Use of electronic Navigation Aids(1), Żeglarstwo, Nawigacja [ Pobierz całość w formacie PDF ]
MARINE GUIDANCE NOTE
MGN 379 (M+F)
Navigation: Use of Electronic Navigation Aids
Notice to all Owners, Masters, Skippers, Officers and Crews of Merchant Ships and
Fishing Vessels
This notice replaces MGN 63
PLEASE NOTE:-
Where this document provides guidance on the law it should not be regarded as definitive.
The way the law applies to any particular case can vary according to circumstances - for
example, from vessel to vessel and you should consider seeking independent legal advice if
you are unsure of your own legal position.
Summary
This note emphasises the need for correct use of navigational equipment by watch-keepers.
Key Points:
-
• Be aware that each item of equipment is an
aid
to navigation.
• Be aware of the factors which affect the accuracy of position fixing systems.
• Appreciate the need to cross check position fixing information using other methods.
• Recognise the importance of the correct use of navigational aids and knowledge of their
limitations.
• Be aware of the dangers of over-reliance on the output from, and accuracy of, a single
navigational aid.
1. INTRODUCTION
Accidents have occurred where the primary cause has been over-reliance on a single
electronic navigational aid. Watch-keepers must always ensure that positional information is
regularly cross-checked using other equipment, as well as visual aids to navigation. In other
cases accidents have occurred where the watch-keeper was not fully conversant with the
operation of equipment or its limitations.
2. PROVISION OF NAVIGATIONAL EQUIPMENT ON SHIPS
The Merchant Shipping (Safety of Navigation) Regulations 2002 (SI 2002 No 1473) implement
the carriage requirements for navigational equipment set out in Regulation 19 of Safety of Life
at Sea (SOLAS) Chapter V. These requirements, together with guidance notes, are contained
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in the 2007 Edition of the Maritime and Coastguard Agency (MCA) publication “Safety of
Navigation – Implementing SOLAS Chapter V 2002”
Ships built before 1 July 2002 may continue to comply with the requirements of SOLAS
Chapter V/74 in force prior to 2002 Regulations, with regard to Signalling Lamps (Reg. 11/74),
Navigation Equipment (Reg. 12/74) and Nautical Publications (Reg. 20/74). However they
must carry a Global Navigation Satellite System (GNSS) receiver or a terrestrial radio-
navigation receiver, Automatic Identification System (AIS) and Voyage Data Recorder (VDR)
or Simplified Voyage Data Recorder (S-VDR) in accordance with the timetables set out in
Regulations 19 and 20 of SOLAS V 2002.
Guidance is also given in Annex 20 of the MCA Safety of Navigation publication on siting and
servicing of the installations.
3. RADAR AND PLOTTING AIDS
3.1 General
Collisions have been frequently caused by failure to make proper use of radar and radar
plotting aids in both restricted visibility and clear weather. Common errors have been deciding
to alter course on the basis of insufficient information and maintaining too high a speed,
particularly when a close-quarters situation is developing. Information provided by radar and
radar plotting aids in clear weather conditions can assist the watch-keeper in maintaining a
proper lookout in areas of high traffic density. It is most important to remember that navigation
in restricted visibility can be more demanding and great care is needed even with all the
information available from the radar and radar plotting aids. Where continuous radar watch-
keeping and plotting cannot be maintained even greater caution must be exercised. A “safe
speed” should at all times reflect the prevailing circumstances.
3.2 Electronic radar plotting aids
Radars must be equipped with plotting aids, the type of which depends upon the size of ship
as follows;
a) Electronic Plotting Aid (EPA)
EPA equipment enables electronic plotting of at least 10 targets, but without
automatic tracking (Ships between 300 and 500 Gross Tonnage (GT)).
b) Automatic Tracking Aid (ATA)
ATA equipment enables manual acquisition and automatic tracking and display of
at least 10 targets (Ships over 500 GT).
On ships of 3000 GT and over the second radar must also be equipped with an
ATA, the two ATAs must be functionally independent of each other.
c) Automatic Radar Plotting Aid (ARPA)
ARPA equipment provides for manual or automatic acquisition of targets and the
automatic tracking and display of all relevant target information for at least 20
targets for anti-collision decision making. It also enables trial manoeuvre to be
executed (Ships of 10000 GT and over).
The second radar must incorporate ATA if not ARPA.
Manual plotting equipment is no longer acceptable except for existing vessels still complying
with SOLAS V/74.
Watch-keepers must be fully conversant with the operation and limitations of these plotting
facilities and should practice using them in clear-weather conditions to improve their skills.
