Technical Aspects of GMDSS Equipments – Propagation, Modulation, Terrestrial Equipment & Antenna

GMDSS Radio Signals

Radio signals are transmitted as electromagnetic radiation and occupy the frequency range 3 kHz ~ 300 GHz. The radiofrequency range is divided into a number of bands. In the GMDSS system both terrestrial and satellite methods of communications are interlocked in order to provide an extremely reliable scheme to alert rescue authorities in the event of a vessel being in distress.

Frequencies in the MF, HF and VHF bands are used for terrestrial communications via Coast Radio Stations (CRS). Frequencies in the SHF bands are used for satellite communications. It’s important to note that communications via INMARSAT satellites take sea place via Coast Earth Stations (CES) which are also known as Land Earth Station (LES) and RCC/MRCC.

The radio frequency spectrum is shown below:

Propagation

Energy is contained in a propagated radio wave in two forms:

• Electrostatic
• Electromagnetic

These are at right angles to each other.

This radiofrequency energy is radiated from an aerial as closed loops of electrostatic energy. The polarisation of a radio wave is in the direction of the electrostatic (electric) Field of the Antenna. A horizontal antenna radiates a horizontally polarised wave whereas a vertical Antenna will radiate a vertically polarized wave. Accordingly receiving Antennas must be erected either vertically or horizontally ( or a combination of both) for the best reception.

Generally, transmissions will be:

When radio waves are propagated from a transmitter antenna system, they form one or more of 3 modes with one mode being dominant. The 3 modes of propagation are

1. Space Wave or Line of Sight
2. Surface Wave or Ground Wave
3. SkyWave

Radio Waves Propagation

Space wave or Line of Sight propagation

Above 30MHz the predominant mode of propagation is by a space wave. If space wave is propagated along the surface of the earth the wave will move in a straight line from Tx to Rx, often called a line-of-sight wave (or direct wave) as well as in the form of ground reflected wave. In practice, however, slight bending of the wave occurs to make the radio horizon somewhat longer than the visual horizon. The radio horizon exceeds the visual horizon by approximately 15 % Radio range on VHF, dependent upon 2 main variables, viz. the Tx and the Rx antenna heights.

UHF signals pass through the atmosphere almost without noticing it. They are hardly refracted or absorbed at all by the ionosphere, so the signals continue in a straight line, which, exactly is needed for sending and receiving signals between a satellite and a ground station. Communications between a satellite and SES or CES is via line of sight propagation.

FM has some advantages over AM. A much better signal-to-noise ratio is possible, so there is much less interference from static or atmospheric noise. FM exhibits what is called “Capture Effect“. If there are two FM signals on the same frequency it will capture the strongest one and ignore the other. Unlike on AM, both signals would be heard.

Marine bands frequency varies from MF (1605 ~ 2850 kHz), on HF ( 4, 6, 8, 12, 16, 18, 22 and 25 Mhz) and on VHF ( 156 ~ 174 Mhz).

Surface wave or Ground wave propagation

This will predominate at all R/F up to approximately 3MHz. They tend to travel along the surface of the earth & propagate within the earth’s troposphere (a band of the atmosphere which extends upward from the Earth to approximately 10 KM). An important phenomenon affecting the surface wave is known as diffraction. There are two effects on the wave one is tilting and the second is absorption by the earth. The range may be increased if the power at the transmitter is increased.

SkyWave propagation

The prime method of propagation in HF between 3MHz and 30 MHz is by skywave. There exist above the earth’s surface a number of layers of ionized energy. The layers are D layer (50-90 kilometres), E layer (120 kilometres), F1 layer (200 kilometres, F2 layer (300-400 kilometres). The F2 layer has a higher level of ionization than the D layer. At night Fl -and F2 layer forms a single layer about 250 KM and the D layer disappears. The natural parameters, which affect the density and height of ionosphere, are the sun, season and eleven-year sunspot cycle. The extent of refraction will depend on the natural parameters as well as the frequency of the propagation and the angle of incidence.

