It’s dark and foggy. You’re in charge of port security. There’s no way to see what’s happening in your port. However, you do have hydrophones placed in the waters throughout your harbor to listen for ship movement. Your screen displays the images of the acoustic waves produced by a vessel entering your port. Although you can’t see the ship, it’s up to you to identify the type of ship and how it’s moving. You must virtually lift the fog to “see” what’s going on in your harbor. The safety of your port and people depend on your skills.
What is acoustic ship tracking?Acoustic ship tracking locates and identifies ships based on the sounds they make. Hydrophones, which are underwater microphones, are used to listen to the ships. The sounds can then be processed to pick out the distinguishing elements from the noise.
How does acoustic ship tracking work?When a sound is detected, a mathematical process can be used to split the sound into its component frequencies. The frequencies that are detected, as well as the volume of these frequencies, are an acoustic 'fingerprint' that identifies the type of ship. There is also usually one frequency that is louder than the others. Some ship tracking devices use computer databases to find the ship that matches the sound, while others use trained operators. The ship can be located because sound does not travel instantly. The hydrophones are spaced throughout the water. The time that it takes for the sound to reach each hydrophone and the location of each hydrophone can be used to calculate the location of the ship. Additionally, Doppler shifts in the pitch of the sound can be detected. If the ship is moving away from the hydrophones, the pitch decreases. If the ship is moving towards the hydrophones, the pitch increases. The faster the ship is moving, the more the pitch changes.
Why do ships sound different from each other?Different types of ships have different engines, which can be identified by sound. A Jet Ski uses a pump-jet instead of a standard propeller. Tugboats use very large, slowly turning propellers. Cavitation, which is when a propeller is turning quickly enough to boil the water around it, is very noisy because of the sound of the collapsing bubbles. The hull of the ship also has a resonant frequency which is the pitch that it amplifies. The resonant frequency is dependent on the size and construction of the hull.
Is acoustic ship tracking a new technology?Acoustic ship tracking, which is also known as passive sonar, was created near the end of World War One to detect submarines. In World War Two, it was fitted to submarines so they could detect surface ships. Passive sonar was again widely used during the Cold War to detect nuclear missile-carrying submarines. This led to a competition between increasing the processing power and sensitivity of the sonar arrays in order to better detect submarines, and making the submarines quieter and harder to detect. Acoustic ship tracking is also used in ports to identify different ships. Shipping traffic is extremely loud compared to a submarine, so the sonar has no problem detecting ships in the port.
What's the difference between passive and active sonar?Active sonar works like a bat's chirping or a dolphin's squeaks: it bounces a sound wave off of objects to 'see' the shape of the objects. Passive sonar listens, and it can't 'see' the shape of an object. It does have certain advantages over active sonar. Passive sonar can identify ships more easily, even sometimes being able to identify a particular ship in a class of ships. It can also sometimes identify activity on the ship. This was particularly important in submarine warfare because of the characteristic sound of torpedoes being loaded into launch tubes.
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