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Use of Electronic Aids to Navigation

Notice to Owners, Masters, Skippers, Officers and Crews of Merchant Ships and Fishing

Vessels

This Guidance Note supersedes Merchant Shipping Notice No. 1158

MARINE GUIDANCE NOTE

MGN 63 (M+F)

Summary

This note emphasises the need for correct use of navigational equipment by watchkeepers.

Key Points:-

• Be aware that each item of equipment is an aid to navigation

• Be aware of the dangers of over-reliance on the output from and accuracy of a single navigationalaid

• Recognise the importance of the correct use of navigational aids and knowledge of theirlimitations

• Appreciate the need to cross check position fixing information using other methods

• Be aware of the factors which affect the accuracy of position fixing systems

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1. NAVIGATIONAL EQUIPMENT

Provision of Navigational Equipment on Ships

1.1 The Merchant Shipping (NavigationalEquipment) Regulations 1993 (SI 1993 No 69)require certain ships to be provided with amagnetic compass installation and other specifiedships to be fitted additionally with a directionfinder, an echo sounder, a gyro compass, radarand ARPA installations, a speed and distancemeasuring installation and a rate of turn indicator.

1.2 Provision is also made in the Regulationsin respect of siting and serviceability of theinstallations and, in the case of radar and ARPA

installations, the qualifications of the radarobservers.

1.3 A number of recent accidents have beencaused by over-reliance on a single electronicnavigational aid. Watchkeepers must always

ensure that positional information is regularlycross-checked using other equipment, as well asvisual aids to navigation.

1.4 Some radars are equipped with Auto-Tracking Aids (ATA) which enable targets to beacquired manually and automatically plotted.Such systems do not provide all the functions of ARPA. Radars for smaller vessels may beprovided with Electronic Plotting Aids (EPA)which require the operator to plot each targetmanually. EPA provides the target calculations

for each manual plot. Operators should be awareof the functional limitations of ATA and EPA.

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2. THE USE OF RADAR AND PLOTTINGAIDS

General

2.1 Col li sions have been caused far toofrequently by failure to make proper use of radar

and ARPA in both restricted visibility and inclear weather. A common error has been alteringcourse on insufficient information and bymaintaining too high a speed, particularly when aclose quarters situation is developing or is likelyto develop. Information provided by radar andARPA/ATA in clear weather conditions canassist the watchkeeper in maintaining a properlookout in areas of high traffic density. It cannot be emphasised too strongly that navigation inrestricted visibility is difficult and great care isneeded even with all the information availablefrom the radar and ARPA/ATA. Wherecontinuous radar watchkeeping and plottingcannot be maintained even greater caution must be exercised. A “safe speed” should at all timesreflect the prevailing circumstances.

Interpretation

2.2 It is essential for the observer to be awareof the current quality of performance of the radar(which can most easily be ascertained by thePerformance Monitor) and to take account of thepossibility that small vessels, small icebergs andother floating objects such as containers may not be detected. When video processing techniquesare employed, caution should be exercised.

2.3 Echoes may be obscured by sea or rainclutter. Correct setting of clutter controls will help but will not completely remove this possibility.When plotting larger targets on a medium rangescale, the display should be periodically switchedto a shorter range, and the clutter controlsadjusted, to check for less distinct targets.

2.4 The observer must be aware of the arcs of

blind and shadow sectors on the display caused by masts and other on-board obstructions. Theymust be plotted on a diagram placed near theradar display which must be updated followingany changes which affect the sectors.

Plotting

2.5 To estimate the degree of risk of collisionwith another vessel it is necessary to forecast theclosest point of approach. Choice of appropriateavoiding action is facilitated by the knowledge of the other vessel’s track. This can be obtained by

manual plotting methods or using EPA, orautomatically, using ATA or ARPA. The accuracyof the plot, however obtained, depends upon

accurate measurement of own ship’s track duringthe plotting interval. Observers should be awarethat an inaccurate compass heading or speedinput will greatly reduce the accuracy of truevectors when using ARPA or ATA, and shouldtherefore treat the apparent precision of thedigital display with caution. This is particularlyimportant with targets on near-opposite courseswhere a slight error of own-ship’s data can makethe difference between a target apparentlycrossing ahead or passing clear.

