337
1 INTRODUCTION
Maritime transport is an important branch of global
economy.Italsocreatesahighriskofenvironmental
pollution because a ship can carry at one time even
more than a hundred thousand tons of hazardous
goods. Therefore, there are a lot of measures
undertaken to prevent ecological disaster caused by
ships.A large ship, beca
use of the huge inertia,
must start a maneuver a few miles in advance,
whereas a small sailing yacht will perform any
maneuverinplace;stillitdependsonthewindforce.
Therefore,thesafetyofsmallcraftsoftendependson
their detection by other vessels. The officer on the
bridge(ataheightof10to30m)isusuallyaloneand
ap
art from observing the vicinity at the same time
he/sheoperatesalotofotherequipment.Ayachtcan
hardlybeseenanditmuchdependsonitslights.The
basisoftheobservationistomonitoraradarimage.
Basicradaroperatesinarangescaleof3cmandthe
vesselsofmoretha
n3000,inaddition,canhaveradar
operatedinarangescaleof10cm.Thelatterdetects
smallcraftsatshorterrangesbutitismoreresistantto
hydro‐meteorologicaldisturbances.
2 RADARDETECTIONOFYACHTECHOES
We have been carrying out radar observations of
small crafts in the Maritime University for ma
ny
years. The observations show that the possibility of
detection of reflected signals from these targets is
limited. The interferenceofradar signal from the
seasurfacetothedirectsignalcausesfluct
uationsand
echo dropouts, and scattered from the sea surface
echo signal masks all the other echoes around their
ownposition.Dependingontheheightoftheantenna
andthe stateoftheseathisrangecanreachupto4
nauticalmiles.Precipit
ationmaymasktheechointhe
whole range of observation. Interference can be
minimized by using a software analyzing visual
signals,or improving the reflective properties ofthe
observed objects, for example. by the use of radar
Influence of Automatic Identification System on
Safety of Navigation at Sea
T.Stupak
GdyniaMaritimeUniversity,Gdynia,Poland
ABSTRACT:Detectionofsmallcraft
s/targets,inparticular,sailingyachtswiththeshipʹsradarisnotalways
possible. Radar reflectors are used to improve their detection. The AIS (Automatic Identification System)
transmitspositioninWGS84geodesysystem,motionvectorandidentificationsignalsofavesseltootherships
and VTS (Vessel Traffic Services) centers. This system significant
ly increases the possibility to detect small
crafts.
The paper presents results of comparative study ofusing information from the Automatic Identification
Systemandfromradartodeterminetheaccuracyoftracki ngthepositionandmotionvectorofships intheGulf
ofGdansk.Possibilitiesandlimit
ationsoftheAISarealsopresented.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 8
Number 3
September 2014
DOI:10.12716/1001.08.03.02
338
reflectors.Figure1shows,thecalculatedbyCARPET
2,theprobabilityofdetectionayachtbyaship’sradar
asafunctionofdistancefromtheantenna.Thezones
ofattenuationofthesignalarevisible[12].
Figure1.Probabilityofdetectionofayacht[8]
We carried out research into the impact of
differenttypesofradarreflectorsonthedetectability
ofayachtintheGulfofGdansk.Radarsinstalledin
the Gdynia Maritime University, on ships of the
Maritime Office and those working in the Vessel
Traffic Service VTS Gdańsk Bay were used
for this
purpose. This corresponds to typical situations
encountered at sea. The reflecting properties of an
objectaredefinedaseffective reflectionsurface.Itis
thesurfaceofametalplategivingthesamedegreethe
reflected signal as a real object. This parameter
depends on the shape, dimensions and
type of
material. Typical effective areas of a small craft are
lessthan50m
2
andofvesselstheyareafewthousand
squaremeters.Metalelementsofthedeckequipment
play a crucial role for a yacht. A standard reflector,
approvedby PRS(Polski RejestrStatków), improves
the detection range of a yacht by about 20%, while
otherreflectors havea minimalimpact on
tracingof
echoesofsmallobjects.Shorebasedradars,working
in traffic control system, have higher performance
andtheexaminedcraftsweredetectedbytheminthe
entirearea[11].Ithasbeenfoundthatradaroperating
inthe X band inthe same conditions is slightly less
resistanttohydro
‐meteorologicaldisturbancesthanS‐
band radar, however, due to the wider radiation
pattern in the horizontal plane the motion vector
definedbytheS‐bandradarislessprecise.Accuracy
of determining the motion vector depends on the
speed of the echo (for speeds below 1.5 m/s ARPA
(AutomaticRadar
Plotting Aids) cannot determinea
motionvector)andoncomputationalalgorithms.On
the basis of the same radar signal, the calculated
parameters of movement of echoes may indicate
different values on radar and on ECDIS (Electronic
Chart Display and Information System) screens (e.g.
