ELECTRONIC AIDS TO NAVIGATION
(Extracts courtesy of A.N.T.A. publications, Ranger Hope © 2008 www.splashmaritime.com.au)
Magnetic Compass and Azimuth Circle
Despite advances in technology the magnetic compass remains a very important navigational aid because of its simplicity and reliability. Although it is susceptible to errors these can be measured and allowed for.
There are many designs of magnetic
compass but the major ones are
“Dry-card” and “Liquid”. The liquid compass is the one most used today, as it is not so susceptible to the vessel’s yawing and pitching.
The liquid in the compass bowl is a mixture of distilled water and alcohol. The liquid dampens the card’s movement allowing it to be read more easily.
The card has magnets attached below it to make it (magnetic) North seeking. The bearing on which it is pivoted is as frictionless as possible. The lubber line at the forward end of the bowl indicates the vessel’s head. The bowl is suspended in gymbals to keep it horizontal in a seaway.
The compass bowl is mounted in the
binnacle which is made of
non-magnetic material. (brass, wood or fibreglass). The housing enables the compass to be located at a convenient location and contains the various correcting devices for the compass.
In small vessels they tend to be desk-top mounted as shown in Figure 1 below.
Figure 1 Desk Top Mounted Binnacle
The effect of the vessel’s own magnetic field is to deviate the compass from magnetic north, hence (deviation). In small compasses designed for non-steel vessels there may be no means of correcting this error. In the type above, particularly on steel vessels, the deviation is corrected by a combination of:
· Flinders Bar (usually forward of the compass)
· Quadrantal correctors (on either side of the compass)
Fore and aft, athwartship and heeling error correction, these all being permanent magnets.
The position of the magnets once set by the adjuster should not be tampered with.
Fitting of the Compass
It should be fitted on the fore and aft centre line of the vessel, be readily visible from the helm and be able to be used to take bearings. This may be achieved by placing the Binnacle on the deck above the wheelhouse and fitting a “periscope” to enable the helmsman to steer by. In small vessels it is more usual to have a hand held compass for bearings.
The compass etc should be such that it receives minimum interference from electrical installations.
Care and Maintenance
· Keep the compass clean and free from salt and grease.
· Check the compass light regularly.
· Keep the gymbals lightly oiled.
· Keep direct sunlight off the card.
Removing a Bubble
In the more expensive compass the bowl may have a reservoir, if so turn upside down and leave it in this position for about three minutes. This action should re-charge the bowl and remove the bubble.
If the above does not work you will have to add more liquid.
Adding extra liquid
You will have to remove the compass from its bracket.
Carefully remove the filling cap.
Adjust the compass carefully so the bubble is exactly under the filler.
With an eye dropper, inject the correct fluid.
Replace cap and check the compass in a working position.
Detecting pivot wear
When alongside, set the compass heading, deflect the card using a steel implement. If the compass does not return to its original setting when the implement is removed, the pivot is defective and the card will tend to produce variable errors.
causes of unusual deviation
· Placing of metal objects, close to the compass eg deck knife, cigarette lighter, pliers.
· Placing of electrical appliances close to the compass eg transistor radio.
· Raw steel work or repairs in the area of the compass.
· Lightning strike.
· Fitting of new instrumentation or wiring in the compass area. “Safe Distance” recommendations should be strictly complied with.
Taking compass bearings
Bearings can be taken using an azimuth circle, which fits over the magnetic compass, a hand bearing compass or a pelorus. The latter is used when the magnetic compass is not conveniently located for taking bearings, and the hand bearing compass is not sufficiently accurate.
The azimuth circle is fitted to the rim of the bowl, and is rotated so that the point of interest is sighted over the top of the prism using the notch, and aligned with the reflected compass bearing. The arrow on the small knurled wheel at the side of the prism should be pointing downwards.
If a shadow pin is fitted, this may be used as a sight line for taking bearings of objects, or as designed for bearings of the sun.
If the positioning of the compass is such that a bearing cannot be taken, or no azimuth circle can be fitted, then bearings can be taking by pointing the vessel at the object in question and noting the vessel’s head when the compass settles.
Alternatively, a pelorus can be used or hand bearing compass if accuracy is not too important.
Errors of the Azimuth Circle
An error in the azimuth circle may be detected by taking a bearing of a distant object with the arrow up, and again with the arrow down. The two bearings should agree. If they do not, the prism must be adjusted by means of its retaining screws.
