Wednesday, August 10, 2016

Ocean Security: Submarine threats

Submarine threats have become the stuff of legends. Ever since WWII when the German Kriegsmarine and the Imperial Japanese Navy made extensive use of submarines to attack allied surface ships - the military utility has never been doubted.

It is not a surprise that narcotraffickers today use submarines to move large quantities of drugs or that most nuclear states employ ballistic missile carrying submarines ("boomers") as part of their deterrent.

It is also not surprising that submarines have played a critical role in gathering intelligence and intercepting enemy communications (ex. Operation Ivy Bells).

Fueled by Hollywood movies, the public imagination of what is possible with a submarine is boundless - the real world has to cope with physical facts like
  • There is not enough air dissolved in the water.
  • As you go deeper under water the pressure on you increases with depth.
  • Water offers significant drag.
  • Sound travels really far under water - especially in the SOFAR channel. 
  • Radio signals and light do not travel far in water.
  • Undersea currents and geography can change with little warning. 
So in the real world - submarines have limitations:
  1. Submarines cannot stay submerged forever - they need air and their crew needs to see the surface. Most submarines are underway for a period of time that is limited by their need to breathe air for the engines - or by the psychological needs of the crew (for AIP or nuclear subs). 
  2. Submarine navigation is quite complex - you really can't see what is around you.  Navigating a submarine requires the use a combination of inertial, stellar and RF/GPS referencing and relies on expensive hydrographic maps to keep clear of undersea obstacles and adverse currents.One can only really operate a submarine properly if one has a sense of where one is relative to the points on the hydrographic map. 
  3. Submarines can operate safely up to a depth of several hundred feet - going below that depth risks hull damage. So submarines rarely go below 700 feet. 
  4. The motion of the submarine under water creates a significant wake and acoustic signature - which can be used to detect the presence of the submarine. If one gets too close to the surface then the wake becomes visible to enemy surveillance platforms. If one goes down deeper and enters the SOFAR channel then even small sounds can be heard by shore based detectors hundreds of miles away. The exact location of the SOFAR channel varies depending on where you are in the ocean.
  5. Weapons can only be released from a submarine at certain depths - if you go too deep, neither the weapon nor the launch mechanism can withstand the pressure.  
  6. Submarine communication is costly - either you have to use an extremely low frequency with low bandwidth or the submarine has to raise a radio mast periodically to be able to receive updates.  
  7. A submarine hull is usually a very large piece of metal (typically steel). This shows up as a massive magnetic anomaly against the water surrounding it. A magnetic anomaly based detection scheme is usually used to sense the exact position of a submarine. 
The sum of all this is a submarine is a very expensive asset, that can operate safely and reliably is a few parts of the ocean.

The amount of supplies of food, fuel and fresh water stowed on board limit the time the submarine can spend underwater. While the theoretical maximums can be quite large - psychological factors can degrade combat effectiveness to unacceptable levels on much shorter timescales.

The exact choice of patrol path itself is limited knowledge of the topology of the ocean floor, by the quality of the inertial navigation systems. Once underwater - the submarine can only operate quietly by using a combination of gradiometry and gyroscopy. The quality of gyroscopes and gradiometers available on the submarine limits its ability to actually follow a specific path laid out on a hydrographic map. All these constraints further limit the combat patrols corridors to a fairly small set.

When travelling large distances for its mission (as for example a land attack submarine like a boomer might do) the submarine will remain in well traveled sea lanes as these sea lanes are deep enough and well charted to allow it to move undetected relatively quickly. In such lanes, it should be relatively simple to replenish the submarine's stores via a tender ship. All the combat patrol corridors will emerge off these established sea lanes.

Each patrol corridor will require a certain degree of accurate hydrographic survey before extensive utilization. The presence of hydrographic survey activities will presage any actual patrol. These are relatively easier to detect than a submarine on a hostile mission.

Any submarine on combat patrol seeking to release a weapon will have to approach the surface. This will leave it vulnerable to detection by enemy systems. Once the weapon is released, the location of the submarine will be compromised and the enemy will be able to send reaction forces to the last known location of the submarine.

A defender who is able to maintain a close watch on the most likely patrol corridors has a good chance of detecting the emergence of a threatening presence in them and react to adverse situations.


At 5:17 AM, Blogger Ralphy said...

only one small nit pick: modern subs make their own fresh water and fresh air. At least the ones I have read about such as the nuclear powered subs of Britain and the US. Others I have admittedly no factual knowledge there of.

At 5:20 AM, Blogger Ralphy said...

The other danger especially to subs is automated anti ship mines.

At 11:30 AM, Blogger maverick said...

Hello Ralphy,

Not everyone has good diesel electric subs. AIPs are more expensive. Very few nations have nuclear subs. I was trying to get all categories in a single sentence.

Good point about mines.


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