OOW Nav & Radar

Variation

The Angular difference between true meridian and magnetic meridian

Deviation

The magnetic field of the vessels structure and contents will cause an additional compass error known as deviation

Characteristics of Permanent magnetism (4)

• Hard Iron

• Heating and banging it in a magnetic field

• High retentuity

• Remains magnetised

How to correct for permanent magnetism

Placing permanent magnets around the compass position, aligned fore and aft, athwartships and vertically

Characteristics of Induced magnetism (3)

• soft iron

• Low retentuity

• Loses magnetism over time

How does iron become induced with magnetism ?

Constant rubbing inside a magnetic field

Water moving along the hull in the earths magnetic field.

How to remove induced magnetism ?

placing soft iron correctors around the compass position. Called kelvins balls

And a flinders bar (Vertical soft iron rod)

When should you complete a Compass Error check ?

• Once a watch

• Sunrise and Sunset

• Whenever the opportunity presents

When should you complete a compass swing?

• When the vessel has experienced a structural change

• When there’s been a large addition of metal or electrical equipment

What to consider when siting a compass(placing) (4)

• easily visible from the helm

• On the centreline

• Clear of magnetic influences

• Clear of electrical wiring

Gyro compass

A free gyroscope is a heavy rotor spinning at a high speed and suspended on three axes, it can be turned in any direction but will continue pointing in the same absolute direction in space (True North) as a result of rigidity in space

Gyro error never more than ….

4 degrees, high or low

How many types of compass are there?

(4)

Gyro (true)

Magnetic

Flux gate

GPS

Gyro compass advantages(5) + disadvantage (2)

Advantage

• Can be placed anywhere there is space

• Repeaters are easily connected

• Can interface with a wide variety of equipment

• More accurate that it’s magnetic counterpart

• Steady reading in far less movement in a Seaway

Disadvantage

• reliant on power

• Requires several hours to start up/settle

Mercator projection features

• All rhumb lines appear straight

• Lines of long are equally spaced and parallel

• Line of lat are equal in length and parallel

• Lines of lat become further apart further north and south

Mercator projection defects

• every latitude has different scales of distance

• Great circles appear as curves

• Areas are distorted

• Impossible to chart polar regions

Types of projection (2)

Mercator projection

Gnomonic projection

Why do tides occur?

The moon and the sun exert attractive forces upon the earth causing the water to rise and fall periodically

• moon effect 2x than the suns

Spring tides are? And occur?

High High waters

Low Low waters

During new moon and full moon

Neap tides are? And occur?

High waters are lower

Low waters are higher

Less of a range

During first and last quarter of the moon phase

Moon cycle (new moon to new moon)

28 days

Springs every 14

Semi Diurnal tides

Two high waters and two low waters in a day

Diurnal tides

One high and one low per day,

Large ocean masses like the pacific respond differently

Mixed tides

Large land masses affect the tides in a great variety due to water being restricted to flow easily

Chart Datum

Chart datum is a reference level on nautical charts that represents zero for both water depths and tide heights, also from which drying heights are measured

Under Keel Clearance

The depth of water between the vessels bottom and the seabed or obstruction.

Height of Tide HOT

Height of water level above chart datum

Drying height

The height of an object, such as a rock, obstruction, shoal, that is above chart datum

HAT

Highest Astronomical tide,

Every 19 years

MHWS

Mean high water springs

Average of the high water springs

On admiralty charts, height of lights, bridges and other land features are measured from here

MHW

Mean High water

MHWN

Mean High water neaps

Sea level

Height of sea

Halfway between MHW and MLW

MLWN

Mean Low Water Neaps

MLLW

Mean lowest low water

US Charts

MLW

Mean Low Water

MLWS

Mean Low water springs

LAT

Lowest Astronomical tide

19 years

Chart Datum

Standard ports

A port with sufficient traffic to warrant the expense and time for specific tide tables

What is a Log?

A log is a device for measuring distance and/or speed

Types of logs available (6)

Towed - Mechanical rotator fin

Impellor - electro-mechanical impeller

Electromagnetic - Electrical

Doppler - Electro acoustical, sound frequency

Pressure - Electro-Hydrostatic, water pressure

Acoustic

Towed/streamed Log

How does it work?

A rotator is towed by means of a plaited logline, connected to a clock or register mounted on the taffrail.

The fins are designed to spin 900 times per nautical mile, the rotation is passed along the logline through a system of gears to dial s that indicate distance.

STW

Errors

Incorrect Logline length

Damaged fins

Being towed in slipstream

Skipping

Impellor log

How it works?

Chernikeef

Small Impellor projecting below the hull, a magnet in the Impellor provides a pulse of electrical energy to a coil as it rotates

STW

Speed and distance are measured

Errors

Incorrect calibration

Could get damaged or growth

Electromagnetic log

How it works?