If the above does not remove the error then it will have to be applied as below.
The observed bearing of an object Arrow Up is 100°
The observed bearing of an object Arrow Down is 104°
The observed bearing of an object Mean Reading is 102°
The error is 2°.
The corrections to apply are +2° to any bearing taken with the arrow down and -2° with the arrow up.
Finding Ranges and Bearings
The range of a target (navigation feature or other vessel) is obtained by either engaging the range rings and estimating the range, or by engaging the variable range marker (VRM) and adjusting it to coincide with the nearest point of the echo. See below for target selection.
To obtain the relative bearing of a target the electronic bearing line (EBL) should be engaged. The normal practice for small targets is to align the EBL with the centre of the echo. When taking bearings of the edge of an echo, as with a headland, it should be remembered that such a target will be expanded on the screen due to the beam width. The average size of this expansion will be half the beam width. For a beam width of 2 degrees, the EBL should therefore be run through a point 1 degree inside the edge of the echo.
Once the relative bearing has been read off the display it must be applied to the ship’s true course to give the true bearing before it can be plotted on the chart. If the bearing is being used for an anti-collision plot, the relative bearing may be plotted.
When using radar to help fix the vessel’s position it should be remembered that as a general rule radar ranges are more accurate than radar bearings. The preferred method for using radar as an aid is therefore to use radar range(s) in conjunction with visual bearing(s). If visual bearings are not available then multiple ranges are preferred to a combination of radar ranges and bearings. Total reliance on radar bearings is the least preferred option.
As with any position fixing, objects should be chosen so that position lines form a good angle of cut. Objects should be readily identified on the chart and there should be no chance of ambiguity.
Racon and Ramark Beacons
There are a number of Racon Beacons around the Australian Coast, scanning the radar frequencies (X and/or S band) when triggered by radar transmission the beacon transmits an identification signal on the same frequency. This appears on the observer’s radar screen as a morse signal, generated from just beyond the target echo towards the edge of the display.
Not only does this make the object highly visible, it also removes the possibility of ambiguity. See Figure 2.
The Ramark is a beacon, transmitting continuously. It shows on the display as a broken radial line extending from the centre to the edge of the screen, through the beacon. See Figure 3.
Racons are more common than Ramarks. The former give bearing and range, while the latter is only suitable for bearings.
They are used for marks such as landfall beacons/buoys, lighthouses and other important targets which would otherwise be difficult to identify amongst similar ones.
Both are subject to giving false echoes, and likely to mask other targets close by.
2 Racon - Morse Code (G) .........................Figure
Errors in Steep-to-coastlines are shown in Figure 4
4 Effect of Tide Change on Different Types of Coastline
When using radar at some distance offshore, care needs to be taken to ensure that the echo received is from the nearest land. See Figure 5.
Figure 5 Potential Error due to Edge of Land being below the Radar Horizon
Range appears to be greater than it is.
Most echo sounders can be adjusted so that the depth of water is shown and not just the under keel depth.
If the set is utilising under keel depth, then the draught needs to be added to the sounding to arrive at the depth of water.
Tide will also have to be taken into account to adjust the sounding shown on the display so as to correspond with the charted depth.
On these displays the different colours represent various signal strengths for returning echoes. This makes it possible to identify the reflecting properties, and therefore composition, of the sea bed etc.
Be aware of multiple echoes in shallow waters.
The alarm control is useful for designating danger soundings in channels etc.
Principles of Operation
Echo sounders usually consist of an oscillator, transducer, amplifier and recorder/display. Refer to Figure 6 below.
Short electrical pulses are generated in the oscillator. A small part of the pulse is fed to the display to indicate the zero base. The major part is passed to the transducer, generally sited on the bottom of the hull. It is transmitted from the transducer as ultra sonic vibrations in a downward direction. The interval between the transmission and subsequent reception of the reflected pulses is measured and indicated in terms of depth on a display.
Signals returning from shallow depths need little amplification compared with those from greater depths, so a variable amplifier is provided in the circuit.
On small vessels the moving paper recorder tends to have been replaced by a digital/video display unit, often in colour.
Figure 6 Sounder Flowchart
The transducer needs to be placed in the optimum position in the vessel’s hull to receive the maximum benefit. The main problem with positioning is due to the aeration in the bubble train under the vessel. This aeration tends to come from the propeller, bow wave or underwater hull projections. Experiments tend to show that the best position is from 1/4 to 1/3 the vessel’s length from the bow.
