CHAPTER5SHORT RANGE AIDS TO NAVIGATIONDEFININGSHORTRANGEAIDSTONAVIGATION500.TermsAndDefinitionses, radiobeacons, racons, Loran C, sound signals, buoysand daybeacons on the navigablewaters of the UnitedStates, its territories,and possessions.Additionally,theTheterm“shortrangeaidstonavigation"encompasseslighted and unlighted beacons, ranges, leading lights,Coast Guard exercises control over privately owned navi-buoys,andtheirassociatedsound signals.Eachshortrangegation aid systems.aidtonavigation,commonlyreferredtoas aNAVAID,fitsA beacon is a stationary,visual navigation aid. Largewithin a system designed to warn themariner of dangerslighthouses and small single-pile structures are both bea-and direct him toward safe water.An aid's function deter-cons.Lighted beacons are called lights;unlighted beaconsmines its color,shape,light characteristic,and sound.Thisare daybeacons.All beacons exhibit a daymark of somechapter explains theU.S.Aids to Navigation System assort.In the case of a lighthouse,the color andtype of strucwell as the international IALA Maritime Buoyage System.ture are the daymarks.On small structures,thesedaymarksTheplacement and maintenanceof marineaids tonavconsisting of colored geometric shapes called dayboardsigation in U.S.watersistheresponsibilityof theUnitedoften have lateral significance.Conversely,the markingsStates Coast Guard.The Coast Guard maintains lighthous-on lighthouses and towers conveyno lateral significance.FIXEDLIGHTSWithfewexceptions,all major lights are operated au-501.MajorAnd MinorLightstomatically.There are also many automatic lights onLights vary from tall, high intensity coastal lights tosmaller structures maintained by the Coast Guard or otherbattery-powered lanterns on single wooden piles.Immov-attendants.Unmanned major lights may have emergencygenerators and automatic monitoring equipment to increaseable, highly visible, and accurately charted, fixed lightsprovide navigators with an excellent source for bearings.the light's reliability.The structures are often distinctively colored to aid in iden-Light structures' appearances vary. Lights in low-lyingtification,SeeFigure50laareas usually are supportedbytall towers,conversely,lightA major light is a high-intensity light exhibited fromstructures on high cliffs may be relatively short.Howevera fixed structure or a marine site.Major lights include pri-its supporttoweris constructed,almostall lights aresimi-mary seacoastlights and secondary lights.Primarylarlygenerated,focused,colored,and characterized.seacoast lights are thosemajor lights established for mak-Some major lights use modern rotating or flashinginglandfallfrom seaand coastwisepassagesfromheadlandlights, but manyolder lights useFresnel lenses.These lens-to headland.Secondary lights are those major lights estab-es consistof intricatelypatterned pieces ofglass inaheavylished at harbor entrances and other locations where highbrassframework.ModernFresnel-typelenses arecastfromintensity and reliability are required.high-gradeplastic:theyaremuchsmallerandlighterthanA minorlight usually displays a light of low to mod-theirglass counterparts.erateintensity.Minor lights are established in harbors,A buoyant beacon provides nearly the positional ac-alongchannels,rivers,andinisolatedlocations.Theyusu-curacyofa light in aplace whereabuoy would normallybeally have numbering,coloring,and light and soundused.SeeFigure5olb.Thebuovantbeaconconsistsofacharacteristicsthatarepartofthelateralsystemofbuoyage.heavy sinker to which a pipe structure is tightly moored. ALighthouses areplacedwhere they will be ofmost usebuoyancy chamber nearthe surface supportsthepipe.Theonprominentheadlands,atharborandportentrances,onlight, radarreflector, and otherdevices are located atop theisolateddangers,or atotherpointswheremarinerscan bestpipe above the surface of the water. The pipe with its buoy-use themtofixtheirposition.Thelighthouse'sprincipaancy chamber tends to remain upright even in severepurpose is to supporta light at a considerableheight aboveweather and heavy currents,providing a smaller watch cir-cle than a buoy.Thebuoyant beacon is most useful alongthe water, thereby increasing its geographic range.Supportequipment is often housednearthetower.narrow ship channels in relatively sheltered water.63
