The document provides information on various bridge equipment used on ships including: - AIS automatically transmits ship information like identification, position, speed to other vessels and coast stations. - Weather facsimile systems receive synoptic charts via radio signals from coastal stations. - Auto pilots control the rudder to maintain a set course using rudder, counter-rudder and yaw controls. - Speed logs like the EMF and Doppler logs measure the ship's speed through and over the water respectively. - GPS uses satellite signals to determine position within 10-15 meters accuracy. DGPS improves this to 3-5 meters. - Radar uses radio pulses to detect targets and their range

1. BRIDGE EQUIPMENTS
AIS:
WEATHER FASCMILE:
ALRS volume 3 gives the time and the
frequencies for the areas where the
stations transmit these signals. All we got
to do is to select the station
frequencies,feed it into the system at the
proper time and a synoptic chart will be
printed.
AIS works on the VHF frequency. The
basic function of the AIS is to transmit
the ships information to other ships and
to the coast stations and also to receive
information from the other ships.
It has 3 kinds of data:

Static data: it is the ships call
sign,IMO number,the length and
the breadth of the ship etc.

Dynamic data: this is the data
which includes the course and the
speed she is doing

Voyage data: this is a data which
needs to be updated every voyage.
This will include,ports of
destination,cargo carried,number
of people onboard etc
AUTO PILOT:
This is used in the open sea’s. It has 4
controls



Rudder control: this is pre-set
earlier. This control,controls the
movement of the rudder to the
pre-set value. Example: if i need to
alter the course from 090 to 120
degrees,and i have my rudder
control set to 10,the rudder will
not move more than 10 degrees to
get my ship to the desired course.
Counter rudder: this is also a preset value. When the desired course
has been achieved,the rudder will
then give a counter rudder to get
the ship back to the desired steady
course
Yaw control: in heavy weather the
auto pilot would have a lot of
wearand tear because of the set
values. This is a pre-set control
and shall not allow the auto pilot
to take over before the set value’s
Example: if iam steering 090 and i have
the yaw control set to 5 degrees....the
auto pilot would not give a rudder
command or the counter rudder
command until the ship’s heading would
change to 085 or to 095degrees.
SPEED LOGS:
EMF log(electro magnetic force log):
this gives speed through water.
This log has a small probe beneath the
ships hull. Electric current is passed
through this probe which generates
magnetic field around it. Water being a
conductor when passes through this ,the
electromagnetic field generates voltage
which is thereby connected to give the
ships speed through water.
Error: the only error that could occur is
that it may not give proper speed as the
boundary layer near to the ships hulls is
not actually moving. To overcome this we
just need to increase the current through
the probe.

2. BRIDGE EQUIPMENTS
DOPPLER log: this gives vessels speed
over the ground. The method used here is
the Doppler shift. This instrument sends
out frequency(pulse) and on receiving the
echo checks how strong the received echo
is. If the frequency (echo) received is
strong then it means that the vessel is
going towards it. Thereby it calculates
speed.
GPS
GPS has 24 satellites and are placed in 6
orbits. The accuracy of the GPS is about
10-15 metres.
Principle: the GPS receiver calculates the
time delays between the transmitted
satellite signals to the signals received by
the receiver. This forms one position line.
Similarly it calculates the time delays
between more satellites to get more such
position lines and the intersection of such
pl’s gives the position.
For 2 dimensional fix,the GPS receiver
calculates such time delays between 3
satellites and gives latitude and longitude.
For 3 dimensional fix,the GPS receiver
calculates such time delays between 3
satellites and gives latitude,longitude and
altitude.
Errors of the GPS as per the ALRS
volume 3


Ionospheric and the tropospheric
errors: these 2 are the layers of
the earths atmosphere. The
satellite signals when they pass
through the earth’s atmosphere
get delayed. This causes the delay
in time at the receiver end.
Signal path errors: the signals of
the GPS satellites reflect several
times before they reach the
receiver and thereby causing the
delay

Receiver clock errors: the receiver
clock may not be a s accurate as
the atomic clock of the satellite.
This can also cause an error.

