forms of EMP, one caused by very high-altitude nuclear explosions, the other by ones close to the ground. (Mid- altitude bursts do not create an EMP.) Nonnuclear EMP devices, a form of modern nonlethal weapon, produce a similar effect locally by vaporizing clusters of tungsten filaments using a high-voltage firing charge. This generates a burst of hard X rays, which are focused by a depleted-uranium reflector to strip electrons from atoms in the targeted area, creating the destructive EMP electrical current. Other nonnuclear EMP weapons use bursts of microwaves emitted from special antennas.
ESGN: the latest submarine inertial navigation system (see
Fathom: a measure of water depth equal to six feet. For instance, one hundred fathoms equals six hundred feet.
Firing solution: exact (or best estimate) information on an enemy target’s location, course, and speed, and depth or altitude if applicable. A good firing solution is needed to preprogram the guidance system of a missile or torpedo so that the weapon won’t miss a moving target.
Floating wire antenna: a long, buoyant antenna wire that is trailed just below the surface by a submerged submarine, for stealth. Such an antenna can receive data at a higher rate (higher baud rate) than ELF radio (see above). Recently, floating-wire-antenna technology has been developed to the point where the wire is able to transmit as well as receive, allowing two-way radio communication while the submarine is completely submerged. (To transmit or receive radio data at a very high baud rate, such as live video imagery of a target, the submarine must come to periscope depth and raise an antenna mast out of the water — which might compromise stealth.)
Frequency agile: a means of avoiding enemy interception and jamming, by very rapidly varying the frequency used by a transmitter and receiver. May apply to radio or to underwater acoustic communications (see
Frequency power spectrum: a display of the relative strength of noise being detected by a sonar array at different sound frequencies. Such data can be valuable in locating and identifying passive sonar contacts, especially when tonals (see below) stand out within the display.
Frigate: a type of oceangoing warship smaller than a destroyer.
Fuel cell: a system for quietly producing electricity, for example to drive a submarine’s main propulsion motors while submerged. Hydrogen and oxygen are combined in a chemical reaction chamber as the “fuels.” The by-products, besides electricity, are water and heat.
Gertrude: underwater telephone. Original systems simply transmitted voice directly with the aid of transducers (active sonar emitters; i.e., underwater loudspeakers) and were notorious for short range and poor intelligibility. Modern undersea acoustic communication systems translate the message into digital high- frequency active sonar pulses, which can be frequency agile for security (see above). Data rates well over one thousand bits per second, over ranges up to thirty nautical miles, can be achieved routinely.
Halocline: an area of the ocean where salt concentration changes, either horizontally or vertically. Has important effects on sonar propagation and on a submarine’s buoyancy.
Hertz (or Hz): cycles per second; applies to sound frequency, radio frequency, or alternating electrical current (AC).
Hole-in-ocean sonar: a form of passive (listening-only) sonar that detects a target by how it blocks ambient ocean sounds from farther off. In effect, hole-in-ocean sonar uses an enemy submarine’s own quieting against it.
Hydrophone: an underwater sound listening device. In essence, a hydrophone is a special microphone placed in the water. The signals received by hydrophones are the raw input to passive (listening-only) sonar systems. Signal-processing computer algorithms then continually analyze this raw data to produce meaningful tactical information — such as a firing solution (see above).
INS: Inertial Navigation System. A system for accurately estimating one’s position, based on accelerometers that determine from moment to moment in what direction one has traveled, and at what speed.
Instant ranging: a capability of the new wide-aperture array sonar systems (see below). Because each wide-aperture array is mounted rigidly along one side of the submarine’s hull, sophisticated signal processing can be performed to “focus” the hydrophones at different ranges from the ship. The target needs to lie somewhere on the beam of the ship(i.e., to either side) for this to work well.
IR: infrared; refers to systems that make it possible to see in the dark or detect enemy targets by the heat that objects give off or reflect.
ISLMM: Improved Submarine-Launched Mobile Mine. A new type of mine weapon for American submarines, based on modified Mark 48 torpedoes and launched through a torpedo tube. Each ISLMM carries two mine warheads that can be dropped separately. The ISLMM’s course can be programmed with way points (course changes) so that complex coastal terrain can be navigated by the weapon, and/or a minefield can be created by several ISLMMs with optimum layout of the warheads.
Isothermal: a layer of ocean in which the temperature is very constant with depth. One example is the bottom isothermal zone, where water temperature is just above freezing, usually beginning a few thousand feet down. Other examples are a surface layer in the tropics after a storm, when wave action has mixed the water to a constant warm temperature; and a surface layer near the Arctic or Antarctic in the winter, when cold air and floating ice have chilled the sea to near the freezing point.
Kampfschwimmer: German Navy “frogman” combat swimmers. The equivalent of U.S. Navy SEALs and the Royal Navy’s Special Boat Squadron commandos. (In the German language, the word
KT: kiloton; a measure of power for tactical nuclear weapons. One kiloton equals the explosive force of one thousand tons of TNT.
LIDAR: Light Direction and Ranging. Like radar, but uses laser beams instead of radio waves. Undersea LIDAR uses blue-green lasers, because that color penetrates seawater to the greatest distance.
Littoral: a shallow or near-shore area of the ocean. Littoral areas present complex sonar conditions because of bottom and side terrain reflections, and the high level of noise from coastal shipping, oil- drilling platforms, land-based heavy industry, etc.
