205
1 INTRODUCTION
AISistheabbreviationsforAutomatic Identification
System, it consists of shore base facilities and ship‐
borne equipment. The basic operations of AIS are
ʺship‐to‐shipʺ andʺship‐to‐shoreʺ information
exchanges, and it can also handle multiple
communications with a fast updating rate. The AIS
dataexchangedaredividedint
othreedifferenttypes
referringtoliterature[1,2]:
Staticdata,.
Dynamicdata,.
Voyage‐relateddata,.
AISbasestationiswidelyusedaroundtheworld,
especially in portsand areas with large traffic flow.
AISdatamaybelostduetoenvironmentalconditions
such as rainor fog, and obstacle condit
ions such as
shadowing caused by landmasses or other vessels,
limitationofVHFtechnology,AISnetworkoverload,
etc. Therefore, the successful reception of ships’ AIS
signalwillbereduced,whichcausedahiddenperilof
marine safety. According to the literature [3]，the
report interval of ship‐borne AIS equipment should
meet the requirements of relevant standards.
However, the data received by AIS ba
se station
seldommeetstherequirementbecauseoftheadverse
impactsfromthetransmissioncharacteristics,power,
the status of the base station, geographical features,
etc. Many scholars have done a lot of researches on
AIS network data link capacity, network congestion
and slot reservation select
ion algorithm, but no one
has solved these problems in the view of practical
application. AIS uses the SOTDMA algorithm to fit
the high density of communications, and to ensure
the reliability and real‐time communications
operations. But due to the restrictions of SOTDMA
whichisdescribedinlit
erature[3],whenthenumber
ofshipsisbeyonditsratednumberorthecoverageof
AISusersignalsoverlapswitheach other,therewill
be a time slot conflict which often causes a large
missingofAISinformationandobstructioninsignal
sending and receiving , thus affect
ing the safety of
navigation. IMO published a model course which
contains an exemplary calculation about the
theoretical capacity of AIS devices. This number is
The Comparison Study on AIS Signal Reception Rate
with Directional Antenna and Omni Antenna
T.Xu,Q.Hu,Z.Xiang,C.Yang&D.Wang
ShanghaiMaritimeUniversity,Shanghai,China
ABSTRACT: With the wide use of AIS system in the world, especially in ports and the waters with heavy
traffic,AISmessagelossmayberelatedtoenvironmentalconditions,obstacle,limitationofVHFtechnology
and AIS network overload, thereby reducing the AIS signal successful reception rate. This pa
per selected
typicalAISdatareceivedbyYagiandOmniantennasatthesametimeinspecificwaters,usesgridandAIS
data recovery technology, establishes the AIS signal coverage model, analyzes signal reception rate of Yagi
antenna and Omni antenna of AIS base station, and verified the superiority of Ya
gi antenna in AIS signal
receptionrate.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 10
Number 2
June 2016
DOI:10.12716/1001.10.02.03
206
basedonidealassumptionsmeaningthateachvessel
is able to reserve time lots whenever needed
according to the reporting intervals .Since AIS data
are often used for manual collision avoidance and
vesselmovementprediction,itisimportantthatAIS
reporting intervals are kept as described in ITU
(2010).On
thebasisofanalysisofthehistoricaldataof
AIS in Shanghai Port about 30% of vessels failed to
update their positions for 2 minutes, 15% for 3
minutes,10%for5minutes,and5%for15minutesin
literature[5].
Currently, most AIS base station antennas are
Omni antennas
which have small gain, the effective
distanceisabout30nmandthesignalreceivingrange
is 360° all around. The signal loss are easily to be
occurredwhenthedensityofshipishuge.However,
directionalantennashavealargegainandeffectivein
a given direction. Directional antennas have been
used in many fields such as mobile communication,
satellite navigation, aviation communication, radio
and television and so on, which overcomes the
shortcomings of the Omni antenna and achieves
satisfactoryresultswhichhavebeenmentionedinthe
literature [6]. But so far directional antenna hasn’t
beenusedinthemarineindustry.
