ORALS

1 | P a g e O r a l s N a v i g a t i o n a n d W a t c h k e e p i n g
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Q. Why are GPS satellites positioned in orbits 20,200 km above earth’s surface?
A. This height of orbits falls within range of orbit heights classified as medium earth orbits. This
height is chosen for GPS satellites for following reasons:
1. Footprint of satellites is large enough to provide global coverage,
2. Because of larger footprints of satellites, the total number of satellites is lesser than would be
needed if satellites were to orbit at lower heights,
3. This height is low enough for earth based transmitters/receivers to have modest sized antennas
and use lower transmission powers for the system to be used effectively.
Q. What is the difference between gross tonnage and net tonnage?
A. Tonnage in general refers to capacity or size of a ship.
Gross tonnage is a function of volume of enclosed spaces of a ship. It is indicative of ship’s size.
Net tonnage refers to volume of cargo carrying spaces. It is indicative of vessel’s earning potential.
The NT cannot be less than 30% of the GT of a ship. Net tonnage is used for ship’s dues.
Both GT and NT are determined by measuring ship’s volume and then applying a mathematical
formula. Both GT and NT are dimensionless numbers and are shown in ship’s International
Tonnage Certificate. They do not have any physical units and should not be confused with unit of
mass, namely ton.
Q. What do you understand by GRT and NRT?
A. GRT stands for Gross Register Tonnage and NRT means NET Register Tonnage. Both these
terms are now obsolete and have been replaced respectively with GT and NT under International
Convention on Tonnage Measurements of Ships.
However, students may note the definitions of GRT and NRT for their reference:
Gross Register Tonnage (GRT) meant a measure of the total internal capacity of the ship. It
consisted of: under-deck volume excluding double-bottoms, volume of tween deck spaces, volume
of superstructures, volume of deck-houses etc. Spaces like navigational areas, galleys, stairways,
light and air spaces were exempted. The total volume thus calculated in cubic feet was divided by
100 (1 gross ton = 100 cubic feet). This was the Gross Tonnage entered in the ship's Register.
Net Register Tonnage (NRT) meant a measure of the available space for the carriage of cargo
and passengers. This was obtained from GRT after making some deductions. These deductions
from GRT included: Master and crew accommodation, safety and storage spaces, water ballast
tanks, allowance for propelling machinery. Again the resulting volume in cubic feet was divided
by 100 (1 net ton = 100 cubic feet). This was the Net Tonnage entered in the Register.
Q. Which radar should be used for long range scanning?
A. 10 cm or S-band radar is better for long range scanning.
Q. Which radar should be used in heavy rain?

A. 10 cm or S-band radar should be definitely on and used in rain, because 10 cm radar gives less
rain clutter. However, in situations like this, it is preferable to keep both radars on and use the
advantage of 10 cm radar accordingly.
Q. How will you ascertain the performance of your radar?
A. The performance of radar can be ascertained with the help of performance monitor.
Q. What is parallel indexing and what is its principle?
A. Parallel indexing is a technique of executing and monitoring ship’s motion along a pre-
determined charted track. While keeping the radar screen under observation it provides instantly
ship’s position with respect to the charted track. It fills the gap between two consecutive plotted
positions and allows for the off-track deviation to be treated well in time. Though the technique is
termed as continuous fixing, it does not absolve OOW of duties of fixing vessel’s position at
regular intervals.
Parallel indexing works on the principle of relative motion, in which the echo of a ground-based
(fixed) target moves across radar display at a speed and in direction which is exact reciprocal of
ship’s motion over ground. The target chosen should be isolated, radar-conspicuous and positively
identified.
Q. Which mode of radar motion should be used for parallel indexing?
A. Both relative motion (RM) and true motion (TM) are equally suitable for parallel indexing.
Q. Radars are interfaced with speed logs as well as GPS. GPS gives SOG, while log is intended to
provide STW. How will you decide which speed to use and when?
A. I will use STW for applying ROR i.e. for taking decisions on collision avoidance. This means to
say that my radar will be switched to STW mode, when at sea.
But, SOG also has its own functions. It is very useful, because it shows the CMG vector as well on
radar screen. So I can use SOG mode when navigating in restricted waters dotted with shoals,
islets, rocks etc. and also while negotiating turns in proximity of hazards.
Q. There is some interface trouble between your radar and speed log and you are unable to get the
STW input on radar. What will you do?
A. I will enter the STW manually by using the manual option on radar and navigate accordingly.
Q. What do you understand by EPA and ATA?
A. Both EPA and ATA are radar plotting aids. EPA stands for electronic plotting aid and ATA
stands for automatic tracking aid.
Q. How are they different from each other?
A. In EPA each target needs to be plotted manually and EPA gives target data for each manual
plot. It is the simplest form of advancement in automation, in which the triangle OAW is solved
electronically. EPA is required to be fitted on all ships of 300 GT and upwards and passenger ships
irrespective of size.

