Naest Theory

(Marine Operations)

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CONTENTS Topic 1 Electromagnetic Propagation 3 Topic 2 Echo Sounders 6 Topic 3 Distance and Speed Measuring Devices (Ships Logs) 14 Topic 4 Radar 22 Topic 5 Radar Plotting and Parallel Indexing 48 Topic 6 Automatic Radar Plotting Aids (ARPA) 63 Topic 7 LORAN-C 76 Topic 8 Global Positioning Systems (GPS) 88 Topic 9 Differential GPS 97 Topic 10 Automatic Identification Systems (AIS) and Long Range 103 Identification and Tracking Topic 11 Voyage Data Recorders (VDR) 110 Bibliography 113

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Topic One Electromagnetic Propagation Radio Wave principles Wavelength: the distance between successive wave crests. (Symbol lambda) Cycle: the movement from one crest through the trough to the next crest.

A specific point within a cycle can be identified by its phase (0 360) Power: the amplitude of the radio wave is a measurement of its power. Once the radio wave leaves the antenna it will lose energy to the atmosphere. Energy loss is proportional to the square of the distance travelled. Frequency (symbol f): the number of cycles which pass a point in a given time, usually a second. One cycle per second is one Hertz. 1,000 hertz = 1 kilohertz (KHz); 1,000,000 hertz = 1 megahertz (MHz) 1,000,000,000 hertz = 1 gigahertz (GHz) Wavelength and frequency are linked by the formula: Speed of radio wave (300 metres/ second) = f

The allocation and use of frequencies is regulated by the International Telecommunications Union (ITU)

Generating radio signals

Applying an alternating current to an aerial creates an electromagnetic field round it.

The induction field remains attached to the aerial. It can be detected by a receiver

only within about two wavelengths of the transmitter.

0 90 180 270 360 360

amplitude

wavelength (1 cycle)

phase

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The radiation field (at the same frequency as the alternating current that generated

it) will be propagated omni-directionally, in a specific plane, in a fixed direction, or

made to rotate, depending on the aerial/antenna design.

All transmitting aerials utilise one or more means of propagation, with one mode

predominating. A brief description of the three modes follows.

Surface (ground) waves travel around the surface of the earth and are modified by

the ground over which they travel. It is the predominant means of propagation for

frequencies up to 3 MHz (approx). The proximity of a surface creates diffraction that

causes the wave to bend towards the surface and allow it to follow the curvature of

the earth. The surface over which the signal travels also affects attenuation and

speed of transmission, depending on the conductivity of the surface and the

wavelength of the transmission. Diffraction will occur when the wave encounters any

large object, like a building, particularly with long wave length transmissions.

These characteristics explain why Loran C requires ASF correction, and why its

reception is possible in urban canyons where GPS reception is not.

Sky waves are the predominant means of propagation in the 3 30 MHz range, but

also occur between 30 KHz and 3 MHz. Sky waves are refracted at the ionosphere and

may be returned to earth over a great distance. They are also subject to attenuation.

The ionosphere, extending from about 60 800 km above the earth, contains a

number of ionised layers, the four major ones being designated D, E, F1 and F2.

Ionisation occurs as a result of the suns ultraviolet radiation. The level of ultraviolet

radiation, and therefore of ionisation, varies with the time of day, the season, and the

sun-spot cycle. The extent of the refraction that occurs further depends on the

density of the ionosphere, the frequency of the propagation, and the angle of

incidence of the wave with each layer.

In total, this refraction can allow for global communication as the radio waves

undergo a number of hops between earth and ionosphere. Fading of the signal can

occur, however, due to changes in attenuation at the ionosphere, reception of out-of-

phase signal that have followed different routes between earth and ionosphere, and

because of variations in attenuation of different frequencies within the transmitted

bandwidth.

Whilst fading is inconvenient, the effect on navigation systems like Loran C, that rely

for their accuracy on measuring the time difference of the received signals from two

or more transmitters, is more critical.

Space (line of sight) waves are not diffracted like ground waves, but within the

troposphere (up to 10 km from the earths surface) they are subject to refraction that

causes some bending towards the earth, but less then the curvature of the earth. This

is the predominant means of propagation for frequencies above 30 MHz and explains

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why VHF and radar horizons are greater than the visual horizon, but only by a

relatively small amount. To call the transmission line of sight is not strictly correct.

Frequency bands summary

Frequency

Band

Wavelength

Mode

Example

3 30 KHz

VLF

10 100 km

Ground Wave

Space Wave

Communications

30 300 KHz

LF

1 10 km

Ground Wave

Sky Wave

LORAN C

300 KHz 3 MHz

MF

100 m 1 km

Ground Wave

Sky Wave

Communications

3 30 MHz

HF

10 m 100 m

Sky Wave

Ground Wave

Communications

(global)

30 300 MHz

VHF

1 m 10 m

Space Wave

Communications

300 MHz 3 GHz

UHF

10 cm 1 m

Space Wave

Satellites

3 30 GHz

SHF

1 10 cm

Space Wave

Marine Radar

30 300 GHz

EHF

0.1 1 cm

Space Wave

Not for mobile

communications

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Topic Two - Echo Sounders

The principles, use and operation of echo sounding equipment

The echo ranging principle utilised by echo sounders is simple. If a short pulse of

ultra-sonic energy is transmitted directly downwards from the hull of a ship, and the

time taken for an echo to return is accurately measured, then the following

relationship applies:

Distance (depth below hull) = speed of sound in water x time

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The speed of sound in seawater (3.4% salinity) at a temperature of 16C is 1505

metres/sec. It increases slightly with higher salinity, temperature and/or pressure.

Echo sounders are generally calibrated for an internationally accepted speed of 1500

metres/sec. Because the speed of sound in fresh water is less than in salt water the

true depth when in fresh water will be approximately 3% less than that indicated.

Since we are unlikely to be aware of the water temperature or salinity at all depths

through which the sound pulse travels it is generally impractical to apply corrections

to the indicated depth.

Components of the echo sounder

The pulse generator creates an electrical

oscillation that is supplied to the

transmitting transducer (oscillator) and

converted into a mechanical vibration. The

vibrating surface of the transmitting

transducer is in contact with the water and

transmits sound vibrations downwards to the

sea bed in a circular or elliptical beam.

The angular size of the beam will depend on

the application but is generally between 12

and 25. The narrower the beam is the

greater the concentration of energy and

therefore the potential range. If the beam

is too narrow it will not indicate true depth

when the ship has a large list. Again there

is a similarity to radar in that the beam

width refers to the half-power limit, and

that side lobes exist.

The receiving transducer is set vibrating by any returning pulse. It converts this

mechanical vibration back into an electrical oscillation that is amplified before being

fed to the depth indicator/recorder, where it produces a visual display or record.

32.4

Pulse

Generator

Transmitting transducer

Receiving transducer

Amplifier

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Refraction and reflection

When the sound pulses generated by the echo sounder encounter water layers of

varying temperatures, salinity or particle content there may be some reflection, with

weak echoes returned to the transducer. Refraction will also occur and effectively

increase or decrease the beamwidth. Since transition between layers is usually

gradual these effects are unlikely to be dramatic.

When the pulse strikes the seabed the

strength of the reflection will depend

on the nature of the seabed (aspect,

composition). A smooth, hard and

sloping seabed will theoretically return

no reflection in the direction of the

transducers. Such reflection is said to

be specular. To ensure that at least

some of the reflected energy returns in

the desired direction requires diffuse

reflection that is achieved by the

normal irregular seabed.To increase

diffuse reflectio