In addition to the advice given above and the instructions contained in the Operating
Manual(s), users of radar plotting aids should ensure that:
(i) performance of the radar is monitored and optimised
(ii) test programmes provided are used to check the validity of the plotting data, and
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(iii) speed and heading inputs to the ARPA/ATA are satisfactory. Correct speed input, where
provided by manual setting of the appropriate ARPA/ATA controls or by an external input,
is vital for correct processing of ARPA/ATA data. Serious errors in output data can arise if
heading and/or speed inputs to the ARPA/ATA are incorrect.
For full details of the carriage requirements and associated guidance see Regulation 19 and
Annex 16 of the MCA Safety of Navigation publication.
3.3 Plotting
To estimate risk of collision with another vessel the closest point of approach (CPA) must be
established. Choice of appropriate avoiding action is facilitated by the knowledge of the other
vessel’s track using the manual or automatic plotting methods (see 3.2 above). The accuracy
of the plot, however obtained, depends upon accurate measurement of own ship’s track during
the plotting interval. It is important to note that an inaccurate compass heading or speed input
will reduce the accuracy of true vectors when using ARPA or ATA. This is particularly
important with targets on near-reciprocal courses where a slight error in own-ship’s data may
lead to a dangerous interpretation of the target vessel’s true track. The apparent precision of
digital read-outs should be treated with caution.
If two radars are fitted (mandatory for ships of 3000 GT and over) it is good practice, especially
in restricted visibility or in congested waters, for one to be designated for anti-collision work,
while the other is used to assist navigation. If only one of the radars is fitted with ARPA then
this should be the one used for anti-collision work and the other for navigation.
3.4 Interpretation
It is essential for the operator to be aware of the radar’s current performance which is best
ascertained by the Performance Monitor. The echo return from a distant known target should
also be checked. Be aware of the possibility that small vessels, ice floes or other floating
objects such as containers may not be detected.
Echoes may be obscured by sea- or rain-clutter. Correct setting of clutter controls will help but
may not completely remove this possibility. When plotting larger targets on a medium range
scale, the display should be periodically switched to a shorter range, and the clutter controls
adjusted, to search for less distinct targets.
The observer must be aware of the arcs of blind and shadow sectors on the display caused by
masts and other on-board obstructions. These sectors must be plotted on a diagram placed
near the radar display. This diagram must be updated following any changes which affect the
sectors.
3.5 Choice of range scale
Although the choice of range scales for observation and plotting is dependent upon several
factors such as traffic density, speed of own ship and the frequency of observation, it is not
generally advisable to commence plotting on a short range scale. Advance warning of the
approach of other vessels, changes in traffic density, or proximity of the coastline, should be
obtained by occasional use of longer range scales. This applies particularly when approaching
areas where high traffic density is likely, when information obtained from the use of longer
range scales may be an important factor in determining a safe speed.
3.6 Appreciation
A single observation of the range and bearing of an echo will give no indication of the track of
a vessel in relation to own ship. To estimate this, a succession of observations must be made
over a known time interval. The longer the period of observation, the more accurate the result
will be. This also applies to ARPA/ATA which requires adequate time to produce accurate
information suitable for assessing CPA / TCPA and determining appropriate manoeuvres.
Estimation of the target’s true track is only valid up to the time of the last observation and the
situation must be kept constantly under review. The other vessel, which may not be keeping a
radar watch or plotting, may subsequently alter its course and/or speed. This will take time to
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become apparent to the observer. Electronic plotting will not detect any alteration of a target’s
course or speed immediately and therefore should also be monitored constantly.
The compass bearing, either visual or radar should be used to assess risk of collision. The
relative bearing of a target should not be used when own ship’s course and/or speed alters, as
risk of collision may still exist even where the relative bearing is changing. Mariners should
also be aware that at close range, risk of collision may exist even with a changing compass
bearing.
Radar displays may be equipped to display AIS target data. Such information may be used to
assist the observer in assessing the situation and taking correct action to avoid a close-
quarters situation. Watch-keepers should be aware that not all vessels transmit AIS data. In
addition it is possible that not all the AIS data displayed will be accurate, particularly data
which is inputted manually on the target vessel.
3.7 Clear weather practice
Radar should be used to complement visual observations in clear weather to assist
assessment of whether risk of collision exists or is likely to develop. Radar provides accurate
determination of range enabling appropriate action to be taken in sufficient time to avoid
collision, taking into account the manoeuvring capabilities of own ship.
It is important that watch-keepers should regularly practice using radar and the electronic
plotting system in clear weather. This allows radar observations and the resulting electronic
vectors to be checked visually. It will show up any misinterpretation of the radar display or
misleading appraisal of the situation, which could be dangerous in restricted visibility. By
keeping themselves familiar with the process of systematic radar observations, and comparing
the relationship between radar and electronically plotted information and the actual situation,
watchkeepers will be able to deal rapidly and competently with the problems which may
confront them in restricted visibility.
