69

1 INTRODUCTION

Korean WA‐DGNSS is a large scale research project

fundedbyMinistryofLand,TransportandMaritime

Affairs Korea [1] [2]. It aims to augment the Global

Navigation Satellite System [4] by broadca sting

additional signals from geostationary satellites and

providing differential correction messages and

integrity data for the GNSS sat

ellites. Korean WA‐

DGNSS uses a network of wide area reference

stations, wide area master station, ground earth

station,andgeostationarysatellites.

Wideareareferencestationsarewidelydispersed

GNSS data collection sites  that monitor and process

satellitedatainordertodeterminesatelliteorbitand

clockdriftplusdelayscausedbytheat

mosphereand

ionosphere. This information is then transmitted to

wide area master station through terrestrial

communication network. Wide area master station

creates correction messages and broadcasts them

through geostationary satellites. The user segment

receivesthecorrectionmessagesandappliesthemin

ordertoimprovepositionaccuracyandreliabilit

y[5].

KoreanWA‐DGNSSprojectisbeingcarriedoutby

a consortium of universities and research institutes.

The research team at Electronics and

TelecommunicationsResearchInstituteisinvolvedin

designanddevelopmentofdataprocessingsoftwares

for wide area reference station and user segments.

Thispaperfocusesonuser segmentsoftwaredesign.

The user segment software uses GPS data broa

dcast

fromeachGPSsatellitetodetermineitsposition and

WA‐DGNSS corrections messages broadcast from

geostationary satellites to improve accuracy and

Korean WA-DGNSS User Segment Software Design

S.C.Shah,W.S.Choi&W.Y.Han

ICTConvergenceAutonomousResearchTeam,ElectronicsandTelecommunicationsResearchInstitute,Korea

H.Yun&C.Kee

echanical and Aerospace Engineering and Institute of Advanced Aerospace Technology, Seoul National

niversity,

Korea

ABSTRACT:KoreanWA‐DGNSSisalargescaleresearchprojectfundedbyMinistryofLand,Transportand

MaritimeAffairsKorea.ItaimstoaugmenttheGlobalNavigationSatelliteSystembybroadcastingadditional

signalsfromgeostationarysatellitesandprovidingdifferentialcorrection messages andintegritydataforthe

GNSS satellites. The project is being carried out by a consortium of universities and research inst

itutes. The

researchteamatElectronicsandTelecommunicationsResearchInstituteisinvolvedindesignanddevelopment

of data processing softwares for wide area  reference station and user segment. This paper focuses on user

segmentsoftwaredesign.KoreanWA‐DGNSSusersegmentsoftwareisdesignedtoperformseveralfunct

ions

suchascalculationofpseudorange,ionosphereandtropospheredelays,applicationoffastandslowcorrection

messages,anddataverification.Itisbasedonalayeredarchitecturethatprovidesamodeltodevelopflexible

andreusable software and isdivided into several independent

, interchangeable and reusable components to

reducecomplexityandmaintenancecost.ThecurrentversionisdesignedtocollectandprocessGPSandWA‐

DGNSSdatahoweveritisflexibletoaccommodatefutureGNSSsystemssuchasGLONASSandGalileo.

http://www.transnav.eu

the International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 7

Number 1

March 2013

DOI:10.12716/1001.07.01.08

70

integrity. It is designed to perform several functions

such as calculation of pseudorange, ionosphere and

troposphere delays, application of fast and slow

corrections messages, integrity monitoring, data

verification,andperformancevisualization.



Figure1.KoreanWA‐DGNSSArchitecture

The user segment software is based on a layered

architecturethatprovidesamodeltodevelopflexible

and reusable software and is divided into several

independent, interchangeable and reusable

components to reduce complexity and maintenance

cost. The current version is designed to collect and

process GPS and WA‐DGNSS data however

 it is

flexible to accommodate future global navigation

satellitesystemssuchasGLONASSandGalileo.



Figure2.UserSegmentSoftwareBlockDiagram

The rest of the paper is organized as follows.

Section 2 provides a high level overview of user

segment software architecture whereas Section 3

describes key data processing algorithms. The

detailed design of user segment software is given in

Section 4. Section 5 discusses the characteristics of

UserSegmentsoftwarewhile

Section6concludesthe

paper.

2 ARCHITECTUREDESIGN

A high level overview of user segment software

architecture is given in Figure 3. This section briefly

describes the key architectural elements. For a

detaileddescription, the readers are referred to User

SegmentSoftwareArchitecturedocument[6].

2.1 Descriptionofkeyarchitectural

elements

2.1.1 GPSEphemerisDataProcessor

This component determines the position of GPS

satellite with respect to user segment and applies

correctionsprovidedbyWA‐DGNSS.Itaccessesdata

from GPS Ephemeris Data Handler, GPS Ephemeris

Data Quality Monitor and Slow Corrections Data

Processor, and provides an interface to SV Azimuth



and Elevation Calculator, GPS SV Clock Data

Processor,ReceiverPositionCalculator,andRawand

Pre‐processedDataBuffer.

