Navigational Instruments & Maritime Regulations

Posted on Mar 14, 2025 in Other subjects

Earth’s Magnetic Field

The Earth itself acts like a huge magnet. The field is thought to be a result of swirling molten rock in the Earth’s core, which causes circulating currents, and thus an electromagnetic field.

Magnetic and Geographic Poles

There are two ‘Norths’: True North and Magnetic North. True North is the center of rotation of the Earth. Bearings and lines of longitude given on charts are based on True North. Magnetic North is the position of the magnetic pole. The Earth acts like a giant magnet, and just like any other magnet, it has different polarities, which attract the opposite and repel the same.

Variation

The difference between the bearing of True North and Magnetic North is called ‘variation’. This changes according to a vessel’s location on the globe.

Deviation

If a magnetic compass is put in a vessel, the presence of iron and steel will cause the compass needle to deviate from the magnetic meridian. The angle between the magnetic meridian and the direction in which the needle actually points is called the ‘deviation’. If compass north lies to the east of the magnetic meridian, the deviation is said to be easterly; if west, westerly. The ship’s hull, keel, machinery, boats, cranes, and electrical machinery affect the magnetic compass because of:

• Permanent ferro-magnetism of ship’s structure caused when the ship was built or when berthed for a prolonged period on one heading.
• Magnetism induced by ship’s “soft iron”.
• Magnetism induced by electro-magnetic equipment (wires carrying an electrical current e.g. aerials).
• Magnetism induced by magnets (loudspeakers, analogue instruments, etc.)

Minimizing Deviation

To correct deviation, or at least minimize it, the compass adjuster breaks down the ship’s magnetism into longitudinal, athwartships, and vertical magnetism. Each component is corrected using the appropriately aligned correctors.

To reduce permanent magnetism, fore-and-aft ‘longitudinal magnets’ are placed beneath the compass to cancel out the effect of the ship acting as a magnet length-wise. ‘Athwartships’ magnets are placed to cancel out the effect of the ship acting as a magnet cross-ways. This will leave the compass needle free to be influenced only by the Earth’s magnetic field.

Reasons for a Change of Deviation in the Long Term:

• Protracted docking or refit period, especially if major works are undertaken causing vibrations to the hull.
• Long repeated voyages on a continuous heading.
• Major changes of latitude.

Reasons for a Change of Deviation in the Short Term:

• Changes in position of large ferrous objects such as a crane trained outboard.
• Changes in electrical equipment, such as new radar or repositioning of same.
• Modifications of a ferro-nature close to the compass or leaving tools, beer cans in vicinity.
• Heavy shocks to vessel such as grounding or collision, or repeated vibration such as pounding in heavy seas.
• Lightning strikes and heavy current short circuits.
• Heavy rolling or leaning for a period in one direction due to weather.
• If the lubber’s line is incorrectly aligned.

Light Ranges

Geographical Range

Is the theoretical range at which a navigational light comes into view. We can find this by taking the height of object from the chart (CD) and adding MHWS then subtracting height of tide. We then take this info along with over height of eye and input it into a Geographical range table, which is found in the Admiralty publications.

Nominal Range

Is the Range at which we see a navigational light according to an Admiralty chart or publication.

Luminous Range

Range at which we see the light when taking Nominal range and Horizontal visibility and applying them to the visibility curve in Admiralty publications.

Elevation of Lighthouse = CD+MHWS-Height of tide

Rate on the day = Rate at Springs x Range on the day / Range at springs.

Safety DATA diagram will tell you on a paper chart when the depth data survey was made. From there to check the liability we will refer to NP100(MARINERS HANDBOOK).

ARPA Plot

Are trails present = NO = No way of telling if display is SEA / GROUND Stabilized

= YES = Do I have trail = YES = Trails are True Motion

(W/O radar set up is true motion)

= NO = Trails are Relative Motion

(O/A radar set up is relative motion)

Are Vectors present = NO = End of story

= YES = Do I have vector = NO = Vectors are Relative

(Forward extension of O-A)

= YES = Vectors are true

(Forward extension of WO – WA)

= Is heading marker and my ship vector out of line = YES = Ground stabilized True Motion misalignment is indicator of SET(tide)

Steering & Sailing Rules (Part B)

Section I – Conduct in any condition of visibility.