Geometric dilution of precision:
this basially depends on the
number of satellites visible and the
elevation of the satellites and how
far apart they are placed. Better
the placing of the satellites better
is the angle of cut and better is the
position.
DGPS: accuracy is 3 to 5 mtrs
Here the coastal station comes into play.
The receiver at the coastal station receives
the signals in the same way. Since the
coastal station position is known
accurately the errors are corrected and
the signals are re transmitted further. If
the ships have a DGPS receiver then they
will get theses corrected signals of the
positions and thus the accuracy is
greater.
RADAR(RADIO DETECTION AND
RANGING)
Principle: the radar works on the echo
principle. It sends out a pulse and waits
for the pulse to hit the target and then
return. Thereby it calculates the distance
of the target.
2 types of radars: X-band and the S-band.
X-band : 9GHz,3cm radar
S-band : 3GHz ,10 cm radar
S-band is very good for long range
scanning and is not affected by the rain
or the sea clutter
Beamwidth of the radars:
Vertical beamwidth: 20 to 30 degrees

3. BRIDGE EQUIPMENTS
Horizontal beamwidth: 0 to 2.5 degrees
Pulses: there are 3 types. Short
pulse,medium pulse and the long pulse.
Radars onboard are usually kept on the
medium pulse(normal pulse). If we
require to acquire the targets very close
then we need to lower the pulse length
and if we require to acquire the targets
over a longer distances then we need to
change the pulse length to long pulse. The
only drawback of the long pulse is that if
the targets are very close to each other
then they appear to be as one smeared
target. (Eg): if there are fishing vessels
fishing in close proximity then they would
appear to be one smeared target and will
not be distinctively shown.
Blind sector: it is that portion of the area
on the ship through which the radar
signal will not pass. Blind sectors of the
radar will be displayed on the
specification manuals
To detect the targets beyond the rain
patch:

Use long pulse and increase the
gain as appropriate.
Stabilization modes of the radar
Ground stabilized: in this mode we get
the vessels speed over the ground. (i.e)
from Doppler log or the gps. Since these
mode does not give the vessels speed on
water we cannot use it for collision
avoidance. This stabilization mode is used
when there is any harbour approach as
the officer would want to know the vessels
speed over ground in relation with the
buoys.
O
A
Shadow sector: Radar signals passing
through this sector are very weak
Sea clutter: sea waves reflect echoe’s. This
gets painted on the radar and the targets
get obscured.usage of the sea clutter
controls basically reduces this effect from
the centre of the screen towards the
circumference. Thus helps in detecting
the small targets.
Rain clutter: when the radar energy
passes through the rain the energy is lost
due to the absorption by the water
particles. Poor responding objects behind
the rain may not be detected because of
this and in addition the precipitation
echoes will also be displayed on the
screen.
To detect if any targets within the rain
patch:

Use gain control and rain clutter
control as appropriate

And reduce the pulse
W
In this triangle WO represents the ships
speed(own ships speed) ,OA represents the
relative motion of the target and WA the
true motion of the target. What happens at
sea is the ships are always affected with
the currents and the winds and therefore
the speed of the ship either decreases or
increases..if we have the ground stabilized
mode set on the radar then the radar will
not take into account these changes of the
ships speed and instead will calculate the
targets approach with our own ships speed
over the ground. This will give a wrong