coursediffersuptoseveraltensofdegrees).
Figure2
illustrates courses of a vessel. Based on numerous
observations,itwasfoundthataradardetectsasmall
targetatadistanceofabout5‐8nauticalmilesbutits
echoontheradarscreenisaverysmall,whatismore,
the operator is not likely to
notice it because of
backgroundnoise. Usuallysmallobjectradarechois
lostin clutters of the sea surface at distances of less
than2NM.Thepossibilityofobservingmovementof
small crafts using ship’s radar is often very limited
[10]. These observations were confirmed during
cruises on ships. It
was found that the vessels have
limited opportunities to detect small crafts at sea.
MaterialsfromthesestudieswereusedtoraiseIMO
(InternationalMaritimeOrganization)interestinthis
problem. (IMO as a specialized United Nations
agencydealingwithmaritimeissuesonly).
3 AUTOMATICIDENTIFICATIONSYSTEM‐AIS
Automatic Identification System‐
AIS uses data
transmissionsintheVHF.AISdevicesoperatewitha
time division multiple access TDMA using for
transmission two AIS1 frequencies 161.975 MHz
(channel 87B) and AIS2 equal to 162.025 MHz
(channel88B),whichmakesitpossibletosendabout
4000 and 2000 reports per minute. The on board
device automatically sends data regarding position
andmotionvectorfromGPSnavigationreceiver,the
gyrocompassandlogcourse,speedthroughthewater
and the identification data. Time interval between
transmissionsdependsonthespeedofthevessel.[9]
AIS was introduced by the end of 2004 for vessels
employed in
international shipping and on ships
takingonlydomesticvoyagesonJuly1in2008.Since
2010 fishing vessels have had such equipment and
leisure‐sportcraftsarealsoplannedtobefittedalike.
Fitting vessels with AIS equipment has changed the
work on the bridge. Navigator gained information
about other ships,
which have so far not been
available to him. The system allows insight into the
instruments of another vessel. Until then, positions,
coursesand speeds were determined on thebasis of
radarobservationorestimatedonthebasisofvisua l
observation.Measuringthedistancebyradarenables
to get the
measurement accuracy of approx. 250 m,
whenechooftheothershipshowsuponthescreen,
whileGPSallowsyoutoobtainthispositionat20m
[10].
Another bit of important information is warning
aboutanother ship maneuver. ARPA helps todetect
another ship in less than 1 min
whereas it is very
difficulttodetectthetargetbyobservingtheimageof
theradar.TheAISsendsthecurrentdataevery2‐3s
duringthemaneuver.Themaneuverisdefinedasthe
changein the course of 5°/30s. Whena shipkeeps a
constant course, the frequency
of sending dynamic
data was chosen so that the distance between the
actualpositionoftheshipandtheobtainedfromthe
system does not exceed 50 m. The AIS provides
informationatamuchgreaterdistancethanradaror
visualobservation,butitisnotsoimportant.