4 Rules in this section apply in any visibility.

5 Proper lookout by all available means appropriate to make full appraisal of the risk of collision.

6 Safe speed in prevailing conditions; affected by:

1. Visibility
2. Traffic density
3. Maneuverability
4. Background lights
5. Weather and nav hazards
6. Draught re depth

7 Use all available means assess risk of collision. Use bearings, and radar.

8 Take avoiding action positive & early. Maintain safe distance. Reducing speed is an option.

9 Narrow channels. Stbd side. Sailing or <20m not to impede vessels using channel. Use sound signals to overtake. Avoid anchoring in <20m

10 Traffic separation zones. See regs. Main points: Avoid crossing if practicable. If crossing zone, heading at right angles. Vessels < 20m & sailing vessels shall not impede vessels following a traffic

Section II – Vessels in sight of one another.

11 This section vessels in sight of one another

12 Sailing vessels

• On opposite tacks Port tack gives way
• On same tack Windward boat keeps clear
• If in doubt Assume other on stbd tack

13 All vessels. If overtaking, keep clear till past & clear. Overrides all other rules in Pt B, Sections I & II. If in doubt, overtaking.

14 2 Power vessels head on both a/c to stbd.

15 2 power vessels crossing vessel with other to starboard gives way. Avoid crossing ahead.

16 Give way vessel to take early & substantial action

17 Action by stand on vessel:

1. Stand on with caution
2. May take action if give way vessel not acting
3. Must take action if collision inevitable

18 Priorities: NUC & RAM, CbD, VEF, SV, PV.

Section III – Conduct in restricted visibility

19 a Rule applies to vessels not in sight of one another

b Safe speed. Engines ready

c Due regard to conditions for Section Id Radar alone take avoiding action, but not:

• a/c to port for vessel fwd of beam unless overtaking
• a/c towards vssl abeam or abaft the beam

e Fog signal fwd of beam slow down or stop, and nav with extreme caution till danger is over.

Sound & Light Signals (Part D)

(SHORT BLAST – 1 SECOND´S DURATION, PROLONGED BLAST – 4-6 SECOND´S DURATION)

When maneuvering:

• I am altering course to starboard 1 SHORT BLAST
• I am altering course to port 2 SHORT BLAST
• I am operating astern propulsion 3 SHORT BLAST
• I do not understand your intentions 5 SHORT BLAST

In a narrow channel:

• I intend to overtake to your starboard 2 LONG 1 SHORT BLAST
• I intend to overtake to your port 2 LONG 2 SHORT BLAST
• Agreement by overtaken vessel 1 LONG 1 SHORT 1 LONG 1 SHORT
• Warning I am coming! (& reply) 1 LONG

In restricted visibility:

• Power vessel making way (every 2 mins) 1 LONG
• Power vessel underway but stopped 2 LONG
• NUC RAM* CbD SV VEF* Tug (*even when at anchor) 1 LONG 2 SHORT
• Towed vessel (last) 1 LONG 3 SHORT
• Vessel at anchor (every 1 min) Bell
• Vessel > 100m at anchor (every 1min): Bell + Gong
• Additionally, to warn approaching vessels 1 SHORT 1 LONG 1 SHORT
• Vessel aground, before and after bell: 3 rings on Bell
• Pilot vessel (optional in addition to above) 4 SHORT

NB. Underway. Towing vessels carry masthead lights, sidelights and sternlights when under way, even if RAM its important to know their heading at all times. Trawling vessels > 50m carry an additional masthead light when under way. Fishing (not trawling) vessels do not.

Making way. Trawling, fishing, NUC, and other RAMs all carry sidelights and sternlights, but only when making way. They may stop in the course of work, so a useful distinction. RAM & dredgers carry masthead lights as well, only when making way their direction of movement is more predictable, so it could be useful to see masthead lights.

IRPCS Questions

Q1. State 5 actions to be taken, in accordance with the IRPCS and the practice of good seamanship, when suspected to be in or near an area of reduced visibility:

A.