Greater the distance between stations higher the frequency (eg. Daytime use 16, 18 or 22 Mhz) and night time (eg. 8 or 12 Mhz)

Critical frequency: for a given ionized layer, there exists a maximum frequency that can be propagated vertically & returned to earth.

Skip Distance and Skip Zone:  The distance where the first sky wave returns is the skip distance. The area where the ground waves end and the first sky wave returns are the skip zone.

Maximum Usable Frequency: The MUF is the highest frequency that can be used to establish communication between 2 points on the earth which puts the receiving station right on the edge of the skip distance. The MUF for a given layer is a function of both the critical frequency & the angle of incidence of the radio wave. As the MUF is dependent upon the electron density of a layer which is varying, it is common to use another frequency which is 15% lower than the MUF that is the OTF. (Optimum Traffic Frequency).

Signal Fading: Fading is a continual variation of signal amplitude experienced at the antenna input to a receiving system. This is because of the continuous ionospheric layer. The use of automatic gain control (AGC) will effectively combat fading.

Modulation

When we talk, we talk at a frequency which is in the band audible to the average human, the human hearing range. For us to send out information to long distances, we modulate Audio Frequency (AF) on to a carrier which traves far greater than AF. i.e we modulate AF on VHF, MF or Satellite frequency.

Information can be sent by either Voice, DSC or Telex. This is called modulation.

Types of Modulation

1. Amplitude Modulation(AM): In this type of modulation, the amplitude of the carrier wave is varied according to the amplitude of the modulating signal.
2. Phase Modulation: The phase of the carrier wave is varied according to the amplitude signal.
3. Frequency Modulation: The frequency of the carrier wave is varied according to the amplitude of the modulating signal.

Because of modulation, side-bands are created which along with the carrier occupy a certain bandwidth, in SSB Tx the side-bands carry the information. To prevent signals taking too much space on the radio band, in amplitude modulation the frequency bandwidth is limited to 3kHz.

Classes of Emission (Mode)

First Symbol: Type of Modulation

• • J, A, H, R – For Amplitude
  • F – For Frequency
  • G – For Phase

Second Symbol: Nature of Signal Modulating Main Carrier

• • 1 – Digital information without modulating sub-carrier
  • 2 – Digital information with modulating sub-carrier
  • 3 – Analogue Information (Voice)

Third Symbol: Type of Information

• • A – Telegraphy for Manual Reception (Morse)
  • B – Telegraphy for automatic reception (Telex or DSC)
  • E – Telephony (Voice)

H3E – This is mainly used for Distress on 2182 kHz.

J3E – This is the preferred mode for R/T and used on all other frequencies.

Simplex and Duplex Operations – Important Aspect of GMDSS

Terrestrial Equipment

1. VHF

Marine VHF – Furuno

1. Volume control, which often incorporates the on/off switch,
2. a control for selecting the channel
3. a squelch control

2. MF/HF

The controls on an MF/HF transceiver vary greatly from one make to another, however, some controls are basic to all sets:

1. Volume: Sometimes called Audio Frequency Gain control. This sets the level of the speaker. The On/Off switch could be incorporated with this control.
2. Radio Frequency Gain: Sometimes called Sensitivity Control. This sets the level of the incoming signal at the antenna side. If the signal is too strong, it will be distorted. The RF control should be set as high as possible without distorting the signal .
3. Automatic Gain Control: The AGC will automatically adjust the level of the incoming signal at the R.F. to a constant level. It is normally switchable and can be put on or off. Under most conditions, it Is best to use the radio with the AGC on, in which case the RF control should be set to maximum. Used to overcome the effects of fading on HF throughout the day and on MF at night.
4. Frequency: Most modern receivers offer a choice of selecting an ITU channel, or the frequencies can be fed directly on Rx and Tx mode.
5. Clarifier: Sometimes called Fine Tuning. If the received signal is getting interference from a nearby strong station then fine adjustment of the frequency can be done by this control.
6. Mode:
   1. J3E (SSB OR USB) Single sideband suppressed carried – used for All R/T communications except 2182 kHz Distress.
   2. H3E:(AM ) Single Sideband Full Carrier Used for Distress R/T- on 2182kHz
   3. Guard Band for MF is 2173.5khz – 2190.5khz
7. Squelch: It Is used to eliminate noise (receiver/atmospheric) when there is no speech signal on the receiver frequency. If the Squelch is ON a speech signal with a signal to noise ratio greater than a certain value is required to pass the signal through the receiver AF- amplifier.
8. Tune: For MF or HF operations the antenna must be tuned the frequency being used. Most modern sets use an automatic tuner. Tuning is simply a case of selecting the frequency and pressing the tune button.
   If there is not an automatic tuner, then the operator must manually tune the antenna, which may consist of using rotary switches. Any manual tuning must be done at low power.
9. Power: This will normally adjust the output power of the transmitter
10. Dummy Load: This is basically an artificial aerial consisting of a resistive load. which dissipates power. This Is used for tuning, testing and repairs without radiation.
11. 2182: Most receivers give instant access to 2182 kHz and the mode switch automatically goes to H3E or AM and full power.

