TRANSVERSE WAVES:
Probably everyone has, at some time or another thrown a stone into a pond or other smooth sheet of water and noticed the circular ripples that spread out from the spot where the stone entered the water. These ripples are an example of a wave motion travelling over a circular wave front.
A somewhat simpler type of wave, called a transverse wave, is seen when one end of a piece or rope or string is moved up and down in a direction perpendicular to its length.
The rope represents the appearance of a series of equidistant crests and troughs that travel forward with a certain velocity.
It is important to realise that it is only the shape or form of the wave that moves forward. The individual particles of the rope merely oscillate up and down. The maximum displacement of a particle from its rest position is called the amplitude of the wave.
The wavelength ( ) is defined as the distance between two successive particles which are at exactly the same point in their paths and are moving in the same direction (from crest to crest, or A to B in diagram).
The number of complete oscillations made in one second is called the frequency (f). The unit of frequency is called the hertz.
1 Hertz (Hz) is defined as 1 cycle (or oscillation) per second.
In the time it takes the particles to make one complete oscillation the whole wave moves forward one wavelength.
Radio waves in the range 10 kHz form the usable radio frequency spectrum, parts of which are used for broadcasting, communications and radio navigation systems throughout the world.
The velocity of radio waves is approximately 300 x 106 metres per second, which is the speed of light. Wavelength and frequency are related:
Wavelength = 300 x 106 in metres
Frequency
ELECTROMAGNETIC/RADIO WAVES:
Magnetic Field:
A current passing through a wire produces a magnetic field around the wire. The strength of the field is proportional to the current. Looking in the direction of the current, the circular magnetic lines of force have a clockwise direction.
Lines of magnetic force around a wire carrying
a DC current away from you.
Induction Field:
The induction field is the magnetic field set up by an alternating current. An alternating current continually changes its magnitude, and periodically changes its direction and its associated magnetic field changes in the same way.
The induction field is only significant up to one wavelength from the wire or transmitting antenna. This can cause deviation in a magnetic compass.
Radiation Field:
Radio frequency current also generates a radiation field. The radiation field is the radio signal and consists of magnetic lines of force of the same circular shape as those of the induction field. Once generated, the radiation field is independent of the antenna and continues to expand.
As the distance from the transmitter increases the signal becomes weaker, until it falls below the background noise level. Increasing the power output (current) in the transmitting antenna can increase the range.
Receiving Antenna:
The radiation field reacts with a receiving antenna and generates a current in the wire.
As the induced field is always at right angles to the magnetic field transmitting and receiving antenna must be in the same plane. This is referred to as vertical or horizontal polarisation.
ELECTRICAL NOISE AND INTERFERENCE:
Very low electrical currents naturally occur in any conductor. This current will increase with temperature. When amplified with the incoming signal this can be heard as a background noise. Noise may also be generated in transistors, resistors, and other components.
External sources of noise include radio waves caused by discharges in the atmosphere. These atmospherics can be reflected by the ionosphere and impede reception of desired stations over a wide area. Precipitation particles (rain, hail) may become electrically charged. When they make contact with the receiving antenna, they discharge to earth via the antenna and the receiver. This so called precipitation static can also cause noise although this occurs very rarely.
A further cause of noise is the sparking of switches when disconnecting apparatus or of direct current motors and dynamos (between commutator and brushes), which induces voltage in the receiver and antenna. This interference usually has a short range. It can be avoided by mounting the receiving antenna in a position located above the level of the noise source. The aerial wire below this level, its conductors (shielding cables, metal cases etc.) which must be connected to Earth.
Noise appears on all frequencies. Most noise can be eliminated if the receiver can be fine tuned. In position –fixing system receivers, noise and interference may partly or completely obscure or distort the signal so that the accuracy of the fix may become unacceptable. Some receivers are equipped with a noise meter or with a display which shows the magnitude of the signal to noise ratio (S/N) to warn the user of the decrease in accuracy.
The stronger the incoming signal the less the effect noise will have. The automatic gain control (AGC) in the receiver decreases amplification when the signal strength increases. As the noise is normally weaker than the signal, it is not heard.
If the received signals are only as strong as, or weaker than, the noise, the signals cannot be separated from the noise. The greater the distance from a transmitter, the weaker its signal, but the noise level is the same everywhere. When the signal is no stronger than the noise increased amplification by the receiver will not help because the signal remains drowned in the noise. Only an increase of the transmitter power could improve the reception.
