So let’s talk for a minute about the environment that ASW takes place in: The Ocean.
Unlike combat on land, or even on the water’s surface, hunting the enemy underwater has a unique set of variables that must be taken into consideration. For starters, you can’t see the target, and he can’t see you. It’s the one element where, for all intents and purposes, both sides are fighting blind. That doesn’t mean we can’t “see” the enemy, but it means we can’t visually ID him. We have to use our sense of hearing as our prime sensor.
Now the Ocean is made up of salt water, and has an enormous number of life forms which call it home. These, too, must be taken into consideration, because they make sounds which will interfere with our target’s sounds. Additionally, there are all sorts of vessels plying their trade up on the surface, and they also radiate noise into the water, which will bounce around and help mask our elusive prey. So what to do, what to do.
As talked about in other posts and comments, you can plop a hydrophone into the water and listen. You can get general information that way, but to get the information in a manner that alolows us to locate, fix, and attack or track a target means we need to refine the data.
Sound in water is affected by a variety of environmental variables. The three general effects are Absorption, Refraction, and reflection. These will muffle, scatter, or bounce the sound waves as they travel between the noise maker and the listener. To these three general effects must be added some physical ones: Temperature, Salinity, and Pressure, known as the “TeaSPoon” effects. The lower the temperature, the faster the sound waves will travel, because the molecules which vibrate are closer together, and therefore use less energy transmitting the sound, thus allowing for more energy to be transmitted. Salinity and pressure also add speed, due to making the water more dense. Considering the speed of sound 4388 feet per second, the equation is thus:
4388 + (11.25 × temperature (in °F)) + (0.0182 × depth (in feet)) + salinity (in parts-per-thousand )
This will give you a close approximation of the actual speed of sound and can be further refined if needs be.
Now, sound is also effected by pressure in two ways: The more deep the sound goes, the more resistance it meets until eventually it is bent upwards. It will then travel upwards until it reaches the thermocline, where the warmer water will bend the sound back down and the process will repeat. The stronger the sound energy, the longer this pattern can repeat. I have personally tracked one intense sound source, traveling in this manner, more than 400 miles away. The name for this effect is called a “convergence zone”. If you could look at it visually, it would appear as a series of ever-expanding donuts. Inside of each donut, the sound will appear to the listener as a “direct path”, but once you reach the donut hole, or get outside the donut, the sound drops off.
When a Convergence Zone, or “CZ” as it’s called in the trade, is not present, then almost always you will have “direct path” contact. that means that the sound radiating from the target is traveling directly to the hydrophone with little to no interference.
More data here
How do we deal with all of this data? How does this come into play? Here follows a general overview for a crew operating a P-3 Orion.
On the way to the station, the crew is developing the basic environmental data. The Op Area has already been defined for them, developed from other data and presented at the briefing. The distance to the Op Area will determine the amount of time spent on station. Missions usually lasted 10-12 hours, and the shorter the transit time, the better.
Once onstation, the crew will drop a BT buoy to garner local temperature data and add that into their equations. The environmental data (coupled with the type of target) will determine what distance to space the sonobuoys, and also what type of pattern (and how many buoys) they will be dropped in. The P-3 can monitor 16 buoys at one time, and has a maximum of 31 separate channels to receive the data on. Technically, you could drop 31 buoys, and monitor them alternately, but that is never done. You use one pattern to generate contact, then add more buoys to refine the data as the situation develops.
The normal pattern is a single line, called a barrier, across the estimated path of the target. These barriers can be many miles long, with the buoy spacing designed so that their detection ranges overlap, not leaving any dead zones for the target to slip through. It’s just like having overlapping fields of fire for machine guns, artillery, or even radars.
One other modifier should be talked about: weather. Weather affects both the target and the crew on the P-3. It isn’t a whole lot of fun bouncing around in turbulence for hours at a time, and the crew’s effectiveness can easily be degraded. If the storm is large enough to increase the sea state and wave height, then it can (and will) rip the hydrophone cables right out of the buoy. Rain will also cause more noise on the surface, which, for the most part only effects shallow water, or littoral areas, and targets operating in the thermocline.
In the old days, Soviet subs kept a careful eye on the weather, and would maneuver whenever possible to be under the local thunderstorm. It used to be a standing joke amongst the flight crews that the easiet way to find a sub was to locate the nearest storm cell in the op area.
Okay, that’s the basics. next post will talk about tactics, and what happens onstation.