RADAR
RADAR, or
Radio Detection and Ranging, is one of the most useful electronic
navigation aids carried aboard all vessels. This instrument
provides information which, if interpreted correctly, can prevent
collision, determine position and can be used to tell the course
and speed of other vessels in the area.
RADAR sets transmit brief pulses of radio waves, which travel
outward in almost straight lines. When these waves strike a
reflecting surface, like another vessel or the shoreline, some
of the energy is returned to the receiver. By measuring the
time difference between the transmission and receipt of these
waves, the RADAR set can calculate an object’s distance
or Range. It also notes the Relative Direction of the object
and graphically plots this information on the screen.
RADAR signals travel along the “line of sight”. The
theoretical maximum range of RADAR depends on the height of
the RADAR scanner and the height of the target. The RADAR horizon
is actually slightly farther than the geographical or visual
horizon because atmospheric conditions can bend the RADAR waves
over the horizon.
The screen is set as a Plan Position Indicator (PPI) where the
vessel is at the center of the screen and the ship’s heading
faces up, whatever direction it might be. Range Rings, or circles
of known diameter, are used for quickly measuring the distance.
The target echo appears as a large blob. The edge of the echo
that is closest to own vessel should be considered the actual
position of the target. As the RADAR scanner is constantly rotating
the navigator can get a 3600 picture of what is going on around.
Automated
Radar Plotting Aid (ARPA)
ARPA is
an acronym for Automatic Radar Plotting Aid. It is a sophisticated
piece of navigation equipment that combines a traditional radar
display with a computer driven plotting device. The system receives
target information input from the ship's radar system, and own
ship course and speed information from the ship's electronic
navigation equipment or from manual inputs from the watch officer.
After either manual or automatic acquisition of radar targets
the ARPA will compute and display a variety of information useful
to the navigator in making collision avoidance and navigational
decisions. It is essential when navigating in areas of high
traffic density such as the Straits of Mollucca or the English
Channel, especially in restricted visibility. ARPA systems are
presently required on most merchant vessels over ten thousand
gross tons.
All ARPA models display basic collision avoidance data: CPA
(closest point of approach), TCPA (time to CPA), range (distance)
and bearing (direction) to the target of interest, and target
true and relative course and speed. The most advanced models
will allow a nautical chart of the area being transited to be
electronically overlayed upon the radar display so that by looking
at one screen the navigator can see where he is with respect
to the charted channel as well as view stationary and moving
targets that may pose a threat to his planned navigation.
GPS
Originally
created for the US national defense, GPS is now available to
all of us. It is a satellite-based radionavigation system that
works 24 hours a day, in any weather, in any visibility, and
to high levels of accuracy and many receivers available today
are capable of providing several observations per second. With
minimal training, users can find their exact position, determine
course and speed, local variation, time to destination, and
cross-track error (how far off course you are). They can also
keep a library of pre determined positions (way points), and
can determine the bearing and range between any two way points.
Next the satellite transmits date and time data as well as orbital
data for ALL of the NAVSTAR satellites.
Now GPS has 24 operational satellites, "man-made stars",
plus several back ups which orbit to within 60° N and 60°
S latitudes. Their flight paths, together with the high altitude
of the satellites, allow GPS signals to reach any point on Earth.
The GPS Navigator displays basic steering and distance to the
next WPT (waypoint - a definite point along the vessel's track
whose coordinates may be stored. A destination waypoint's position
is the basis for heading, range, steering and ETA calculations).
BRG (bearing), COG (course over ground), UTC, TWA (true wind
angle), TWD (direction), TWS (speed), drift, depth, etc. are
also displayed. The displays in the Plotter give graphical presentations
of the vessel's movement, sailplan, track and other information.
It shows the actual sailplan the navigator is executing, the
waypoints in the sailplan, the number of remaining waypoints,
distances and ETA's. An Optimize Performance Monitor helps you
improve your sailing efficiency and fuel economy.