3.8 Operation
Radar if fitted should be operating at all times. When weather conditions indicate that visibility
may deteriorate, and at night when small craft or unlit obstructions such as ice are likely to be
encountered, both radars if fitted should be operating, with one dedicated to anti-collision work.
This is particularly important when there is a likelihood of occasional fog banks, so that vessels
can be detected before entering the fog. Radars are designed for continuous operation and
frequently switching them on and off could damage components.
3.9 Parallel Index technique
Investigations into cases where vessels have run aground have often shown that, when radar
was being used as an aid to navigation, inadequate monitoring of the ship’s position was a
contributory factor.
Parallel Index techniques provide the means of continuously monitoring a vessel’s position in
relation to a pre-determined passage plan, and would in some cases have helped to avoid
these groundings. Parallel indexing should be practised in clear weather during straightforward
passages, so that watch-keepers remain thoroughly familiar with the technique and confident
in its use in more demanding situations (in confined waters, restricted visibility or at night).
The principles of parallel index plotting can be applied, using electronic index lines. A number
of index lines may be pre-set and called up when required on all modes of display: electronic
index lines remain at the set cross index range (CIR) enabling the operator to change range
without corrupting the range of the index line. Care should be exercised when activating pre-
set parallel index lines that the correct line(s) for the passage are being displayed.
a) Parallel indexing on a relative motion display
On a relative motion compass-stabilised radar display, the echo of a fixed object will move
across the display in a direction and at a speed which is the exact reciprocal of own ship’s
ground track: parallel indexing uses this principle of relative motion. Reference is first made to
the chart and the planned ground track. The index line is drawn parallel to the planned ground
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track at a perpendicular distance (cross index range or offset) equal to the planned passing
distance off an appropriate fixed target. Observation of the fixed object’s echo movement
along the index line will indicate whether the ship is maintaining the planned track: any
displacement of the echo from the index line will immediately indicate that own ship is not
maintaining the desired ground track, enabling corrective action to be taken.
b) Parallel indexing on a true motion display
The use of a true motion radar presentation for parallel indexing requires an ability to ground-
stabilise the display reliably. Parallel index lines are fixed relative to the trace origin (i.e. to
own ship), and consequently move across the display at the same rate and in the same
direction as own ship. Being drawn parallel to the planned charted track and offset at the
required passing distance off the selected fixed mark, the echo of the mark will move along the
index line as long as the ship remains on track. Any displacement of the fixed mark’s echo
from the index line will indicate that the ship is off track, enabling corrective action to be taken.
c) Integration with ECDIS
Where the radar display is integrated with an Electronic Chart Display and Information System
(ECDIS) the practice of parallel indexing continues to enable the navigator to monitor the
ship’s position relative to the planned track and additionally provides a means of continuously
monitoring the positional integrity of the ECDIS system.
d) Precautions
Some older radars may still have reflection plotters. It is important to remember that parallel
index lines drawn on reflection plotters apply to one range scale only. In addition to all other
precautions necessary for the safe use of radar information, particular care must therefore be
taken when changing range scales.
The use of parallel indexing does not remove the requirement for position fixing at regular
intervals using all appropriate methods available including visual bearings, since parallel
indexing only indicates if the ship is on or off track and not its progress along the track.
When using radar for position fixing and monitoring, check:
(i) the identity of fixed objects,
(ii) the radar’s overall performance,
(iii) the gyro error and accuracy of the heading marker alignment,
(iv) that parallel index lines are correctly positioned on a suitable display, and
(v) the accuracy of the variable range marker, bearing cursor and fixed range rings.
3.10 Chart Radar
Some radars are provided with electronic chart overlays. These charts may have a limited
amount of data and are not the equivalent to an Electronic Navigational Chart (ENC) used in
the ECDIS or paper charts. They should not therefore be used as the primary basis for
navigation.
3.11 Regular operational checks
Frequent checks of the radar performance must be made to ensure that the quality of the
display has not deteriorated.
The performance of the radar should be checked using the Performance Monitor before sailing
and at least every four hours whilst a radar watch is being maintained.
Misalignment of the heading marker, even if only slightly, can lead to dangerously misleading
interpretation of potential collision situations, particularly in restricted visibility when targets are
approaching from ahead or fine on own ship’s bow. It is therefore important that checks of the
heading marker should be made periodically to ensure that correct alignment is maintained. If
misalignment exists it should be corrected at the earliest opportunity. The following procedures
are recommended:
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