Jive's Sub Brief: Russia's Secret Sonar System

Broadly speaking there are 2 major types of fixed sonar arrays:

1. the very long, large transoceanic Sound Surveillance System (SOSUS which still exist in all oceans! eg. see Section B.) type that can detect and locate submarines (by triangulation) 1,000s of km away. This type is better for deep, open ocean where there is less sound distortion caused by shallow seabeds, rocks, islands and other landforms.

and

2. Reliable Acoustic Path/Fixed Distributed System RAP/FDS systems (like "Harmony" in the video) which have many more sensor-receivers (and active sonars) per km permitting more acute location of close submarines/UUVs. They are much better suited to sound-distorted conditions (eg. shallow water, littorals, and relatively narrow waters between landforms). They can be combined with fixed or mobile mines/UUVs (which may use lightweight torpedos) to defend high value assets (eg. one's submarine bases). This may involve destruction of snooping enemy submarines or AUVs/UUVs in wartime and perhaps on the approaches to, or in, one's bases/ports/harbours even during cold wars.

Both sonar array types can work together and can detect, then locate, enemy submarines/AUVs/diver delivery vehicles/small boats/or USVs in order to "cue" friendly submarines or other naval, air assets or land-based missiles onto the enemy vessels.