63 CHAPTER 5 SHORT RANGE AIDS TO NAVIGATION DEFINING SHORT RANGE AIDS TO NAVIGATION 500. Terms And Definitions The term “short range aids to navigation” encompasses lighted and unlighted beacons, ranges, leading lights, buoys, and their associated sound signals. Each short range aid to navigation, commonly referred to as a NAVAID, fits within a system designed to warn the mariner of dangers and direct him toward safe water. An aid’s function determines its color, shape, light characteristic, and sound. This chapter explains the U.S. Aids to Navigation System as well as the international IALA Maritime Buoyage System. The placement and maintenance of marine aids to navigation in U.S. waters is the responsibility of the United States Coast Guard. The Coast Guard maintains lighthouses, radiobeacons, racons, Loran C, sound signals, buoys, and daybeacons on the navigable waters of the United States, its territories, and possessions. Additionally, the Coast Guard exercises control over privately owned navigation aid systems. A beacon is a stationary, visual navigation aid. Large lighthouses and small single-pile structures are both beacons. Lighted beacons are called lights; unlighted beacons are daybeacons. All beacons exhibit a daymark of some sort. In the case of a lighthouse, the color and type of structure are the daymarks. On small structures, these daymarks, consisting of colored geometric shapes called dayboards, often have lateral significance. Conversely, the markings on lighthouses and towers convey no lateral significance. FIXED LIGHTS 501. Major And Minor Lights Lights vary from tall, high intensity coastal lights to battery-powered lanterns on single wooden piles. Immovable, highly visible, and accurately charted, fixed lights provide navigators with an excellent source for bearings. The structures are often distinctively colored to aid in identification. See Figure 501a. A major light is a high-intensity light exhibited from a fixed structure or a marine site. Major lights include primary seacoast lights and secondary lights. Primary seacoast lights are those major lights established for making landfall from sea and coastwise passages from headland to headland. Secondary lights are those major lights established at harbor entrances and other locations where high intensity and reliability are required. A minor light usually displays a light of low to moderate intensity. Minor lights are established in harbors, along channels, rivers, and in isolated locations. They usually have numbering, coloring, and light and sound characteristics that are part of the lateral system of buoyage. Lighthouses are placed where they will be of most use: on prominent headlands, at harbor and port entrances, on isolated dangers, or at other points where mariners can best use them to fix their position. The lighthouse’s principal purpose is to support a light at a considerable height above the water, thereby increasing its geographic range. Support equipment is often housed near the tower. With few exceptions, all major lights are operated automatically. There are also many automatic lights on smaller structures maintained by the Coast Guard or other attendants. Unmanned major lights may have emergency generators and automatic monitoring equipment to increase the light’s reliability. Light structures’ appearances vary. Lights in low-lying areas usually are supported by tall towers; conversely, light structures on high cliffs may be relatively short. However its support tower is constructed, almost all lights are similarly generated, focused, colored, and characterized. Some major lights use modern rotating or flashing lights, but many older lights use Fresnel lenses. These lenses consist of intricately patterned pieces of glass in a heavy brass framework. Modern Fresnel-type lenses are cast from high-grade plastic; they are much smaller and lighter than their glass counterparts. A buoyant beacon provides nearly the positional accuracy of a light in a place where a buoy would normally be used. See Figure 501b. The buoyant beacon consists of a heavy sinker to which a pipe structure is tightly moored. A buoyancy chamber near the surface supports the pipe. The light, radar reflector, and other devices are located atop the pipe above the surface of the water. The pipe with its buoyancy chamber tends to remain upright even in severe weather and heavy currents, providing a smaller watch circle than a buoy. The buoyant beacon is most useful along narrow ship channels in relatively sheltered water