2.1.2 SlowCorrectionsDataProcessor

This component processes slow corrections

messages for GPS ephemeris and clock errors. It

accesses data from Slow Corrections  Data Handler

and provides  an interface to GPS Ephemeris

Data

Processor,GPSSVClockDataProcessorandRawand

Pre‐processedDataBuffer.

2.1.3 IndependentDataVerifier

This component independently  verifies the

integrityofactivedata.ItaccessesthedatafromData

Processors, Pseudorange Calculator and Receiver

Position Calculators, and provides an interface to

RawandPre‐processedDataBuffer.



2.1.4 GPSSVClockDataProcessor

ThiscomponentcalculatesSVclockcorrections.It

accessesthedatafromGPSSVDataHandler,GPSSV

DataQualityMonitor,GPSEphemerisDataProcessor

and Slow Corrections Data Processor, and provides

aninterfacetoPseudorangeCalculator.

3 DATAPROCESSINGALGORITHMS

Thissectiondescribesthe

keyalgorithmsrequired to

process fast and slow corrections messages. A

detailed description of data processing algorithms is

availableintheSBASUserSegmentSoftwareDetailed

Designdocument[7].

3.1 Algorithmtoprocessfastcorrections

IF(MessageType==0)

 SystemTesting

IF(MessageType==1)

 SavePRNmaskandIssueofDataPRN(IODP)

 providedinMessage

IF(MessageType==2‐5OR24)

 IF(IODP



 IODPprovidedinMessageType1)

Return

 IF(UDREI==14ORUDREI==15) 

InitializeRRCcalculationandsetRRC=0

SetsatellitestatusandReturn

 IF(PRCisnotvalid)Return

 IF(precisionapproach)

IF(UDREI>=12)

Satellitestatus=“donotuse”and

Return

 IF(

==0)SetRRC=0

 ELSE

Selectpreviouslyreceivedpseudorange

71

correctionPRCprevious

 IF(IODFofPRC

current==3)

SelectPRC

previouswhichisclosestto



secondspriortothePRC

current

 IF(IODF<3)

SelectmostrecentlytransmittedPRC

previous

 Calculaterangeratecorrections

 Calculatefastcorrections:

 CheckRRCstatus

IF(RRCstatusisvalid)

Calculatefastcorrections

 IF(MessageType==6)

 IF(IODF

j==3)

IF(messageisnotvalid)Return

 ELSE

DecodeandapplyUDREtocorrections

RecalculatePR

 IF(IODF

j<3)

MatchIODF

jwithIODFjprovidedinMessage

Typej(2‐5and24)

IF(notmatched)Return

IF(messageisnotvalid)Return

ELSE

DecodeandapplyUDREboundstocorrections

providedinMessageTypej

RecalculatePR

IF(MessageType==7)

 IF(IODP!=IODPprovidedinMessage

Type1)

Return

 ELSE

StoreUDREdegradationandtime‐outintervals



Figure3.UserSegmentSoftwareArchitecture



3.2 Algorithmtoapplyfastandslowcorrections

IntegrityCheck:

IF(UDREI==15)

 SVisunhealthy

 IF(UDREI==14ORSBASdatanotvalidORIOD! =IODE)

SVunmonitored

 IF(Precisionapproachmode)

IF(UDREIinMessageTypes2‐5and24>=12)

Donotusesatellite



IF(Precisionapproachmode)

 IF(Anyofcorrectionmessages

aremissedAND

previouslyreceivedcorrectionsarevalid)

Applydegradationmodels[7]

Applyfast,ionospheric,troposphericandclock

corrections:



measured SBAS SBAS

PR PR FC IC TC CC



  



Applyslowcorrections:



corrected GPS

yyy































IF(CorrectionmessagesarenotmissedAND

 previouslyreceivedcorrectionsareavailable)

 Applyfastandslowcorrections[7]

ELSE

 Precisionapproachnotpossible

Exit

72

IF(Non‐precisionapproachmode)

IF(Fastandslowcorrectionsareavailable)

Applyfastcorrections:

measured

PR PR FC CC

Applyslowcorrections:

corrected GPS

yyy



























IF(SBASionosphericcorrectionsareavailable)

ApplySBASionosphericcorrections:

measured SBAS

PR PR FC IC CC

ELSEIF(GPSionosphericcorrectionsavailable)

ApplyGP

Sionosphericcorrections:

measured GPS

PR PR FC IC CC

ELSE

Donotapplyionosphericcorrections

IF(SBAStroposphericcorrectionsareavailable)

ApplySBAStroposphericcorrections:

measured SBAS SBAS

PR PR FC IC TC CC

ELSE

Donotapplytroposphericcorrections

ELSE

Donotus

etheSV

3.3 Algorithmtocalculatepseudorange

Pseudorange residual implies the remaining error

obtained after eliminating errors such as satellite

clockbias,ionosphericdelay,troposphericdelay,and

receiver clock error from the smoothed pseudorange

ofsatellitej.