· Gain: This controls the amplification of the received pulse, and should be adjusted to give the clearest recording. On a colour sounder, its use is more critical as the colours will change.
· Range and phase control: The range control allows for different range scales to be selected in order that the maximum depth can be detected. Phase controls shift the zero line off the display, without changing the scale. This means the upper limit shown on the display may not in fact be zero. If the sounding is tending towards the upper limit of the display, do not assume the vessel is likely to go aground.
· Metre to fathom switch.
· Paper speed control to select the paper advance speed.
· Minimum depth alarm control.
· Zero adjustment or draught setting control.
The sounder should always be initially operated with a zero line showing and then phased, bottom locked etc. This avoids the possibility of observing a second or third multiple echo and thinking it is the seabed.
With a hard seabottom and maximum sensitivity especially in shallower waters, there are often numerous echo lines visible. This is generally due to the pulses being reflected a number of times between the seabottom and the hull or surface of the water. Reducing sensitivity will help remove these.
When using the phased range, the first echo may not appear, but the second and/or third echo are indicated, with the consequent risk that the second or third echo is taken to be two or three times its true value.
Second trace return
In deep water it may occur that during the time the first pulse is on its way to the seabed and back, the transducer has sent a second pulse, and while open to receive this second pulse, receives the original pulse. It will be displayed as though it was from the later (second) pulse, and at a much shallower depth than the seabed. This type of return may often appear with radar.
This occurs when the vessel is in rough seas and moving over a smooth seabed. It is caused by the vessel pitching, giving rise to the apparent up and down movement of the seabed. The minimum reading should be taken as the depth therefore erring on the safe side.
Other effects are side lobes from the beam. These may return echoes, particularly over a sloping bottom that may be displayed at a sorter range than the true seabed.
· A smooth bottom is usually mud or sand. Mud often producing a weaker echo.
· A bottom with sudden steep changes in depth is likely to be coral or rock.
· A hard or rocky bottom can give rise to second and third echoes.
· It can occur that mud will overlay hard rock giving rise to separate echo lines. This generally shows as a light trace over the rock contour below, which is a harder line.
Practical Activity - requires access to a vessel’s operational echo sounder.
Read the manufacturers operating instructions before using the set.
Familiarise yourself with all the controls provided.
While under instruction operate the set on your vessel.
Speed through the water can be estimated from current engine or propeller speed. Experience will suggest the amount of “slip” expected under various conditions. Various mechanical and electronic logs have been devised to provide other means of measuring speed and/or distance travelled.
The Patent log is a rotator streamed from the stern on a plaited line. The rotator should be clear of the wash. The turning of the rotator is transferred through log line and governor to a clock on the taffrail which converts the revolutions to distance run (900 revs per nautical mile). There may be a repeater on the bridge.
The Impeller type log works on the same principle as the streamed log by measuring revolutions of a propeller or paddlewheel projecting below the hull. Magnets implanted into the rotating shaft of the propeller or paddlewheel cause an induced alternating current in the coil. This is transmitted to the recorder and displayed as speed. Due to turbulence near the hull, impeller type logs suffer inaccuracy, particularly in rough weather. They are not designed to record when going astern.
Figure 7 Impeller Log
The pressure (Pitometer) type of log measures pressure created in tubes projecting below the hull. One tube points vertically and measures the pressure due to depth. The other points ahead and measures the pressure due to depth and speed. The difference in pressure between the two gives a measurement of speed. The pressure log will not register at low speeds, nor going astern. Because the sensor must project well below the hull it is susceptible to damage and can be withdrawn.
Figure 8 Pressure Log
The Doppler log transmits sound pulses in a similar way to an echo sounder but at an angle ahead and astern (Janus configuration) towards the seabed at an angle of 60° from the horizontal. The difference in frequency of the reflected echo is measured indicating speed.
The change in frequency is the Doppler effect. It will indicate speed through the water like other logs, but can measure speed over the ground in depths up to around 200 metres.
Figure 9 Doppler Log
The Electro Magnetic log consists of a probe in the vessel’s hull. A magnetic field is generated which induces a current in the moving conductor (seawater). This current is proportional to the speed of movement of the conductor. A probe measures this induced current which is converted to speed in the display. As with the impeller type logs, the electro-magnetic log is subject to error caused by turbulence. Its advantage is the lack of moving parts.