In ATA target is acquired manually and plotting is automatic. This is the next step ahead where
target once acquired manually is tracked automatically. All ships of 500 GT and above are required
to be fitted with ATA.
Q. What are the minimum parameters of a radar plot?
A. The seven minimum parameters are:
Target identity, bearing, distance, CPA, TCPA, target course and target speed.
Q. Is there any other type of plotting aid?
A. Yes. It is known as automatic radar plotting aid (ARPA) and it is required to fitted on all ships
of 10,000 GT and above.
Q. As a cadet you have sailed on ships upwards of 10,000 GT. You are about to join as 3rd
mate a
ship whose GT is less than 10,000 and it is not fitted with ARPA. What does it mean to you as an
OOW?
A. It means that the ship will be fitted with ATA. I will be aware of the limitations of ATA as
compared to ARPA. ATA does not provide functions like automatic acquisition and plotting of at
least 20 targets, trial maneuver etc. available on any ARPA. I will make myself fully familiar with
these limitations and operational procedures by referring to the operator’s manual and carry out
navigational duties accordingly.
Q. What are the two additional navigational equipments required to be fitted on ships of 50,000
GT and upwards?
A. They are:
1. Rate of turn indicator (ROTI) to determine and indicate the rate of turn for facilitating the
execution of curved segments of planned passage.
2. Speed and distance measuring equipment to determine and indicate speed of ship over ground
(SOG) in the forward and athwartships direction. This requirement is usually met by fitting
Doppler speed log.
Q. What do you understand by wind rose?
A. Wind rose is graphical depiction of wind data as regard to direction and force of wind likely to
be encountered in an area, in a particular period or month. With the help of the wind rose, it can be
known at a glance the likelihood of encountering wind from a particular direction at a given force.
Wind roses are shown in red on routeing charts, in the form of a circle having arrows pointing
radially towards centre. The arrows fly with the wind and this fact provides the direction. The
length of the arrow indicates the percentage frequency of occurrence on the scale provided on the
chart (2inches equal 100%). The thickness of the arrow indicates the wind force. The arrows cross
the circle by an amount which equals 5% frequency and provides a quick estimation.
The circle also encloses three figures placed one above the other. The top figure represents the total
number of observations. Middle figure represents the %age frequency of variable winds
encountered. And, the bottom figure is indicative of %age frequency of calms.

