• Have an elementary knowledge of frequency and frequency bands;
• Be aware of the characteristics of frequencies/radio propagation;
• Have an elementary knowledge of different types of modulation and classes of emission;
• Knowledge of the role of the various modes of communication
• Frequency allocations to the Maritime Mobile Service.
RESOURCES
•An Introduction to GMDSS; T.R. Kristensen; 8th Edition; Poseidon•Handbook for Marine Radio Communication; G.D. Lees & W.G. Williamson; 3rd Edition; Lloyds of London Press.•Model Course 1.25; 2004 Edition; IMO•Template from www.presentationmagazine.com
Frequency and Frequency Bands
• What is the propagation speed of radio waves?
• The formula to find frequency or wavelength is:
λ×= fC
Presenter
Presentation Notes
The propagation speed of radio waves is 300,000,000 meters/second (We will use meters instead of kilometres so that the wavelength will come to a understandable value). The formula use symbols were C = propagation speed of radio waves, f is the frequency of the radio wave and is the wavelength. The formula can be transposed to find the unknown value, either frequency or wavelength.
RADIOWAVE
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Presentation Notes
The diagram illustrated comes from “An Introduction to GMDSS” and shows the components of a radio wave. Although the same in magnitude, period and wavelength refer to two different features. The period refers to the unit of measurement for radio waves, the Hertz. A Hertz is the number of periods, or cycles, per second. Wavelength and amplitude are parts that are also further discussed in this unit.
UNIT OF MEASURING FREQUENCY
The unit used for measuring frequency Is the Hertz (Hz). Radio transmissions are usually very large numbers. Therefore these large numbers are written as:
• Using the formula C = f x L , find the wavelength for each of the following frequencies:• 100 kHz• 500 kHz• 2182 kHz• 156.8 MHz• 1227.6 MHz
Presenter
Presentation Notes
The 100 kHz frequency is that used for the Loran C system The wavelength for this frequency is 3,000 meters. The 500 kHz frequency was the original Distress and hailing frequency for Morse Code. This frequency is no longer used in the maritime world, having been replaced with GMDSS . The wavelength for this frequency is 600 meters. 2182 kHz is the distress and hailing frequency for the Medium Frequency radio system in GMDSS. The wavelength for this frequency is 137.488 meters 156.8 MHz is the distress and hailing frequency for the VHF radio. The wavelength for this frequency is 1.9 meters. 1227.6 MHz is one of the frequencies used by GPS. The wavelength of this frequency is 2.4 centimeters
RADIO FREQUENCY SPECTRUM
• The radio frequency spectrum is divided into several different bands. They are:• ELF (Extremely Low Freq) 3 Hz - 3 kHz• VLF (Very Low Frequency) 3 kHz – 30 kHz• LF (Low Frequency) 30 kHz – 300 kHz• MF (Medium Frequency) 300 kHz – 3 MHz• HF (High Frequency) 3 MHz – 30 MHZ• VHF (Very High Frequency) 30 MHz – 300 MHz• UHF (Ultra High Frequency) 300 MHz – 3 GHz• SHF (Super High Frequency) 3 GHz – 30 GHz• EHF (Extremely High Freq.) 30 GHz - 300 GHz
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Presentation Notes
With the different spectrum designations, there is a relationship between frequency and wavelength. The lower the frequency, the longer the wavelength. So for instance, in the ELF band, the wavelength varies form 100,000 km to 100 km while at the other end of the spectrum, the EHF band has wavelengths of 1 cm to 1 mm.
PROPAGATION OF RADIO WAVES:THE IONOSPHERE
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Presentation Notes
When considering radio propagation, the first consideration should be the medium through which the radio signal passes. This is the troposphere and directly above it lies the ionosphere. The ionosphere becomes electrically charged by the radiation from the sun. During the daytime, the ionosphere divides into different layers which react with radio signals. The reaction to the sun is called ionization. The level nearest the earth is the D Layer which varies in height from 40 – 90 km (about 20- 50 miles) but the ionization is low but can refract signals of low frequencies. At sunset the D layer disappears. The next layer shown on the diagram is the E layer. Its height is approximately 90 – 145 km from the earth. This layer can refract signals with frequencies as high as 20 MHz. The highest layer is the F Layer which has a height of between 145 – 400 km. During the day, the F layer divides into two layers, the F1 and F2 layer The height of the F1 layer is about 145-210 km and the height of the F2 layer is about 210 km to 400 km. The ionization of these layers are quite high and varies widely during the course of a day. Further, the ionization will vary with season, latitude and sunspot activity. This layer will refract radio signals of up to 30 MHz. Thus looking at the previous slide, radio signals from ELF to HF will be refracted by the various layers of the ionosphere.
