549
1 INTRODUCTION
NavigatorwhosailsnarrowchannelattheInlandSea
requireshighaccuracyoffixedpositionwhichis5m
(2drms) or less.In FRP 2008 (DoD, DoH.S. & DoT
2008), therequirement of position accuracy is 2 5 m
for the inland waterway phase.Therefore the
positionaccuracy
ofstandaloneGPSisinsufficiency.
ConsequentlyitneedstouseDGPS(DifferentialGPS)
fornavigatorsailingtheInlandSea.
Decreased reliability of fixed position using GPS
means that the reliability of GPS signal information
decreases and there is a possibility of abnormal
propagationofDGPScorrectiondatasignal,andsuch
case occurs in the Inland Sea.Decreased reliability
of GPS signal information means that there is
decreased reliability of transmitting signal including
satelliteconditionandsomechangesofGPSreceiving
condition around user including GPS receiver.
Decreasedreliabilityoftransmittingsignalshouldbe
compensatedbyRAIM(RadioAutonomousIntegrity
Monitoring)which
isabletoconfirmitautomatically.
However, if abnormal propagation of DGPS
correction data signal occurs, it is possibility to be
affectedbyit.Inthis paper, weresearchas part of
investigatingreceivedconditionintheInlandSea,but
sailingcircumstanceinEuropewheretherearemany
river ports,
large ships sailing on river around the
Great Lakes have common receiving conditionsame
astheInlandSea.Itconsidersthat changes of GPS
receiving condition are caused by incident of multi
path wave by not only sea reflection but also large
offshorestructuresorotherships.Inthispaper,the
main subject is to analyze the propagation
characteristic of DGPS correction data signal, so to
analyzechangesofGPSreceivingconditionisfuture
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
M
arineTechnicalCollege,Japan
ABSTRACT:UserattheInlandSearequires highpositionaccuracywhichis5m(2drms) orless.Thereforethe
positionaccuracyofstandaloneGPSisinsufficiency.ConsequentlyitneedstouseDGPSfornavigatorsailing
the Inland Sea. We executed numerical simulation of the propagation characteristic
on the extended line of
bridgepieratoppositesidefromDGPSstation,andalreadyconfirmedthatbiterrorinDGPScorrectiondata
signal occurs, and that correction data could not form by bit error. Furthermore, we carried out numerical
simulationofthepropagationcharacteristicofDGPScorrectiondatasignalreceived
atsailingthroughcenterof
thebridge,andsolvedreceivingconditionofDGPScorrectiondatasignalbeforeandafterpassingthroughthe
bridge.Inthispaper,weexecutedtoinspectmutuallyresultsofelectricfieldintensitysimulationforoversea
andoverlandpropagationonsomeseaareaoftheInlandSea
andmeasuringresultsofelectricfieldintensity
forDGPScorrectiondatasignal,andevaluatedthepossibilityofabnormalpropagationcomprehensively.
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
propagationand/orsomeeffectsbyoffshorestructure
suchasabigbridge.InpreviouspapersatITM2011
andITM2012(Okudaetal.2011&2012),we
executed
numericalsimulationofthepropagationcharacteristic
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 bybit error.In this
paper, we
carried out numerical simulation of the propagation
characteristicofDGPScorrectiondatasignalreceived
atsailingthroughcenterofthebridgethatwasfuture
task in previous paper, and solved receiving
conditionof DGPS correction datasignal before and
after passing through the bridge.This simulation
calculated a variation
of signal strength by
composition of superior reflection wave and surface
wave.Whenthetrailshipapproachesfromopposite
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
increasesanddecreasesaccordingtocombinedphase
becausereflectionwavefromthebridgeiscombined
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
November2010andJuly2012.
2 DGPSINJAPAN
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
receivesthecorrectiondatausingMFbeaconreceiver.
Position accuracy is improved by fixed calculation
using the correction data.At present nominal
positionaccuracybystandaloneGPS is 9 m (2drms)
in FRP 2008, on the other hand JCG (Japan Coast
Guard)
announcesthatpositionaccuracybyDGPSis
1m(2drms)orless.Whenusingdifferentialsystem,
accuracy decreases depending on distance from the
referencestation.Thenitisappropriatethatposition
accuracy by DGPS is 15 m (2drms) depending on
distance.DGPS has not only a function of
improvement
ofpositionaccuracybutalsoafunction
ofintegritymonitor.Afunctionofintegritymonitor
informssomechangesofGPSsatellitehealthinessor
decreasingaccuracyofpseudorangemeasurementto
user quickly and break off use of the satellite data.
ThissystemisexactlythesameasDGPSoperatedby
USCG(United
StatusCoastGuard).
At present in Japan there are 27 DGPS stations
whichisthereferencestation,andthecoverageisall
coastal area except a few isolated islands.Table 1
shows DGPS specification operated by JCG.JCG
callsuser’sattentionconcerningDGPScoverage(JCG
DGPScenter).
