549

1 INTRODUCTION

NavigatorwhosailsnarrowchannelattheInlandSea

requireshighaccuracyoffixedpositionwhichis5m

(2drms) or less.In FRP 2008 (DoD, DoH.S. & DoT

2008), therequirement of position accuracy is 2 ‐5 m

for the inland waterway phase.Therefore the

positionaccuracy

ofstandaloneGPSisinsufficiency.

ConsequentlyitneedstouseDGPS(DifferentialGPS)

fornavigatorsailingtheInlandSea.

Decreased reliability of fixed position using GPS

means that the reliability of GPS signal information

decreases and there is a possibility of abnormal

propagationofDGPScorrectiondatasignal,andsuch

case occurs in the Inland Sea.Decreased reliability

of GPS signal information means that there is

decreased reliability of transmitting signal including

satelliteconditionandsomechangesofGPSreceiving

condition around user including GPS receiver.

Decreasedreliabilityoftransmittingsignalshouldbe

compensatedbyRAIM(RadioAutonomousIntegrity

Monitoring)which

isabletoconfirmitautomatically.

However, if abnormal propagation of DGPS

correction data signal occurs, it is possibility to be

affectedbyit.Inthis paper, weresearchas part of

investigatingreceivedconditionintheInlandSea,but

sailingcircumstanceinEuropewheretherearemany

river ports,

large ships sailing on river around the

Great Lakes have  common receiving conditionsame

astheInlandSea.Itconsidersthat changes of GPS

receiving condition are caused by incident of multi‐

path wave by not only sea reflection but also large

offshorestructuresorotherships.Inthispaper,the



main subject is to analyze the propagation

characteristic of DGPS correction data signal, so to

analyzechangesofGPSreceivingconditionisfuture

task.

The Propagation Characteristic of DGPS Correction

Data Signal at Inland Sea - Propagation Characteristic

on LF/MF Band Radio Wave

S.Okuda,M.Toba&Y.Arai

arineTechnicalCollege,Japan



ABSTRACT:UserattheInlandSearequires highpositionaccuracywhichis5m(2drms) orless.Thereforethe

positionaccuracyofstandaloneGPSisinsufficiency.ConsequentlyitneedstouseDGPSfornavigatorsailing

the Inland Sea. We executed numerical simulation of the propagation characteristic

on the extended line of

bridgepieratoppositesidefromDGPSstation,andalreadyconfirmedthatbiterrorinDGPScorrectiondata

signal occurs, and that correction data could not form by bit error. Furthermore, we carried out numerical

simulationofthepropagationcharacteristicofDGPScorrectiondatasignalreceived

atsailingthroughcenterof

thebridge,andsolvedreceivingconditionofDGPScorrectiondatasignalbeforeandafterpassingthroughthe

bridge.Inthispaper,weexecutedtoinspectmutuallyresultsofelectricfieldintensitysimulationforoversea

andoverlandpropagationonsomeseaareaoftheInlandSea

andmeasuringresultsofelectricfieldintensity

forDGPScorrectiondatasignal,andevaluatedthepossibilityofabnormalpropagationcomprehensively.



http://www.transnav.eu

the International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 7

Number 4

December 2013

DOI:10.12716/1001.07.04.10

550

Abnormal propagation of DGPS correction data

signal is caused by single or multiple actions which

are increase of propagation loss by overland

propagationand/orsomeeffectsbyoffshorestructure

suchasabigbridge.InpreviouspapersatITM2011

andITM2012(Okudaetal.2011&2012),we

executed

numericalsimulationofthepropagationcharacteristic

on the extended line of bridge pier at opposite side

from DGPS station, and confirmed that bit error in

DGPS correction data signal occurs.We also

confirmed that there is a possibility that correction

data could not form bybit error.In this

paper, we

carried out numerical simulation of the propagation

characteristicofDGPScorrectiondatasignalreceived

atsailingthroughcenterofthebridgethatwasfuture

task in previous paper, and solved receiving

conditionof DGPS correction datasignal before and

after passing through the bridge.This simulation

calculated a variation

 of signal strength by

composition of superior reflection wave and surface

wave.Whenthetrailshipapproachesfromopposite

side of using DGPS station, around just under the

bridge reflection and/or scattering wave of bridge

girder become to be superior, and signal strength

increases because it is combined with surface

wave.