Thispaperfocuses
on the AIS antenna and analyses the advantages of
AIS directional antenna in practical application. To
identify the antenna influence of AIS data loss the
following aspects are being evaluated within our
work:
General comparison and analysis of the AIS
messagereceptionrateofbothAISantennas
atthe
sametimeandplace
Evaluating AIS dynamic data reception rate in
differentdistance
DetailedevaluationofAISsignalcoverage.
2 TECHNICALBACKGROUND
AISutilizesSOTDMAaccessmethodtobroadcastand
receive ships’dynamic and static data automatically
for the purpose of realizing the identification,
monitoring and
communication. AIS base station
system receives ships’ data and navigational status
andbroadcasthydrological,meteorologicalandother
informationreleasedbythemanagementdepartment
to ships at the same time, to manage and provide
servicefortheshipswithinthearea.
2.1 PrincipleofSOTDMA
SOTDMA algorithm AIS is introduced in
literature[3].SOTDMA algorithm is the core
technologyofAIS,itbasesonTDMAtechnology.The
TDMA technology dividestime into periodic
frames, each frame is divided into several time slot,
andtheinformationoftheshipistransmittedtoeach
mobilestation
in a specified time slot of a frame on
principle of certain time slot allocation. Every ship
can choose a time slot which does not conflict with
othershipswithoutthecontrolofthebasestationto
release own information independently. Any ship
withintheAISrangewillbe able
to send or receive
message reports from all ships (shore stations)
without interfering with each other. According to
IMO model course in the literature [4], theoretically
AISsystemcanaccommodatethedataof450shipsat
thesametimeandwhenthesystemisoverloaded,the
furthertargetsfromownship
willbeabandonedsoas
toensurethepriorityofcloserangetarget.AISusers
canusethefreetimeslotselectionalgorithmtowrite
theobtainedreservationtimeinformationintotheslot
reservation information message to actualize the
informationsharingwithotherusers.Everyusercan
send their own
time slot reservation information
periodicallyandreceivethemessagefromotherusers
toachieveinformationsharing, so the status of time
slotisconstantlyupdated.Whenthecommunication
channelisfree,theidletimeslotiseasytofind.But
when the channel is busy, there may be a slot
collision,
because several AIS terminal may make
synchronouslyanappointmentofthesametimeslot.
UndertheSOTDMAprotocolitisinevitablethatthe
situation of overlapping occur when many users
choosingtimeslots,leadingtofailureforasingleor
several users to transmit‐receive messages. With the
wideapplication
ofAISsystem,theloadofAISdata
link are increasing, and all those have resulted in a
significantincreaseinthecollisionrateoftimeslots.If
the ratio of time slot collision increases to a certain
degree, the reliability of AIS system will be greatly
affected.
Currently,existingAIS
equipmenthasbeenunable
to solve the problem of AIS slot collision. It is
necessary to explore new methods to improve the
receiving efficiency of AIS signals. Therefore, we
intend to increasing AIS reception rate by replacing
Omni antennas with directional antennas within
experiment.
2.2 AntennatypeofAISbase
station
Accordingtodifferentcoverageofpowerorradiation
field,AISantennacanbedividedintoOmniantenna
and directional antenna, of which the biggest
differenceisthedirectionalgain.Themostimportant
indicatorofmeasuringantennaistheperformanceof
the gain. The way to measure antenna gain is to
polymerize the electromagnetic waves beam emitted
bytheantenna,andthendividethevalueatthesame
pointonthereceivedpower,ortheratioofthemajor
lobeoftheradiationdensityandtheradiationdensity
in isotropic when the output power equal to each
other. Antenna gain
is related to the width of the
majorbeamof electromagnetic waves.Thenarrower
themajorbeamofantenna,thesmallerthesidelobe
and tail lobe and the higher the gain of major lobe
direction. The gain of the antenna is inversely
proportionalwiththeradiationrange.
1 AISOmni
antenna
Omni antenna means homogeneous radiation in
horizonindirectionsof360°,whichusuallymeans
non‐direction. Because antenna gain is inversely
proportionaltotheradiationrangeoftheantenna,
Omniantennagainisworse,usuallybelow9dB.
