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where d = distance to objectin (NM); h
e = height of
the eye of the observer; h
o = height of the observed
object; and k = constant calculated from the Earthʹs
mean radius and distance conversion in nautical
miles,k=1.9274.
Sincetheseahorizonisgreaterthangeometric,it
canbe stated thatk = 2. Thisleads to final distance
determination(Koset
al.2010):
2
eo
dhh (2)
Although aged, described positioning method
providesthenavigatorwithazimuth(directionline)
and distance (circle), the same lines of position on
whichcontemporarycoastalnavigationrelieson.
One of the first and sustained positioning
methodsis DeadReckoning(DR).In thismethod,a
knownposition is advanced on the
basis of vessel’s
course and distance prevailed regarding vessel’s
speed.Thismethodcanbeemployedalways,under
condition that there is one known position, known
course and known speed of the vessel. DR method
developed from manual plotting on navigational
charts to sophisticated methods embedded in
navigational devices, by employing various
differential and other algorithms. It can provide
reasonable accuracy, however it becomes fairly
inaccurate over time (Nav 2013). An Estimated
Position (EP) can be viewed as DR upgrade, where
outereffectson vessel’scourse and speed are taken
intoconsideration(e.g.setanddriftandotherleeway
effects), providing corrected
Course Over Ground
(COG) and Speed Over Ground (SOG) (Nav 2013,
Bowditch2002).
Celestial navigation positioning methods are not
reliantonanyelectronicsystems,andthepositionin
dueaccuracy(≤1NM)canbeobtainedbyusingthe
sextant, compass and nautical almanacsolely. Lines
ofposition(azimuthanddistances
again)arederived
by known locations and movements of celestial
bodies–stars,planets–whichareactingasreference
objects. Astronomical positioning methods are still
valid check in open sea navigation. However, with
the rise of new technologies and real‐time
positioning, there appears a possibility that these
methods
willslowlygointooblivion.Thenumberof
vessels where sextant is not anymore obliged
increases, distancing celestial navigation methods
fromthecommonusage.
Radar represents an enhanced eye of the
navigator, providing visibility in different parts of
the frequency spectrum. Vessel’s position can be
obtained in several ways. LOPs from
reference
objects can be derived thusmanually providing the
position.Inanautomatedway(EchoReference‐ER),
the reliable object in vessel’s vicinity can be used,
providingvessel’scontinuouspositionrelativetothe
object.Radarisautonomousdevice,meaningthat it
is not dependent on any other source except
electricity.
Even in the case of connected sources
failure (e.g. heading and speed sources) it can be
used with satisfying positional accuracy (e.g. in
RelativeMotionmodeandHeadUporiented).Radar
positioningisconfinedtocoastalnavigation,whilein
opensearemainsaprimarycollisionavoidancetool.
As for (and not
only) ocean navigation, satellite
positioning provides the navigator continuous
service of positioning, navigation and velocity
determination, as well as time standard service
provision (Parkinson & Spilker, Jr. 1996, IS‐GPS
2013). Among all GNSSs, the Global Positioning
System (GPS) is most used, fully operable system.
Satellitenavigation isembedded in
various systems
which are based on its services. In relevant
navigational equipment, such as Automatic
IdentificationSystem(AIS)andECDIS,GPSnotonly
provides its services, but acts as anintegrated part.
With the development of multi‐frequency satellite
receivers (either employing several dedicated
frequencies from one GNSS system, or employing
various GNSS system’s frequencies) and local and
global differential services, satellite positioning
services are reaching great levels of accuracy and
reliability; however they are, as any other system,
susceptible and vulnerable to effects of number of
externalcauses(Chapter5).
A number of supplementary positioning and
situationalmethodscanbe
usedinordertoobtaina
positionandinterpretnavigationalsituation.Someof
them can be used as direct positioning means (e.g.
InertialNavigationSystems,orhyperbolicnavigation
systems where available) while other are used as
complementary navigation tools (e.g. Echo
Sounders).
This short positioning overview was given in
order to
present the choices and possibilities OOW
canemployinordertoobtainaposition,butwhatis
more important, to provide the OOW with
supplementary positioning means in redundancy
terms. Emphasizing the need for secondary
positioning source, every position has to be double
checked or multi checked. Moreover, theoretical
knowledge
ofpositional erroranduncertainty areas
isessential.
3 ECDIS
ECDISsystemunitscanbedividedinhardwareand
softwarecomponents,uninterruptablepowersupply,
official/updated databases (electronic charts) and
required sensor ports formandatory and additional
navigation and aiding devices (IMO MSC 232(82)
2006, IMO A 817(19)1995). Thesedevices in
ECDIS
contextactasasensors,and they are not treated as
standalone devices anymore. This is important fact.
Once the regulated requirements are fulfilled
(SOLAS 1974, IMO MSC 232(82), IMO A 817(19),
IMOA694(17),IMOMSC191(79),IHOSPS‐662010,
IMO SN/Circ. 248), the system can
be ‘…accepted as
complying with the up‐to‐date chart’. This fact
determines further way of understanding and
conceptionofECDISasaprimary navigationmean,
butentailsnumberofconsequences.
3.1 Generalreview
According to (IMO MSC. 232(82), IMO A. 817(19)
1995) performance standards, at least three
mandatory devices
should be connected in ECDIS;