Choice of range scale

2.6 Although the choice of range scales forobservation and plotting is dependent uponseveral factors such as traffic density, speed of own ship and the frequency of observation, it isnot generally advisable to commence plotting ona short range scale. Advance warning of theapproach of other vessels, changes in trafficdensity, or proximity of the coastline, should beobtained by occasional use of longer range scales.This applies particularly when approaching areasof expected high traffic density when informationobtained from the use of longer range scales may be an important factor in determining a safespeed.

Appreciation

2.7 A single observation of the range and bearing of an echo will give no indication of track

of a vessel in relation to own ship. To estimatethis, a succession of observations must be madeover a known time interval. The longer the periodof observation, the more accurate the result. Thisalso applies to ARPA/ATA which requiresadequate time to produce accurate informationsuitable for assessing collision risk anddetermining appropriate manoeuvres.

2.8 Estimation of the target’s true track is onlyvalid up to the time of the last observation and thesituation must be kept constantly under review.The other vessel, which may not be keeping aradar watch or plotting, may alter its courseand/or speed. This will take time to becomeapparent to the observer on own ship. NeitherARPA nor ATA will detect any alterationimmediately and therefore should also bemonitored constantly.

2.9 It should not be assumed that because therelative bearing of a target is changing, there is norisk of collision. Alteration of course and/orspeed by own ship may alter the relative bearing.A changing compass bearing is more reliable.

However, account should be taken of the target’srange because, at close quarters, risk of collisioncan exist even with a changing compass bearing.

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2.10 Radar should be used to complementvisual observations in clear weather to assistassessment of whether risk of collision exists or islikely to develop. It also provides accuratedetermination of range to enable appropriateaction to be taken in sufficient time to avoidcollision, taking into account the manoeuvringcapabilities of own ship.

Clear weather practice

2.11 It is important that all using radar andARPA/ATA should obtain and maintainexperience in its operation by practice at sea inclear weather. This allows radar observations andARPA/ATA vectors to be checked visually. Thusmisinterpretation of the radar display or falseappreciation of the situation, which in restrictedvisibility could be potentially dangerous, ishighlighted. By keeping themselves familiar withthe process of systematic radar observation, andthe relationship between radar and electronicallyplotted information and the actual situation,watchkeepers will be able to deal rapidly andcompetently with the problems which willconfront them in restricted visibility.

Operation

2.12 The radar display should be kept on at alltimes when weather conditions indicate thatvisibility may deteriorate, and at night wherever

fog banks, small craft or unlit obstructions suchas icebergs are likely to be encountered. This isparticularly important when there is a likelihoodof occasional fog banks so that vessels can bedetected before entering the fog. The life of components, and hence the reliability of theradar, will be far less affected by continuousrunning, than by frequent switching on and off.

Radar watchkeeping

2.13 In restricted visibility the radar displayshould be permanently on and observed. The

frequency of observation will depend on theprevailing circumstances, such as own ship’sspeed and the type of craft or other floatingobjects likely to be encountered.

Parallel index techniques

2.14 Investigation of casualties where radar was being used as an aid to navigation prior to thevessel grounding have indicated that inadequatemonitoring of the ship’s position contributed tomany of the accidents. Parallel index techniquesprovide valuable assistance to position

monitoring in relation to a pre-determined

passage plan, and would have helped to avoidthese groundings. Parallel indexing should bepractised in clear weather during straightforwardpassages, so that watchkeepers becomethoroughly familiar with the technique beforeattempting it in confined difficult passages, or atnight, or in restricted visibility.

2.15 The principles of parallel index plottingcan be applied, using electronic index lines, to both relative and true motion displays. Theseindex lines can be stored and called up whenrequired on all modes of display. Electronic indexlines also enable the operator to switch ranges.With such a facility, care must be taken duringpassage planning to ensure that the correctparallel index lines for the intended voyage areavailable for retrieval.

2.16 On a relative motion display, the echo of afixed object will move across the display in adirection and at a speed which is the exactreciprocal of own ship’s ground track. Parallelindexing uses this principle of relative motion, andreference is first made to the chart and the plannedground track. The index line is drawn parallel tothe planned ground track with a perpendiculardistance (cross index range or offset) equal to theplanned passing distance off the object.Observation of the fixed object’s echo movingalong the index line will provide a continuousindication of whether the ship is maintaining the

planned track. Any displacement of the echo fromthe index line will immediately indicate that ownship is not maintaining the desired ground track,enabling corrective action to be taken.