AISOmniantennahasthesamegainindifferent
directions. It
causes a lot of technical
shortcomings,suchasthehighprobabilityofslot
collision, transmit power usage insufficient and
interferencebetweensendandreceivesignalsand
so on. Currently, AIS base station basically uses
Omniantenna.
207
2 AISdirectionalantenna
Directional antenna means radiation in a certain
directioninhorizon,whichisusuallydescribedas
directional. Directional antenna has better
electromagneticeffectfromoneorseveralspecific
directions, and the electromagnetic waves from
other directions is very tiny. Similar to Omni
antenna,thesmallerthebeam
widthis,thegreater
the gain benefit is. The directional antenna was
generally applied in the circumstance of long
distance communication, small coverage, high
targetdensity.
Usually, the main purpose of directional antenna
istoenhancetheanti‐jammingcapability.Comparing
with Omni antenna, directional antenna is more
sufficient in
utilizing power. Under the same
circumstances, directional antenna can transmit and
receive signals further than Omni antenna does in
given direction. It will reduce the probability of the
slot collision when transmitting message by using a
directional antenna. And the high gain beam can be
generated in a certain direction so
as to reduce the
probability of occurring time slot interference and
increase greatly the throughput capacity of network
and the number of subscribers. All these unique
advantages listed above provide a new approach in
solvingtimeslotcollisioninterference.
3 AISDYNAMICDATAPROCESSING
TECHNOLOGY
Intotal,thereare27
typesofAISmessage. Eachtype
of AIS message serves a specific purpose, e.g.
message type one, two and three represent the
dynamicdataofClassAsystemincludingtherateof
turn,heading, speedover groundorthenavigational
status. The AIS data are also used as an additional
overlay,
e.g. on radar or ECDIS screens. The AIS
reporting intervals may vary depending on the AIS
messagetypesandvessel’sstate.Thedynamicdataof
ClassAsystemaresentintheformofpositionreports
withintheAISmessagetypesone,two,andthreeas
showninTable1
accordingtoITU(2010).
Table1. Reporting intervals of class A system position
reports
_______________________________________________
VesselstateInterval
_______________________________________________
Atanchor/mooredandnotmovingfasterthan3kn 180s
Atanchor/mooredandmovingfasterthan3kn 10s
0kn≤SOG≤14kn10s
0kn≤SOG≤ 14knandchangingcourse4s
14kn＜SOG≤ 23kn6s
14kn＜SOG≤ 23knandchangingcourse2s
SOG＞23kn2s
SOG＞23knandchangingcourse2s
_______________________________________________
Because of the delay and message loss of AIS
dynamicdataupdating, itisnecessarytoprocessthe
data, evaluate the difference of received AIS signal
from the AIS base station. The corresponding
geographic grid structure should be established for
the specific waters to adopt a certain algorithm to
evaluate the
AIS data reception rate in different
regions.
3.1 InterpolationalgorithmforshipʹsAISmotionstate
Shipʹs AIS motion interpolation can recover AIS
dynamicdata,whichisofmuchimportanceforshipʹs
traffic data analysis. Making use of the vector
functiontoexpresstherelationshipbetweentimeand
spacecoordinates,itstangentlineanditschangerate
canreflectthedirectionandspeedoftheshipmotion
as well as their acceleration and other physical
characteristics.Accordingtotheknownshipdynamic
data to optimize the ship motion vector parameters,
then the ship motion state interpolation model is
established
torealizetheinterpolationofthedynamic
data of the ship at any given time. Refer to the
literature[5],many factors such as vector function of
Shipʹs motion state, interpolation model of Shipʹs
motion state and calculation on shipʹs acceleration
mustbeconsideredinto.
3.2 Gridofresearch
waters
InordertoevaluatetheeffectofAISsignalreceived
byAISbasestationinagivenwater,allthedatashall
be classified accurately according to geographical
positioninresearchedarea.Quadtreeindexisoneof
the commonly used spatial indexes in geographic
informationsystem. Thispaperrefersto
theQuadtree
datastructureindex,anddividesthestudiedareainto
anumberofsubregionsofequalsize.
