The UK Royal Navy (RN) successfully developed a hydrophone system and "indicator loop" magnetic anomaly technology during WWI. By 1918 it was an effecient system defending the RN Grand Fleet's base at Scapa Flow. The German submarine UB-116 was detected by hydrophones on 28 October 1918 attempting to enter the base. Two hours later UB-116 was detected by a magnetic anomaly system defending the base. Unfortunately for UB-116 that system was interlaced with a remotely controlled minefield which was exploded by electical impulse from on-shore RN personnel - thereby destroying UB-116.
By 1941 the RN had deployed magnetic anomaly and hydrophone systems to defend strategic harbours all over the British Empire (see this long list) including several Australian harbours. The RN also shared all this technology with the US Navy to defend many US harbours (also on list) during WWII. The US then developed the much more extensive Sound Surveillance System (SOSUS) since against the Soviet Navy from around 1950. The UK and presumably Empire/Commonwealth countries benefitted from SOSUS networks.
China with land wars and/or Mao to cope with for decades was a latecomer - likely having magnetic anomally and SOSUS networks since the 1990s. China had "Passive bottom arrays protect harbor entrances, extending out to sea about 20-30 nm" by 1997.
Now Chinese SeaWeb (Chinese sensors of all types on all platforms orientated to sea targets). Chinese SeaWeb is quicker reacting due to increased computer processing power and data storage capacity. China's NSA provides the essential network backbone. Such capacity makes Chinese SeaWeb better able to store and recognise the sonic (and other signatures) of foreign submarines. For example it is important to be able to distinguish between similar submarines such as Chinese, Vietnamese and Russian versions of the Kilo SSK. Once a Western (including Japanese) submarine is detected China is moving toward the ability to positively identify the nationality of a submarine or even identify an individual sub by its sonar signature or other signatures (including the submarine motion habits-routines of particular Captains).
The US-Western SEAWEB is assumed to be more advanced.
Non-acoustic (active or passive sonar) sensors may include:
- magnetic anomaly
- water pressure
- non-natural water anomaly
- IR light
- visible light (underwater CCTV)
- light emitting diodes (LED) bouncing light off the submarine hull
- anti-submarine nets or lines
- water temperature (from engine and hull heat)
- unnatural chemical levels from AIP (sniffers)
- unnatural radiation
- electronic emissions including sigint.
In a wartime situation China's SeaWeb would be transformed from just an intelligence tool to being the trigger for various types of anti-submarine weapons.
Around 2 minutes after detection a torpedo carried by missile might destroy the submarine. The weapon to destroy the detected submarine could be an Anti-Submarine Rocket (ASROC). ASROCs have existed for decades. They can carry Common Very Light Weight Torpedo (CVLWT) which may weigh less than 100kg. Multiple CVLWT could be launched with the onboard "intelligence" to strike particularly vulnerable parts of the sub.
China has been steadily developing Anti-Submarine missiles with:
- a rocket engine, such as the CY-1 (supersonic flight out to 20km)
- or jet engines. The CY-2 uses a C-802 missile for subsonic flight out to 55 km.
At the upper end of the anti-submarine spectrum China's DF-21D "carrier killer" ballistic missiles could deploy one or more light weight torpedos or depth bombs with conventional or nuclear warheads against high value submarine targets.
As Bryan Clark indicated in May 2015 a logical response to this greater sensitivity and lethality of anti-submarines sensors is increased use of Unmanned Underwater Vehicles (UUVs). This saves manned submarines from harm or capture. UUVs can be launched and serviced by manned submarines. The relatively small size of UUVs makes them harder for undersea sensors to detect. Manned submarines can also install recharging and data download depots on the seafloor (one depot is called OceanWorks "sub-sea dock").
Types of UUVs
UUVs with sufficient range for a 100 km (round trip) reconnaisance mission, towards an opponents coast, might be the size and weight of a Mark 48 heavyweight torpedo. Hence launchable from current 533mm torpedo tubes. Such ranges are possible because UUVs do not have to move quickly (against higher water resistance) to perform a mission. Their propulsion would ideally be different from a Mark 48's.
Shorter range missions could be performed by lightweight torpedo sized UUVs - including the Bluefin range of Autonomous Underwater Vehicles (AUVs). These can also be launched in 533mm torpedo tube fitting containers.
Near static missions can be performed by Wave Gliders and static pods have been used by superpowers since Operation Ivy Bells.
It does not necessarily follow that UUVs larger than heavyweight torpedos are necessary. These "large diameter" or "large displacement" LDUUVs may the latest "must have" according to some American corporations but electronics are increasing in capabilities while shrinking into smaller packages.
There are always tradeoffs between weapon system choices. Rather than Bryan Clark's suggestion that a very large LDUUV launches several CVLWTs a very long range (even if slow moving) heavyweight torpedo would also make sense. Such a torpedo could pass over the undersea sensor danger zone and hit ships/subs in harbour or leaving harbour.