PROPAGATION OF RADIO WAVES:TYPES OF PROPAGATION
• There are three types of propagation methods for radio waves:• Line of Sight• Ground Wave• Sky Wave
Presenter
Presentation Notes
The line of sight propagation is essentially for radio communication above the 50 MHz frequency. This is also called space wave propagation. For these frequencies (higher than 50 MHz) it is possible for the radio signal to bounce off of the ground which could pose a cancellation affect at the receiving antenna. However, the ground is a poor conductor so over land this effect is minimal, but over water it is possible since water is a good conductor. Therefore, general practice is to mount the antenna as high as possible above the water. Ground Wave propagation depends on the frequency of transmission and the ground conductivity. Since water is a good conductor ground wave propagation can travel great distance with less power than over land. The ground wave will follow the curvature of the earth and this will occur predominantly at MF, LF and VLF frequencies. Sky Wave propagation occurs in the frequency range below 30 MHz. Radio signals will reflect off the ionosphere thus increasing the range of communications. Since ionization is caused by the sun the time of day will affect ionization, and remembering the shape of the earths orbit around the sun, the amount of ionization will also depend on the season of the year. A further factor is the nature of the sun itself. The sun gives off radiation which is closely tied in with the sunspot activity on the surface of the sun. Therefore the ionospheric storms will affect radio communications which may cause radio blackouts. These blackouts may last for brief periods of time or for days at a time.
MAXIMUM USABLE FREQUENCY (MUF)
• The maximum frequency that will be reflected off the ionosphere over any particular path.
Presenter
Presentation Notes
The MUF varies according to which layer of the ionosphere is reflecting the radio signal back to earth. The highest MUF is obtained when the radio signal leaves the earths atmosphere tangentially such as the path followed by “C” in the diagram. The diagram shows that the maximum range of a radio wave is about 2200 n miles, a signal that follows path “A” and is reflected off the F2 layer. The radio path “B” gives a range of about 1200 n miles and is reflected off of the E layer.
MAXIMUM USABLE FREQUENCY
• The true range of communication is not the entire distance between the transmitter and receiver.
Presenter
Presentation Notes
The signal will leave the transmitter following the ground wave as well as the sky wave path. Receivers along the ground wave path will pick up the signal, but as the distance increases the signal will become weaker. The signal that follows path 2 or 3 will be received again, however there is an area which will not receive the message. This is termed the skip zone which is sometimes also called the “dead” zone (no radio reception). The term skip distance is total distance that is between the transmitter and receiver.
CRITICAL FREQUENCY
• The highest frequency for which reflection off any layer of the ionosphere can be achieved. The Critical Frequency is written as fo
MUF = fo X Cos A
A = Angle of Incidence
Presenter
Presentation Notes
On the previous slide, the diagram show an arrow at P3, this arrow represents the angle of incidence. Generally speaking, we will not need to calculate this angle at all, but it can be seen that when “A” is greater than 450, reflection off of the ionosphere will occur. When the frequency is less than the critical frequency no skip will occur. This is generally for frequencies below 8 MHz.
LOWEST USABLE FREQUENCY
• As the radio frequency is reduced, the reflection will occur in the lower layers of the ionosphere. But the lowest levels of the ionosphere will also absorb the radio signal. Therefore the frequency will reach a lowest usable frequency level that can be reflected off of the ionosphere.
Presenter
Presentation Notes
The D layer does not reflect radio signals, but instead will absorb it. Thus lower frequencies during day time will only follow the ground path and explains why we need to use higher frequencies during daylight hours.