3.3 AISCoveragealgorithm
International scholars has carried out a lot of
researchesworkforAIScoverage.Inliterature[7]and
[8], they put proposed a new method of estimating
AIS
signalcoveragefromthereceiverterminal,using
interpolationmethodbasedtorecovererrormessages
in AIS transmission;. In literature[9], the author
proposed the Grid‐Count algorithm, using AIS
historical data, interpolation algorithm and grid
technology,whichcanbemoreefficientincalculation
ofAISbasestationsignalcoverage.
Grid‐Countalgorithmisadoptedintheanalysisof
AISsignalcoverage.GridofthewaterareanearaAIS
basestation,selectthe
targetship,getthegridofthe
AISdatasent,andcalculatethenumberofthesingle
ship which has been sent to the grid map. Using
interpolation algorithm in literature[5], calculate the
AISmessagethatshouldbesentfromthetargetship,
then obtain the grid map of
number of signal;
CompareeachthegridmapofnumberofAISsignal,
finally form the AIS signal coverage grid frequency
map of the waters near the base station; AIS base
stationsignalcoverageisdeterminedbythenumber
ofsignalssentortobesentfromthenearbasestation;
Fillinwithcolorsaccordingtothefrequencyvalueof
thesizeofthegridfrequencymap.
In order to obtain the message coverage grid
graph, we can calculate the total number of data
messages sent or to be sent in each grid, on the
premisethatthenumberof
datamessagessentorto
besentby several vessels is known. Assume that in
theigrid，thetotalnumberofpacketssentisS
i，the
totalnumberofpacketsshouldbesentist
iafterthe
interpolation，and then the signal coverage rate in
theareaisindicatedasr
i：ri=si/ti..Fillinthegridwith
208
colors according to the size of the coverage rate to
create a map of grid coverage. Utilizing Flex
technologytocorrespond the gridmaptotheactual
geographic coordinates, we can directly observe
signalcoverageoftheAISbasestationintherelated
waters.
The experiment and performance are
described
withinthefollowingsection.
4 EXPERIMENTANDPERFORMANCE
To precisely evaluate AIS message received by
directional antenna and Omniantenna , we select
thetypicaldatareceivedbyAISbasestationinTianjin
，China as object of study, and evaluate the signal
receivingeffectofAISbasestationin
aspectsoftotal
quantity of message, dynamic data and signal
coverage. In the experiment, we select Yagi antenna
asdirectional antenna.
The AIS antenna installations
have been done according to IMO guidelines. To
minimizenegativeeffectsonthereceptionrate,the
two sets of AIS equipment are the same condition
exceptAISantenna.
4.1 CharacteristicsofAISbasestationantenna
Table2.ParameterofAISshorestationantenna
_______________________________________________
parametersYagiantennaOmniantenna
_______________________________________________
Frequency162M162MHz
VSWR1.151.2
Inputimpedence50Ω50Ω
Gain16.6dBi6.7dBi
Signaldirection 40°（majorlobe） 360°
_______________________________________________
Figure1. Horizontal directional signal radiation of Omni
antenna
Figure2 Horizontal directional signal radiation of Yagi
antenna
ThecharacteristicsofAISshorestationantennais
showedinTable2,Fig.1andFig.2,itisobviousthat
thegainof Yagi antenna ismuchbetterthanthatof
Omniantenna, but signal coverage scope is much
less than the latter. In general， higher gain will
resultin
higherreceptionratethanlowerone.
5 ANALYSISOFEXPERIMENTRESULTS
The results of the experiment can be divided into
three aspects. The first one deals with general
comparison about all the message reception rates of
the two antennas. The second one is about the
evaluation the AIS dynamic data reception rate in
different range. The last
one has a detailed research
onAISsignalcoverage.
5.1 Generalcomparisonquantityofdatareceivedbyboth
antennas
TheAISmessagesreceivedfrom00:00to24:00LTon
24
th
,Oct,2015byAISbasestationintheexperiment.
All the messages have been received by two AIS
antennasasshowninTable3.
Table3. AIS message count of Yagi antenna and Omni
antenna.