• Assume possibility of poor visibility to landward
• Start a fog signal, switch navigation lights on
• Call Master
• Bring engine room to standby
• Reduce Speed
• Post lookout
• Close up helmsman with steering in hand mode (follow up)
• Commence plotting of radar contact
• VHF: listen Ch 16 and Coastal/Port Control Channel

Q2. A target is detected on the radar at 6 miles on your port bow and appears to be on a steady bearing but this vessel cannot be seen or heard. State the action that should be taken?

A.

• Don’t assume that he can see you.
• Take early action to avoid a close quarters situation developing.
• Call the Master
• Reduce speed
• Sound fog signal and take actions as for a) above
• Rule 19 applies

Echo Sounder

How it works:

• Voltage across coils (or quartz) in transducer causes magnetic field.
• Core expands/contracts.
• Pulse of sound transmitted through water at 1500m/sec.
• Return pulse moves coil creates voltage. Return is timed.
• Depth processed and displayed
• Depth = ½ distance = ½ speed x time

IMO requirements:

• IMO require the echo sounder to record depths from 2 meters to 400 meters.
• Scale must always start at zero.
• Range scales 0 30, 0 400 meters, etc.

Errors

Instrument errors:

• Index Error: faulty offset correction
• Separation Error: on old echo sounders where there is separation between transmitter and receiver (consult separation correction tables)

Propagation effects:

• Aeration: bow wave causes air bubbles under hull from 1/4 length from fwd to 1/4 length from aft, blocking signals. Reversing propellers has same effect. Pounding and high speed may aggravate problem, as well as shallow water.
• Propagation speed error: Readings are less in hot saline seas than in cool fresh water.
• Water layers of differing salinity/density/temperature. DSLs

False echoes:

• Multiple echoes: may bounce a number of times between seabed and surface/hull. Turning down sensitivity will usually remove the weaker signals (or automatic).
• The cone effect
• Side echoes, reflections
• Fish shoals
• Fresh water submarine springs
• Kelp or seaweed
• Turbulence from own or other ships wash.

GPS

Geographical datum and datum shift

A horizontal datum is a base reference system for specifying position in Lat and Long it is a mathematical model of the earths surface. Nearly every country had its own datum, to be found in the chart title panel. The World Geodetic System 1972 (WGS72) was introduced, and was replaced by WGS84 datum.

System description

1. There are 3 segments: SPACE, GROUND(CONTROL), USER
2. Satellites send signal to user and integrity monitor
3. Integrity monitor measures time signal and informs control center of any errors
4. Control center tells differential reference station to transmit correction to ships
5. Also tells broadcast transmitter to tell satellite to correct orbit
6. WAAS (wide area augmentation system) + EGNOS (european geodetic navigation overlay system) satellites receive corrections from differential reference station and transmit corrections to all stations.

The control segment

The control segment consists of five ground monitoring stations for tracking all visible satellites. They also receive data concerning the conditions of the atmosphere and clock data obtained from the caesium frequency atomic clock. This data is then transmitted to the master control station where the ephemeris data and clock status predictions for each satellite are generated. This data is then used in formulating the navigation messages to be uploaded to the satellites every eight hours from the three ground antennas at earth stations.

The space segment

The space segment consists of the 21 satellites plus 3 spares. The height of the satellites are approximately 20200km, and have a sidereal period of 12 hours.

Each satellite transmits data on two frequencies onto which is superimposed a coded signal. This signal contains information on satellite positions.

User segment range position fixing

Signals with extremely accurate time and satellite position are broadcast by each satellite. The GPS receiver takes 3 signals, and can use them to triangulate an approximate position. The position must be approximate because the clock in the receiver on board is relatively inaccurate. However, the receiver applies corrections to the on board clock so that all 3 position lines come together at one point. The on board clock is then in time with the satellite clocks, and an accurate position is obtained. So 3 position lines are required for a 2D fix. A 4th satellite is required for a 3D fix.

Differential GPS (DGPS)

This is a technique for eliminating many of the errors in the system. This is done by comparing the result from GPS signals with a known position on land, and broadcasting the differences, or errors.