.
Wind rose as shown on a routeing chart
Q. What is fog detector light?
A. Fog detector light is a light which is fitted on structure of light station. It may, however, be
fitted some distance away from the station. Fog detector light detects fog and switches on fog
signals automatically. It may also transmit the range of visibility to a data center, which in turn
broadcasts the information to mariners. Fog detector lights operate both by day and night.
Q. What is the difference between ‘aid to navigation’ and ‘navigational aid’?
A. ‘Aid to navigation’ is a mechanism or arrangement designed, established and operated
externally to ship in order to enhance navigational safety of marine traffic. For example: beacons,
buoys, light houses, leading lights etc.
‘Navigational aid’ means a gadget, instrument or equipment internal to ship and is intended to
assist navigation. For example: echo sounder, radar, AIS, chart, compass etc.
Q. What kind of information is available from routeing charts?
A. Routeing charts provide following categories of information and play significant role in passage
planning:
1. Wind direction, strength and probability of encountering (%age frequency) same in a particular
area. This is done in the form of wind roses. Wind rose also gives %age frequency of variable
winds and calm seas.
2. Direction and strength of sea currents with %age constancy in quadrants and octants.
3. Shipping routes and distances in nautical miles between ports.
4. Mean air pressure
5. Mean air temperature
6. Mean sea water temperature
7. Mean dew point temperature

8. Percentage frequency of fog occurrence (visibility < 0.5 M)
9. Percentage frequency of low visibility (visibility < 5.0 M)
10.Percentage frequency of winds of force 7 and above
11.Selected tracks of TRS experienced in the past years in the area
12.ILL zones
13.Ice limits
Q. What is the difference between ‘observation spot’ and benchmark?
A. Observation spot represents a position on chart, where accurate astronomical observations have
been made to obtain latitude and longitude. On charts, it is shown as:
Benchmark is a mark the height of which above a known datum is accurately and exactly known. It
may consist of bolt head or a rivet fitted into concrete or an arrow mark cut into masonry.
On charts, it is shown as:
Q. What is the difference between rectilinear stream and rotary stream?
A. Rectilinear stream is that type of tidal stream which travels almost in straight line alternately in
opposing directions separated by a period of slack water.
Rotary stream is that tidal stream whose direction changes gradually through 3600
completing a tidal cycle.
The change in direction may be in clockwise or counter clockwise direction.
Q. Compare pebbles cobbles and boulders?
A. All three are water rounded stones and are named according to their respective sizes as follows:
Pebbles: 4 mm – 64 mm
Cobbles: 64 mm – 256 mm
Boulders: > 256 mm
Q. What do you understand by ‘situational awareness’?
A. Situational awareness comprises three components in a situation:
a. Perception (hearing, seeing, feeling etc.) of technical and/or navigational information available to
OOW.
b. Understanding unambiguously the meaning of such information.
c. Development of the present status in the near future and the adequate preparedness for the effective
handling of the development.
Q. In the context of ECDIS, BNWAS etc., what do you understand by ‘single operator action’?
A. ‘Single operator action’ means procedure attained by ONLY one hard-key or soft-key action. There is
no involvement of any voice actuation using programmed codes or cursor movements.
Q. Why is moon not used for taking amplitude?
A. Amplitude is taken at the time of geometric rise or set of a celestial body. This occurs when the centre
of body coincides with rational horizon. However, all the bodies except the moon appear well above the
horizon due to the influence of refraction.

Refraction elevates the moon as well, but this effect is outweighed by the depressing effect of horizontal
parallax. As a result moon remains invisible at the time of its geometric rise or set and its amplitude cannot
be calculated.
Q. What is the objective and importance of day light signaling lamp?
A. The objective of day light signaling lamp is to ensure, in suitable and reliable manner, conveyance of
information between ships or between ship and shore by light signals, both by day and night.
Carriage of day light signaling lamp is important for safety reasons. It is necessary for signaling in different
situations as required by:
 COLREGS
 IAMSAR and
 International Code of Signals
Q. What is the least visibility of light emitted by day light signaling lamp, under normal atmospheric
conditions?
A. Two nautical miles.
Q. What are the power requirements for the signaling lamp?
A. The signaling lamp is not to be dependent only on ship’s main or emergency supply of electricity. In
addition, it should be provided with a portable battery having sufficient capacity to ensure the lamp’s
operation for a period of not less than 2 hours.
Q. What are the requirements of day light signaling lamp’s bulb?
A. The lamp should be designed such as will allow the replacement of bulb even in dark. The use of
screwed sockets should be avoided.
At least three spare type-approved bulbs are required to be provided with the signaling lamp.

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