you can please download GMDSS CLASSES VIDEOS from YOUTUBE
key word: GMDSS VASU
MODULATION
What is DSB? What is SSB? What is USB? What is LSB?
DSB – Double Side
Band
SSB- Single Side
Band
USB-Upper Side
Band (this is SSB) –Marine Side Band
LSB- Lower Side
band (this is SSB)
What is Modulation?
What is Demodulation?
Intelligence
(AF) is super imposed on carrier (RF) .
This is called Modulation
This is done in
transmission (transmitter)
Intelligence is
separated from RF carrier wave. This is called Demodulation.
This is done in
receiver.
What is Amplitude modulation?
Amplitude of the
carrier wave is modulated as per the strength of AF signals.
Frequency is
constant.
What is Frequency Modulation?
Frequency of the
carrier wave is modulated as per the strength of AF signals.
Amplitude is
constant.
What is Phase modulation?
Angle of the
carrier wave is modulated as per the strength of AF signals.
What is the bandwidth of NBDP and VOICE?
NBDP = 170 Hz VOICE = 3 KHz
Propagation
What is propagation?
Radio wave travel
over the space/above earth
What type of propagation is used for MF communication?
Ground wave
propagation
What type of propagation is used for HF communication?
Sky wave
propagation
What type of propagation is used for VHF
communication?
Line of sight
(direct wave propagation)
Explain Ground wave propagation.
MF Radio waves
travel along the surface of the earth. (Follow the surface of the
Earth) This is called ground wave propagation
Explain sky wave propagation.
HF Radio waves
travel towards the ionospheric layers and reflect back to earth
And again refract
back to ionospheric layers and reflect back to earth and so on.
HF Radio waves
travel for a long distance with the help of ionospheric
Layers
Explain line of sight/direct wave propagation.
VHF radio waves
travel for a short distance
What is skip distance?
The distance
between the transmitting point and the first reflecting point is called skip
distance.
What is skip zone?
No reception of
signals area is skip zone.
What is the structure of Ionosphere?
Day time, there
are four layers
50 – 95 km above
the earth - D layer
90- 150 km above
the earth – E layer
150 - 200 km above
the earth – F1 layer
250 – 450 km above
the earth – F2 layer
Night time, it
became two layers E layer and F layer
What is attenuation?
Loss of energy in
radio waves
What is ducting?
This happens in
VHF communication.
What is MUF?
Maximum Usable
Frequency – The highest frequency used for communication
Between two
stations at a given angle of incidence
What is OUF?
Optimum Usable
Frequency – 85% of MUF
What is the length of aerial is required for
transmission?
It should be half
of the wave length of the frequency which is tuned
AERIALS
Where are the aerials installed?
As high as
possible on the superstructure
In the ship, on the monkey island
Types of
aerials:
Old type: L or T
type aerial= fitted on the masts
MF/HF aerial –
long wire aerial
WHIP aerial –
flexible vertical antenna
Dipole aerial –
short vertical half wave aerial
Yagi antenna-
multi element VHF aerial/ Directional aerial/For Tv receiving ( old type)
Active aerial –
Built in amplifier at its base – NAVTEX receiver
Parabolic antenna
- for SAT-B and Fleet 77
MF/HF receiver aerial – 6 – 8 meters
Mf/HF Transmitter aerial – 8 – 10 meters
f = frequency in
Hertz
l = wave length in
meters
f x l
= 3 x 108
Wave length is
inversely proportional to the frequency
Aerial Tuning Unit
ATU change the
electrical length of the antenna for every transmission.
ATU consists of a
no.of.capacitors, a no.of.Inductors, a small
motor and a micro processor
What is Transceiver?
A combined unit of
transmitter and receiver
What is sensitivity?
Sensitivity is the
ability of receive to receive the weak signal of the weakest signal
What is selectivity?
Selectivity is the
ability of a receiver to receive the tuned frequency and push off the nearby
frequency
What is stability?
What is fidelity?
Fidelity is the
ability of a receive the transmitted signal without any distortion
What is fading? How to overcome fading?
It occurs in HF
communication. When the radio waves travel for a long distance, there will be a
lot of fluctuations in the incoming signal at receiver.
Sometimes signal
level will go down. Sometimes it is steady.
No constant output in the receiver. This is called fading
AGC is used to
overcome the fading.
By using AGC,
there will be a constant output in the receiver.
What is the power supply to the Transceiver?
Main supply 110v
ac/220 v ac
Battery supply 24v
DC
What is the power source to the GMDSS equipments?
Ship’s main,
What is the reserve source of power supply?
Battery
If main supply is failure, what is the additional
power source?
Battery and
emergency generator
EIRP – Electrically ISOTROPIC Radiated
power
Decibel
watts (dbw) – emissions
10
dbw = 10 w
20
dbw = 100 w
30
dbw = 1000 w
Frequency bandwidth
3
– 30 KHz \VLF –VERY LOW FREQUENCY -
MYRIAMETRIC WAVES
30
– 300 KHz LF – LOW FREQUENCY –
KILOMETRIC WAVES
300
– 3000 KHz MF – MEDIUM FREQUENCY –
HECTOMETRIC WAVES
3
– 30 MHz HF – HIGH
FREQUENCY - DECAMETRIC WAVES
30
– 300 MHz VHF – VERY HIGH
FREQUENCY – METRIC WAVES
300
– 3000 MHz UHF – ULTRA HIGH
FREQUENCY – DECI METRIC WAVES
3
GHz – 30 GHz SHE – CENTIMETRIC
WAAVES
30
GHz – 300 GHz EHF – MILLIMETRIC WAVES
AIR
frequency
720
KHz
Rx.
Tune : 722.000 KHz – 720.120 KHz
What is the range of communication
possible in MF ( 1.6 MHz to 4 MHz)?
More
than 150 NM in day time
What are the HF marine bands?
4
MHz, 6 MHz, 8 MHz, 12 MHz, 16 MHz and 22 MHz
How much time it takes to power ON
(pressing on/off button) the MF HF equipment?
4
seconds
Power output levels?
Low
power 20 watts and high power 250 watts.
Permitted level 400 watts
(In
DSC and TELEX, automatically it is on high power)
What are the details appear in the
display?
MF HF transceiver carriage requirement
MF HF SCANNING
UP TO 10 CHANNELS
ONE
CHANNEL SCANNING PER 2 SECONDS
MULTIWATCH
1
DSC FREQUENCY AND UPTO 10 SSB CHANNELS
DUAL
WATCH
DSC
FREQUENCY and CURRENCT VOICE(TELEPHONY) FREQUENCY
SCANNING
– EACH 2 SECONDS
DSC WATCH
UPTO
6 DSC FREQUENCIES
SCANNING
- EACH 2 SECONDS