The impeller log

The log is operated by the flow of water passing along the vessels hull. The force of water rotates an impeller situated at the base of a retractable log tube, which extends about 0.3m below the hull plating when in use. The impeller is integral with the revolving magnet fitted at end of a pick-off coil. The pulses induced in the coil are counted by an electronic counter and fed to a speed indicator. The speed pulses are integrated and the output shown on a distance recorder. The equipment is intrinsically safe as the maximum output of the log is less than one volt. The log is retracted and supported by a check tube when the vessel enters shallow water or dry dock. The impeller is practically loadless and has very little slip or variable error. The submerged mechanism being the only mechanically working part, the remainder being electronic. These logs are vulnerable to fouling and damage from jetsam they are obsolete, and not used on yachts. Most smaller yachts are equipped with an impeller log with a paddlewheel rotator on an athwartships axis. These are much smaller and hardly protrude at all; they are still affected by fouling, but less prone to damage.

The electromagnetic log

The flow of a liquid past a sensor is measured by electromagnetic induction. Faradays principle is that an e.m.f. (electro motive force = voltage) will be induced if the conductor (sea water) remains stationary and the magnetic field (provided by the ship) moves with respect to it. In the case of the electromagnetic log, the conductor consists of water moving in a horizontal plane across the vertical magnetic field of a coil carrying alternating current. Two electrodes detect the induced electromagnetic field signal in the water. The coil and sensors are contained in a housing, which can be flush with hull in modern systems or may protrude up to 0.5m below the keel (it can, however, be sited aft of the turn in the bilge and thus need not protrude below the lowest part of the keel.) The signal is sent via a comparison transformer and amplifier to drive the speed indicator by means of a servomotor. This may be electronically integrated to display the distance run. These logs can be either single or twin axis. If the log is twin axis then a second pair of electrodes is fitted to sense the athwartships movement of the vessel. This permits the water track distance to be obtained. The effects of pitch and roll may be overcome electronically. However, difficulty may be encountered when calibrating the athwartship component, so care should be taken in ascertaining the actual water track.

Errors

• The induced voltage and hence the speed indication will vary with the conductivity of water (salinity and temperature). A graph displayed on bridge will indicate adjustment corrections for fresh water.
• The device measures the speed of water flowing past the hull of the ship. This flow can vary due to the non-linearity of a hull design. Non linearity can be corrected by calibration (on some log models) so that the log reads correct water speed at all speeds.
• Ocean currents may introduce errors (due to salinity and temperature changes.
• Pitching and rolling will affect the relationship between the water speed and the hull (this effect may be compensated for by reducing sensitivity of the receiver).
• Object caught around probe, or damaged probe; fouling.

The Doppler log

The Doppler Effect is the frequency shift of sound (and other) waves resulting from the relative motion between a transmitter and receiver. The Doppler log measures the change in frequency between the transmitted ultrasonic waves and those reflected from the seabed or water layer. The frequency shift results from the movement of the transmitter (wave source), and the shift is proportional to the vessels velocity.

Errors

• Propagation speed, determined by sea temperature and salinity, has the greatest effect on accuracy. The error can be negated by mounting salinity and temperature sensors in the transducer array, or by using a phased beam.
• Damaged unit (transmitter and receiver)
• Heavy pitching and rolling
• Water layer moving at different velocity to surface layer.
• Aeration (from bow thrusters, screws) Aeration does affect ultra sound waves (although some systems use a phased transmission system to nullify the effect). Transponders should be mounted level, forward of area of aeration, away from effects of bow thrusters, projections, inlets, outlets all of these may cause aeration.

Advantages of Doppler log over EM log:

• Yields STW as well as SOG
• Not susceptible to hull shape irregularities
• Better in pitching & rolling
• More resistant to fouling & aeration
• Better at low speed, berthing, reversing

GPS Errors

Ionosphere error

The ionosphere is the layer of the atmosphere ranging in altitude from 50 to 500 km. It consists largely of ionized particles, which can slow the passage of GPS signals, affecting range calculations. While much of the error induced by the ionosphere can be removed through mathematical modelling, it is the most significant error source, giving errors of up to 5m.