64SHORTRANGEAIDS TONAVIGATIONLIGHTFOCALPLANE15PLATFORMWATERLINEBUOYANCYCHAMBERLZAPLABASINKERBOTTOMFigure 501a.Typical offshore light stationFIG25sec"15Figure 501b. Typical design for a buoyant beacon.502.RangeLightslighting.Installation of lightpipes is proceeding on severalrange markers throughout the country.The Coast Guard isRange lights are light pairs that indicate a specific linealsoexperimenting with longrangesodiumlightsforareasrequiring visibility greater than the light pipes can provide.of position when theyarein line.Thehigherrear light isplaced behind the front light. When the mariner sees theTheoutput from a lowpressure sodium light is almostentirely at onewavelength.This allows theuse of an inex-lightsverticallyinline,heisontherangeline.Ifthefrontpensive band-pass filter tomake the light visibleeven duringlightappearsleft ofthe rear light,the observer isto the rightoftherangeline; ifthefront appears to the right oftherearthe daytime.This arrangement eliminates the need for highthe observer is left ofthe rangeline.Range lights are some-intensitylightswiththeirlargepowerrequirementstimes equipped with high intensity lights for daylight use.Range lights are usually white,red, or green.They displayThese are effective for long channels in hazy conditionsvarious characteristics differentiating them from surroundingwhen dayboards might not be seen.The range light struc-lights.tures are usually also equipped with dayboards for ordinaryA directional light is a single light that projects a high in-daytime use.Some smaller ranges,primarily in the Intrac-tensity,special characteristic beam in a given direction. It isoastal Waterwayand other inland waters,have justtheused incases whereatwo-lightrange maynot bepracticabledayboards with no lights.SeeFigure 502.Adirectional sectorlightisadirectional lightthatemitstwoTo enhancethe visibility of range lights, the Coastormorecolored beams.Thebeams haveapreciselyorientedGuard has developed 15-foot long lighted tubes called lightboundarybetweenthem.Anormalapplicationofasectorlightpipes.They are mounted vertically,and the mariner seeswouldshowthreecoloredsections:red.white,andgreenthem as verticalbarsoflightdistinctfrom backgroundThewhitesector would indicatethatthevessel is onthe
64 SHORT RANGE AIDS TO NAVIGATION 502. Range Lights Range lights are light pairs that indicate a specific line of position when they are in line. The higher rear light is placed behind the front light. When the mariner sees the lights vertically in line, he is on the range line. If the front light appears left of the rear light, the observer is to the right of the rangeline; if the front appears to the right of the rear, the observer is left of the rangeline. Range lights are sometimes equipped with high intensity lights for daylight use. These are effective for long channels in hazy conditions when dayboards might not be seen. The range light structures are usually also equipped with dayboards for ordinary daytime use. Some smaller ranges, primarily in the Intracoastal Waterway and other inland waters, have just the dayboards with no lights. See Figure 502. To enhance the visibility of range lights, the Coast Guard has developed 15-foot long lighted tubes called light pipes. They are mounted vertically, and the mariner sees them as vertical bars of light distinct from background lighting. Installation of light pipes is proceeding on several range markers throughout the country. The Coast Guard is also experimenting with long range sodium lights for areas requiring visibility greater than the light pipes can provide. The output from a low pressure sodium light is almost entirely at one wavelength. This allows the use of an inexpensive band-pass filter to make the light visible even during the daytime. This arrangement eliminates the need for high intensity lights with their large power requirements. Range lights are usually white, red, or green. They display various characteristics differentiating them from surrounding lights. A directional light is a single light that projects a high intensity, special characteristic beam in a given direction. It is used in cases where a two-light range may not be practicable. A directional sector light is a directional light that emits two or more colored beams. The beams have a precisely oriented boundary between them. A normal application of a sector light would show three colored sections: red, white, and green. The white sector would indicate that the vessel is on the Figure 501a. Typical offshore light station. Figure 501b. Typical design for a buoyant beacon