_______________________________________________
AISID MessagemeaningYagi Omni
_______________________________________________
1,2,3 PositionreportclassA4528547 1311878
5 Staticandvoyagerelated 331231 98909
dataclassA
18 StandardclassBposition 467980 112666
report
19 Extendeddynamicdata 23999 6836
classBpositionreport
24 Staticandvoyagerelated 132554 32014
dataclassB
Other 521998 150340
_______________________________________________
Total6006309 1712643
_______________________________________________
Comparing the count ofAIS message the Yagi
antennahasreceived2‐4timesmoredatathanOmni
antennadidindifferentperiod.Theresultshowedin
Fig.3.
The quantity of data received by Yagi antenna is
always larger than that of Omniones in different
periods for all the
day. During period 0900‐2100,
Omni antenna performs well relatively, but Yagi
antenna’sreceptionis2‐3timesmore,whichis4times
at other period of the day. As expected, the higher
gain the antenna is, more AIS messages have been
received.
AIS message count comparison in different time
0
20000
40000
60000
80000
100000
120000
140000
160000
180000
0000-0100
0100-0200
0200-0300
0300-0400
0400-0500
0500-0600
0600-0700
0700-0800
0800-0900
0900-1000
1000-1100
1100-1200
1200-1300
1300-1400
1400-1500
1500-1600
1600-1700
1700-1800
1800-1900
1900-2000
2000-2100
2100-2200
2200-2300
2300-2400
time
message count
Ragi Omni
Figure3.AISmessagecountcomparisonindifferenttime
209
5.2 EvaluationoftheAISdynamicdatareceptionratein
differentdistance
Since the dynamic data contains latitude and
longitudedata,itispossibletocalculatethereception
distanceforboth antennas. According to all Class A
positionreportsasshowninFig.3.Toexplainfurther
the difference between Yagi
antenna and Omni
antenna in receiving AIS message , we divided all
positionreportsofClassAintodifferentranges,and
thereisabigdifferenceamongthem.
messages count received by Ragi antenna in different range
0
10000
20000
30000
40000
50000
60000
70000
0000-0100
0100-0200
0200-0300
0300-0400
0400-0500
0500-0600
0600-0700
0700-0800
0800-0900
0900-1000
1000-1100
1100-1200
1200-1300
1300-1400
1400-1500
1500-1600
1600-1700
1700-1800
1800-1900
1900-2000
2000-2100
2100-2200
2200-2300
2300-0000
R≤1 1＜R≤5 5＜R≤10 10＜R≤15 15＜R≤20
20＜R≤25 25＜R≤30 30＜R≤35 35＜R
Figure4.Dynamicdatareceptionrateindifferent rangesby
Yagiantenna
messages count received by omni antenna in different range
0
10000
20000
30000
40000
0000-0100
0100-0200
0200-0300
0300-0400
0400-0500
0500-0600
0600-0700
0700-0800
0800-0900
0900-1000
1000-1100
1100-1200
1200-1300
1300-1400
1400-1500
1500-1600
1600-1700
1700-1800
1800-1900
1900-2000
2000-2100
2100-2200
2200-2300
2300-0000
R≤1 1＜R≤5 5＜R≤10 10＜R≤15 15＜R≤20
20＜R≤25 25＜R≤30 30＜R≤35 35＜R
Figure5.Dynamicdatareceptionrateindifferent rangesby
Omniantenna
Indifferentrangesmeasuredbetweentheposition
ofvesselandAISbasestation, the quantityreceived
byYagiantennaisalwaysmorethanOmniantenna
did.TheresultisshowedinFig.6.
Comparation Quantity of messages received by Ragi and Omni antenna
0
200000
400000
600000
800000
1000000
1200000
R
≤
55
＜
R
≤
10 10
＜
R
≤
15 15
＜
R
≤
20 20
＜
R
≤
25 25
＜
R
≤
30 30
＜
R
≤
35 35
＜
R
Range
Message count
Ragi Omni
Figure6.AISdynamicdatacountcomparison
With the increase of the distance between the
vessel and AIS base station antenna, the amount of
AIS data received by Yagi antenna does not
significantly decrease. But with the increase of the
distance, the AIS datareceived by omni antenna
gradually decreased, and the AIS shore station
antennaisalmost
unabletoreceivetheAISdataofthe
shipwhenthedistanceismorethan35nm.Andwith
the increase of distance, the performance advantage
of Yagi antenna receiving AIS signal is more and
moreobvious.ThecountofdatareceivedbytheYagi
antennaisabout2times
asmuchasthatoftheOmni
antennawithin10nm..Between10nmand30nm,the
countofdatareceivedbytheYagiantennaisabout4
timesasmuchasthatofOmniantenna.Atadistance
ofmorethan30nm,thenumberofdatareceivedby
theYagiantennaisabout10timesasmuchasthatof
theOmniantenna.