Ephemeris error

Ephemeris (or orbital) data is constantly being transmitted by the satellites. Receivers maintain an “almanac” of this data for all satellites and they update these almanacs continuously as new data comes in. In the small intervals between updates errors can creep in which will have an effect on accuracy. The errors can be greater if there is a long period without update for any reason. Errors up to 2.5m.

Satellite clock error

Accurate time on board the satellite is clearly essential for a timing and ranging system. Drift in the satellites cesium clocks can produce errors of up to 1.5m.

Multi-path error

Mountains, buildings, bridges and other vessels can all reflect the signal and cause the unit to receive more than one signal. The reflected signal will interfere with the direct signal causing errors to 0.6m.

Tropospheric delay

The troposphere is the lower part of the earth’s atmosphere that encompasses our weather. It’s full of water vapor and varies in temperature and pressure. Messy as it is, it is quite thin, so causes relatively small errors, to 0.5m.

Receiver noise

Interfering radio transmissions can cause very small errors to arise. These can be reduced by mathematical means. The quality of the equipment also has a bearing on the degree of receiver noise.

DOP (Dilution of Precision)

DOP is the measure of the factor by which any system errors are magnified by fix geometry. Two position lines at right angles give a better fix than position lines at an acute angle. Two position lines at right angles give a theoretical minimum DOP of unity (1). This is the maximum dimension across the error diamond created by two position bands at right angles. The position bands are of a width dependent on their system errors a position could thus be anywhere within the position band, and a position fix anywhere within the error diamond. On the right below, DOP is 1, yielding a good fix. In the scenario to the left below, the position lines are not well arranged, and the measure across the error diamond is 4. DOP is therefore 4, giving a poor fix. If system errors added up to 10m, positional error would be 4 x 10 = 40m.

Various different measures of DOP exist:

• GDOP Geometric dilution of precision DOP in 3 dimensions
• HDOP Horizontal DOP DOP in 2 dimensions, Lat & Long
• VDOP Vertical DOP DOP in the vertical dimension
• PDOP Position DOP

AIS

AIS is a shipboard marine VHF transceiver system. It broadcasts information about the transmitting ships identity, as well as position and movement from its own GPS. This information is received by AIS equipped ships, and displayed on radar, ECDIS, or dedicated AIS display. Targets can be interrogated for more detailed information.

AIS is required by SOLAS on all vessels over 300gt on international voyages, some other vessel categories, and as required by flag states. AIS adds to radar function by providing additional target information so can be useful for collision avoidance. Thus AIS data is logically displayed on radar, although it may give conflicting position and movement information, and it may not be present for all target echoes. Radar systems are now generally AIS compatible. AIS also provides vessels name and callsign, and so has an identification function. As this information, and much more besides, is broadcast, this does have security implications.

Information broadcast by AIS

Class A AIS meets the IMO carriage requirements, and transmits the information below. Class B AIS does not meet IMO requirements and is for voluntary installation. They have a lower reporting rate and will not transmit all of the information listed below.

A Class A unit broadcasts every 2-10 seconds while underway, and every 3 minutes while at anchor:

• MMSI number
• Navigation status; at anchor, under way using engine, NUC
• Position Lat & Long
• Speed over ground, Course over ground True
• True Heading 0 to 359°
• Rate of turn right or left, 0-720°/min
• Positional accuracy assessment
• Time stamp UT information generated

Class A AIS units broadcast the following additional information every 6 minutes:

• Ships name, type, and dimensions, including draught
• Destination & ETA
• Radio call sign
• MMSI number; IMO number (unique, related to ship’s construction)
• Type of position fixing device and location of reference point on ship

Advantages of AIS

AIS can enhance radar in its collision avoidance role as:

• It gives a name and a call sign to a radar target.
• It may indicate targets invisible to radar e.g. behind low headlands, or in clutter.
• It may clarify a targets intentions.
• It may improve target path prediction using rate of turn indication.