4 USEOF
AUTOMATICIDENTIFICATION
SYSTEM
Another very important issue related to the
introduction of the AIS is the registration of vessel
trafficbyservicesonlandwhich,amongotherthings,
demands more diligent work from watchkeeping
officers.Itallowsmonitoringtrafficandsupportsthe
rescueoperations. However,sending data to all in
a
radius of 50km can be dangerous (activities of
pirates), so the ship has the right to turn off this
339
device.Theshipʹscrewknowingly,ormoreoftennot,
canfalsifythedatasentbythedevice,becausedespite
of the supply of very important and more accurate
datatheAISisnotusedasobservationmeans.Ifthe
targets located by radar and the AIS are sufficiently
close
to each other, the provisions of the IMO
recommends the use of data from the AIS. An
experiment carried out by the author does not
confirmthat.AISmeasurementwasfirstusedduring
testingtherangeofradarsoperatinginVesselTraffic
VTS Gulf of Gdansk [13]. Positions inWorld
Geodetic System‐1984 from the GPS receiver were
recordedonthe‘Zodiac’shipandthenweregivento
AISdevice.
TheWorldGeodeticSystem1984(WGS84)datum
isthenominaldatumusedbyGPS[16].Itisbasedon
the WGS84 ellipsoid which only exhibits a small
differenceintheflattening
parametercomparedtothe
GRS80andthereforebothellipsoidscanbeassumed
identical for most practical purposes. The WGS84
datum has been refined several times to be closely
aligned with the International Terrestrial Reference
Frame‐ITRFinordertopreventdegradationofthe
GPS broadcast ephemerides due to plate tectonics.
TheITRFisthemostpreciseearth‐centredearth‐fixed
terrestrial datum currently available, realised by an
extensive global network of accurate coordinates
basedontheGeodeticReferenceSystem1980(GRS80)
ellipsoid and derived from geodetic observations
using GPS, Very Long Baseline Interferometry,
Satellite Laser Ranging, Lunar Laser Ranging and
Doppler
Orbitography and Radiopositioning
IntegratedbySatellite.TheITRFisadynamicdatum
and changes according to temporal variations of
coordinates and their velocities due to the effects of
crustal motion, earth orientation, polar motion and
other geophysical phenomena such as earthquakes
andvolcanicactivity.TheITRFisstillrefineto
ensure
thatthechangebetweensuccessiverealisationsinthe
order of 1–2 cm. Similar redefinition of World
Geodetic System, known as WGS84 (G1150), was
implemented in GPS week 1150 (20 January 2002).
AfterthisalignmentwiththeITRF2000,itwasshown
thattheWGS84coincideswiththeITRFwithina few
centimeters at the global level [15]. For all mapping
andchartingpurposes withaccuracyrequirementsat
the10cmlevel,theWGS84andthemostcurrentITRF
can therefore be assumed identical [16] then GPS
measurementscouldberealizedinthissystem.
ItturnedoutthattheAISdeviceretrieves
thedata
of the ship equipment to the buffer memory and
sendsthemaccordingtothescheduleofthesystem.
Asaresult,itwasfoundthattheerrorsofthesystem
aregreaterthanthoseimpliedbytheassumptions.
Furtherresearch showedthattheGulfofGdansk
dynamic data are
received less frequently than it
results from the assumptions of the system. The
differences between the position of the vessel
perceived by means of AIS and that recorded on
board the ship reached 250 m [2]. There are usually
about 100 vessels in the Gulf of Gdansk covered by
the
AIS, mainly at anchor (sending dynamic data
every 3 minutes). One problem is static data sent
everysixminutesastheycanoccupymorethanone
frame. The author registeredsimilar problems in
otherEuropeanwatersduringthevoyagesonboard
thetallship‘DarMłodzieży’.Therewerefew
ships
on the waters covered by the research within VHF
communication, so there could be no problem with
the lack of free frames. Fewer transmissions were
received than expected [1]. TheAIS device does not
signalifthedataaresent,soitisnotpossibletostate
ifthe
systemallocatesframesfortransmissionornot,
or whether as a result of interference they are not
received. The author also registered information
receivedfromtheAISonboardashipatanchor.Data
are provided by AIS every 3 minutes, which is
enough to know how many ships are at
anchorage,
buttheir current status cannot be defined basingon
them, or whether by the action of wind the ship is
swinging or dragging anchor [3]. The frequency of
broadcasts by ships at anchor should be at least 10
timesmore,butthismayblockthesysteminareasof
veryhightraffic(e.g.Singapore).Thesolutionmaybe
decreasingthestrengthofthesignaloradjustingthe
frequencyofbroadcastsbytrafficcontrolinthearea.
