Aids to navigation

Aids to navigation are special structures like lighthouses, lightships, beacons, buoys, etc that are used to enhance safety by providing more opportunities to obtain LOPs.

These lights and marks are prescribed across the world by the International Association of Lighthouse Authorities (IALA). In 1977 this IALA endorsed two maritime buoyage systems putting an end to the 30 odd systems existing at that time. Region A - IALA A covers all of Europe and most of the rest of the world, whereas region B - IALA B covers only the Americas, Japan, the Philippines and Korea. Fortunately, the differences between these two systems are few. The most striking difference is the direction of buoyage.
All marks within the IALA system are distinguished by:
 

• Shape

• Colour

• Topmark

• Light

Light identification

During daytime, the identification of aids to navigation is accomplished by observing: location, shape, colour scheme, auxiliary features (sound signals, RACON, RC, etc) or markings (name, number, etc).

During the night, we use the features of the aid to navigation's light to both identify it and ascertain its purpose. There are three features to describe the light:

• Colour: Either white, red, green or yellow. If no colour is stated in the chart, default is white.

• Period: The time in seconds needed for one complete cycle of changes.    The arrow indicates the 10 second period of this flashing light “Fl(3) 10s”.

• Phase characteristic: The particular pattern of changes within one complete cycle (hence, within one period). Below are the most common types:

• Fixed   F
     This light shines with an unblinking and steady intensity and is always on. In this example a yellow fixed light is shown.

• Flashing   Fl:
     The duration of the light is always less than the duration of the darkness. The frequency does not exceed 30 times per minute.

• Quick Flashing   Q:
     Again, the duration of quick flash is less than the darkness. The frequency is at least 60 times per minute.

• Very Quick Flashing   VQ:
     Also here, the duration of very quick flash is less than the darkness. The frequency is at least 100 times per minute.

• Interrupted Quick Flashing   IQ:
     Like Quick Flashing with one moment of darkness in one period.

• Isophase   Iso:
     This Light has equal duration between light and darkness. A period consists of both a light and a dark interval. Also called Equal Interval (E Int).

• Group Flashing   Gp Fl(x+x):
     This is actually a combination of two patterns in one period. In this example the first 2 flashes followed by the pattern of 3 flashes result in: Gp Fl(2+3).

• Occulting   Occ:
     Occulting is the opposite of flashing, the light is more on then off.

• Alternating   AL:
     An alternating light changes colour. This special purpose light is typically used for special applications requiring the exercise of great caution. In this example ALT.WG is shown, alternating between green and white.

• Morse U   Mo (U):
     This light shows two flashes and a longflash, which is equivalent to the letter “U” in Morse code.

• Long-Flashing   LFl:
     This light has one long flash in a period. A long flash is at least 2 seconds long.

Let's look at some examples using colour, period and phase characteristics. The arrows mark the periods:
 

•   Fl (4) 8s

•   Oc (2+3) 10s

•   Iso G 4s

All lighted aids to navigation are either major or minor lights, where major lights are used for key navigational points along sea-coasts, channels and harbour and river entrances. These lights are normally placed in lightships, lighthouses and other permanently installed structures, providing both high intensity and high reliability of the lights. Major lights are then subdivided in primary lights (very strong, long range lights used for the purpose of making landfalls or coastal passages) and secondary lights (shorter range lights found for example at harbour and river entrances). Important details of (especially) primary lights can be found in a reference called the Light List where information (about pedestals etc.) can be found which is not included in the chart.
Minor lights on the other hand are likely to be found within harbours, along channels and rivers. These have a low to moderate intensity and sometimes mark isolated dangers.

Five types of navigation buoys

• Lateral

• Cardinal

• Isolated danger

• Safe water

• Special

Lateral Buoys and Marks

The location of lateral buoys defines the borders of channels and indicates the direction. Under IALA A red buoys mark the port side of the channel when returning from sea, whereas under IALA B green buoys mark the port side of the channel when sailing towards land. Red buoys have even numbers and red lights; green buoys have odd numbers and green lights. Lateral lights can have any calm phase characteristic except FL (2+1).

 

Generally, when two channels meet, one will be designated the preferred channel (i.e. most important channel). The buoy depicted on the right indicates the preferred channel to starboard under IALA A. The light phase characteristic is R FL (2+1):
 

The buoy depicted on the left indicates the preferred channel to port under IALA A. These buoys are marked with the names and numbers of both channels. The light phase characteristic is G FL (2+1):

For an example of lateral buoys used to mark a (preferred) channel, see direction of buoyage below.