Block diagram for Transmitter & Reciever

Ship’s Antennas

A ship’s antenna arrangement consists of several different transmitter and receiver antennas, and as the available space on board is very limited, the Antenna placement design locations are often a result of various compromises.

The design of the antenna arrangement is of major importance when it comes to the amount of radiated power available and reception of different radio signals.

Antennas at sea:

1. The Whip Antenna:

MF-HF-Antenna Whip Antenna

2. Vertical half-wavelength dipole antenna.

VHF FiberGlass Whip Antenna

3. The Yagi-Uda antenna :

Yagi Uda Antenna

4. Microwave parabolic antenna

Satcom Parabolic Antenna, Credits: SAILOR

This is popularly known as the dish antenna. An SES uses a prime focus system, whereby the received signal from the satellite is reflected by the perfect parabola formed by the dish, to be collected at the prime focus point. The antenna works in a reciprocal way to this when used on transmit.

For Satcom ‘B’ (Old Tech). These are directional antennas, they are gyro stabilised and housed in a low loss protective Radome. Omnidirectional Antenna is used for Satcom -‘C’.

5. The long wire antenna

This used to be a standard fitting on merchant ships because it provides excellent radiation properties at the medium frequencies. It is relatively easy to install. Two types of the long-wire antenna may be found, the ‘T’ type and the inverted ‘L’ type. An antenna is considered to be a long wire only when it is long in terms of wavelength. The antenna is directive with maximum radiation occurring off the ends of the assembly and little broadside to it.

Long Wire Antenna

Antenna Maintenance

On Whip Antennas check for cracks. Salt and dirt will act as a fairly effective in stopping the radio waves from reaching the wire antenna inside. On Wire Antennas, the insulators must be kept clean and freshwater washed. Any shackles, bulldog grips and thimbles should be in good condition. Check to see the safety link is in place (breaking strength of the safety link is 1/3 the strength of the main wire antenna ). Also check for proper connections, brackets, mountings and clamps.

Safety Precautions to be taken when working with antenna:

Great care must be taken that the Transmitter cannot be activated with anybody is near the ends of the antenna. This is especially important for Telex as in the ARQ mode the transmitter activates automatically. So switch off the equipment and ground all aerials.

Radiation hazards exist around Satcom Antenna. The minimum sate distance from Satcom ‘A’ (Obsolete now) or ‘B’ antenna is 7 meters and Satcom ‘C’ is 1 meter. When working with the Sat antennas makes sure the equipment is switched off.

Antenna Tuning Unit (ATU)

Long Wire Antenna – PC: egmdss.com

To have an efficient transmission, the physical length of an antenna would have to be altered for each transmitted frequency. This is impractical and as an antenna of fixed length is normally used. The Antenna Tuning Unit alters the electrical length of the antenna by changing the electrical characteristics to match the wavelength of the frequency being transmitted. Adding an inductance in series increases the length of the antenna. A capacitance in parallel decreases the length.

The ATU is mounted as close to the main antenna as possible, it should be remembered that dangerously high voltages and RF currents are present in the area. Ideally, the ATU and the link to the main antenna should be protected to prevent anyone from touching the feeder.