Calculatedistancebetweenuserandsatellitej:

222

jjjj

D(xx)(yy)(zz)

(x,y,z)

 Usersegmentcoordinates

jjj

(x , y ,z )

 Satellitejcoordinates

CalculatePseudorange:

j j iono tropo sv p

PR D d d t B e  

 Measurementnoise

Bisreceiverclockerror

SmoothPseudorange:

jC1

SPR PR





 isavailablefromnavigationmessage

Mismovingaveragepoints

CalculatePseudorangeResiduals:

j j j iono tropo sv

PRR SPR D d d t B



  



Figure4.ClassesofUserSegmentSoftware

4 CLASSDIAGRAMS

The class diagrams are used to describe  the static

structureofasystem[3].Eachclassinaclassdiagram

has attributes, operations and the relationship with

other classes. The classes are represented by

rectangles which show the name of the class and

optionallythe nameof theoperations and a

ttributes.

Compartments are used to divide the class name,

attributesandoperations.

The classes used in WA‐DGNSS User Segment

softwarearelistedinFigure4.Theclassesaredivided

intofivecategories,eachrepresentedwithadifferent

color. The classes in light red color are used to hold

raw and pre‐processed data whereas classes in light

yellowcolorareusedtoholdcorrectionsdata.Classes

in light green and purple colors are data processing

classes tha

t are used to process raw GPS data and

WA‐DGNSS corrections, respectively. The classes in

light blue color are used to pa

rse RINEX navigation

and observation files.The class diagramof raw GPS

data processing classes is depicted in Figure 5. A

detailed description of each class is specified in the

SBAS User Segment Software Detailed Design

document[7].

73



Figure5.ClassDiagramofGPSDataProcessingClasses



4.1 EphemerisDataClass

EphemerisData class is used to hold satellite orbit

parametersthatareusedtocalculatesatelliteposition

and velocity. It is also used to hold parameters for

calculating satellite clock corrections. For each

parameter, a property is defined that is used to

provide a flexible mechanism to read,

write, or

computethevalueofparameter.

4.2 RawPre‐proccssedDataBufferClass

A RawPre‐proccssedDataBuffer class is a container

class that is used to contain objects such as

ephemerisData, ionosphericData and observationSet.

It also provides methods for adding and removing

objectsaswellaswaystoiteratethroughthem.

4.3 FastCorrections

FastCorrections

 class is used to hold fast corrections

datasuch aspseudorange corrections  andrange rate

corrections which are used to remove pseudorange

errors. It also provides a mechanism to set and get

valueofeachfastcorrection.

4.4 EphemerisDataProcessorClass

EphemerisDataProcessor class is used to process

ephemerisdatainorder

todeterminesatelliteposition

andclockerrors.

4.5 FastCorrectionsProcessorClass

FastCorrectionsProcessorclassisusedtoprocess fast

corrections data in order to determine valid

pseudorangeandrangeratecorrections.

4.6 IonosphericDataProcessorClass

IonosphericDataProcessor class is used to process

ionospheric data and observation data in order to

determineionosphericcorrectionsforsingle

anddual

frequencyusers.

4.7 PseudorangeCalculatorClass

PseudorangeCalculator class is used to calculate

pseudorange and pseudorange residuals. The

calculated pseudorange includes corrections for

ionosphere,troposphereandsatelliteclockerrors.

74

5 USERSEGMENTSOFTWARE

CHARACTERSITICS

5.1 ModularSoftwareDesign

WA‐DGNSS User Segment software is modular in

design. It is divided into several independent,

interchangeable and reusable components to reduce

complexity and maintenance cost. The components

canbeaddedorreplacedintousersegmentsoftware

without or with minimum affect

 on the rest of the

components. The current software is designed to

collectandprocessGPSandSBASdatahoweveritis

flexibletoaccommodatefutureGNSSsystemssuchas

GLONASSandGalileo.

Thenewcomponentcanbeincorporatedintouser

segmentsoftwarethrough interfacesthat express the

elements that

are provided and required by the

component.Ifnewcomponentadherestointerface,it

can be added or replaced without affecting other

components.

5.2 LayeredArchitecture

A layer represents a group of related functionality.

Layered architecture provides a model to create

flexible and reusable software. The user segment

softwareisdivided

intodifferentlayerssuchasData

CollectionandData Processing  where eachlayercan

beeasilyreplacedornewlayercanbeaddedwithout

orwithminimumaffectonotherlayers.

6 CONCLUSION

This paper describes the detailed design of Korean

WA‐DGNSSUserSegmentsoftwareintermsofUML

architectural, activity and class diagrams. The user

segment software is designed to perform several

functions such as calculation of pseudorange,

ionosphere and troposphere delays, application of

fast and slow correction messages, and data

verification.Itisbased ona layered architectureand

adoptsamodularsoftwaredesignapproach.

ACKNOWLEDGEMENT

This research

 was a part of the project titled “WA‐

DGNSS Development” funded by the Ministry of

Land,TransportandMaritimeAffairs,Korea.

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