Limitations of AIS

• Bridge Workload. Another system to learn, maintain and monitor. Information must be entered into, and assimilated from the AIS, especially at busy moments. User must be familiar with how the AIS is displayed, and the effects of the AIS signal on ARPA operation. Any conflicts must be resolved.
• Information overload. Allied to the above, if the information given is not useful, it can be obstructive and confusing.
• Inaccuracy. AIS information may not be accurate. This applies to position and movement information due to GPS errors, as well as user entered information, such as destination. The AIS set may be switched off, out of use, or not even fitted. One documented incident records a ship leaving harbor with the AIS signal continuing to transmit at its berth. The transmitter was ashore being tested in a workshop.

Radar

The word Radar is derived from the acronym Radio Direction and Ranging. The system works at the super high frequencies within the X or S band. 3cm radar transmits in the X band having a frequency of between 9300 and 9500 MHz with a wavelength of about 3 cms, hence the name 3cm radar. The 10cm radar has a frequency of between 2900 and 3100 MHz, working in the S band and having a wavelength of about 10cms. The radar system is fundamentally a precision clock which measure the time taken for a radio pulse to travel from the transmitter and return after being reflected by an object, commonly called a target. Knowing this time interval and the velocity of the radio wave the distance can be found from the formula: distance = velocity x ½ travel time.

Horizontal beam width (often Beamwidth)

The width of the scanner together with the transmitted frequency determines the width of the beam, the wider the scanner the narrower the beam. Likewise the higher the frequency the narrower the beam. Hence a 3cm X-Band radar will give a narrower beam than a 10cm S Band radar with the same width scanner. A narrower beam gives better definition and hence better bearing accuracy and discrimination.

Bearing discrimination – the ability of the radar to differentiate between two targets on different bearings, but at the same range. Wide horizontal beam width gives poor bearing discrimination. Narrow horizontal beam width gives good discrimination. The beam width tends to extend the edges of land laterally more to seaward. Poor bearing discrimination can render small gaps, such as harbor entrances, invisible on the screen. It also makes radar bearings less accurate. Radar bearings are essentially inaccurate due to horizontal beam width. Radar bearings are also heavily reliant on fast processing, accurate and spontaneous heading input, and accurate radar alignment. As it is generally not possible to ensure that all these errors are absent, radar bearings are generally not relied on.

Side lobes and side echoes – Not all the energy is focused into the main beam, but some escapes to form a side lobe pattern. These side lobes are very weak and will only detect the very strongest targets. A large vessel at close range may show as a number of targets in a curve either side of the true target. They can join together to form a complete circle around the origin.

Vertical beam width

To allow for rolling of the vessel the vertical beam width is made much greater. The IMO requires vertical beam width to be no less than 20°. The problem with this is that the lower parts of the beam are likely to be reflected from the surface of the sea and coincide with the nonreflected energy. If these two beams interfere out of phase, there will be no energy radiating in this area.

The result is a layered pattern of propagation known as lobes. This can cause low objects to be missed at certain ranges, but shown at other ranges. A 3 cm X-Band radar produces 3 times as many lobes as a 10cm S-Band radar for a given scanner height. The effect will only occur if the sea surface is smooth, or reflective to the radar beam. A choppy sea will often break up the pattern and fill in the low energy areas, while a rough sea may still be smooth to the radar beam due to the very short wavelength used. The IMO requires that specific target types are clearly displayed to a minimum range of 50 meters. The least range that a radar set can receive a return signal is half the pulse length, and that relies on the TX/RX switch to change quickly enough from transmit to receive. A high antenna increases both maximum and minimum range, because the beam cannot depress low enough to reach objects near to. Sea clutter is also worse with a high antenna. A low one has the smallest minimum range provided the superstructure does not cause a shadow area.

Pulse length and range discrimination

The length of the pulse is governed by the transmission time. The longer the pulse the more energy is transmitted and the better the chance of detecting a target. This is particularly important at longer range or when trying to detect a target within or beyond rain. The pulse length determines the minimum detection range. This range is half the pulse length, so that a short pulse should be used when detecting targets close to the vessel.

The pulse length also affects the Range Discrimination of the radar – The ability of the radar to differentiate between two targets at different ranges, but on the same bearing. If two contacts are close together on the same bearing the radar will show them as one unless they are more than half a pulse length apart, which is 150 meters if long pulse is selected. Long pulse gives poor range discrimination; short pulse gives good range discrimination, so a short pulse should be used if one suspects a second target behind the first.