After passing through the bridge, signal strength

increasesanddecreasesaccordingtocombinedphase

becausereflectionwavefromthebridgeiscombined

with surface wave.Furthermore, we investigated

validity of numerical simulation by checking the

result of electric field measurement and also

investigated effects of oversea and/or overland

propagation by

 measuring electric field intensity

every adequate distance at the Inland Sea on

November2010andJuly2012.

2 DGPSINJAPAN

DGPS detects pseudorange error between GPS

satellite and the reference station whose position is

known, converts the error into correction data, and

broadcasts the correction data to user on

board

around the reference station.Each user on board

receivesthecorrectiondatausingMFbeaconreceiver.

Position accuracy is improved by fixed calculation

using the correction data.At present nominal

positionaccuracybystandaloneGPS is 9 m (2drms)

in FRP 2008, on the other hand JCG (Japan Coast

Guard)

announcesthatpositionaccuracybyDGPSis

1m(2drms)orless.Whenusingdifferentialsystem,

accuracy decreases depending on distance from the

referencestation.Thenitisappropriatethatposition

accuracy by DGPS is 1‐5 m (2drms) depending on

distance.DGPS has not only a function of

improvement

ofpositionaccuracybutalsoafunction

ofintegritymonitor.Afunctionofintegritymonitor

informssomechangesofGPSsatellitehealthinessor

decreasingaccuracyofpseudorangemeasurementto

user quickly and break off use of the satellite data.

ThissystemisexactlythesameasDGPSoperatedby

USCG(United

StatusCoastGuard).

At present in Japan there are 27 DGPS stations

whichisthereferencestation,andthecoverageisall

coastal area except a few isolated islands.Table 1

shows DGPS specification operated by JCG.JCG

callsuser’sattentionconcerningDGPScoverage(JCG

DGPScenter).

1 Exceptionof

someareaatInlandSeaabout200km

coverage.

2 Existenceofdifficultcasetousebyeffectofterrain

etc.

HoweverJCGmakenomentionaboutanareaora

phenomenonconcretely.

Table1.DGPSSpecificationinJapan

_______________________________________________

transmissionrate200bps

transmittingpower75W

coverage200kmfromDGPSstation

transmissionformatITU‐RM.823‐1(RTCMSC‐104)

messagetypeType3,7,9,16

_______________________________________________



Next,wedescribeaboutthepresentconditionfor

integrity monitor.Each header of DGPS message

includes operating condition of DGPS station. If

positionaccuracyoftheDGPSstationbecomestobe

1.5 m or more, DGPS system demands to change

DGPSfixintostandaloneGPSfix.InType9,

DGPS

systemdemandstobreakofftouseonfixcalculation

whencorrectionvalueis8mormore.

Before now, it was reported about an effect of

interference between nighttime ionospheric scatter

propagation wave and surface wave of MF beacon

wave for DGPS (Yagitani et al. 2004).They

discussed some

 effects about distance from DGPS

station and other station transmitting the same

frequency,butitisdifferentfromoursubject.

In this study, one of triggers is the phenomenon

thattransmissionofdifferentialcorrectionvaluewas

interruptedbecauseofpropagationtroubleonspecific

area.AtpresentDGPSinJapanbroadcasts

Type 3,

7,9,16basedonRTCM‐SC104format(Kalafusetal.

1986).Update rate for all satellites in view is 4‐5

seconds so that number  of satellites in view varies.

When bit error of transmission data occurs  by

propagation trouble and the correction data is not

completed,it

isshownthatdifferentialcorrectiondata

isnotupdateinthecaseofonetimedatalost.

3 ANALYSISFORPROPAGATION

CHARACTERISTIC

3.1 InCaseofBridgePier

In previous paper, we analyzed the propagation

characteristicofDGPScorrectiondatasignalnearbya

big bridge in order to investigate abnormal

propagation of MF beacon signal for DGPS.