Cardinal Buoys

The four cardinal buoys indicate the safe side of a danger with an approximate bearing. For example, the West cardinal buoy has safe water on its West and the danger on its East side. Notice the “clockwise” resemblance of the light phase characteristics. The top marks consist of two black triangles placed in accordance with the black/yellow scheme of the buoy. When a new obstacle (not yet shown on charts) needs to be marked, two cardinal buoys - for instance a South buoy and an East buoy - will be used to indicate this “uncharted” danger. The cardinal system is identical in both the IALA A and IALA B buoyage systems.

 

Marks indicating isolated dangers

This type of buoy indicates the position of an isolated danger, contrary to cardinal buoys which indicate a direction away from the danger. Body: black with red horizontal band(s); Topmark: 2 black spheres. The light (when present) consists of a white flash: Fl(2).

 

Marks indicating safe water

Notice that whereas most horizontal striping spells “danger”, this safe water buoy is vertically striped. These marks are for example seaward of all other buoys (lateral and cardinal) and can be used to make landfall. Body: red and white vertical stripes; Topmark (if any): single red sphere. Lights are typically calm and white: Morse A, Iso, Occ or LFl 10s.
 

Special Buoys and Marks

I saved these buoys for last since they have not an actual navigation purpose. Most of the time these yellow buoys indicate pipelines or areas used for special purposes. I have drawn the five official IALA shapes, from left to right: conical, spar, cylindrical, pillar and spherical.

Chart symbols

The seafaring nations of the world - members of the International Hydrographic Organization - agreed in 1982 on an universal set of chart symbols, abbreviations, colours, etc to be used in the nautical chart, in order to obtain uniformity.

On regular charts a white, red, yellow or green lights will be indicated by , and on GPS displays and modern multi-coloured charts in specific colours: , with the yellow coloured lobe indicating a white light.
The precise position of a chart symbol is its center, or is indicated with a line and circle , the “position circle”.

Two distinct types of sea mark are drawn differently in the chart:

• beacons - fixed to the seabed; drawn upright;

• buoys - consisting of a floating object that is usually anchored to a specific location on the sea floor; drawn at an oblique angle and with oblique numbering, descriptions of colours and light characteristics.

Full example of a light description in the chart:

Fl(3)WRG.15s21m15-11M
Class of light: group flashing repeating a group of three flashes;
Colours: white, red, green, exhibiting the different colours in defined sectors;
Period: the time taken to exhibit one full sequence of 3 flashes and eclipses: 15 seconds;
Elevation of light : 21 metres;
Nominal range(s): white 15 M, green 11 M, red between 15 and 11 M, where “M” stands for nautical miles.

Lateral Marks - direction of buoyage

Lateral marks are generally for well-defined channels and there are two international Buoyage Regions - A and B - where these Lateral marks differ. Where in force, the IALA System applies to all fixed and floating marks except landfall lights, leading lights and marks, sectored lights and major floating lights.

The standard buoy shapes are cylindrical (can) , conical , spherical , pillar and spar , but variations may occur, for example: minor light-floats . In the illustrations below, only the standard buoy shapes are used.
In the case of fixed beacons - lit or unlit - only the shape of the topmark is of navigational significance.

Visibility of lights

It is important to know at what distance we may (begin to) see a certain light, and when we can expect to lose sight of it, especially when making landfall. Several practical ranges are used to the describe the visibility of lights in navigation:

• The meteorological range is based on the current atmospheric conditions. The table below shows that the atmosphere immensely influences the visibility of light travelling through it.

• The geographic range is based on the elevation of the light. A higher light means that its horizon is farther away, see distance of horizon.
  Moreover, if the observer's height of eye is higher than sea level the light can been seen beyond its geographic range, the dipping range. However, on sailing yachts this potential is limited.

• The nominal range of a light is based on its candlepower, and is typically the range mentioned in the chart. The nominal range is the maximum distance at which a light can be seen in weather conditions where visibility is 10 nm.

So, a minor light - perched on a 70m high cliff - with a geographic range of 20 nm will not be detectable by the human eye at a distance of 6 nm

1. if the nominal range is just 5 nm.

2. if the meteorological range is just 5 nm due to a light haze.

Because of the limiting factor of the geographic range, most major lights will never be seen from a sailing yacht 20 nm away. Yet, it is sometimes possible to take a bearing on the loom of the light: its reflection against the clouds.