Signal to Noise Ratio:
The ratio between signal strength and noise level is the signal to noise ration, and gives an indication of how clear reception will be.
PROPAGATION OF RADIO WAVES:
Radio waves travel outwards in all directions. They will continue expanding until meeting an obstruction. Radio waves transmitted over land or sea follow different tracks depending on their frequency.
Space Wave:
When the transmitter and receiver are in sight of one another the radio waves may travel between them by two methods: the Direct Space Wave and the Reflected Space Wave. The direct space wave and the reflected wave are usually grouped together and called the Space Wave.
Ground Wave:
The Surface or Ground Wave is produced by the signal travelling close to the earths surface and being diffracted or bent to follow the surface beyond the horizon. Often the surface waves and space waves are grouped together and known as the Ground Waves.
The ground wave follows the earths surface due to friction slowing the lower part of the wave (known as diffraction). The amount of slowing depends on the electrical conductivity of the surface. Sea water is a good conductor and slows the radio waves less than land.
Attenuation is the decrease in the amplitude of radio signals as they travel outwards from the transmitter i.e. the signals become weaker due to:
1) absorption by the medium through which they travel;
2) scattering due to water and ice in the atmosphere.
Sky Wave:
The ionosphere sometimes refracts Sky Waves so that they return to the earths surface. In simple terms the ionosphere reflects the radio waves back to earth. There will be an area around the transmitter outside ground wave coverage where no signals can be received, known as the Dead Zone. The Skip Distance is the distance between the transmitter and the first return of the sky wave.
The Ionosphere:
At altitudes between 50 and 500 km above the earths surface there exists a number of bands containing electrically charged atoms called ions. In the upper atmosphere ions are formed by the action of ultra violet light. The extent of ionisation depends mainly upon the intensity of UV light from the sun reaching our atmosphere. Ionisation is therefore less during the hours of darkness and under constant change during the transitional periods of sunrise and sunset.
Four distinct ionised layers exist during the day reducing to two at night. Each layer has the property to attenuate and, under certain conditions, refract a radio sky wave. Global communications rely on this phenomenon.
At the greatest distance above the earths surface is the F layer, which during the day effectively consists of two levels of ionisation designated F2 and F1. At night the two layers combine to form a single layer. At an altitude of approximately 90 to 150 km there exists a less intensely ionised layer called the E layer. This layer maintains its altitude throughout diurnal changes although at night it becomes weaker by a factor of two. Lowest and weakest of the layers is the D layer, which disperses at night. It is this layer which is the principle source of absorption of HF radio waves. LF and VLF radio waves are reflected from this layer to provide long distance operations during the day.
Method of Propagation:
The method of propagation depends on the frequency:
Below 500 kHz surface wave
500 kHz to 1.5MHz surface wave for short distances
sky wave for long distances.
1.5 MHz to 30 MHz sky waves
above 30 MHz space wave within line of sight
Fading:
If the ground waves and sky waves from a transmitter are received simultaneously, the two signals may not be in phase because the sky waves have travelled over a longer path than the ground waves. The same applies if two or more sky waves from the same transmitter, having followed different paths through the ionosphere, are received.
The phase differences will vary, depending on the paths, and may even become opposite. The resulting signal strength will vary and may be reduced to below the noise, making it impossible to receive the signals. This phenomenon is called fading. Normally, receivers compensate for fading by automatically varying the amplification to stabilise the output of the receiver.
Phase variations cause significant errors in some radio navigation systems, especially Decca.
NON-STANDARD PROPAGATION WITHIN THE TROPOSPHERE:
The troposphere is the layer of the atmosphere about 10 km deep closest to the surface of the earth. Variations of temperature and humidity can cause non – standard propagation of electromagnetic waves. Meteorological variations have the most effect in the VHF and Radar bands, either reducing or generally extending the range. This is dealt with in more detail in the radar notes.
