The primary source of information about the world outside the hull of a submerged submarine is sound, detected by instruments and translated into visual data by computers. Sonar—an acronym for sound navigation and ranging—allows submariners to locate and track their targets, identify potential threats, and determine their own position. The sonar dome, a spherical array of several hundred sound detectors (hydrophones) mounted near the bow, is often supplemented (for improved accuracy) by a towed array—a series of hydrophones mounted on a cable towed behind the submarine.

Active sonar bounces sound waves off the target and detects the reflected echoes; it is rarely used because it can also easily be detected by the target. Passive sonar detects sounds generated by the target, such as clanking machinery or noisy propellers. Sonar can be used at any depth. All other means of observation and communication are more dangerous because they require bringing the submarine up to periscope depth. Visual observation, radio communication, and navigational updates all require running near the surface, where submarines are most vulnerable.

Submarines can receive radio waves of very or extremely low frequency (VLF/ELF), which can penetrate seawater deeply; this one-way communication allows submarines to remain in constant contact with the outside world. The ability to receive VLF/ELF was, and still is, especially important to boomers, which must have presidential authorization to launch their missiles.

Two-way communication requires submarines to come to periscope depth and break the surface with an antenna, risking detection. Communications satellites have helped minimize the risk by greatly reducing the time spent at periscope depth to exchange data.

Radar is an acronym for radio detection and ranging. A radar system transmits a radio beam, then detects and measures the echo when the beam bounces off an object. This provides information about both the target's direction and range. Like periscopes and radio antennas, however, radar masts protruding above the water provide vital information at the expense of increased risk of detection. Submarines generally use radar only on the surface when leaving or entering port.

AN/BQQ 5 Sonar System
Each of these three consoles displays data from a different sector (direction) around the boat. The external sounds converted into graphic images, called "waterfall" displays, are compared to holdings in the library of sound signatures in order to identify individual from potential targets. A sound signature is the unique, repetitive noise created by every ship's rotating machinery, such as engines, propellers, and pumps. Submarine sonar technicians analyze and record these signatures for the sound library. They also record background noises, such as "biologics" (shrimp, whales, and fish), for comparative purposes. Basic training for sonar technicians takes three months on shore; there can be up to another 15 months of advanced schooling as well.

AN/BQH-1 Speed of Sound Measuring System
This device above the sonar console continuously measures and displays the depth, temperature, and salinity of the surrounding seawater to compute the speed of sound in the ocean. These data are essential to the interpretation of incoming sonar data and the avoidance of detection by active or passive sonar. A similar recorder is located in the attack center.

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