65SHORTRANGEAIDS TONAVIGATIONRICHTOFRANE CINEONRANGELINERANCELINE.OFigure 502.Range lightsulations and the applicable Coast Pilot. Certain bridges maychannel centerline;thegreen sector would indicate that thevessel is off the channel centerline inthe direction of deepalso beequipped with sound signals and radar reflectors.water, and the red sector would indicate that the vessel isoff the centerline in thedirection of shoal water.505.Shore LightsShore lights usually have a shore-based power supply503.AeronauticalLightsLights on pilings,such as thosefound in the Intracoastal Wa-Aeronautical lights may be the first lights observed atterway,are batterypowered. Solar panels may be installed tonight when approaching the coast.Those situated near theenhancethelight'spower supply.Thelights consistofa powercoast and visible from sea are listed in the List of Lightssource,aflashertodeterminethecharacteristic,a lampchangThese lights are not listed in the Coast Guard Light List.erto replaceburned-out lamps,andafocusing lensThey usuallyflash alternating white and green.Various types ofrotating lights are in use.They do notAeronautical lights are sequenced geographically inhaveflashers but remain continuouslylitwhile a lens orre-the List of Lights along withmarine navigation lights.How-flector rotatesaround thehorizon,ever, since theyare not maintained formarine navigation,The whole light system is carefully engineered to pro-videthemaximum amount of lighttothemarinerfor thetheyaresubjecttochangesofwhichmaritimeauthoritiesmay not be informed.These changes will be published inleast power use.Speciallydesigned filaments and specialNoticeto Airmen butperhaps not in NoticeToMariners.grades of materials are used in the light to withstand theharshmarineenvironmentThe flasher electronically determines the characteris504.Bridge Lightstic by selectively interrupting the light's power supplyRed, green,and white lights mark bridges across navigaaccordingtothechosencycleble waters ofthe United States. Red lights mark piers and otherThe lamp changer consists of several sockets arrangedparts ofthe bridge.Red lights are also used on drawbridges toaround a central hub. When the circuit is broken by ashowwhenthey arein theclosed position.Green lights markburned-outfilament,a newlamp is rotated into position.Almost all lights have daylight switches whichturn theopendrawbridges and mark the centerlineofnavigable chan-nelsthroughfixedbridges.Thepositionwillvaryaccordingtclightoffatsunriseandonatduskthe type of structure.Navigational lights on bridges in the U.SThe lens for small lights may be one of several types.Thecommon ones in use are omni-directional lenses ofareprescribedbyCoastGuardregulations.Infrequently-used bridges may be unlighted. In foreign155mm,250mm,and300mm.Inaddition,lightsusingpar-waters, the type and method of lighting may be different fromabolic mirrors or focused-beam lenses are used in leadingthose normallyfound inthe United States.Drawbridges whichlights and ranges.The lampfilaments must be carefullymustbeopenedtoallowpassageoperateupon sound and lightaligned with the plane of the lens or mirror to provide themaximum output oflight.The lenssize is chosen accordingsignals given by the vessel and acknowledged by the bridge.Theserequired signals are detailed in the Code ofFederal Regtothetypeofplatform,powersource,andlampcharacteris-
SHORT RANGE AIDS TO NAVIGATION 65 channel centerline; the green sector would indicate that the vessel is off the channel centerline in the direction of deep water; and the red sector would indicate that the vessel is off the centerline in the direction of shoal water. 503. Aeronautical Lights Aeronautical lights may be the first lights observed at night when approaching the coast. Those situated near the coast and visible from sea are listed in the List of Lights. These lights are not listed in the Coast Guard Light List. They usually flash alternating white and green. Aeronautical lights are sequenced geographically in the List of Lights along with marine navigation lights. However, since they are not maintained for marine navigation, they are subject to changes of which maritime authorities may not be informed. These changes will be published in Notice to Airmen but perhaps not in Notice To Mariners. 504. Bridge Lights Red, green, and white lights mark bridges across navigable waters of the United States. Red lights mark piers and other parts of the bridge. Red lights are also used on drawbridges to show when they are in the closed position. Green lights mark open drawbridges and mark the centerline of navigable channels through fixed bridges. The position will vary according to the type of structure. Navigational lights on bridges in the U.S. are prescribed by Coast Guard regulations. Infrequently-used bridges may be unlighted. In foreign waters, the type and method of lighting may be different from those normally found in the United States. Drawbridges which must be opened to allow passage operate upon sound and light signals given by the vessel and acknowledged by the bridge. These required signals are detailed in the Code of Federal Regulations and the applicable Coast Pilot. Certain bridges may also be equipped with sound signals and radar reflectors. 