THE RADIO SPECTRUM:
Abbreviation Band Frequency Range Wavelength Uses
AF Audio 20 Hz to 20 kHz 15000 km to 15 km Sound
RF Radio 10 kHz to 300 GHz 30 km to 0.1 cm
VLF Very Low 10 Hz to 30 kHz 30 km to 10 km
LF Low 30 kHz to 300 kHz 10 km to 1 km
MF Medium 300 kHz to 3000 kHz 1 km to 100 m Navigation aids
HF High 3 MHz to 30 MHz 100 m to 10m Marine Radio
VHF Very High 30 MHz to 300 MHz 10 m to 1 m
UHF Ultra High 300 MHz to 300 MHz 1 m to 10 cm
SHF Super High 3 GHz to 30 GHz 10 cm to 1 cm Radar & Satellite
EHF Extreme High 30 GHz to 300 GHz 1 cm to 0.1 cm
VLF (very low frequency)
VLF radio signals propagate using a combination of both the ground and space waves, which are guided, over great distances, between the lower edge of the ionosphere and the surface of the earth. Very large antenna systems are required to match the very long wavelength of VLF signals. With a wavelength of 30 km, at 10 kHz, highly efficient large antennae are only possible on land installations, often erected between mountain peaks. Good ground wave range up to 4000 – 8000 miles but requires great power.
LF (low frequency)
Communication is mainly by ground wave which suffers greater attenuation as frequency is increased. Range therefore depends upon the amplitude of the transmitted power and the efficiency of the antenna system. Wavelength has decreased to the point where suitable antennae of a practical size can be produced. Range of the ground wave for a given power is 1500 to 2000 km (Loran C). The sky wave is returned from the ionosphere, particularly during the hours of darkness, producing errors in some navigation systems.
MF (medium frequency)
Ground wave attenuation rapidly increases. Range, for a given transmitter power, is therefore reduced as frequency is increased. Ground wave range is typically 1500km to under 50km for a transmitted signal with a peak output power of 1kW correctly matched into an efficient antenna system. In the band below 1500 kHz sky waves are returned both day and night although communication using these waves is unreliable. Above this figure the returned sky wave has greater reliability but is affected by changes of the ionosphere due to diurnal changes, seasonal changes and the sunspot cycle. By taking these factors into account and carefully selecting the frequency, reliable communications up to a range of 2000 km can be achieved.
HF (high frequency)
This frequency band is widely used for terrestrial global communications. Ground wave range is insignificant being only a few km. For satisfactory communications using sky waves the frequency must be carefully selected. Sky waves of frequencies at the lower end of the band are absorbed by ionospheric layers during the hours of daylight and are not returned to earth. Under these conditions communication can be established by selecting a frequency in the centre or upper end of the HF band. At night the lower frequency sky waves are returned, whereas the higher frequencies are not refracted sufficiently to be returned and are lost. The choice of frequency in this band for reliable communication over great distances is therefore a compromise.
VHF (very high frequency)
Both ground waves and sky waves are practically non existent and can be ignored. Communications is via the space wave which may be ground reflected. Space waves effectively produce line of sight transmission and consequently the height of the antenna becomes important. The antenna must also be directional.
Large objects in the path of a space wave will produce a blind spot in which reception is extremely difficult.
UHF (ultra high frequency)
Space waves and ground reflected waves are used with highly directional efficient antenna systems. Signal fading is minimal although wave polarisation may be affected as the wave is ground reflected, resulting in a loss of signal strength. Blind spots are still a problem.
SHF (super high frequency)
Transmissions in this band have very short wavelengths and are known as microwaves. No sky waves exist, propagation is by direct and ground reflected waves. Wavelengths are in centimetres, therefore compact highly efficient antennas can be used. This band is used for maritime radar and satellite communications.
EHF (extreme high frequency)
This band although at present not used in the maritime service and forms the upper end of the usable radio frequency spectrum.
HYPERBOLIC NAVIGATION:
The diagram on the following page shows two points of foci, A and B, joined by a Base Line. The Base Line Extensions extend outwards from A and B. The line at right angles to, and in the centre of the base line, is known as the Centre Line. Circles expand outwards equally from A and B, for convenience imagine the first circle being one mile from the foci, the second two miles, the third three miles etc.
A hyperbola is a line which joins points which have a constant difference in distance from two fixed points, or foci. The centre line is a hyperbola – the difference in distance from point A and B is zero.
Now identify on the diagram the hyperbola where the difference in distance is two. You should find that there is one to the right and also one to the left of the centre line. Now identify on the diagram the hyperbola for a difference in distance of four and six.