• Faraday maxwell theory

• When a conductor(water) passes through an electromagnetic field, a voltage is created

• The amount of voltage created is proportional to the speed of the conductors movement

• STW

1. AC current to the coil

2. Coil creates an electromagnetic field around sensors

3. Current set up between the two electrodes

4. Ship begins to move, speed of water increases, increasing voltage between electrodes

5. Ship speed can be converted to distance run

Advantages :

Highly accurate

No moving part to get fouled

Measures speed astern

Errors

Variations in Temperature

Be mounted clear of propeller streams or intake/discharges

Doppler Speed log

How it works?

• the Doppler effect

• A shift in frequency between transmitted signal and received signal caused by movement of the sound source

• The speed of the source can be determined by measuring the shift in frequency between the transmission frequency and received frequency

• SOG / STW

• Extremely accurate in ideal circumstances

1. a beam of ultrasound energy is projected forwards from the transducer down at an angle of 60 degrees below horizontal

2. Energy is reflected back from the seabed or from water layers (DSL)

3. Second transducers are fitted to overcome vertical motion of the vessel. Same angle aft (60) called the Janus configuration

4. Combined readings eliminate vertical motion error

SOG to 200m deep

After this depth signals are bouncing of Deep scattering layers(DSL) colder water layers giving STW

Big disadvantage, You do not know when it has switched from SOG to STW

Which speed log gives STW or SOG?

Towed?

Pressure?

Acoustic?

Impellor?

Electromagnetic?

GPS ?

Doppler?

Towed - STW

Pressure - STW

Acoustic - STW or SOG

Impellor - STW

Electromagnetic STW

GPS - SOG

Doppler - STW or SOG

Janus configuration

60-60-60

It eliminates the error of pitching

4 Principle errors to consider for the Doppler speed log

VOMS

• Velocity of sound errors, due to density/temp/salinity

• Transducer Orientation error, excessive pitching and rolling

• Motion error, excesssive vibration

• Signal loss error, attenuation during transmission and reception

Pressure speed log

How it works?

• a double walled tube and two orifices leading to a chamber with a diaphragm

• Speed is calculated by measuring the difference between dynamic and static pressures at the diaphragm

• only measure speed, not distance

• Less prone to damage, but can become clogged

• STW

Echo sounders

How they work?

Use the principle of sound waves travelling through water and being reflected from other materials saucy as rock, sand and steel.

The Device basically measures the time interval between transmission of pulse of sound energy to reception after being reflected from seabed.

DSL

Deep Scattering Layer

A zone of higher density That reflects sound waves

State two common frequencies used by an echosounder?

24 to 55 kHz for deep water soundings

200kHz for shallow water

Range Formula

Depth Formula

Fixing with a bearing + Depth

Echo sounder Controls (4)

• Gain - controls the sensitivity of the device and works on the amplifier

• Range - Determines the scale in use

• Alarm - some echo sounder are fitted with a preset alarm to warn if you reach a depth less than it

• Pulse - altering pulse length

Short pulse for shallow

Long Pulse for deep

Echo sounder Errors

DDEESSI

• Density - Salinity, Temp, Depth

• Datum

• Multiple Echoes

• Second trace Echoes

• Fish Shoals

• Seaweed/Kelp

• Interference - Noise & Aeration

Echo Sounder error DDEESSI

Density

• Salinity

• Temperature

• Depth

Effect the density of the water

Denser = Signal travels faster

Error, denser water may show inaccurate readings, less than actual depth

Speed of sound through water ?

1500m/s

Speed of light

300 000 000 m/s

Echo Sounder errors DDEESSI

Multiple Echoes

Same pulse may travel a number of times between the vessel and the seabed

Use the closest reading

Echo Sounder errors DDEESSI

Second trace Echoes

In very deep water, the time taken for a pulse to ravel to the seabed and return may be greater than the time between pulses

The transducer is receiving the echo from the last pulse and therefore giving an Innacurate depth that is shallower than true depth

Echo Sounder errors DDEESSI

Datum

The depth recorded will be in the depth below the transducer. This can be lead to error if the user is unaware whether the depth is DBKeel or DBSea or DBTransducer

Echo Sounder errors DDEESSI

Fish shoals

Some fish have swim bladders, these bladders being filled with gas can reflect sound waves making it an echo reading incorrect to the actual depth to seabed

Echo Sounder errors DDEESSI

Seaweed/Kelp

Very difficult to identify on an echo sounder, the bottom is usually clearly visible through the kelp.

Echo Sounder errors DDEESSI

Interference

Mechanical Noise - noise from engines and machinery

Electrical Noise - noise from electrical sources or radios

Water noise - rough weather or splashing waves on the hull

Aeration - bubbles passing the transducer reflect the sound waves, going astern, transducer placement etc

Electromagnetic wave?

A current passing through a wire produces a magnetic field around the wire

If its AC, its a ripple, acting as a transmitting signal.

Speed of light ?

300 Million m/s

300 000 000 m/s

Wave terminology

Amplitude - the height of antenna

Wavelength

Cycle

Frequency formula

Frequency equals Velocity over Wavelength

Attenuation ?

The term used to describe the weakening of a signal strength with distance from the transmitter,

Happens less in space, more through the earths atmosphere.