Structure around  bridge pier of big bridges at the

Inland Sea is regarded as metal screen (Araki 1977)

showninFig.1,transmittivityaroundthereisa little

over 1 %.Electric field intensity on propagation

path from DGPS stationto big bridge is

 obtained in

caseofoverseaandoverlandindependentlybecause

of complex terrain (Nishitani 1980).In addition,

thereisdiffractionlossthatbridgepierisregardedas

knife edge (Shinji 1992) shown in Fig. 2, and the

diffraction loss is 10dB or more depending on

distancefrombridgepier.

551



MetalScreen

Figure1.Structureofmetalscreen



Figure2.Knifeedgediffraction

Table2.Totalelectricfieldintensity

_______________________________________________

bridge DGPS distanceelectric diffractiontotal

station (km) field loss  intensity

intensity

(*)  (*)(*)

_______________________________________________

Kanmon Wakamiya118.2 56  10.2  45.8

kyo

Kanmon Seto  130.8 55  10.3  44.7

kyo

Akashi‐ Oohama 195.6 50  12.3  37.7

Kaikyo

Akashi‐ Muroto‐ 170.4 52  12.1  39.9

Kaikyo Misaki

_______________________________________________

(*):dBμV/m



Figure3. Electric field intensity when passing through the

bridge(leveldowncase)

ModulationsystemofMFbeaconwaveforDGPS

is MSK (Minimum Shift Keying).Electric field

intensityisneeded40dBμV/mnottooccurtwicebit

errorper1wordthatbiterrorcorrectingcannotwork

(Saito 1996).Table 2 shows calculation results of

electricfieldintensitynearbybridgepier.In

thecase

of Akashi‐Kaikyo Bridge, if beacon receiver uses

DGPS station in Table 2, electricfieldintensityis 40

dBμV/morlessandalwaysthereisapossibilitythat

DGPScorrectiondataismissedonetime.

Inpreviouspaper,thereisapossibilitythatDGPS

correction data signal

causes bit error, and it is

confirmed that there is a possibility that adequate

position accuracy cannot be obtained (Okuda et al.

2011).Fig.3 shows measuringdata of electricfield

intensitymeasurementnearbyAkashi‐KaikyoBridge

on November 2010.It  was measured approximate

3dB decrease data at passing through the

 bridge.

BecausemeasuredsignalfromEsakistationwhichis

very near from Akashi‐Kaikyo Bridge has large

intensity, 3dB reduction does not occur some

problem. However, when Esaki station signal is not

utilizedbysomereasonswhichismissingtotransmit

etc., in case of using Muroto‐Misaki  station signal

user’sshipsailsontheextendedlineofbridgepier,so

thatitmaybeoccurredsomeproblemtofixposition.

These relationship and directions of transmission

fromDGPSstationsareshowninFig.4.



Figure4.GeometricrelationwithDGPSstationandAkashi‐

KaikyoBridge

3.2 InCaseofCenterofBridge

Now, we analyze the propagation characteristic

nearby a center of the bridge that was unsolved in

previous paper (Okuda et al. 2011).In order to

simplifythis problem, it is assumedthat in addition

surface wave the superior reflection wave which

reflectsatstructure

ofthebridgeexists.Fig.5shows

outlineofdirectionofMinamiBisan‐SetoBridgeand

Oohama DGPS station used on this simulation, and

trial ship sailed eastward.Fig. 6 (a) and (b) show

plane view and side view respectively indicated

propagationpathofreflectionwave.Fig.6(c)shows

an aspect of reflection around under the bridge.

Antennaheightoftrialshipusedthissimulationis10

mand bridge girder ofMinami Bisan‐Seto Bridge is

65mabovesealevel.Fig.7showoneofnumerical

simulation results for relative value of composite

signal strength (corresponding  to electric

 field

intensity) with surface wave (0dB) and reflection

wave on a condition shown in Fig. 6.It was

calculatedthatreflectioncoefficientwas0.5.



552



Figure5. Bisan‐Seto Bridge and direction of

transmission

Relative value of signal strength to exceed 0 dB

meansthatelectricfieldintensitybecomestobelarge

(toincreasesignalgain).Lessthan0dBmeansthat

electric field intensity becomes to be small (to

decrease signal gain).If it

is assumed that all of

reflection wave work as noise, maximum signal to

noise ratio is 6 dB.In this case, it is satisfied with

advicebyITUwhichrequiresthatsignaltonoiseratio

is7dBormoreinsideDGPScoverage.