1 Exceptionof
someareaatInlandSeaabout200km
coverage.
2 Existenceofdifficultcasetousebyeffectofterrain
etc.
HoweverJCGmakenomentionaboutanareaora
phenomenonconcretely.
Table1.DGPSSpecificationinJapan
_______________________________________________
transmissionrate200bps
transmittingpower75W
coverage200kmfromDGPSstation
transmissionformatITURM.8231(RTCMSC104)
messagetypeType3,7,9,16
_______________________________________________
Next,wedescribeaboutthepresentconditionfor
integrity monitor.Each header of DGPS message
includes operating condition of DGPS station. If
positionaccuracyoftheDGPSstationbecomestobe
1.5 m or more, DGPS system demands to change
DGPSfixintostandaloneGPSfix.InType9,
DGPS
systemdemandstobreakofftouseonfixcalculation
whencorrectionvalueis8mormore.
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,butitisdifferentfromoursubject.
In this study, one of triggers is the phenomenon
thattransmissionofdifferentialcorrectionvaluewas
interruptedbecauseofpropagationtroubleonspecific
area.AtpresentDGPSinJapanbroadcasts
Type 3,
7,9,16basedonRTCMSC104format(Kalafusetal.
1986).Update rate for all satellites in view is 45
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
isshownthatdifferentialcorrectiondata
isnotupdateinthecaseofonetimedatalost.
3 ANALYSISFORPROPAGATION
CHARACTERISTIC
3.1 InCaseofBridgePier
In previous paper, we analyzed the propagation
characteristicofDGPScorrectiondatasignalnearbya
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)
showninFig.1,transmittivityaroundthereisa little
over 1 %.Electric field intensity on propagation
path from DGPS stationto big bridge is
obtained in
caseofoverseaandoverlandindependentlybecause
of complex terrain (Nishitani 1980).In addition,
thereisdiffractionlossthatbridgepierisregardedas
knife edge (Shinji 1992) shown in Fig. 2, and the
diffraction loss is 10dB or more depending on
distancefrombridgepier.
551
MetalScreen
Figure1.Structureofmetalscreen
Figure2.Knifeedgediffraction
Table2.Totalelectricfieldintensity
_______________________________________________
bridge DGPS distanceelectric diffractiontotal
station (km) field loss intensity
intensity
(*) (*)(*)
_______________________________________________
KanmonWakamiya118.2 56  10.2 45.8
kyo
KanmonSeto 130.8 55  10.3 44.7
kyo
Akashi‐Oohama195.6 50  12.3 37.7
Kaikyo
Akashi‐Muroto‐170.4 52  12.1 39.9
Kaikyo Misaki
_______________________________________________
(*):dBμV/m
Figure3. Electric field intensity when passing through the
bridge(leveldowncase)
ModulationsystemofMFbeaconwaveforDGPS
is MSK (Minimum Shift Keying).Electric field
intensityisneeded40dBμV/mnottooccurtwicebit
errorper1wordthatbiterrorcorrectingcannotwork
(Saito 1996).Table 2 shows calculation results of
electricfieldintensitynearbybridgepier.In
thecase
of AkashiKaikyo Bridge, if beacon receiver uses
DGPS station in Table 2, electricfieldintensityis 40
dBμV/morlessandalwaysthereisapossibilitythat
DGPScorrectiondataismissedonetime.
Inpreviouspaper,thereisapossibilitythatDGPS
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 measuringdata of electricfield
intensitymeasurementnearbyAkashiKaikyoBridge
on November 2010.It was measured approximate
3dB decrease data at passing through the
bridge.
BecausemeasuredsignalfromEsakistationwhichis
very near from AkashiKaikyo Bridge has large
intensity, 3dB reduction does not occur some
problem. However, when Esaki station signal is not
utilizedbysomereasonswhichismissingtotransmit
etc., in case of using MurotoMisaki station signal
user’sshipsailsontheextendedlineofbridgepier,so
thatitmaybeoccurredsomeproblemtofixposition.
These relationship and directions of transmission
fromDGPSstationsareshowninFig.4.
Figure4.GeometricrelationwithDGPSstationandAkashi
KaikyoBridge
3.2 InCaseofCenterofBridge
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
simplifythis problem, it is assumedthat in addition
surface wave the superior reflection wave which
reflectsatstructure
ofthebridgeexists.Fig.5shows
outlineofdirectionofMinamiBisanSetoBridgeand
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
propagationpathofreflectionwave.Fig.6(c)shows
an aspect of reflection around under the bridge.
Antennaheightoftrialshipusedthissimulationis10
mand bridge girder ofMinami BisanSeto Bridge is
65mabovesealevel.Fig.7showoneofnumerical
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
calculatedthatreflectioncoefficientwas0.5.