Propogation?

Sound needs a medium to travel through

Light does not, electro magnetic waves are self propagating

Ground waves?

• Frequencies that follow the curvature of the earth

• Medium frequencies

Sky waves?

• Very high frequencies

• Travel in straight lines, some penetrating through the ionosphere, others being reflected back

• Line of sight

GNSS?

Global Navigation satellite systems

Satellite based positioning systems

• very accurate clock

• Sending signals, calculating speed and time interval for travel

• Creating pseudo ranges

• Multiple satellites repeating the process allow for a position fix

Why are Pseudo ranges innacurate?

The calculation includes a time value with an error, that’s why they use multiple satellites to fix the position

GNSS 3 components/segments ?

1. Space

2. Control

3. User segment

GNSS Space Segment

How many Satellites?

Orbital elevation? Above what?

How many can you see at all times?

What frequency?

• Consist of 24 Satellites In 6 different orbits

• Orbital elevation 20 200 Km above WGS84(Datum)

• Orbital time - 12 Hours

• The constellation ensures that at all times the earth stations can view 4 - 6 satellites more than 5 degrees above the horizon

UHF Frequency = Line of sight = little refraction

GNSS Control segment

How many earth stations?

Earths stations responsibilities?

How often is new information uploaded?

Almanac contains?

• 5 Earth stations

• Constantly tracking the satellites and their position

• Informations passed to the main control station

• Main control station - Colorado Springs

• This is where all the calculations are made for their next orbits

• Every 8 hours the satellites come into view of Colorado and new information is uploaded to them, clocks adjusted.

• Almanac includes , Time, Position, elevation

GNSS User segment

4 pieces?

What does the user do?

Antenna

Receiver - amplifier

Decoder

Display

• Downloads Almanac of information from satellite

The Almanac that is downloaded from the satellite Contains…

The position, course and speed of the satellite

The time that the satellite will transmit the signal

WGS84 what is this datum ?

World geodetic system 1984

The horizontal datum relating to lat and long, the reference point for gps positions to be calculated

GNSS System errors

Errors that can affect range measurement.

MIDSEG

• Multipath error

• Ionospheric Error

• Datum - WGS84

• Satellite clock bias error

• Ephemeris errors

• GDOP - Geometric Dilution of Precision

GNSS System errors - Multipath Errors

MIDSEG

Multipath errors are caused by reflected signals from surfaces near the receiver

GNSS System errors - Ionospheric errors

MIDSEG

when the UHF signal passes through the Ionosphere and troposphere, the signal gets refracted slightly, giving an error by as much as 5m

GNSS System errors - Datum

MIDSEG

WGS84

World Geodetic System 1984

GPS positions will differ if the chart is dot using the same datum, up to 300m error, 6NM in extreme cases

GNSS System errors - Satellite Clock Bias

MIDSEG

Our clock is not as good as the clock onboard the satellite

GNSS System errors - Ephemeris error

MIDSEG

Positional errors

A result of incorrect orbital predictions of satellites

GNSS System errors - GDOP

MIDSEG

GDOP

Geometric dilution of precision

• The orientation of the satellites around the receiver

• Satellite geometry

• Shallow angle = poor accuracy

What type of Dilution of Precision DOP is the mariner interested in?

HDOP

Lat and Long

= GDOP

= PDOP + TDOP

= HDOP + VDOP + TDOP

Position Dilution of position (Lat/Long/Altitude)

Time Dilution of position

Horizontal Dilution of position (Lat/Long)

Vertical Dilution of Position (Altitude)

GNSS System error Probability basis

• 95% probability

• 19 out of 20 occasions the error will be within the value

• The remaining 5%, the error could be put to 3x the stated value

GNSS Selective availability

US Department of defense denied the civilian the same accuracy as the military.

Deliberately corrupting the time and position data signals

+ - 100m accuracy

System has been discontinued

GNSS

Why were Augmentation systems created ?

What Augmentation systems are there?

To provide corrections to the (SA) Selective availability Positions

GBAS and SBAS

Ground based And Satellite Based

GBAS

How it works?

• DGPS - Differential GPS

• satellites would send incorrect positional data to the vessels and ground stations

• The ground station would already know the true position of its own location

• Calculating the innacuracy in the data and passing that information to vessels in the vicinity through VHF

• Resulting In a fix accuracy of 5-10 meters

SBAS

How it works?

• DGPS - Differential GPS

• SBAS is far superior

• greater coverage

• better accuracy(3m)

• UHF, no range limitations

• satellites would send incorrect positional data to the vessels and geostationary satellites

• The geostationary satellite would already know the true position of its own location

• Calculating the innacuracy in the data and passing that information to vessels in the vicinity

Multiple satellites would be used to create a fix, with pseudo ranges

SBAS

WAAS?

N America : Wide Area Augmentation System

RADAR acronym?

Radio Detection and Ranging

Main components (6)

Transmittter

TX/RX Cell

Scanner

Receiver - Amplify

CPU

Display