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.