Figure6(a). Propagation path of reflection wave (plane

view)



Figure6(b).Propagationpathofreflectionwave(sideview)



Figure6(c)Propagationpathofreflectionwaveunderthe

bridge



Figure7. Result of calculation for electric field intensity

(relativevalue)nearthebridge

Infact,reflectionpointisnotoneplace.Thereis

not only mirror reflection, and effects of reflection

wave decrease depending on increase of distance

fromthebridge. Ifitisassumedthattherearethese

phenomena, it is considered that composite signal

strengthhassomevariationshowninFig.8.



Fig. 9 (a) shows measuring data of electric field

intensitymeasurementwhosesituationisthesameas

thissimulation.Shiptocollectdatasailedeastward

on South Bisan‐Seto Traffic Route.It was observed

that signal strength increased and decreased at the

frontofthebridge, andsignalstrengthincrease

5dB

above minimum value around under the bridge.

This simulation calculated using single reflection

wave, so that measured data indicated the same

tendency.To summarize, in case of sailing passing

through a center of the bridge, it is observed some

variationofelectricfieldintensityaroundthebridge.

However, when

 magnitude of the va riation is less

than 7 dB against received surface wave directly,

thereisnopossibilitythatbiterroroccurs.



Figure8. Corrected result of calculation for electric field

intensity(relativevalue)nearthebridge

553



Figure9 (a). Electric field  intensity at passing through the

bridge (Minami Bisan‐Seto Bridge, using Oohama station

signal)



Figure9 (b). Electric field intensity at passing through the

bridge (Akashi‐Kaikyo Bridge, using Oohama station

signal)

Asmoreexamples,Fig.9(b)showsthatshipsailed

westward on Akashi‐Kaikyo Traffic Route, distance

fromOohamaDGPSstationbecametobecloseafter

passing through the Akashi‐Kaikyo Bridge.

However,comparedwithFig9 (a),periodicvariation

of electric field intensity was not remarkable.It

might be the

 boundary distance as DGPS MF radio

beaconwave.

3.3 SituationofOverseaPropagation

Weinvestigatedelectricfieldintensityperdistanceto

research an effect of relationship with oversea and

overland propagation.We measured electric field

intensity every adequate distance during sailing on

traffic route or waterway.So, its propagation was

almostoversea.Fig.10showsthatitwasappeareda

relationship with distance to intensity although

differenceat4‐90 km places from DGPS station was

smaller than reference (Nishitani 1980). In future

study,wewillcollectelectricfieldintensityincluded

overland propagation, and try to characterize an

effectofoverland

propagation.

-30

-20

-10

110100

Distance (km)

Electric Field Intensity

per each distance



Figure10.Electricfieldintensitypereachdistance

3.4 DiscussforOtherBridge

First, we discuss about nearby Kanmonkyo Bridge.

In the vicinity of Kanmonkyo Bridge which locates

midwaybetweenWakamiyastationandSetostation.

DGPS user utilizes either of these stations.

Wakamiya station is 12 km closer than Seto station

fromKanmon‐KyoBridge.Fig.11showsthat

incase

ofusingWakamiyastationon westward sailing and

incaseofusingSetostationoneastwardsailingthere

isapossibilitythataproblemoccursontheextended

line of bridge pier.Electric field intensity of DGPS

correctiondatasignalisover40dBμV/mrequiredby

DGPS

system according to estimation in previous

paper, but there is a possibility to occur bit error

whenoverlappingeachotherfactor.

Next,wediscussaboutnearbyKurushima‐Kaikyo

Bridge.In case of using Oohama station which is

veryclosefromKurushima‐KaikyoBridge,thereisno

problemevenifdiffractionloss

bybridgepierexists.

Consequently, when it cannot be used Oohama

station,thereisapossibilitythatsameproblemoccurs

incaseofusingSetostationorEsakistation.

Itisconsideredthatabove‐mentionedsomeeffects

of the big  bridge are same as DGPS operated by

USCGwhichis

thesamesysteminJapan.Existence

of big bridge, complex terrain etc., details of DGPS

operated by USCG is not clear, but it is considered

that the Golden Gate Bridge and the Bay Bridge in

SanFranciscoBaymaybeexistedthesameproblem.