Errors in transducer alignment
· The beam should be exactly fore and aft.
· Trim will cause misalignment of the beam.
· Ship motion, roll, pitch, surge and heave all contribute to the speed which the log is measuring. For the fore and aft axis log, pitch is the most significant error. For a cross track log roll can be significant.
· Speed of sound varies with water temperature, salinity and pressure. Temperature being the most significant.
There are a large number of automatic pilots available for small vessels. They are designed to steer a vessel along a course line with only minor fluctuations, over long periods of time.
Correctly adjusted it tends to steer a much straighter course than a helmsman can, reducing the effect of drag by using less helm. This drag effect ultimately reduces the vessel’s speed.
It is also able to be interfaced with other electronic navigational equipment and relieves a crew member from steering duties.
Auto Pilot Controls
These will vary between different auto pilots but the following is an indication.
The on/off control does exactly that.
standby switch will normally be engaged when the system is switched
on, i.e. the system is receiving information from the compass but the
auto pilot is not engaged.
The lock switch engages the auto pilot. It begins sending messages to the drive to maintain the current course (or the course entered with some models). To return to hand steering at any time, the standby switch is re-engaged.
Course trim switches allow adjustments to the course or small alterations to be made without disengaging the pilot. For larger alterations the standby mode would normally be engaged and the alteration made by hand before re-locking.
This setting determines the amount of rudder the pilot will use to maintain course. Heavy laden vessel require more rudder.
Calm seas require less rudder than heavy seas.
This control counteracts the tendency of the vessel to swing through its course. When the vessel begins to swing back towards its course, the helm is brought back to midships, when nearly on course, opposite rudder is applied to stop the swing.
The weather setting is adjusted for existing conditions. It allows the vessel to move easily with the seas without applying excessive helm i.e. delays the auto pilot’s response.
The bias or permanent helm setting allows the operator to input some amount of “permanent” helm to overcome the vessel’s tendency to steer to one side of the course or the other.
The off-course alarm may be pre-set on some auto pilots so that a visible/audible alarm operates when the vessel wanders more than a certain number of degrees from the set course. On many auto pilots this number can be changed by the operator.
Establishing rules for steering in auto
The Master should establish rules for use of the auto pilot and ensure that they are followed at all times. Some examples might be:-
1. Auto pilot is not to be engaged within harbour limits.
2. Auto pilot to be disengaged in any close-quarter’s situation.
3. Course alterations over 20 degrees to be executed in hand steering.
4. Rudder, weather and bias settings to be adjusted after any major course alteration/weather change.
5. Auto pilot to be disengaged in heavy weather to overcome danger of broaching.
6. During heavy traffic density and fog.
7. Complex navigation areas due to reefs, shoals etc.
There may be numerous alarms fitted within the wheelhouse, covering equipment in remote positions, also alarms within localised equipment used for navigational purposes.
The following are by no means exhaustive, but tend to be common on most small vessels.
Bilge level alarm (engine room)
This is required by NSCV for vessels under 24 metres and fully decked. The requirement is that it must be heard at the steering position when the vessel is under full power.
The simplified form of alarm is a float switch. When the bilge rises to a pre-set level the float reaches such an angle that mercury will flow along a sealed tube to complete an electrical circuit between two contacts.
Navigation light failure alarm
The purpose of the alarm is to indicate that one of the navigation lights is incorrectly functioning. Each light has a dedicated circuit containing a switch indicator light and fuse.
If the alarm is set off the indicator light will go out and the first thing to check is the fuse, as this may be the cause. If a bulb has been blown it should be immediately replaced or a secondary source turned on.
Refrigeration compartment alarm
Where personnel are required to enter a refrigeration space it shall be provided with an alarm audible outside the compartment. The cancellation of any such alarm being only from within the compartment.
The most common detection sensors are heat and smoke and must activate the alarm by making or breaking an electrical circuit.
When a sensor is activated, alarm bells are sounded automatically and a light on the indicator panel will indicate the fire location.
Whenever the alarm bells are activated always check for the cause and never re-set until it is confirmed that there is no emergency.
Machinery control and monitoring equipment
Although they do not necessarily give an audible alarm, the various indicators often show danger zones in red. It is the duty of the watchkeeper to keep a watchful eye on these monitoring devices. Whenever these devices are suspect, immediate checking should be carried out and repairs made at the earliest opportunity.