To find position, the GPS instrument takes the satellite data,
determines the exact time and calculates the exact predicted
position of the satellite. It measures the amount of time that
the satellite's signal takes to reach the instrument and calculates
the distance to the satellite. By doing this with at least three
different satellites, a 2 dimensional fix (latitude and longitude)
can be triangulated by the GPS R/C. With data from 4 or more
satellites, a 3 dimensional fix can be obtained showing lat/long
as well as altitude. The positional variation is from 0 to 300
feet. Where a greater accuracy is needed, Differential GPS (DGPS)
technology can achieve an accuracy of within 3 metres or better.
Differential
GPS is an extension of the GPS system that uses land based radio
beacons to transmit position corrections to GPS receivers. It
involves the cooperation of two receivers, one that's stationary
and another that's roving around making position measurements.
The stationary receiver is the key. It ties all the satellite
measurements into a solid local reference. Some of the limitations
of DGPS are the added cost of the equipment and the limited
coverage area.
Electronic
chart systems, which only became a reality within the last ten
years, have emerged as a new navigation aid that should result
in significant benefits to maritime navigation, safety, and
commerce. They consist of a computer based video display screen
on which charted areas of the world, coasts, harbors, and rivers
may be displayed. More than simply a graphic display, EC Systems
combine both geographical and textual data into a readily useful
operational tool. As an automated decision aid capable of continuously
determining a vessel’s position in relation to land, charted
objects, aids to navigation, and unseen hazards, ECs can be
a real-time navigation system that integrates a variety of information
that is displayed and interpreted by the Mariner.
There are two basic types of electronic charts in terms of system
components, features and functional stability. The most advanced
form of electronic chart is the Electronic Chart Display and
Information System (ECDIS). All other types of EC can be regarded
as Electronic Chart Systems (ECS).
As defined in the IMO Performance Standard, ECDIS is a “navigation
information system which with adequate back-up arrangements
can be accepted as complying with the up-to-date chart required
by Regulation V, Chapter 20 of the 1974 SOLAS Convention.”
By displaying selected information from an electronic navigational
chart (ENC) and positional information from navigation sensors
ECDIS should assist the mariner in route planning and route
monitoring, and if required, display additional navigation-related
information.” ECDIS is an automated decision aid, capable
of continuously determining a vessel’s position in relation
to land, charted objects, aids-to-navigation, and unseen hazards.
As a real-time navigation system that integrates a variety of
information from a variety of sources, ECDIS will lead to dramatic
changes in the type of navigation that can be performed on the
bridge of a ship by mariners in crowded or unfamiliar waters,
or during periods of reduced visibility.
This general category can be sub-divided into electronic charts
that use either raster or vector data. In a vector-based system.
Electronic chart data is composed of: a series of lines (vectors)
in which different layers of information may be stored or displayed.
With vector ECS, the user has some flexibility in the amount
of information that is displayed. For raster-based systems,
the data is stored as picture elements (pixels). Each pixel
is a minute component of the chart image with a defined colour
and brightness level. Raster-scanned images result in a computer
photograph of existing paper charts. Although a raster ECS does
not have the same intelligence” as a vector ECS, they are
easier to produce.
Electronic charts represent a revolutionary advance in maritime
navigation and safety. ECDIS offers enormous potential to improve
the safety and efficiency of maritime navigation. Since ECDIS
is capable of continuously displaying own ship’s position
on the electronic chart, there is increased benefit in having
other real-time information available that can be used to increase
the safety and efficiency of the voyage. Timely information
on water levels and current flow can be of significant benefit
to a mariner in terms of optimising the timing of vessel transits,
or the amount of vessel cargo loading. However it should be
noted that none of these units or their chart databases are
IMO or USCG approved for replacing the existing paper chart
systems required aboard commercial vessels.
Automatic
pilot, also called Autopilot, or Autohelmsman, is a device for
controlling a ship or other vehicle without constant human intervention.
Automatic pilots consist of four major elements: (1) a source
of steering commands (such as a computerized guidance program
or a radio receiver), (2) motion and position sensors (such
as gyroscopes, accelerometers, altimeters, and airspeed indicators),
(3) a computer to compare the parameters specified in the guidance
program with the vessel’s actual position and motion, and
(4) servomotors that actuate the ship’s engines to alter
its course when corrections or changes are required.
