The smallest is active sonar (ping), the 2nd would be passive sonar, the 3rd visual, the 4th radar... usually maybe - because mods might change them... There used to be an illustration in the Silent Hunter Manual - I think... might have been a dream...
- but I do know that somewhere on the site here is a rather good description of it, but I wouldn't have a clue what to "search" on to find it (them)...
