GLOSSARY OF MARINE NAVIGATION 866 unit, n. A value, quantity, or magnitude in terms of which other values, quantities, or magnitudes are expressed. In general, a unit is fixed by definition and is independent of such physical conditions as tem- perature. See also STANDARD, definition 2; INTERNATIONAL U SYSTEM OF UNITS. United States Coast Pilot. One of a series of SAILING DIRECTIONS published by the National Ocean Service, that cover a wide variety Ulloa’s ring. See BOUGUER’S HALO. of information important to navigators of U.S. coastal and intrac- ultra high frequency. Radio frequency of 300 to 3,000 megahertz. oastal waters, and waters of the Great Lakes. Most of this informa- tion cannot be shown graphically on the standard nautical charts ultra quick light. A navigation light flashing at a rate of not less than 160 and is not readily available elsewhere. This information includes flashes per minute. See also CONTINUOUS ULTRA QUICK navigation regulations, outstanding landmarks, channel and anchor- LIGHT, INTERRUPTED ULTRA QUICK LIGHT. age peculiarities, dangers, weather, ice, currents, and port facilities. ultrashort wave. A radio wave shorter than 10 meters. A wave shorter Each Coast Pilot is corrected through the dates of Notices to Mari- than 1 meter is called a MICROWAVE. See also WAVE. ners shown on the title page and should not be used without refer- ultrasonic, adj. Having a frequency above the audible range. Frequencies ence to the Notices to Mariners issued subsequent to those dates. below the audible range are called INFRASONIC. See also SU- United States National Map Accuracy Standards. A set of standards PERSONIC. which define the accuracy with which features of U.S. maps are to ultrasonic depth finder. A direct-reading instrument which determines be portrayed. 1. Horizontal accuracy: For maps at publication scales the depth of water by measuring the time interval between the emis- larger than 1:20,000, 90 percent of all well-defined features, with sion of an ultrasonic signal and the return of its echo from the bot- the exception of those unavoidably displaced by exaggerated sym- tom. A similar instrument utilizing signals within the audible range bolization, will be located within 0.85 mm of their geographic po- is called a SONIC DEPTH FINDER. Both instruments are also sitions as referred to the map projection; for maps at publication called ECHO SOUNDERS. scales of 1:20,000 or smaller, 0.50 mm. 2. Vertical accuracy: 90 umbra, n. 1. The darkest part of a shadow in which light is completely cut percent of all contours will be accurate within one-half of the basic off by an intervening object. A lighter part surrounding the umbra, contour interval. Discrepancies in the accuracy of contours and el- in which the light is only partly cut off, is called the PENUMBRA. evations beyond this tolerance may be decreased by assuming a 2. The darker central portion of a sun spot, surrounded by the lighter horizontal displacement within 0.50 mm. Also called MAP ACCU- PENUMBRA. RACY STANDARDS. uncorrecting, n. The process of converting true to magnetic, compass, or universal plotting sheet. See under SMALL AREA PLOTTING SHEET. gyro direction, or magnetic to compass direction. The opposite is Universal Polar Stereographic grid. A military grid system based on the CORRECTING. polar stereographic map projection, applied to maps of the earth’s polar regions north of 84° N and south of 80° S. uncovered, adj. & adv. Above water. The opposite is SUBMERGED. See Universal Time. Conceptually, time as determined from the apparent di- also AFLOAT; AWASH. urnal motion of a fictitious mean sun which moves uniformly along undercurrent, n. A current below the surface, particularly one flowing in the celestial equator at the average rate of the apparent sun. Actual- a direction or at a speed differing from the surface current. See UN- ly, Universal Time (UT) is related to the rotation of the earth DERTOW, SUBSURFACE CURRENT, SURFACE CURRENT. through its definition in terms of sidereal time. Universal Time at under the lee. To leeward. any instant is derived from observations of the diurnal motions of undertow, n. Receding water below the surface of breakers on a beach. the stars. The time scale determined directly from such observations See also UNDERCURRENT, SUBSURFACE CURRENT, SUR- is slightly dependent on the place of observation; this scale is des- FACE CURRENT, BACKRUSH, RIP CURRENT. ignated UT0. By removing from UT0 the effect of the variation of underway, under way, adv. Not moored or anchored. See also ADRIFT. the observer’s meridian due to the observed motion of the geo- See also MAKING WAY. graphic pole, the scale UT1 is established. A scale designated UT2 results from applying to UT1 an adopted formula for the seasonal undevelopable, adj. A surface not capable of being