Navigational equipment alarms
Alarms are generally fitted to navigational equipment to warn of impending dangers. The alarms are set by the operator to provide an early warning system.
Echo sounders may be fitted with a depth alarm which sounds when a predetermined depth is reached. Radars may be fitted with range alarms, also warning of possible collisions.
The purpose of this alarm is to alert the watchkeeper to the vessel not maintaining the required heading. It is generally associated with the automatic pilot and the limits can be set by the watchkeeper. During adverse weather, with the vessel’s yawing, the deviations off course can be allowed for by selecting a higher alarm setting.
Practical Activity - Requires access to a wheelhouse
Inspect the vessel’s wheelhouse and:
1. Identify alarms and monitoring equipment
2. Identify the purpose of the equipment in 1.
3. Where appropriate, under guidance, test and re-set alarms.
Original Global Positioning System (G.P.S.)
Understanding the initial Global Positioning Systems
GPS is essentially a way to find your position using ranges from several objects in known locations. It is an old idea based on the navigational concept of Triangulation.
GPS is a space-based satellite navigation system composed of 24 to 32 satellites in medium Earth orbit that provides location and time information on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. The system provides critical capabilities to military, civil, and commercial users around the world. The United States government created the system, maintains it, and makes it freely accessible to anyone with a GPS receiver.
Figure 10 GPS Satellites in Orbit
The maritime user requires only 3 of the satellites to give latitude and longitude. The position can be updated at about 1 second intervals.
Figure 11 Two Dimensional Positioning on Sea
The vessel is positioned by the meeting point of three spheres formed by the three satellites.
12 The GPS Fix
The errors can be classified as -
Some system errors are caused by the changing satellite geometry, and is a phenomena familiar to piloting, celestial or Loran techniques. It is referred to as Dilution of Precision (DOP).
For a three dimensional fix, you require four satellites. The ideal position is for one to be directly overhead, and the other three on the horizon separated by 120o of azimuth. Any arrangement of satellites that is not like this will cause DOP (Dilution of Precision) in accuracy.
For a two dimensional fix only three satellites are needed, and the ideal position should be on the horizon separated by 120o in azimuth.
The HDOP (Horizontal Dilution of Precision) describes how an uncertainty in the range measurements from the selected satellites will affect the vessel’s horizontal position in latitude and longitude.
HDOP Horizontal Dilution of Precision
Figure 13 HDOP and Deviation
Figure 14 HDOP
Horizontal Dilution Of Precision has an enormous effect on the accuracy of the ‘fix’ result, even when Selective Availability (SA) is turned on. This latter is to some extent overcome by Differential Global Positioning System.
A good value of HDOP is between 2 and 4. Values in excess of 6 are of poor accuracy.
Selective availability is the ability of the military to scramble the GPS readings, hopefully to confuse the enemy during hostilities.
DGPS was developed to reduce Selective Availability (SA). It is limited to approximately 50 miles from a beacon. The error increases with distance. However, the position of accuracy offered by DGPS is in tens of metres not hundreds. The official accuracy of GPS for commercial use is 100 metres at 95% probability level.
· Ionospheric delay:- signals from satellites bend, on entering the ionosphere (refraction) and their speed will vary.
· Moisture and salt in the air
· Poor weather conditions, lightning activity
· Multipath error: In effect the satellite signal may be reflected off the water or generally, metal objects on board the vessel. This gives rise to the receiver antenna receiving the signal by two or more paths. It can be avoided by careful siting of the antenna.
· This is mainly due to set programming. The computation it performs can be carried out to ridiculous lengths or rounded off for efficient operation.
· Mis-matching of satellite signals.
The GPS operates on a spheroid called World Geodetic Spheroid 84 that best models the entire globe.
In general the position obtained on the GPS will have to be corrected before being plotted on the chart.
In the past, charts have been based on various spheroids, some being Clarke’s 1858, Australian Geodetic Datum 1966 (AGD66), but the Australian Hydrographic Office is now producing all new charts on the WGS 84 Spheroid.
Developments in satellite Positioning Systems
In addition to American Global Positioning System (GPS), other systems are in use or under development including the Russian Global Navigation Satellite System (GLONASS), the European Union Galileo positioning system, China's BeiDou Navigation Satellite System, the Japanese Quasi-Zenith Satellite System, and India's Indian Regional Navigation Satellite System.