Note:
The distance between two hyperbolas increases as the distance from the baseline becomes greater.
The distance between the hyperbolas at the base line is equal.
The accuracy of a hyperbolic system is greatest on the base line. In contrast, the accuracy is very poor in the vicinity of the base line extensions and at extreme range. Hyperbolic systems should be used with caution in these areas. The sites of the stations are chosen to give the greatest degree of accuracy in areas where it is most necessary, for example, where there is a high concentration of ships.
In the following diagram, two patterns of hyperbolas are shown from transmitters, A, B and C. In the shaded area the angle of intersection is very good and the lanes are narrow, so a fair degree of accuracy can be expected. The accuracy will be less outside this area because it is either close to the base line extensions or at extreme range, the angle of intersection is unfavourable and lanes are wider.
Two hyperbolic position lines may have two points of intersection, which may give rise to ambiguity in ascertaining the position. In practise this does not usually cause difficulty, as the two possible positions are normally far apart. If it is necessary to make use of a hyperbola quite near the base line, the distance between the two possible positions may become so small that doubt arises. In such cases the correct position may be determined by consulting the chart, taking into account the course of the vessel, and determining, for each of the two positions, whether the readings should increase or decrease. Another possibility is to determine a third line of position by any method, this ambiguity always arises near a baseline extension.
Each measurement has an error, which consists of both a fixed or systematic error and a variable or random error. Fixed errors are those that have a constant value and thus can be eliminated by corrections. All other errors are termed variable errors.
Fixed Errors:
Fixed errors may be defined as the difference between the chart co-ordinates of a given position and the average of a large number of readings by day for the same position. This may be caused by the actual speed of propagation being different from the speed assumed in the computation of the hyperbolic lines in the chart. The effective speed varies widely with the electrical characteristics of the terrain over which the signals pass, being slowest over dry land.
Variable Errors:
Variable errors are due to uncontrollable causes and there is an equal chance that they will be positive or negative. Corrections cannot be applied to variable errors, but they can be taken into account by allowing for an error around each position line.
Suppose that a great number of measurements have been made and that the error of each measurement has been determined by comparison with the correct value.
To use the largest of these as a measurement of the accuracy is misleading, because a single large error may give a wrong impression of the accuracy. For this reason the 95% error is normally applied. This is the error that is exceeded in only 5% of all cases. The 68% error is also used for Decca errors; statistically the 68% error is half the 95% error.
GEOMETRIC DILUTION OF POSITION:
If the value of the 95% error of a line of position is known, it is possible to draw parallels on either side of this line at a distance equal to the error. It may be assumed that the observer is between these two parallels. For the other line of position, two parallels can be drawn in the same way. The size of the parallelogram so formed is a measure of the accuracy.
If the two lines of position intersect at a small angle the parallelogram will become larger and so the accuracy will be less. It should be noted that the accuracy is still good in the direction of the short axis but poor along the long axis.
The most favourable angle of intersection for two lines of position is 900. Calculation shows that the figure inside which the position can be assumed to lie is actually an ellipse rather than a parallelogram.
WAYPOINT NAVIGATION:
Electronic systems determine automatically the position of a ship. This enables the receiving system, equipped with a computer, memory, display and keyboard, to calculate and display continuously additional navigation information – course and speed made good, time of arrival, distance to destination etc. Because this equipment is more and more often incorporated in position-fixing systems, a short description of its may extra facilities is appropriate.
Normally, the route of a ship consists of a number of legs. The first leg begins at the start position of the ship. At the end of each leg is a turning point or way point. To sail such a planned route is called waypoint routing, or waypoint navigation. The track to be followed should first be plotted on the chart. The latitude and longitude of the waypoints, obtained from the chart, have to be keyed in; they will be memorised by the computer and shown on a display. For ocean navigation the legs will be very long. In such cases some waypoint equipment has the ability to calculate and display the rhumb line or great circle, or even a combination of both, between two waypoints.
AUTOMATIC DEAD RECKONING NAVIGATION:
Most integrated navigation systems will revert to dead reckoning mode (DR) if they lose signal for any reason.
There is a danger that the change in mode may not be noticed. The set of the vessel caused by wind, current and sea conditions will n...
dariusz.lipinski