Since 2001, the author has been carrying out a
comparativestudyoftheuseofinformationfromthe
Automatic Identification System and from radar
to
determine the accuracy of tracking the position and
motionvectorofshipsinthe Gulf ofGdansk. These
studies confirm that the AIS system enables quicker
accesstoinformationabouttheshipʹsmaneuverand
usually enables to determine the motion vector of
anothervesselmoreaccurately.However,cases
were
recorded when the AIS indications were subject to
errors,forexampleshiftingofthepositionsbyseveral
cables.Sucherrorscanbeidentifiedbytrafficcontrol
systemsbutonboardtheshipisnotalwayspossible
[7].
The possibility of introducing compulsory AIS
equipmentofClassBonboardleisure
andsportcrafts
is currently being considered. The development of
new modulation of signals (CSTDMA) for the
equipment means that it does not overload the
network system [5]. According to the simulation
research,evenverylargenumberofsmallcrafts(over
1000)inoneseaareawillnotblockthe
system.Itis
very important to increase safety of small crafts
becausetheyare often not visible atseaandarenot
detectedbyship’sradar,andtheirmaneuverabilityis
dependentonthedirectionandforceofthewind[12].
The chance of detecting targets using radar
depends on their reflective
properties which in the
case of small crafts are insufficient. While using the
Automatic Identification System the conditions of
propagationintheVHFbandandtheparametersof
thedeviceshaveimpactonthedetectabilitytherefore
thissystemallowsthedetectionofsmallcrafts atsea
regardlessofthehydro‐
meteorologicalconditions.
Figure2.CoursesobtainedfromARPAandAIS
340
Figure2showscourseofashipsailingtotheport
in Gdansk registered with the AIS device and
calculated by the radar tracking systems. The radar
signal comes from a single block of transmitter –
receiver and was calculated by the new NSC34
Raytheon,olderversionMK2,byECDIS
systemand
Transas 3000 [14]. The least accurate data were
obtainedfrom theelectronicchart system.The main
problemisnotthatthedataaredifferentbutthatthe
navigatorreceivesvariousdatatobeanalyzed.
The introduction of the ships AIS equipment
allowsthenavigatortoobtainnewinformationabout
thevesselsinthevicinity.Previously,thisinformation
was provided by a lookout, visual observation
dependent on lighting and visibility conditions and
wasassisted by acousticsignals.Theintroduction of
radar made it possible to assess the risk of collision
and traffic parameters of other vessels also during
limitedvisibility.
However,the observation ofabout
20 km around the vessel on a small screen and the
limitationsofthedeviceitselfcanproducetrafficdata
withlimitedaccuracy.Radarallowsdetectinganother
vessel from a distance of 10‐20 km and then
determining its position with an accuracy of about
200m.
Courses and speed of other vessels are
calculated on the basis of changes in the position
during subsequent observation and knowledge of
own course and speed. Therefore, changes in the
courseofanothervesselcanbedetectedbyradarwith
adelay–intheAISsystemthesedataare
transmitted
directly from the shipʹs equipment so the data are
available on other ships with a minimum delay of
severalseconds,whiletheARPAshould,onthebasis
ofradarobservations,detectthemaneuverwithinone
minute. Other data are also available, such as the
name of the vessel, IMO
and MMSI numbers,
dimensions, port of destination. The AIS allows
sendingshorttextmessagetoashipselectedfromthe
listandknowingitsname,communicationusingVHF
radio can be established. The system of automatic
identification of vessels made some data that were
previouslyunavailable,availableandincreasedtheir
accuracy.However,therearealsodisadvantagesand
limitationsofthesystem.Wehavenocontrolwhether
thereceiveddataaretrue.Notalwaysthecooperation
of the vessel’s equipment with the devices of AISis
correctorcertainactionscancausefalsificationofthe
transmitteddata.