Gyrocompass
and Repeaters
All ships
are outfitted with a master gyrocompass which indicates the
true north. Connected to the master compass are numerous gyro
repeaters which are strategically placed in order to properly
navigate the vessel. For example, repeaters are located at the
vessel's helm/steering stand(s), on each bridge wing, at the
after or emergency steering stand, and any other location as
deemed necessary by the vessel’s master and owners.
ELECTRONIC
CHARTING
1. The advent
of the electronic chart with global positioning system (GPS)
input offers significant benefits in advancing the accuracy
and safety of navigation at sea. Such advantages will also bring
with them benefits of reduced ship operating costs.
There are two distinct categories of digital charts available:
the raster chart which is simply a scanned image of a paper
chart and the vector chart, where each element or feature of
navigational data is geographically referenced and has its own
attributes e.g. a buoy and a colour. The electronic navigational
chart (ENC) is a vector chart produced to a definite standard.
The raster chart is relatively simple technically and widely
available. It presents features exactly as they are seen on
the paper version and thus has the greatest advantage of familiarity
of presentation to the mariner. Raster charts are excellent
tools for passage planning and pre-planned routes can automatically
be made available in all chart scales covering a given area.
GPS will be the normal means of plotting position with a back
up of gyro course and log speed input together with radar or
visual fixing should the GPS fall.
2. ENCs
ENCs can be considered as geographic object-oriented databases
of each equivalent chart area. They possess flexibility in terms
of choice of what is displayed, thus enabling the navigator
to benefit from an uncluttered display, providing him, if desired,
with the minimum of information on which to conduct safe navigation.
The mariner is able to select the use of data from the ENC to
meet the safe navigational needs of his own particular vessel,
allowing for its draft and the requisite under-keel clearance.
The ENC thus can be used to clearly indicate the navigable water
for any given vessel, and lends itself to being utilized to
provide automatic warnings of impending danger.
ENC updates, using CD-Rom or telecomm channels, combine with
the baseline ENC data in the ECDIS to create the composite updated
picture. The intention is the ENCs will include temporal data
such as tides, currents and ice limits.
ENCs must be seen as providing the navigating system of the
future and in many areas of the world raster will be only an
interim system in the short or medium term. Since the presentation
and the layout of the ENCs are unfamiliar this underlines the
need for training.
3. Chart
accuracy
As the ENCs are based upon the vector capture of existing paper
chart editions they may replicate their limited accuracy. The
mariner will have to be aware that electronic charting does
not equate automatically to charts containing data to full modern
survey standards. Another consideration s that the accuracy
of the GPS/DGPS may well exceed the positional accuracy of the
charted data and due allowance have to made for this in navigation
using ENC. Absolute reliance upon GPS with no relative reference
to land or other features through radar, echo-sounder or visual
fixing, risks calamity should there be any significant error
in the positioning of the vector data of a hazard.
4. Benefits
of ENCs
The following are the most significant benefits of digital charts
for the mariner:
-Safer and easier navigation
-The ability to integrate the navigation function with collision
avoidance radar display.
-More accurate navigation, leading to better passage planning
and the use of less fuel.
-The availability of watchkeeping effort to contribute toward
the general ship management functions
-Instantaneous and more accurate chart updating
-The ability to receive chart information by telecomm, thus
minimising the number of charts which need to be carried
5. Disadvantages
of ENCs
-Digital charts are more expensive and require investment in
both equipment and the purchase of digital versions
-Special mariner/operator training will be required. This crucial
to successful transition from paper to digital
-Back-up charting facilities will be necessary, to guard against
system failure.
6.Legislative
background
In 1998 the IMO set the performance standards required for ECDIS.
IN 2000 there were made IMO amendments to these standards, which
permit ECDIS to operate in a raster mode using (hydrographic
office produced) raster charts, where ENCs are not available
in order to encourage familiarization with digital versions.
The task is the hydrographic offices to develop their ENCs together
with the requisite reliable update service. No doubt there clearly
will be a steady growth in the number of approved ECDIS-fitted
vessels in the coming years.