2.17 Electronic parallel index lines are drawnand used in the same way on true motiondisplays in both sea-stabilised and ground-stabilised mode. Parallel index lines are fixedrelative to the trace origin (ie to own ship), andwill consequently move across the display at thesame rate and in the same direction as own ship.Being drawn parallel to the planned chartedtrack, and offset at the required passing distanceoff the selected fixed mark, the echo of the markwill move along the index line as long as the shipremains on track. Any displacement of the fixedmark’s echo from the index line will indicate thatthe ship is off track enabling corrective action to be taken.

2.18 Parallel indexing is an aid to safenavigation and does not replace the requirementfor position fixing at regular intervals using allappropriate methods available including visual

checks.

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2.19 When using radar for position fixing andmonitoring, check:

(a) the radar’s overall performance,

(b) the identity of fixed objects,

(c) the gyro error and accuracy of the headingmarker alignment,

(d) the accuracy of the variable range marker, bearing cursor and fixed range rings,

(e) that parallel index lines are correctlypositioned on a suitable display.

2.20 Some older radars may still have reflectionplotters. It is important to remember that parallelindex lines drawn on reflection plotters apply toonly one range scale. In addition to all otherprecautions necessary for the safe use of radar

information, particular care must therefore betaken when changing range scales.

Regular operational checks

2.21 Frequent checks of the radar performancemust be made to ensure that the quality of thedisplay has not deteriorated.

2.22 The performance of the radar should bechecked before sailing and at least every fourhours whilst a radar watch is being maintained.This should be done using the performance

monitor.

2.23 Mis-alignment of the heading marker, evenif only slight, can lead to dangerously misleadinginterpretation of potential collision situations,particularly in restricted visibility when targetsare approaching from ahead or fine on ownship’s bow. It is therefore important that checksof the heading marker should be madeperiodically to ensure that correct alignment ismaintained. If misalignment exists it should becorrected at the earliest opportunity. Thefollowing procedures are recommended:

(a) Check that the heading marker is alignedwith the compass heading of the ship.

(b) Ensure that the heading marker line on thedisplay is aligned with the fore-and-aft line of theship. This is done by selecting a conspicuous butsmall object with a small and distinct echo whichis clearly identifiable and lies as near as possible atthe edge of the range scale in use. Measuresimultaneously the relative visual bearing of thisobject and the relative bearing on the display. Anymisalignment must be removed in accordancewith the instructions in the equipment manual.

2.24 To avoid introducing serious bearingerrors, adjustment of the heading marker shouldnot be carried out by using the alignment of the berth on a ship which is alongside in harbour; norshould it be carried out using bearings of targetswhich are not distinct, close to the vessel or havenot been identified with certainty both by radarand visually.

Electronic radar plotting aids (ARPA and ATA)

2.25 In addition to the advice given above andthe instructions contained in the OperatingManual, users of ARPA /ATA should ensurethat:

(a) the test programmes are used to check thevalidity of the ARPA/ATA data,

(b) the performance of the radar is at itsoptimum,

(c) the heading and speed inputs to theARPA/ATA are satisfactory. Correct speed input,where provided by manual setting of theappropriate ARPA/ATA controls or by anexternal input, is vital for correct processing of ARPA/ATA data. Serious errors of output datacan arise if heading and speed inputs to theARPA/ATA are incorrect. Users should be awareof possible hazards of using ground stabilisedmode with ARPA/ATA when assessing risk of collision with approaching vessels, particularly in

areas where significant tidal streams and/orcurrents exist. When course and speed inputs arederived from electronic position fixing systems(eg LORAN, GPS and DGPS) the display isground-stabilised. The output data of trackedtargets will relate to their ground track and,although accurate, may be highly misleadingwhen assessing target aspect and determiningcollision-avoidance manoeuvres. In cases of gyrofailure when heading data is provided from atransmitting magnetic compass, watchkeepersshould remember to determine and apply theerrors of the magnetic compass.