505. Shore Lights Shore lights usually have a shore-based power supply. Lights on pilings, such as those found in the Intracoastal Waterway, are battery powered. Solar panels may be installed to enhance the light’s power supply. The lights consist of a power source, a flasher to determine the characteristic, a lamp changer to replace burned-out lamps, and a focusing lens. Various types of rotating lights are in use. They do not have flashers but remain continuously lit while a lens or reflector rotates around the horizon. The whole light system is carefully engineered to provide the maximum amount of light to the mariner for the least power use. Specially designed filaments and special grades of materials are used in the light to withstand the harsh marine environment. The flasher electronically determines the characteristic by selectively interrupting the light’s power supply according to the chosen cycle. The lamp changer consists of several sockets arranged around a central hub. When the circuit is broken by a burned-out filament, a new lamp is rotated into position. Almost all lights have daylight switches which turn the light off at sunrise and on at dusk. The lens for small lights may be one of several types. The common ones in use are omni-directional lenses of 155mm, 250mm, and 300mm. In addition, lights using parabolic mirrors or focused-beam lenses are used in leading lights and ranges. The lamp filaments must be carefully aligned with the plane of the lens or mirror to provide the maximum output of light. The lens’ size is chosen according to the type of platform, power source, and lamp characterisFigure 502. Range lights
66SHORTRANGEAIDS TONAVIGATIONtics. Additionally,environmental characteristics of thestalled inside the lens to provide the proper characteristic.location are considered.Varioustypes of light-condensingA special heavy 200mm lantern is used in locationspanels, reflex reflectors,or colored sector panels may be in-where ice and breaking water are a hazard.LIGHTCHARACTERISTICS506.CharacteristicsTherefore,thetransition from one colorto another is notabrupt.The colors change through an arc of uncertainty ofA light has distinctive characteristics which distin-2°orgreater, depending onthe optical designof the lightTherefore determining bearings by observing the colonguish itfrom otherlightsor convey specific information.Alight may show a distinctive sequence of light and dark in-change is less accurate than obtaining a bearing with an az-tervals.Additionally,a light may display a distinctive colorimuth circle.or color sequence.In the Light Lists, the dark intervals arereferred to as eclipses.An occulting light is a light totally508.Factors AffectingRangeAnd Characteristicseclipsedatregular intervals,thedurationoflightalways beTheconditionoftheatmospherehasa considerableeffecting greater than the duration of darkness.A flashing lightis a light whichflashesat regular intervals,theduration ofupon a light's range. Sometimes lights are obscured by fog.lightalwaysbeinglessthanthedurationofdarkness.Anhaze,dust, smoke,or precipitation.On theother hand,refrac-isophase light flashes at regular intervals, the duration oftion may cause a light to be seen farther than under ordinarylightbeing equal to the duration ofdarknesscircumstances.AlightoflowintensitywillbeeasilyobscuredLight phase characteristics (Figure 506a and Figurebyunfavorable conditions of theatmosphere.Forthisreason506b)are the distinctive sequences of light and dark inter-the intensity ofa lightshould always be considered when look-vals or sequences in the variations ofthe luminous intensitying for it in thick weather.Haze and distance may reduce theof a light.The light phase characteristics of lights whichapparentduration ofa light'sflash.In some conditions ofthechangecolordonotdifferfromthoseof lightswhichdonotatmosphere,whitelights may have a reddish hue.In clearchange color.Alight showing different colors alternately isweather green lights may have a more whitish hue.described as an alternating light.The alternatingcharacter-Lightsplacedatgreatelevationsaremorefrequentlyisticmaybeused withotherlightphasecharacteristicsobscuredbyclouds.mistandfogthanthosenearsealevelLight-sensitive switches extinguish most lighted navi-In regions where ice conditions prevail, an unattendedlight'slanternpanesmaybecomecoveredwithiceorsnowgation aidsduring