4. BRIDGE EQUIPMENTS
aspect of the approaching target.(explain
the above only if asked why ground
stabilization mode cannot be used for
collision avoidance)
Sea stabilized: this stabilization mode we
have the vessels speed over water and
therefore it is used for the purpose of
collision avoidance.
Question:
Can AIS be used for the purpose of
collision avoidance?
No. Because AIS gets the information feed
of the speed from the GPS which gives the
speed over ground.
Head up (unstabilized mode: )
unstabilzed mode because there would be
always a smear on the screen when the
ship alters. Here there would not be any
gyro inputs and the target vessels would
be on the relative bearings.
In this mode the ships head will always be
pointing up. As we alter course the entire
picture on the screen would move as seen
outside the window.
RADAR START UP PROCEDURE:
 Check scanner (to make sure that
there is no one working there)
The other vessel would get the wrong
aspect of my vessel.
What do you mean by a vector and
trail?
Vector means the path that the target
would follow with time,whereas the trail
means the history of the target.
Check transmitter room ( no
person working in there)

Switch to transmit (wait for
warming up)

Press run

Choose range scale

Choose orientation (head up/north
up/ course up)

Choose mode (relative or true
motion.....head up is always by
default relative motion while north
up or course up can be relative or
true)

If your vessel is on the sea stabilized
and the other vessel has set his radar
on the ground stabilized mode. What
would happen?

Set gain to correct level

Reduce range and set sea clutter

Use rain clutter if raining

Check the tuning of the radar (left
hand green rectangular box)
RADAR ORIENTATIONS:
There are 3 orientations of the radars....
North up(stabilized mode): in this we
would always have the north pointing up
and the picture on the radar screen would
be similar to that what we have on the
chart.
Disadvantage is that it could create a
confusion as to where the target is
especially when we are doing a southerly
course
Course up(stabilized mode):here the
course would always be pointing up. The
circumference of the radar screen would
move for ever.y alteration of the course.
Note: this start up procedure needs to be
told in the same order. It cannot be
interchanged.

5. BRIDGE EQUIPMENTS
ECHO SOUNDER:
The principle of the echo sounder is that
it sends a pulse to the sea bed and
calculates the time required for that pulse
to return,thereby calculating the depth.
Controls on the equipment are:

Range switch: to vary the range

Scale selector: to select the choice
of scale

Gain: to set the gain in order to get
better echo’s

Dimmer
Errors on the echo sounder:

Transmission line error:a zero
depth indication is displayed on
the display screen after sampling a
small portion of the si.gnal. if this
does not occur at zero then a false
reading will be got

Propagation error: the velocity of
the sound in water is 1500m/sec.
This can differ with the water
temperature,depth,salinity and
pressure. These sound waves can
also reflect a lot of other false
bottom layers and give a wrong
depth (could even be big fish)

Aeration : can get false echoe’s due
to the bubbles in the water caused
by the propeller wash

Multiple echoe’s

Pythagoras errors: this is because
the transmitter and the receiver
are separate. In the areas where
the water is not shallow,this error
can be neglected. But in shallow
waters the reading on the display
scale will be greater than the
actual depth which is dangerous.

6. BRIDGE EQUIPMENTS
ECHO SOUNDER:
The principle of the echo sounder is that
it sends a pulse to the sea bed and
calculates the time required for that pulse
to return,thereby calculating the depth.
Controls on the equipment are:

Range switch: to vary the range

Scale selector: to select the choice
of scale

Gain: to set the gain in order to get
better echo’s

Dimmer
Errors on the echo sounder:

Transmission line error:a zero
depth indication is displayed on
the display screen after sampling a
small portion of the si.gnal. if this
does not occur at zero then a false
reading will be got

Propagation error: the velocity of
the sound in water is 1500m/sec.
This can differ with the water
temperature,depth,salinity and
pressure. These sound waves can
also reflect a lot of other false
bottom layers and give a wrong
depth (could even be big fish)

Aeration : can get false echoe’s due
to the bubbles in the water caused
by the propeller wash

Multiple echoe’s

Pythagoras errors: this is because
the transmitter and the receiver
are separate. In the areas where
the water is not shallow,this error
can be neglected. But in shallow
waters the reading on the display
scale will be greater than the
actual depth which is dangerous.