2.26 The use of audible operational warningsand alarms to denote that a target has closed on arange, transits a user-selected zone or breaks a pre-set CPA or TCPA limit does not relieve the userfrom the duty to maintain a proper lookout by allavailable means. Such warnings and alarms, whenthe ARPA is in automatic acquisition mode,should be used with caution especially in thevicinity of small radar-inconspicuous targets.Users should familiarise themselves with theeffects of error sources on the automatic trackingof targets by reference to the ARPA OperatingManual.

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2.27 Information on detection and use of Searchand Rescue Transponders (SARTs) is provided inChapter 4 of Volume 5 of the Admiralty List of Radio Signals.

3. TERRESTRIAL HYPERBOLICPOSITIONING SYSTEMS

General

3.1 With world-wide coverage by satellitenavigation systems, the use of hyperbolicpositioning systems at sea is declining. The Omegasystem has ceased operation, and under presentplans the Decca Navigator System will cease tooperate in Europe around the year 2000. LORANC, however, is to be retained for the time being incertain areas. It will be available to maritime usersas the terrestrial electronic position fixing serviceto back-up global satellite systems.

3.2 The use of lattice charts with hyperbolicpositioning systems has declined, because mostreceivers convert the readings to latitude andlongitude. These receivers display positionsreferred to a particular horizontal datum (egWGS 84). This may not be the datum of the chartin use. The user must still remember thathyperbolic systems have inherent errors, and thatthe apparent accuracy of the displayed positionsshould be treated with caution.

3.3 Some equipment processes data f romseveral electronic positioning systems (eg Decca,LORAN and GPS) and computes the bestpossible position, so providing a valuable checkof one system against another. The use of suchequipment does not remove the responsibility of the navigator to check the position periodicallyusing other means, including visual aids.

3.4 Users should be vigilant when receiversare capable of reverting to dead reckoning (DR)mode. Serious accidents have occurred whenfaults in sensors and antennae connections havecaused the receiver to switch to DR mode

undetected by the watchkeepers.3.5 Some terrestrial hyperbolic navigationreceivers give a numerical indication of positionalaccuracy in the form of values of Horizontal andPositional Dilution of Precision (HDOP andPDOP). Users should refer to the equipmentmanual, as the receiver will not necessarily allowfor fixed or variable errors in the system.

3.6 Further information on hyperbolic positionfixing systems as well as up to date details of their operational status and coverage can befound in the Ad mira lty Li st of Radi o Si gnal s,Volume 2.

The Decca Navigator System

3.7 Decca Marine Data Sheets give the fixederrors for geographical areas where these areknown. Where no errors are given, it should not be assumed that no error exists. In areas where nofixed errors are given, Decca positions should be

treated with caution, especially when near thecoast and in restricted waters. Receivers whichconvert positional data to latitude and longitudemay not take fixed errors into account.

3.8 Decca is also subject to variable errorswhich depend on the time of day, season anddistance from the transmitters. The error in agiven location is not constant, and the DeccaMarine Data Sheets give diagrams and tableswhich can be used to predict an approximateerror based on a 68% probability level, (ie theyare not likely to be exceeded on more than one in

three occasions).

Lane Slip

3.9 Particularly at night, there is a possibilityof slipping lanes due to interference such asexcessive Decca skywave signals, external radiointerference and electric storms. The possibility of this happening is small at short range, butincreases towards the edge of Decca coverage.Fouling of the Decca antenna and disruptions tothe power supply can also cause lane slip. It can best be detected by plotting the ship’s position at

regular intervals and comparing with fixesobtained by other means.

The LORAN C system

3.10 LORAN C has a greater range than Deccaand is based on the measurement of timedifference between the reception of transmittedpulses. The ground-wave coverage is typically bet we en 800 and 120 0 mi les , al th ou gh th eaccuracy of positional information will dependupon the relative position of the transmitters.

3.11 When entering the coverage, or when

passing close to transmitters on the coast, thereceiver may have difficulty in identifying thecorrect ground-wave cycle to track. Care should be taken to ensure that it is tracking on the correctcycle.

3.12 The fixed errors of the LORAN C systemare caused by variations in the velocity at whichthe pulses travel. Additional Secondary Factor(ASF) corrections are provided to allow for theseerrors. Account should be taken of ASF correctionswhich may be very significant in some areas. Somereceivers automatically allow for calculated ASFvalues and display a corrected position.