daylighthours.However, owingtothevarioussensitivityofthelightswitches,alllightsdonotThis may reduce the light's luminous range and change thecomeon orgooff at the sametime.Mariners should ac-light'sobservedcolor.count for this when identifying aids to navigation duringThedistancefrom a light cannot be estimated by its ap-twilight periods when some lighted aids are on while othersparent brightness.There are too many factors whichcanare not.change the perceived intensity.Also,a powerful, distantlightmaysometimesbeconfusedwithasmaller,closerone507.Light Sectorswith similar characteristics.Every light sighted should becarefullyevaluatedtodetermine if it is the one expectedSectorsof coloredglass orplastic aresometimesThe presence of bright shore lights may make it diffi-placed in thelanterns ofcertain lights to indicatedangerouscult to distinguish navigational lights from backgroundwaters.Lights so equipped showdifferentcolorswhen ob-lighting.Lights mayalso beobscured byvarious shoreob-served from different bearings.A sector changes the colorstructions,natural and man-made. The Coast Guardof a light, but not its characteristic,when viewed from cer-requests mariners to report these cases to the nearest Coasttain directions.For example, a four second flashing whiteGuard station.light havinga red sectorwill appearas a four second flash-A light's loom is seen throughhaze or the reflectioning red light when viewed from within thered sector.from low-lying clouds when the light is beyond its geo-Sectors maybe only a fewdegrees in width or extendgraphic range.Onlythemostpowerful lights cangeneratein a wide arcfrom deepwatertoward shore.Bearings refer-aloom.Theloommaysometimesbesufficientlydefinedtoring to sectors are expressed in degrees true as observedobtain a bearing.If not, an accuratebearing on a light be-fromavessel.yond geographic range may sometimes be obtained byInmostcases,areascovered byred sectors shouldbeascending to a higher level where the light can be seen,andavoided.Thenatureof thedanger canbedetermined fromnoting a star directly over the light.The bearing of the starthe chart. In some cases a narrow sector may mark the bestcan then be obtained from the navigating bridge and thewater across a shoal, or a turningpoint in a channel.bearingtothe lightplotted indirectlyAtshortdistances,someofthebrighterflashinglightsSectors generated by shadow-casting filters do nothave precise boundaries as directional sector lights domayshowafaintcontinuous light,orfaintflashes,between
66 SHORT RANGE AIDS TO NAVIGATION tics. Additionally, environmental characteristics of the location are considered. Various types of light-condensing panels, reflex reflectors, or colored sector panels may be installed inside the lens to provide the proper characteristic. A special heavy 200mm lantern is used in locations where ice and breaking water are a hazard. LIGHT CHARACTERISTICS 506. Characteristics A light has distinctive characteristics which distinguish it from other lights or convey specific information. A light may show a distinctive sequence of light and dark intervals. Additionally, a light may display a distinctive color or color sequence. In the Light Lists, the dark intervals are referred to as eclipses. An occulting light is a light totally eclipsed at regular intervals, the duration of light always being greater than the duration of darkness. A flashing light is a light which flashes at regular intervals, the duration of light always being less than the duration of darkness. An isophase light flashes at regular intervals, the duration of light being equal to the duration of darkness. Light phase characteristics (Figure 506a and Figure 506b) are the distinctive sequences of light and dark intervals or sequences in the variations of the luminous intensity of a light. The light phase characteristics of lights which change color do not differ from those of lights which do not change color. A light showing different colors alternately is described as an alternating light. The alternating characteristic may be used with other light phase characteristics. Light-sensitive switches extinguish most lighted navigation aids during daylight hours. However, owing to the various sensitivity of the light switches, all lights do not come on or go off at the same time. Mariners should account for this when identifying aids to navigation during twilight periods when some lighted aids are on while others are not. 507. Light Sectors Sectors of colored glass or plastic are sometimes placed in the lanterns of certain lights to indicate dangerous waters. Lights so equipped show different colors when observed from different