A top-down game of submarine hunting. You never see the enemy on the plot. You hear it: a bearing, a sound, an echo. Turn those into a position before they do the same to you.
SE = SL − TL − NL + DI − DT
signal excess = radiated noise − transmission loss − background noise + array gain − detection threshold
Positive means you hear them. Every knot of speed, every foot of depth, and the weather move these numbers.