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