bearings. A sector changes the color of a light, but not its characteristic, when viewed from certain directions. For example, a four second flashing white light having a red sector will appear as a four second flashing red light when viewed from within the red sector. Sectors may be only a few degrees in width or extend in a wide arc from deep water toward shore. Bearings referring to sectors are expressed in degrees true as observed from a vessel. In most cases, areas covered by red sectors should be avoided. The nature of the danger can be determined from the chart. In some cases a narrow sector may mark the best water across a shoal, or a turning point in a channel. Sectors generated by shadow-casting filters do not have precise boundaries as directional sector lights do. Therefore, the transition from one color to another is not abrupt. The colors change through an arc of uncertainty of 2° or greater, depending on the optical design of the light. Therefore determining bearings by observing the color change is less accurate than obtaining a bearing with an azimuth circle. 508. Factors Affecting Range And Characteristics The condition of the atmosphere has a considerable effect upon a light’s range. Sometimes lights are obscured by fog, haze, dust, smoke, or precipitation. On the other hand, refraction may cause a light to be seen farther than under ordinary circumstances. A light of low intensity will be easily obscured by unfavorable conditions of the atmosphere. For this reason, the intensity of a light should always be considered when looking for it in thick weather. Haze and distance may reduce the apparent duration of a light’s flash. In some conditions of the atmosphere, white lights may have a reddish hue. In clear weather green lights may have a more whitish hue. Lights placed at great elevations are more frequently obscured by clouds, mist, and fog than those near sea level. In regions where ice conditions prevail, an unattended light’s lantern panes may become covered with ice or snow This may reduce the light’s luminous range and change the light’s observed color. The distance from a light cannot be estimated by its apparent brightness. There are too many factors which can change the perceived intensity. Also, a powerful, distant light may sometimes be confused with a smaller, closer one with similar characteristics. Every light sighted should be carefully evaluated to determine if it is the one expected. The presence of bright shore lights may make it difficult to distinguish navigational lights from background lighting. Lights may also be obscured by various shore obstructions, natural and man-made. The Coast Guard requests mariners to report these cases to the nearest Coast Guard station. A light’s loom is seen through haze or the reflection from low-lying clouds when the light is beyond its geographic range. Only the most powerful lights can generate a loom. The loom may sometimes be sufficiently defined to obtain a bearing. If not, an accurate bearing on a light beyond geographic range may sometimes be obtained by ascending to a higher level where the light can be seen, and noting a star directly over the light. The bearing of the star can then be obtained from the navigating bridge and the bearing to the light plotted indirectly. At short distances, some of the brighter flashing lights may show a faint continuous light, or faint flashes, between
67SHORTRANGEAIDSTONAVIGATIONTypeDoscrintionAtrevisticnmtastrationF1.FIXEDAligntshowing continuouslyendsteadily2OCCULTINGAightin wtichthetotalduraticn of light inaperiod is longer than thetotalduration ofdarkness and the intervals otdarknese (ecipses)are usually cfequelduraticn.O2.1Single-occulting.An occulting light in which an eclipse is regularly repeated.pertod2.2Group-occulting.Oc(2)Ancculingghtinwhichgroupofcipses,specifiedinnubssgulaperiodrepeated.Oc(2+1)2.3Compositegroup-occulting.Alight,sinlar to sgroup-occutting Tight,except thaf sucoessivegroups in aperiedperiod have different numbers of eclipses.3.ISOPHASE.IsoAlight in wtich all durations of ight and darkress are equalperod4.FLASHINGA ight in which the total duration of light in a period is shorter than the totalduraticn otdariness and the sppesrancesoflight (lashes) areususily ofequalduration.FI4.1Single-flashing.88Afiashing light in which a fiash is regularly repeatec (frequency notexceeding 30perios,fashes per minute)===THISFIGUREHASTOBEREPAIRED!!==Figure 506a.Light phase characteristics.===-.=
SHORT RANGE AIDS TO NAVIGATION 67 Figure 506a. Light phase characteristics. = = = = = = THIS FIGURE HAS TO BE REPAIRED!! = = = = = =