Greenwich hour angle of the mean sun, expressed in time units plus 12 hours. GUIDE—Vessel designated in a formation or disposition as the one for others to keep station on. GUN SALUTE—Blank shots fired to honor a dignitary or in celebration. H-HOUR —The term used to designate the time for an operation to commence. HEAD-ON VESSEL—One vessel meeting another on a reciprocal or nearly reciprocal course involving risk of collision. HOIST—To move an article vertically upward by means of some hoisting rig. HULL DOWN—Said of a vessel when, because of distance and curvature of Earth, only the superstructure is visible. INFRARED —Transmission of signals by light outside the visual spectrum. This method, which may be directional or nondirectional, necessitates the use of special equipment and affords greater security than normal visual means. IRISH PENNANT—A loose end of line carelessly left dangling. JANAP—Joint Army-Navy-Air Force Publication. LATITUDE—Distance north (N) or south (S) of the equator, expressed in degrees and minutes. LONGITUDE—Distance east (E) and west (W) of the prime meridian, which runs through Greenwich, England. LORAN—An electrical navigation system by which hyperbolic lines of position are determined by measuring the differences in the time of reception of synchronized pulse signals from two fixed transmitters. MANEUVERING BOARD—A polar coordinated plotting sheet devised to aid in the solution of problems involving relative movement. MASTHEAD LIGHT—The white running light placed over a vessel's fore-and-aft centerline showing an unbroken light over an arc of the horizon of 225°, fixed to show the light from right ahead to 22.5° abaft the beam on either side of the vessel. MEAN TIME—Time based upon the rotation of Earth relative to the mean sun. MERCATOR PROJECTION —A conformal cylindrical map projection in which the surface of a sphere or spheroid, such as Earth, is conceived on a cylinder tangent along the equator. MESSENGER—(l) A line used to haul another heavier line across an intervening space; (2) One who delivers messages. NEED-TO-KNOW —A criterion used in security procedures that requires the custodians of classified information to establish, prior to disclosure, that the intended recipient must have access to the information to perform his/her official duties. NIGHT VISION DEVICES—Precision instruments that use electronic optics for observation, surveillance, and navigation. Also referred to as Night-Vision Sights. NTP—Naval Tactical Publication. NWP—Naval Warfare Publication. OCCULTING LIGHTS—A navigational aid in which the period of light is equal to or more than the period of darkness. OCCUPATIONAL STANDARDS—The minimum requirements for enlisted occupational skills of a certain rate or rating. OFFICIAL INFORMATION—Information that is owned by, produced by, or subject to the control of the United States Government. OFFICIAL VISIT—A formal visit of courtesy requiring special honors and ceremonies. OMEGA —An electronic navigational system. PELORUS—Device for taking relative bearings. PERSONNEL QUALIFICATION STANDARDS— Qualification for officers and enlisted personnel to perform certain duties. PHYSICAL SECURITY—That part of security concerned with physical measures designed to safeguard personnel; to prevent unauthorized access to equipment, installations, material and documents; and to safeguard them against espionage, sabotage, damage, and theft. POSITION ANGLE—The number of degrees an object seen in the sky is above the horizon. POWER-DRIVEN VESSEL—Any vessel propelled by machinery. AI-2