TTU usually stands for through transmission. I would like to see a TTU immersion unit. I've used the fokker. It's a nice setup but didn't care much for the a/b scale numerical display. However; it is about as good as you get with UT for carbon fiber honeycomb structures.
If you've got that background, I suggest looking into Power Utilities for Phased array UT. OR PAUT. You may not have done it directly but it should'nt be to much of a stretch for you to understand and implement as I suspect your already dealing with constructive,destructive interference issues, and resonance manipulation. It would'nt be much shorter stretch to understanding how it all forms into a single wave front/steers etc for PA.
What I suggest you do is this. While your at work, consider the physics of what your doing, refresh you mind on the specifics of the exam your doing, then load up this link.
http://www.olympusndt.com/en/ndt-tutorials/intro/whatispa/Read through it, if you click on the reasoning right away, you'll be good to go for PA.
My opinion for what it's worth,
Gerald