Thissystemchangedthepossibilities
ofobtaining
data on other ships in maritime transport. The data
shouldbeaccurateanduptodate,becausetheycome
directlyfromtheshipʹsequipment.However,theyare
storedinabufferofthedeviceandtheyaresentwith
some delay. Not all transmissions are received (or
maybe they are not sent at all?), neither isknown
theirreliability.Theycanbefalsifiedintentionallyor
inadvertently.
A vessel is equipped with a SART transponder
(SearchandRescueTransponder)thatallowslocating
the distress position of a survivor. Once activated,
after receiving a signal from shipboard radar, the
device
sends a response, which is displayed on the
radar screen as a special echo. Since 2010 the AIS
transponder has been an equivalent to a radar
transponderasitsendsitsnumberandpositionfrom
thebuilt‐inGPSreceiver.
This device is more resistant to interference than
theonesworkinginradarband.Aspecialsymbolis
reserved for it but older devices do not know this
signal.Inthissituationthereisnoguaranteethatthe
crew will identify the signal of the distress position
[4].Figure3showsthesignalsofaSARTbuoy.
Figure3.SignalsoftheAIStransponder
5 CONCLUSIONS
TheAIS,aseachoftheelectronicsystemsusedatsea,
has many limitations, knowledge of which is
necessaryforitsproperusage.Inparticular,theuser
oftheAISshouldbearinmindthefollowing:
DataobtainedfromtheAISdevice areasaccurate
asaccurateand
correctwerethedataenteredinto
thesystem.Inparticular,thedataenteredinto
the AIS device manually should be interpreted
withgreatcaution.
NotallshipsareequippedwiththeAISoritcan
be switched off. In particular, non‐conventional
vessels, such as sport, leisure, fishing boats
and
warshipsneednotbe,andforvariousreasons,are
oftennot,equippedwiththeAIS.
It should be taken into account that not all
installationsareproperlymadeandthedevicecan
be turned on /operational when transmitting the
datamaybeagainstsafetyregulations.
Operationof the
AISisbasedon the use ofVHF
bandandissubjecttothesamelimitationsasany
othersystemoperatinginthisband.
In summary, the AIS system is a useful and
moderntoolthatisstillbeingdevelopedandusedin
moreandmorenumerousapplications,atool,
which
usedintherightway,cancontributetoasignificant
increaseinthesafetyofpassengers,crew,cargoand
ships,aswellasthemarineenvironment.
Theresearchhasshownthatobservationofsmall
crafts such as yachts, fishing boats is significantly
restricted. The detection of such objects is possible,
mainly in addition sea clutters area, and their radar
echoes are small and difficult to notice. Radar
observationissupportedbythedatafromAutomatic
IdentificationSystem‐theAISandvisualobservation
–a lookout. The AIS is the system that allows
detecting small crafts from a proper distance,
regardlessof
thehydro‐meteorologicalconditions.
The training of ship’s crews and programs of
study should incorporate information on new
technologies and system functions especially those
connected with the safety of navigation and saving
life at sea. This can be done effectively by entering
such information into the program of individual
rescue techniques
trainings, as each crew member
undergoesthiscourseevery5years.
Forthesereasons,smallcraftsduringseapassages
shouldmandatorybeequippedinaClassBoftheAIS
device.Thiswillallowfortheirdetection,positioning
and tracking regardless of the hydro‐meteorological
conditionsandsignificantlywillincrease
thesafetyof
341
millionsofsmallcraftssailingatseasandoceansall
overtheworld.
The satellite navigational systems (GPS Navstar,
GLONASS) give possibility of objects continuous
coordinates calculate. Additionally differential or
Sbasse methods enable getting better accuracy of
positions. Radio transmission of object positions in
World Geodesy System WGS84 helps to
presents
themonthecharttosteersafetynavigation.
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