Learn the simple block diagram of a radar. You wont be expected to know what each component does but if you do, all the better. Know the differences between primary and secondary and what information we can get from them. You wont be expected to know a radar mile, sufficient to know that the distance is calculated by the time it takes for a pulse to hit a target and return.
You may get a circuit board put infront of you and asked to name the components and values etc. They may also throw in a daft thing like a plug wired wrong (they have in the past).
All Nats managers have done the same interview techniques course so you will be marked for simple things like seeming keen, eye contact and for Gods sake dont slouch down in your seat, sit up. These daft marks could be the difference between getting selected and not.
It's a tough interview process so good luck