an onboard computer to run ROS and Slam, and cooperate with Dronekit to send some commands to Pixhawk to control the drone. The image recognition aspect intends to use OpenMV. But these are not in-depth, I will seriously study the various techniques you said and improve my drone.

Finally thank you again for your guidance!

No problem. Another suggestion it might be challenging to run slam in realtime on a raspberry Pi, so when you start implementing that, make sure to look if it runs in realtime or if there is some delay, since it can causes some significant problems if not taken into account.