GLONASS is a space-based satellite navigation system of 24 satellites enabling full global coverage operated by the Russian Aerospace Defense Forces. It provides an alternative to GPS and is the second alternative navigational system in operation. Smartphones can receive GLONASS positioning information along with GPS for a more accurate reception of up to 2 meters.
Galileo, the global navigation satellite system (GNSS) of 24 operational and 6 active spare satellites is currently being created by the European Space Agency Galileo. It is intended to provide position to 1-metre, and better at high latitudes. It will start offering from 2016 with the complete 30-satellite Galileo system expected by 2020.
BDS, the BeiDou Navigation Satellite System is a Chinese satellite navigation system of two separate satellite constellations – a limited test system that has been operating since 2000, and a full-scale global navigation system that is currently under construction. The first BeiDou system consists of three satellites and has offered limited coverage in China’s neighboring regions since 2000. The second generation BDS known as COMPASS or BeiDou-2, will be a global satellite navigation system consisting of 35 satellites, and is under construction as of January 2015. In-mid 2015, China started the build-up of the third generation BeiDou system (BDS-3) in the global coverage constellation.
QZSS (Quasi-Zenith Satellite System), is a proposed Japanese three-satellite regional system. The construction of three satellites is slated for launch before the end of 2017 and the basic four-satellite system is planned to be operational in 2018. QZSS can only provide limited accuracy on its own and is not currently required in its specifications to work in a stand-alone mode. As such, it is viewed as a GNSS Augmentation service.
IRNSS, the Indian Regional Navigation Satellite System or is a Navigation Satellite System consisting of 3 satellites in GEO orbit and 4 satellites in GSO orbit providing positioning extending to 1500 km around India. The requirement of such a navigation system is driven because access to foreign government-controlled global navigation satellite systems is not guaranteed in hostile situations.
Automatic Identification System (AIS)
identification system (AIS) works automatically and continuously, regardless
of where a vessel is located.
There are two dedicated frequencies used for AIS:
AIS 2 (channel 88B).
Each frequency is divided into 2250 time slots that are repeated every 60 seconds. The AIS units send packets of information which are transmitted in these time slots. Shipborne AIS units autonomously broadcast different AIS messages including:
'dynamic data' which includes latitude, longitude,
position accuracy, time, course, speed, navigation status.
· 'static data' which includes name, dimensions, type, draft, destination and estimated time of arrival.
· Position reports are broadcasted frequently (between 2–10 seconds depending on the vessel’s speed, or every 3 minutes if at anchor), while static and voyage related reports are sent every 6 minutes.
Some publicly available AIS websites are a crowd-based approach to AIS information. The International Maritime Organization (IMO) does not support the display of AIS on public websites. The AIS receivers used for this purpose are not always certified AIS base stations and may not provide accurate or valid data.
Under SOLAS and the relevant IMO guidelines, the master of any vessel has the discretion to turn off the AIS unit if its continual operation might compromise the vessel's safety or security.
Long range identification and tracking
Long range identification and tracking (LRIT) is not the same as AIS and does not replace AIS. AIS cannot be used for LRIT.
LRIT is an international system used to monitor the location of vessels travelling within 1000 nautical miles off the Australian coast. Vessels must willingly provide their location to the LRIT system and its use of satellites mean it can be used anywhere in the world. LRIT was adopted by the International Maritime Organization as an amendment to Chapter V of SOLAS and came into force on 1 January 2008.
The mariner is advised to refer to the Chart Title under the heading “Positions” and in particular “Satellite - Derived Positions”.
These will say to plot direct or give corrections to apply before plotting.
EXAMPLE 1: CHART AUS 21 MELVILLE ISLAND -
Positions are related to the World Geodetic System 1984
SATELLITE - DERIVED POSITIONS
Positions obtained from satellite navigation systems are referred to the WGS Datum and can be plotted directly onto this chart.
EXAMPLE 2: CHART AUS 830 RUSSEL ISLAND TO LOW ISLETS
Positions are related to the Australian Geodetic Datum 1966
SATELLITE - DERIVED POSITIONS
Positions obtained from satellite navigation systems are normally referred to the WGS Datum; such positions should be moved 0.09 minutes SOUTHWARD and 0.05 minutes WESTWARDS to agree with the chart.