Propagation Anomalies

Radar propagation speed is increased in lower density air (warm & dry) and vice versa. The radar beam thus tends to curve away from warmer drier air towards colder wetter air relative to the standard temperature/humidity/height profile. Such beam bending leads to subrefraction, super refraction, and ducting.

Subrefraction

Cold polar air over a warmer sea deflects the beam upwards, causing range reduction of up to 40%, but also a loss of signal at short ranges due to the uplift.

Super refraction

An increase in radar range can be caused by relatively warmer air over cooler sea, as found in high pressure areas in temperate regions, when insolation is high and sea temperatures relatively low. Common, generally in the Tropics, and the Red Sea, Gulf of Aden and Med, when warm desert air blows over cooler sea.

Ducting

is caused by temperature inversion (air temperature rising with altitude). This can reflect the beam so the radar waves travel in the space between the inversion and the surface, called a surface duct. Can increase radar range to 400 miles if antenna is in the duct. Common in Red Sea, Gulf of Aden, and the Med in the summer.

Reductions in detection/range

Rain: 1 inch in 1 hour gives reduced detection of up to 50%.
Fog: 45m visibility gives reduced detection up to 30%.
Hail & snow: reduced detectio n up to 50% in heavy snow.
Dust and sand: reduced detection up to 30% in sand storms.

Factors affecting minimum range of radar:
ß Pulse length (min range = half a pulse length)
ß Vertical beam width and height of aerial
ß Blind area from superstructure
Choosing objects for parallel indexing:
ß Conspicuous and stand proud of general coastline
ß On the beam rather than ahead / astern
ß Steep to
ß Use 3nm range scale in pilotage waters. Preferred to 6nm.

Second trace echoes
Caused by energy from one pulse being received after the next pulse has been sent. It is therefore possible to get a second trace echo on the screen even though the true target is well outside the screen range setting.
Factors affecting radar contacts
Materials which are good conductors of electricity are considered to be good reflectors of radar energy. (Steel, aluminium and carbon fibre in composites) GRP is completely transparent to radar energy so it is most important that GRP boats are fitted with a radar reflector. Wood is porous to radar energy and gives a weak
reflection.
Factors influencing radar refelectivity are:
Material; Aspect; Size; Texture; Shape. (MASTS)

Factors determining the practical detection range of the radar:
ß Height of the scanner
ß Height and reflective properties of the target
ß Atmospheric conditions.

ECDIS

Explain the differences between an ECS and an ECDIS:
ECS is a chart plotter which does not conform to the IMO requirements and standards laid down in the S-52 standard (Symbology library, pixel size, colourings, etc), and may use private chart data which is neither to S-57 requirements nor corrected on a weekly basis. An ECS may be used for passage planning purposes, but the ship must only be navigated on up-to-date official paper charts. An ECDIS using private ENC map data (not conforming to S-57) is considered to be an ECS. An ECDIS conforms to the S-52 standard for its hardware and manufacturers software, and uses type approved S-57 ENC (vector) charts, which are corrected weekly and produced by a recognized Government Authority. An ECDIS may also be used in the RCDS mode with S-61 raster charts.

Both ENC an RNC charts are available on ECDIS. Explain the precautions that need to be taken with both types of charts to ensure accuracy of the positional information displayed:

RNC may be on a different Datum to WGS84 and a datum shift may be applied. Some charts are still on their original country regional datums that may vary from WGS84 positions by hundreds of metres, or even miles. In every case a suitable margin of safety must be allowed for. With both systems it is important to consult the Source Diagram (RNCs) or the Zones of Confidence (ENCs) to establish quality of chart data. The survey may be old and inaccurate, silting or coral growth may have occurred in the meantime, storms may have moved bottom contours, or perhaps the chart may not be fit for purpose. In both cases the position of the ship must be regularly checked by all available means toverify the position given by the GPS. GPS can be very reliable most of the time, but it is a one source position system and, for instance, an offset position could have been incorrectly or accidently applied. Methods to check the position are: A Transit, Rising and Dipping Distance, 2 or 3 Lines of Position fix, 2 or 3 radar ranges fix,  Position from a second GPS or the Russian GLONASS system, Parallel Indexing method, Placing the Radar Overlay over the ECDIS, Selecting a Navigation Mark as an ARPA Target, Looking out of the window when passing close to a known Navigation Feature, Checking against the Echo Sounder.