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4. GLOBAL POSITIONING SYSTEM (GPS)

4.1 The NAVSTAR GPS Standard PositioningService (SPS) now provides a global positioningcapability giving a 95% accuracy in the order of 100 metres. The system is capable of much greateraccuracy, but the commercial service is

deliberately degraded by Selective Availability(SA). Differential GPS (DGPS) is also becomingmore widely available. DGPS receivers applyinstantaneous corrections to raw GPS signalsdetermined and transmitted by terrestrialmonitoring stations. Positional accuracy of betterthan 5 metres may be possible.

4.2 The GLONASS system is fully operationaland available to commercial users. The system issimilar to GPS and also provides globalpositioning for 24 hours a day. Some receiversuse both GPS and GLONASS signals to computea more precise position. The repeatable accuracyof GLONASS is higher than GPS as there is nodegrading of signals by SA. When navigating inconfined waters, navigators must bear in mindthat the displayed position from any satellitepositioning system is that of the antenna.

4.3 Serious accidents have occurred because of over-reliance upon global satellite positioningequipment. In one case a passenger vesselgrounded in clear weather because thewatchkeepers had relied totally upon the GPS

output which had switched to DR mode becauseof a detached antenna. The switch to DR modewas not detected by the watchkeepers. Checkingthe position using other means, including visualobservations, would have prevented the accident.

Datums and Chart Accuracy

4.4 GPS positions are referenced to the globaldatum WGS 84. This may not be the same as thehorizontal datum of the chart in use, meaning thatthe position when plotted may be in error. Thereceiver may convert the position to other datums.

In this case the observers must ensure that they areaware of the datum of the displayed position.Where the difference in datums is known, a noteon the chart provides the offset to apply topositions referenced to WGS 84, but where this isnot given the accuracy of the displayed positionshould be treated with caution. DGPS positions arenormally referenced to WGS 84 though localdatums may be used (eg NAD 83 in the USA).Also, when using DGPS, it is possible that thepositioning of charted data may not be as accurateas the DGPS position. Mariners should thereforealways allow a sensible safety margin to accountfor any such discrepancies.

4.5 From April 1998, a new Volume 8 of The Admiralty List of Radio Signals, entitled SatelliteNavigation Systems will contain full descriptions of all satellite systems, including GPS and DGPS, aswell as notes on their correct use and limitations.Also included will be descriptions and examples of over-reliance on GPS, together with theadvantages and disadvantages of using DGPS, anda full account of the problems caused by differinghorizontal datums. Mariners using satellitenavigation systems are strongly advised to studythe information and follow the advice contained inthis publication.

5. ELECTRONIC CHARTS

5.1 A number of vessels now use electroniccharts. Mariners should be aware that the onlytype of electronic chart system with performance

standards adopted by IMO is the Electronic ChartDisplay and Information System (ECDIS). Onerequirement of an ECDIS is that it must only useofficial vector data produced by a nationalhydrographic office. At present, this ElectronicNavigational Chart (ENC) data is not widelyavailable and the use of ECDIS is limited. AnECDIS using official ENC data satisfies theSOLAS Chapter V requirement for vessels tocarry up to date charts.

Vector charts

5.2 The ENC is a database of individual itemsof digitised chart data which can be displayed asa seamless chart. ENCs of appropriate detail areprovided for different navigational purposes suchas coastal navigation, harbour approach and berthing. The amount of deta il displa yed isautomatically reduced when the scale of aparticular ENC is reduced, in order to lessenclutter. Individual items of data can be selectedand all relevant information will be displayed(for instance, all the available informationrelevant to a light or navigation mark.) ECDIS is

therefore very much more than an electronicversion of the paper chart. With vector charts thedata is “layered”, enabling the user to de-selectcertain categories of data, such as a range of soundings, which are not required at the time.This facility, as well as reducing chart clutter,enables the user to select a depth contour soproviding an electronic safety contour which mayautomatically warn the watchkeeper whenapproaching shallow water. Mariners should usethe facility to de-select data with extreme cautionas it is possible accidentally to remove dataessential for the safe navigation of the vessel.

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