Security Clearance Question

Looking for some specific information regarding the security investigation process.

Surprisingly, everything online is extremely vague and no direct answers are given. On the FAA site, it states that you need to pass a "security investigation." Would that be a public trust? standard background investigation? secret clearance?

I've read from unofficial sources that enroute is the only facility that requires a secret clearance. Can someone confirm this?

The reason I ask is because I've recently been denied a security clearance for my current job. The reason in which I've been denied is extremely ridiculous and I'm currently appealing the government's decision. Assuming I end up not getting a clearance, does that mean I have zero chance at becoming an air traffic controller?

The FAA states that they run a security investigation to ensure that they hire people with clean records, good ethics, etc. I've been denied a security clearance for reasons outside their reasoning of conducting security investigations.