If the radar target of a single navigational buoy is overlaid on the electronic chart and doesnt coincide with the displayed position of the charted buoy, state 4 possible causes of the error:

a)Gyro error, b)Heading Marker mis-alignment, c)Chart correction not applied,
d)buoy has dragged, e)wrong Datum, f)the 2 systems not aligned N, or C or
H Up.
Describe the main differences between Vector and Raster Electronic Charts,
and state TWO advantages and TWO disadvantages of each type:
An RNC is a facsimile or digitized photograph of an already type approved official chart, whose features are easily recognizable. An ENC is a vectored chart where each item has its own individual data base. This allows layers of information to be removed or added to produce the most suitable image of chart as required by the Operator. It is therefore intelligent and can give automatic warnings and it is easy for information to be demanded, changed or added. An RNC is an inert pixel image, although items can be added over the top of the image.
Advantages of an RNC:

a) They are already type approved official charts and there is complete world coverage, and the operator does not have to keep changing
between RNC and ENC.

 b) The features on an RNC are easy to identify as they are identical to those on standard charts.

Advantages of an ENC:

a) They are intelligent and will automatically give the operator a warning when the Ships Safety Domain is approaching a navigational hazard.

b) Layers of information can be added or removed to allow better clarity. The chart is seamless and when zooming in or out the depths and symbols will remain the same size. The system will always ensure the best scale of chart cell is shown.

Disadvantages of RNC:

a) The best scale of chart may have to be selected, and when zooming in or out the symbols will also change in size. Information cannot be added or removed, or interrogated.

b) The RNC is not intelligent and cannot automatically give warnings of impending navigational dangers, although it is
possible to draw danger areas manually over the surface of the chart which will then set off alarms.
Disadvantages of ENC:

a) Worldwide coverage does not yet exist, so the operator may find himself having to alternate between official S-57 ENCs (where they are available) and RNCs (where ENCs are not available).

b) It is possible for an operator to remove layers of information for purposes of clarity, and to forget to add essential ones when required; for instance, when anchoring, it is important to know if there is foul ground on the seabed that could foul the anchor.
Describe three differences when navigating using Vector and Raster electronic charts:
When navigating with an RNC the Datum may be based on a Regional Datum and not be compatible with WGS84; an Offset would have to be applied to correct the position. When navigating with an RNC it is a requirement that a fix is regularly placed on an official up-to-date paper chart that is out on the chart table for use. When navigating with an ENC a warning will automatically be given if the ships Safety Zone is approaching a navigational hazard. This is not the case with an RNC, where no automatic alarm is triggered. However, with an RNC, a dangerous area can be manually drawn over the chart by the operator and an alarm will be sounded if the ships position approaches this delineated area. One can demand information about objects on ENC, but not on RNC. It is possible to hide information on an ENC for clarity; not so on an RNC. An ENC is a seamless chart, whereas with RNC it is important to check that the chart with the most suitable scale has been selected.
Describe the dangers of using the wrong datum and list three methods of determining the possible error:
Each country had its own local datum and although many charts have now been converted to the WGS84 datum system used by GPS, some charts still remain on their original local country datums. These datums may not be compatible with WGS84 and may vary by several hundred metres or, in some cases, several miles. This could easily place a ship in danger of running aground, if the horizontal difference in position between the local datum and WGS84 has not been allowed for. To determine the possible error:
a) check position of ship when close to a known navigation mark,
b) overlay the radar image on the ECDIS,
c) place an ARPA target on a navigation mark,
d) take a 2 or 3 point visual bearing fix; a 2 or 3 point radar range fix; a VSA or HSA fix; or a fix/position line using: Transit/s, Rising and Dipping distances, Soundings, another GPS or GLONASS or using parallel indexing.