Different coloured lights with equal candlepower have different ranges. White light is the most visible followed by yellow, green and then red. Therefore, at extreme ranges an “AL WG” can resemble a “Fl W”.

The range of a lit buoy is never indicated - with the exception of a LANBY - but on a clear night the maximum range is 3 nm, yet often considerably less.
There are 2 visual clues to determine your distance from a buoy: at about 0.5 nm, the light will rise up from the horizon, and at about 200m, the light will reflect in the surface.

Glossary

• Navigation aid: An onboard instrument, device, chart, method, etc., intended to assist in the navigation.

• Aid to navigation: A device or structure external to the ship, designed to assist in determination of position, to define a safe course, or to warn of dangers or obstructions.

• Mark, seamark, navigation mark: An artificial or natural object of easily recognizable shape or colour, or both, situated in such a position that it may be identified on a chart. A fixed artificial navigation mark is often called a Beacon.

• Light characteristics: The sequence and length of light and dark periods and the colour or colours by which a navigational light is identified.

• Topmark: One of more objects of characteristic shape placed on top of a buoy or beacon to aid in its identification.

• Lateral Mark: An aid to navigation intended to mark the sides of a channel or waterway.

• Cardinal Marks: An IALA aid to navigation intended to show the location of a danger to navigation based on its position relative to the danger using the “cardinal point”: north, east, south, west.

• Isolated danger Marks: An IALA aid to navigation marking a danger with clear water all around it; it has a double ball topmark and is black with at least one red band. If lighted its characteristic is Fl(2).

• Sector light: A light having sectors of different colours or the same colour in specific sectors separated by dark sectors.

• Light sector: As defined by bearings from seaward, the sector in which a navigational light is visible or in which it has a distinctive colour difference from that of adjoining sectors, or in which it is obscured.

• Lighthouse: A distinctive structure exhibiting a major navigation light.

• Light List: A publication giving detailed information regarding lighted navigational aids and fog signals.

• Landfall: The first sighting (even by radar) of land when approached from seaward.

• Range: Two or more objects in line. Such objects are said to be in range. An observer having them in range is said to be on the range. Two beacons are frequently located for the specific purpose of forming a range to indicate a safe route or the centerline of a channel.

• Leading line: On a nautical chart, a straight line, drawn through leading marks. A ship moving along such line will clear certain dangers or remain in the best channel.

• Range lights, leading lights: Two or more lights at different elevations so situated to form a range (leading line) when brought into transit. The one nearest to the observer is the from light and the one farthest from the observer is the rear light. The front light is at a lower elevation than the rear light.

• Lights in line: Two or more lights so situated that when observed in transit they define a position: the limit of an area, an alignment used for anchoring, etc. Not to be confused with range lights, which mark a direction to be followed.

• Light-float : A buoy having a boat-shaped body. Light-floats are nearly always unmanned and are used instead of smaller lighted buoys in waters where strong currents are experienced.

• Primary (sea-coast) light: A light established for purpose of making landfall or coastwise past from headland to headland.

• Secondary light: A major light, other than a primary (sea-coast) light, established at harbour entrances and other locations where high intensity and reliability are required.

• Major light: A light of high intensity and reliability exhibited from a fixed structure (lighthouse) or on marine site (except range lights). Major lights include primary sea-coast and secondary lights.

• Minor light: An automatic unmanned light on a fixed structure usually showing low to moderate intensity. Minor lights are established in harbours, along channels, along rivers, and in isolated dangers.

• Visual range: The extreme distance at which an object of light can be seen.

• Geographic range: The extreme distance limited by the curvature of the earth and both the heights of the object and the observer.

• Bobbing a light: Quickly lowering the height of eye and raising it again when a navigational light is first sighted to determine if the observer is at the geographic range of the light.

• Luminous range: The extreme distance limited only by the intensity of the light, clearness of the atmosphere and the sensitiveness of the observer's eye.

• Luminous range diagram: A diagram used to convert the nominal range of a light to its luminous range under existing conditions.

• Charted or Nominal Range: The nominal range is indicated in the chart next to the light or can be found in the Light List. This is the maximum distance at which a light may be seen at night based upon intensity and 10 nautical miles of visibility.

• Meteorological Range: The nominal range is indicated in the chart next to the light or can be found in the Light List. This is the maximum distance at which a light may be seen at night based upon intensity and 10 nautical miles of visibility.