SINGLE HOP RADIO SIGNALS
• It is possible for radio signals to do multiple hops. This phenomenon is not always useful since each hop will absorb some energy until finally no voice signal is received.
Presenter
Presentation Notes
In general, the number of hops that are useable for telephony is 2 hops. when radio communication is received, the signal will experience “fading” which is due to the differing parts of the ionosphere which reflect part or all of the signal. This fading occurs during part of one particular transmission and is very noticeable at the receiving station.
OPTIMUM TRAFFIC FREQUENCY
• To determine what this would be, the following is provided as a guide for normal conditions:• Daytime MUF is higher than night time MUF• Winter MUF’s are both lower than and vary more than
summer MUF’s• Radio circuits less than 600 n miles normally use
frequencies below 15 MHz• Radio circuits greater than 600 n miles normally use
frequencies above 15 MHz• MUF’s are higher when the sunspot number is high• MUF at night is approximately half of the daytime MUF
Presenter
Presentation Notes
Basic rule of usage is that the shorter the distance the lower the frequency used. Secondly, during the daytime, the longer the range of communication wanted, the higher the frequency under normal atmospheric conditions.
CLASSES OF EMISSION
• “Emission” is defined as radiation produced, or the production of radiation, by a radio transmitting station.
• There are two basic types of emission (the transmittal of radio signals):• Amplitude Modulation• Frequency Modulation
Presenter
Presentation Notes
In its original form, radio communications was done by Morse code, a simple turning on/off of the signal. This limited the speed of message transmission from 20-25 words per minute. As radio knowledge expanded, two other forms of emissions were discovered, amplitude modulation and frequency modulation. Using modulation, speech could now be transmitted and communication was now at the rate of oral speech.
AMPLITUDE MODULATION
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Presentation Notes
It was found that changing the amplitude of the radio signal required a great amount of power which was not always feasible. However, the information being transmitted could be modulated by a second frequency close to the original frequency could provide the same signal at considerably lower power levels. Since a carrier frequency is used and another frequency nearby, a single communication channel takes up frequency space which is termed “bandwidth.” Fro amplitude modulation, the bandwidth is 2.8 kHz. When the bandwidth is narrow there is less noise and interference (both man made and natural) at the receiver. The total effect of this is that the effective range of transmission will be greatly extended by using a narrow bandwidth.
FREQUENCY MODULATION
Presenter
Presentation Notes
Frequency Modulation uses a frequency bandwidth of 16 kHz, however, FM provides between quality reception because the demodulation process can reject weak interfering transmissions as well as substantial amounts of amplitude varying interference from electrical storms, electrical machinery and spark ignition systems. The diagram under the letter “B” shows that FM is achieved by Phase modulation of the carrier frequency and the modulated frequency. The end result at the receiver is the same, speech is received.
SIDE BANDS
• The modulating signals are known as side bands, that is on both or either side of the carrier frequency.
• Sometimes, the sidebands are both either above or below the carrier frequency, in which case, there is an “assigned frequency.”
Presenter
Presentation Notes
The use of sidebands now provides the definition of upper and lower sideband if they are on either side of the carrier frequency. Both sidebands provides the same information for modulation, so now it is possible to use only one side band away from the carrier frequency. The benefit of using a single sideband is that more power can be directed to the modulating signal which then provides long range radio communications. The other benefit of single sideband is that less of the radio spectrum is being used, and so more users can use this form of communication. The radio license will specify whether the station is using the upper or lower sideband of the carrier frequency.
MODES OF EMISSION
• A1A
• A2A
• H2A
• J2B
• A3E
• H3E
• R3E
• J3E
• F1B
• F3E
Presenter
Presentation Notes
The Radio Aids to Marine Navigation (RAMN) provides a list of the types of emissions from a coast station. Common ones are shown on the slide: A1AUnmodulated Morse Code A2ADouble Sideband (DSB) modulated Morse Code H2ASingle Sideband (SSB) modulated Morse Code J2BSSB Telex A3EDouble Sideband Telephony (Commercial Broadcast) H3ESSB Full Carrier telephony (2182 kHz) R3ESSB Reduced carrier telephony J3ESSB Suppressed carrier telephony F1BTelex F3EFrequency modulated telephony
THE USES OF DIFFERENT EMISSIONS
• In the MF/HF voice communications bandwidth, the J3E will be the primary emission for use in GMDSS . At present, H3E and R3E may still be used as a mode of emission.