Figure7.Vessels’AIStrajectoryreceivedbyYagiantenna
Figure8.Vessels’AIStrajectoryreceivedbyOmniantenna
Overlayvesselmotiontrajectoryontheelectronic
chartasisshowninFig.7andFig.8
,thequantityand
distance of data received by Yagi antenna are much
largerandlongerthanthatofOmniantenna.
5.3 AISsignalcoverage
We select AIS base station in Tianjin port as the
experimental object. It is the center on position of
exact geographical location Lat:38°58′32″8N,
Long:117°47′14
″E, the water is divided into 60×60
grids, 2 nm×2 nm each grid. Use Grid‐Count
algorithm to obtain all the single ship signal grid
maps,andfinallygeneratethegridfrequencymapof
the base station signal coverage. The major lobe of
AIS Yagi antenna direction is 110°.The color
of the
grid in Fig. 10 and Fig. 12 is different, which
represents signal coverage rate in different location.
Thescaleofsignalcoveragecorrespondstodifferent
color.InFig.9andFig.11,theverticalcoordinatesis
the AIS coverage, the horizontal coordinates is the
corresponding number of grid. The non
‐grid range
indicatesareaswhere no ships passing by, of which
thesignalcoveragerateis0.
210
Figure9. AIS signal coverage grid frequency chart of the
Yagiantenna
Figure10. Yagi antenna signal coverage trend chart （
horizontalsectiondirectionis110° ）
AccordingtoFig.9andFig.10,YagiantennaAIS
signal coverage increases with the change of the
distance.Within20nm,theAISsignalcoveragerateis
36%‐22%; between 20 nm to 40nm, the signal
coveragerate is 20%‐25%; and between 40 nm to 60
nm, the signal
coverage rate is 20%‐40%. It proves
thattheAISYagiantennahasobvious directivity.Itis
restricted by the direction of the major lobe. The
coverage rate of AIS signal in short range is low,
whereaswiththeincreaseofdistance,thedirectional
advantage is obvious. With the increase
of the
distanceanddecreaseofship’sdensity,thecoverage
rate of AIS signal is increased within the AIS signal
coveragerange.
Figure11. AIS signal coverage grid frequency chart of the
Omniantenna
Figure12. Omni antenna signal coverage trend chart
(horizontalsectiondirectionis110°)
AccordingtoFig.11andFig.12,withtheincrease
of distance, the coverage rate of AIS Omni antenna
gradually reduced. Within 20nm, the AIS signal
coveragerateis8%‐18%;between20‐40nmthesignal
coveragerateis4%‐18%;andthesignalcoveragerate
is 1%‐4% beyond 40
nm, Due to the influence of
mores ships in short range and the slot collision of
AISsignals,theAIScoveragerateofOmniantennain
close range is very low. And with the increase of
distance, AIS signal intensity is weakened further,
andthusAISsignalcoveragerateis
furtherreduced.
ThecoveragerateofAISsignalreceivedover60nmis
lessthan1%.
6 CONCLUSION
ThroughtheevaluationofAISdatareceivedby two
AIS antennas in Tianjin AIS base station, we found
the fact that the directional antenna has more
advantages in receiving AIS message in the
count,
distance and signal coverage than Omni antenna
under the same conditions. The directional antenna
maymakeupthedeficiencyoftheOmni antennain
theAISdatareceptionrate,reducesthemessageloss
andefficientlyincreasethesignalcoveragerangeand
reduce the AIS signal blind area in ports
and the
waterswithheavytraffic.
211
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