Figure11. Geometric relation with DGPS station and

KanmonkyoBridge

554

3.5 EffectofOverlandPropagationandMulti‐path

Signal

Experimental navigation using our training ship

Kaigi‐Maru in July 2012 could be collected some

electric field intensity data including overland

propagation.12‐13 % of propagation path are on

land, which are about 6 n.m. against 50 n.m. which

are

allof propagationpath.Inthecase of300KHz

band,ifallofpropagationpathisland,itisestimated

that electric field intensity of50 n.m. path decreases

about 10 dB (Nishitani 1980).Because decreasing

electric field intensity regarding obtained data by

experimental navigation is approximately 1 dB,

normalsailingontrafficrouteisnoproblem.

Itwas compared with datafor mooring atFukae

pierofKobeportandsailingonKobeairportoff,soit

was observed that electric field intensity decreased

about 2 dB and data fluctuation is slightly large on

mooring at Fukae shown in

 Fig. 12(a), (b).It is

considered that 2 dB decreasing of electric field

intensityare to be 5 n.m. distance longer than Kobe

AirportofffromEsakistationwhichwasusedstation

ofDGPSandratioofoverlandpropagationare59%,

thatincludes80 %urbanarea.Butdata

fluctuation

atFukaeisalittlelessthan3dBbecausethereisno

large construction near Fukae pier.Nearby

Kanmon‐Kyo, electric field intensity of Diffraction

lossgeneratedarea44‐6dBμV/mshowninsection3.1

andTable2.Consequently,iftherearesomeeffects

of multi‐path signal,

 then electric field intensity

becomestobelessthan40dBμV/m,sothatbiterror

occurs and data of DGPS will have a trouble to

receivebyoccurringbiterror.Thereisapossibility

to be large effect of multipath by gantry crane at

containeryardexceptaneffect

ofbigbridge. Inthe

caseofinsideport,therearesomeproblemwhenpier

docking and undocking.However, investigating

minutely of effect of multi‐path by calculation for

intensity ratio of direct and reflection signal is

physicallyimpossible.Inthecaseofexistingmulti‐

path, it is considered that receiving

 signal level is

more than 1 Hz frequency, so we need to review

methods for data collectionandanalyzing,and then

theyarefuturetasks.



Figure 12(a) Sample of Including Much Overland

Propagation



Figure12(b)SampleofNormalSailing

4 CONCLUSION

Weleadedtothepropagationcharacteristicarounda

center of the bridge to apply simplified reflection

wave.The result almost corresponds to measuring

data of electric field intensity.To utilize this

simulationresultsandsomeeffectsofdiffractionloss

by bridge pier becomes to be able to examine

receiving

 condition of DGPS station.When DGPS

user cannot utilize the DGPS station which is close

andhassufficientsignalstrength, or when to obtain

position is insufficiency from the beginning, it is

confirmedthattoenterontheextendedlineofbridge

pier is that bit error may occur and

there is a

possibility that it cannot fix accurate position.

Furthermore we measured electric field intensity at

everyeachdistancefromDGPSstationtoresearchan

effectofoverlandpropagation.

Finally, as stated above, there is some possibility

that DGPS correction signal have been occurred bit

errorcausedbydecreasingelectric

fieldintensityand

level fluctuation by multi‐path, etc. nearby large

construction such as big bridge including overland

propagation, so there are some case that normal

DGPS position accuracy cannot obtain.Hereafter,

high accuracy position fix on pier docking and

undockingisessentialtoexecutesafetyandefficiently

navigation for

navigator using DGPS.

Consequently, we have toclarify these phenomenon

and consider countermeasures.In this paper, it is

suggestedthattherearenotonlyeffectofbigbridge

butalsoeffectoflargeconstructionnearpier,andwe

show necessity to investigate minutely an effect of

multi‐path.

REFERENCES

Araki,T.1977,DenjiBougaitoBousiTaisaku,TokyoDenki

UniversityPress,138‐139

DepartmentofDefense,DepartmentofHomelandSecurity

and Department of Transportation: 2008 Federal

RadionavigationPlan,2008

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