• The Bandwidth of J3E is 2.8 kHz.
THE USES OF DIFFERENT EMISSIONS:CONTINUED
• Double Side Band (A3E) transmissions are used by LF/MF/HF broadcasting stations with a bandwidth of 9-20 kHz.
• Narrow Band Direct Printing or Telex is the standard mode of transmission for MF/HF to send data information. The bandwidth is 170 kHz. Telex can be transmitted using SSB in which case the emission is J2B. It can also used the F1B mode of transmission.
THE USES OF DIFFERENT EMISSIONS:CONTINUED
• VHF normally uses the F3E mode of emission, but sometimes it can also use the G3E
Presenter
Presentation Notes
The G3E frequency modulation is achieved by shifting the phase of the modulating and carrier frequencies. For the user, there is no discernable difference in the communication.
ANALOGUE VERSUS DIGITAL
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Presentation Notes
Not much has been said about the differences between analogue and digital forms of communications. This diagram is found in my Student Manual of the GOC GMDSS course I took at BCIT, the PMT Campus. This explains why the range of communications for digital is further than analogue. For instance, there have been many times I heard the International Signal Alarm on 2182, but never received the voice communication giving the nature of distress etc. Thus digital information gives a better range before the communications fades.
ITU CHANNELS
• The International Telecommunications Unions has divided up the frequency bands into “Channels.” These channels are numbered in a simple manner. For example Channel 402 indicates:• “4” is the 4 MHz band• 02 indicates the second channel within that band
• When the ITU Channel is 1604, what is the frequency band being used?
Presenter
Presentation Notes
The frequency being used is the 16 MHz band.
TELEPHONY COMMUNICATIONS
• Communications sometimes comes in two forms, simplex and duplex.• A simplex channel has one frequency used to both transmit
and receive communication. To know when to transmit, the word “over” is used to indicate the end of that part of the telephony communication.
• A duplex channel uses two frequencies for communication and the communication can occur simultaneously, just like the telephone.
• For a duplex conversation with a shore station, how do we ensure that the communication is possible?
Presenter
Presentation Notes
Many Intership frequencies are simplex, but communications between ship and shore occurs most of the time with a duplex frequency. When speaking with a coast station, the ship will transmit on one frequency and receive on another frequency. So if frequency 1 is used to transmit from the ship to the shore, frequency 2 is used to receive. The shore station will transmit on frequency 2 and receive on frequency 1.
GMDSS MODES OF COMMUNICATION
• Digital Selective Calling (DSC) is for communication between ship and shore or ship to ship using digital methods. Primary use is for Distress, Urgency and Safety messages.
• Radio telephony is used for voice communications between stations and is an important part of safety communication.
• Both DSC and radio telephony is used on the MF/HF and VHF bands of communications.
Presenter
Presentation Notes
With DSC, the range of communication is extended. For normal VHF propagation, range is 30-50 miles, with a loss in signal strength as the distance increases. The use of power of the DSC signals and the range is extended to 70 miles before there is a significant signal fading.
GMDSS MODES OF COMMUNICATION:CONTINUED
• Narrow Band Direct Printing (NBDP) is also known as NAVTEX. This type of data communications is the sending of Maritime Safety Information (MSI) in the LF/MF/HF frequency bands.
• Telex is used for ordinary communications not related to safety.
Presenter
Presentation Notes
NBDP can also be used for distress communications on 2174.5 kHz and 4177.5 kHz
GMDSS FREQUENCIES
• The Distress, Urgency and Safety frequencies for MF/HF is:
• NAVTEX frequencies are:• 490 kHz for National language broadcasts• 518 kHz for English language broadcasts• 4209.0 kHz for National language broadcasts
GMDSS FREQUENCIES
• Maritime Safety Information is available on the following HF frequencies:
