301
1 INTRODUCTION
At the beginning of the 21
st
century a major shift
occurred in the field of naval navigation. The
navigatorwasnowinpossessionofdatapertainingto
ships in their vicinity which was previously
unavailable. Utilizing the Automatic Identification
Systemshipstransmittheirname,size,etc.aswellas
current navigational readings. This opens new
possibilities
of situation assessment and allows for
direct radio communications. One can obtain more
precisedataaboutothervesselsthantheonegivenby
anon‐boardradar,but,atthesametime,theycanbe
blocked or incorrect. As a result of that, the
comparativeresearchofresultprecisionbetweenship
radar and the Automatic Identification System was
undertaken. It was conducted within Gdańsk Bay
using radars installed in AM laboratories and on‐
boardcommercialvessels.
2 TRACKINGPRECISIONREQUIREMENTS
On the 6th of December 2004 the Maritime Safety
Committee adopted a new resolution entitled
Adoption of the Revised Performance
Standards for
Radar Equipment[2, which pertains to the radar
equipmentinstalledon‐boardseavessels starting on
1stJuly2008.Thisdocumentstatestherequirements
for radar tracking devices and the precision with
which object parameters must be presented during
acquisition and tracking. The International
Electrotechnical Union presented a norm which
preciselystatesinwhatwaythisequipmentmustbe
tested. It is the IEC 60872‐1 norm: Maritime
navigationandradio‐communicationequipmentand
systems–Part1:Shipborneradar‐AutomaticRadar
PlottingAids‐Performancerequirements.Methodsof
testingandrequiredtestresults[1].
Automatictracking is basedonthe relative
radar
echo position measurement and inner vessel
movement parameters. Other available sources of
informationmaybeusedasasupportintheprocess
ofautomatictracking.Echoesclearlyvisiblefor5out
of 10 concurrent cycles of antenna rotation, or a
period equal to that, should be tracked. For vessels
travelling
with real speeds up to 30 knots, the
tracking device should give results with error
marginsnotgreaterthanthosegivenintable1(with
95%probability):
The Assessment of Drafting Ship Movement
Parameters Using Radar and the Automatic
Identification System
T.Stupak
GdyniaMaritimeUniversity,Gdynia,Poland
M.Wąż
PolishNavalAcademy,Gdynia,Poland
ABSTRACT:ThisarticlepresentsthemovementvectorresearchconductedintheradarlaboratoryofGdynia
MaritimeUniversityandduringvesselcruises.Theprecisionofdesignatingthevesselsʹlocation,course,speed
andCPAwereresearchedusingon‐baordradarsandAISdata.Itisconcludedthatthe
precisionofdesignating
theresearchedparametersisgreaterthantheInternationalMaritimeOrganizationrequires.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 12
Number 2
June 2018
DOI:10.12716/1001.12.02.10
302
Table1.Trackingdeviceprecision(for95%probability)[2]
__________________________________________________________________________________________________
TimeofsetRelative Relative Closestpoint Timeofclosest Real Real
trackingprocess course speedofapproach pointofapproach course speed
Minutes
Degrees KnotsNauticalmile MinutesDegrees Knots
__________________________________________________________________________________________________
1
111.5or10%
1
1.0‐‐‐
330.8or1%
1
0.30.550.5or1%
1
__________________________________________________________________________________________________
relative movement tendencies of the echo in one
minutetrackingintervals,
movementparametersoftheechointhreeminute
trackingintervals.
The trackingprocess isset whenown vesseland
the tracked object do not manoeuvre and the
precisionsareasfollows:
radarmeasurementsare
within2oand50mor+/‐
1% observational range (bigger margin of error
decides),
the information about movement parameters is
lesssufficientthanthatrecommended by IMOin
resolutions.
Device testing leading to vessel installation is
performed on a simulator, which allows the
introductionofechoeswiththerequiredparameters.
Those parameters are kept with precision, so the
objectmovementconditionsarestableandunchanged
instableenvironmentalconditions
3 VESSELMOVEMENTVECTORTESTINGIN
GDAŃSKBAY
Systematic surveillance of vessel movement in
Gdańsk Bay isperformed usingequipment installed
in the radar laboratory. Data given by the
radars is
compared with data given by the Automatic
Identification System, through which vessels send
current navigational parameters obtained from their
equipment.Inaddition,vesselsmovewithinGdańsk
Bay on designated waterways and thus their real
courses are known. Measurement conditions are
better than on open sea because the ships
ʹ own
movementdoesnotplayarole.
TheAutomaticIdentificationSystemdeliversdata
from ship equipment, and allows for source
information gathering, the same information the
shipʹsofficer obtains.Thisisthebestsource of data,
howevernotalways.WecanswitchtheAISoffwhen
it endangers our
safety. It being deactivated means
thevesselisnolongervisiblewithinthesystem.We
obtain one‐time data from a given device (GPS
receiver,gyrocompass)inregularintervalsdependent
ontheshipʹsspeed.However,theradiotransmission
isnotalwaysreceivedandouractionscancausethe
data sent
to be invalid. That is why the main
information source for manoeuvre planning is the
radar.In the AISsystem theposition ofthe receiver
antenna is transmitted, while the echo is created
where themicrowave signal is reflected (thehull or
otherelementsofthevesselsconstruction)andsuch
a
signalisprolongedwiththeradarstransmittedsignal
and widened by the radar antenna radiation angle.
That is why the shift between both positions will
occur.Duetothefactthatthedataiscalculatedfrom
subsequentpositions,differencesinotherparameters
willoccur,thatisthecourseofthe
trackedvessel,its
CPA and TCPA speed. Measurements are recorded
everyminute.Everyvesselisusuallysurveilledfora
periodofseveralminutestoonehour.[3]
Radars with the X Raytheon Mk2 i NSC34, and
Decca AC 1659 spectrums were used in the
laboratory. Both Raytheon radars worked with one
transmitter,butthesamesignalwhichcanbeseenon
the diagram, was calculated differently due to
differentsoftware.
On Gdańsk Bay about 100 vessel cruises were
recorded. The radar measures the distance and
direction of the echo. The CPA, the time it has
achieved,aswellasthecourse
andspeedofthevessel
arecalculatedbasedonthefollowingmeasurements.
Distancemeasurementsinthe10Nmrangeare95%
probable to within a circle of 0.03 Nm in diameter.
The distances between radar echo position and that
obtained through AIS are within a 0.03 to 0.06 Nm
range.
Thevaluesarewithinthewidthrangeof0.5o
(from0.2oto0.7o).Devicesfromdifferentproducers
calculatethoseparametersdifferently.TheARPAby
Raytheon always showsa greater distance than that
showedonECDIS300byTransas,mostoftenby0,04
Mm.
The AIS sends temporary values of the course,
whichwereearlierobtainedfromthememorybuffer
of the device. The tracking system in the radar
calculatesitonthebasisofconcurrentmeasurements
of distance and direction. The course presented by
different devices can differ up to a couple degrees.
The lowest fluctuations can be found in the
AIS
systems.ThehighestintheDecca systems,andabit
lower in Raytheon. This is the result of different
trackingalgorithms.[5]
For radar surveillance performance it is essential
todesignateacourseandspeedofanothervesseland
the closest point of approach and time of its
achievement. The distance
and direction
measurementdependsontheradarʹsprecisionandin
practice, due to the fact that tracked vessels give a
strongreflectedsignal,thesemeasurementsaremore
precisethanthenormsrequire.Therestofthedatais
averaged from concurrent measurements and
depending on the algorithms assumed, the
calculationsmaygivedifferentresults.Dataobtained
from different devices differs form one another. At
sea the vessel is always under the influence of
different ever‐changing forces, thatʹs why its speed
and course is constantly but ever‐slightly changing.
The radar signal is also changing due to condition
changes
and the changes of surface calculations of
trackedobjects.Thismeansthattheconcurrentradar
signalsilluminatedifferentpartsofthevesselandas
suchtheparametersobtaineddiffer.[4]