I want to make a ground to ground non-line-of-sight (NLOS) data link. As I understand SDRs are perfect for this because I can specify frequency, bandwidth, channel size, etc., making it suitable for my use.
Where could I start? Is there a standard library for programming them? I would like to transmit ~200kbps for a few km in city and forest.
Maybe I could use SDR for NVIS?
To be honest, I think your question is too broad – "where do I start" really has no "correct" answer. However, I choose to interpret your question as
Please point me to a standard reference on learning how to do SDR.
There's plenty of them. It really depends on what background you bring and how deep you want to dive. To get a rough estimate on what people deem helpful references, GNU Radio's wiki has a page "Suggested Reading" with a couple dozen references.
I think a good entry point might be PySDR: A Guide to SDR and DSP using Python by Marc Lichtman. It's very hands-on, you get very practical results, and afterwards you're well-equipped to understand other people's and do your own software-defined radio implementations. It requires you to understand python, but StackOverflow says you've been using Python for two years – you might feel capable enough.
I don't think much less gets you far. And if you don't have a solid engineering-level background in math, much more might probably overload you.
Maybe I could use SDR for NVIS?
The direction you send your signals and how you generate them are really independent problems. You might want to read the wikipedia pages on both SDR and NVIS!
