Wheezer posted:

1

There was a write-up about Target Motion Analysis a while ago, but someone mentioned the perfect firing solution and I'd like to point out getting it with passive sensors is a bitch.

For TMA you need:
A) several target bearings collected over time and
B) speed of the target or
C) distance to it over time

From enough data you can compute the missing variables. Heading is difficult to determine immediately unless your target is on the surface and you can just look at it, so TMA used. To an extent TMA becomes more difficult the more the target is manouvering, but ship's own manouvers can reduce uncertainty as we saw.

Target bearing is usually the first bit of information available. Submarines have several hydrophones in each sonar array and bearing can be determined pretty accurately from just the contact. It works by calculating from the time difference it takes for the sound to arrive to each hydrophone. A single directional hydrophone can also give an educated guess.

The towed array is a great asset for firing solutions. Since it isn't built in to the ship's hull it's very sensitive. Also, since it doesn't have a noisy propeller behind it, it clears the huge blind spot behind the submarine. If a submarine can hear the target on another array as well as the towed array, target distance can be determined, just not exactly. Every bit of information helps with TMA however, and the towed array is a separate sonar array.

One way to determine target speed is to listen to the propeller sound. Propellers always make a sound when turning and much more when cavitating. The WWII method was to listen to the propeller sound and count the beats per minute, then estimate speed based on ship category (merchant, warship slow or warship fast.) Submarines use demodulation of noise (DEMON.) DEMON gives more accurate information. For DEMON to work you need to know how many blades the target's propeller has. DEMON then tells you how fast the propeller is turning. If you know what ship you are listening to you can calculate its speed.

For figuring out what exactly the target is submarines listen to the narrowband sounds. Each ship has a distinct sound profile: Ship propellers, engines, coolant pumps and other systems each make a sound and each combination is unique. The narrowband frequencies can be used to distinquish between ships. Frequency analysis is pretty accurate, given you can hear the target clearly enough.

As an example, the diesel electric engine of a Kilo will make a sound in the 130-150 Hz range (the motor slot passage frequency.) The ships transmission will also make a sound unique to the ship. Listen hard enough and have a good enough system for analysis and you can determine what exactly you are listening to without giving away anything about yourself.