In the case of chart AUS 830, the correction would be as follows:-
GPS position 170° 00.00 S 146° 05.00 E
Correction +0.09 -0.05
Chart Position 17° 00.09 S 146° 04.95 E
Electronic Chart Displays
Video chart plotters like other navigation devices should not be relied upon as a single approach. Always use alternative means to cross-check your work.
Before using any system always consult the manufacturers operational guide etc.
Some capabilities of Plotters are listed below:-
· Instantly being able to find the position of, and range and bearing to any point on the chart
· Lay a course with as many Waypoints as you like, instantly reading the position, range, and bearing of each.
· Track and record your vessel’s passage so as to compare against the plotted course while under way.
· Store courses and Waypoints in memory for future voyages.
· Set alarms to warn you when you reach Waypoints or enter into hazardous water.
· Access a far larger chart library, or database, than possible with paper charts.
· Steer your boat by the instruments when visibility is impossible.
At present it may be considered in some cases to be a high cost duplication of paper charts, especially as the Electronic Chart System of the Hydrographic Office is not considered to be the equivalent of the paper chart for International Maritime Organisation requirement.
The paper chart show as much or more detail and data than most video charts. Paper chart scales are obvious, however with the ability to zoom in and out, scale and accuracy limitations are not so immediately apparent. Over-scaling may appear to provide accuracy greater than that available from the data base.
Present RAN charts are Raster-Scanned and when buying any chart make sure it is scanned from the official chart of suitable scale and most up to date.
Be aware that they can be conducive to sloppy navigation. Due to being easy to use, mariners may not bother to back them up with paper plots or log entries.
Many video plotters display much less chart information and can cause you to get into dangerous places by relying on them exclusively.
All GPS functions can be combined with an Electronic Chart system and produce a constantly updated position and intended movement of the vessel.
The plotter can be used for piloting, where the radar doesn’t define location well.
Low lying land, similar looking channels, heavy rain blocking solid targets such as buoys, can make coastal navigation with radar uncertain.
With the plotter, you have the radar’s dynamic updating, and paper chart’s mapping accuracy.
With a GPS interfaced, you can watch your position on screen in relation to well charted features and nav aids.
You can enable yourself to be in the centre of the plotter screen; - giving relative information, or place yourself to travel across the screen giving a true picture.
On a paper chart it is common to set a course directly from Buoy to Buoy. With a plotter plugged into a navigation receiver and auto pilot, it is advisable to set your Waypoints well clear of the objects you are going to use as turning marks.
Although alarms can be set for a preset distance off the Waypoint. If the first alarm doesn’t activate a damaged vessel and buoy may be the result.
Some plotters allow you to draw lines on screen paralleling your course - useful for narrow channels where depth is important. An alarm can be pre-set for the vessel’s screen marker hitting one of the lines.
Simple Integrated Navigational System
Plotters - Autopilot - G.P.S. Can Be Linked
The controls may all be on the plotter control panel. You may be able to set your course on the screen and then engage the pilot without first having to transfer the route elsewhere.
Being able to do all from one set of controls limits confusion and the possible inputting of incorrect commands into GPS., plotter or pilot.
The above may mean using one manufacturers equipment.
The simplest function is when you plug in the GPS, your vessel’s position moves across the screen at a heading and speed in scale with the vessel’s speed over the ground.
The plotter generally performs in one of 3 modes.
Range and Bearing Navigation
and Voyage Planning
You can show vessel’s track versus planned path, watch for steering irregularities and effects of wind, current and tidal streams.
When observing the Lat/Long position make sure you know whether it is the vessel’s position, cursor’s position or some Waypoint, buoy or other position.
Normal default is vessel’s position.
To avoid mistakes develop a routine for using the plotter.
Start by keying in the basic chart and data display, note position, course, heading, speed, time and other essentials in your paper log.
Waypoints can be entered by Latitude and Longitude, and also by a ‘save’ function which simply memorises the current position as a Waypoint.
Waypoints once entered are given a reference number in a required sequence. Upon recall, the position is displayed with bearing and distance to it, shown as a straight track.
As the vessel moves towards the Waypoint, its actual course is plotted, enabling the mariner to identify cross track error, which with the autopilot synchronised with the set will adjust the course as required. See Figure 15
As the vessel arrives at a Waypoint, signalled by a set alarm the auto pilot will change course to the reset sequence Waypoint.
The requirements for a good Electronic Chart and Display Informational System is being developed as a joint effort of the (IMO) and (IHO).
Electronic Chart and Display Information System (ECDIS)