flattened without dis- variation in the rate of the earth’s rotation. UT1 and UT2 are inde- tortion. The opposite is DEVELOPABLE. pendent of the location of the observer. UT1 is the same as Green- undisturbed orbit. See NORMAL ORBIT. wich mean time used in navigation. See also TIME SCALE. undulating, adj. Having the form of more or less regular waves. Universal Transverse Mercator (UTM) grid. A military grid system undulating light. See under FIXED AND FLASHING LIGHT. based on the transverse Mercator map projection, applied to maps undulation of the geoid. See GEOIDAL HEIGHT. of the earth’s surface extending to 84°N and 80°S. undulatus, adj. Having undulations, referring to a cloud composed of unlighted buoy. A buoy not fitted with a light, whose shape and color are elongated and parallel elements resembling ocean waves. the defining features; may have a sound signal. unlighted sound buoy. See under SOUND BUOY. unfavorable current. A current flowing in such a direction as to decrease unmanned light. A light which is operated automatically and may be the speed of a vessel over the ground. The opposite is FAVOR- maintained in service automatically for extended periods of time, ABLE CURRENT. but with routine visits for maintenance purposes. Also called UN- unfavorable wind. A wind which delays the progress of a craft in a de- WATCHED LIGHT. sired direction. Usually used in plural and chiefly in connection unperturbed orbit. See NORMAL ORBIT. with sailing vessels. A wind which aids the progress of a craft is unsettled, adj. Pertaining to fair weather which may at any time become called a FAIR or FAVORABLE WIND. See also FOLLOWING rainy, cloudy, or stormy. See also SETTLED. WIND, HEAD WIND. unstabilized display. A radarscope display in which the orientation of the Uniform State Waterway Marking System. An aids to navigation sys- relative motion presentation is set to the ship’s heading and changes tem developed jointly by the U.S. Coast Guard and state boating ad- with it. ministrators to assist the small craft operator in inland state waters unstabilized in azimuth. See under STABILIZATION OF RADAR- marked by states. It consists of two categories of aids to navigation. SCOPE DISPLAY. One is a system of aids to navigation, generally compatible with the unwatched light. See UNMANNED LIGHT. Federal lateral system of buoyage, to supplement the federal system upper branch. That half of a meridian or celestial meridian from pole to in state waters The other is a system of regulatory markers to warn pole which passes through a place or its zenith. the small craft operator of dangers or to provide general information upper culmination. See UPPER TRANSIT. and directions. upper limb. The upper edge of a celestial body, in contrast with the LOW- unipole antenna, n. See ISOTROPIC ANTENNA. ER LIMB, the lower edge. unique sanctuary. A marine sanctuary established to protect a unique upper transit. Transit of the upper branch of the celestial meridian. Tran- geologic, oceanographic, or living feature. See also MARINE sit of the lower branch is called LOWER TRANSIT. Also called SANCTUARY. SUPERIOR TRANSIT, UPPER CULMINATION. 867 GLOSSARY OF MARINE NAVIGATION uprush, n. 1. The rush of the water onto the foreshore following the break- vardar, n. A cold fall wind blowing from the northwest down the Vardar ing of a wave. 2. See RUN-UP. valley in Greece to the Gulf of Salonica. It occurs when atmospher- upstream, adj. & adv. Toward the source of a stream. The opposite is ic pressure over eastern Europe is higher than over the Aegean Sea, DOWNSTREAM. as is often the case in winter. Also called VARDARAC. up-the-scope echo. See CLASSIFICATION OF RADAR ECHOES. vardarac, n. See VARDAR. upwelling, n. The process by which water rises from a lower to a higher variable, n. A quantity to which a number of values can be assigned. depth, usually as a result of divergence and offshore currents. Up- variable parameters of satellite orbit. See under FIXED AND VARI- welling is most prominent where persistent wind blows parallel to ABLE PARAMETERS OF SATELLITE ORBIT. a coastline so that the resultant wind-driven current sets away from variable range marker. An adjustable range ring on the radar display. the coast. Over the open ocean, upwelling occurs whenever the variable star. A star which is not of constant magnitude. wind circulation is cyclonic, but is appreciable only in areas where variance, n. The square of the standard deviation. that circulation is relatively permanent. It is also observable when variation, n. 1. The angle between the magnetic and geographic meridians the southern trade winds cross the equator. at any place, expressed in degrees and minutes east or west to indi- upwind, adj. & adv. In the direction from which the wind is blowing. The cate the direction of magnetic north from true north.