425

1 INTRODUCTION

Over the years, Fugro has developed a number of

augmentationservicesforGlobalNavigationSatellite

Systems (GNSS) from code and carrier‐phase

differentialsystemstostate‐of‐the‐artPPP[1].Precise

Point Positioning [2,3] is based on precise orbit and

clock estimates for GNSS satellites together with

precise observation modelling at the user‐end. The

in advantage of PPP with respect to differential

positioningistheprovisionofhomogenousaccuracy

withindependenceofdistancetoareferencestation,

and the need for a less dense reference station

network to maintain a given level of position

accuracy.

In this context, Fugro int

roduced in 2009 the G2

service, which was at that time the first global

realtimePPPservicesupportingGPSandGLONASS

[4]. Following the GNSS evolution, the system later

addedsupportforBeiDouin2015andGalileoin2016,

thusbecomingaG4serviceandsupportingallglobal

GNSSsystems[5].

In addition, lat

est advances in PPP technology

have shown the possibility to fix so‐called carrier

phase ambiguities to their integer values, provided

that additional corrections (namely Uncalibrated

PhaseDelays–UPDs–whicharehardwaredelaysin

theGNSSsatellites)areprovidedtotheend‐user[6,7].

FugroiscurrentlygeneratingUPDsforGPSsat

ellites

[8], therefore allowing the users to obtain higher

levelsofpositionaccuracy.G4+denotesG4solutions

whencarrier‐phaseambiguitiesarefixed.

G4 Multi-constellation Precise Point Positioning Service

for High Accuracy Offshore Navigation

.Tegedor,O.Ørpen&T.Melgård

FugroNorwayAS,Oslo,Norway

D.Łapucha

FugroUSAMarineInc.,Lafayette,US

H.Visser

FugroIntersiteB.V.,Leidschendam,Netherlands

ABSTRACT: Fugro is operating a global GNSS infrastructure for the delivery of high‐accuracy multi‐

constellationPrecisePointPositioning(PPP)service,namedG4.Preciseorbitandclockforallglobalsatellite

navigationsystemsareestimatedinreal‐timeandbroadcastto theusers usinggeostationary satellites.End‐

userswithaG4‐enabledreceiverareab

letoobtainsub‐decimeterpositioningaccuracyinreal‐time.Thesystem

hasbeentailoredforoffshoreapplicationswhereanearbyGNSSstationisnotalwaysreadilyavailable.

G4 offers seamless integrationofGPS,GLONASS,Galileo and BeiDou in the navigation solution, therefore

allowing the userto obtain a reliable andaccurate positionevenin challenging environments, especially in

presence of interference, scint

illation or partial sky visibility. In addition, carrier‐phase integer‐ambiguity

resolution(IAR)issupported,forthoseusersrequiringthehighestpossiblenavigationaccuracy.

Thispaperpresentsthe G4system architectureand currentperforma

nce.Thebenefits ofmulti‐constellation

PrecisePointPositioning(PPP)areshownintermsofincreasedavailability,robustnessandaccuracy.

http://www.transnav.eu

the International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 11

Number 3

September 2017

DOI:10.12716/1001.11.03.05

426

2 GNSSSYSTEMSSTATUS

Atthetimeofwritingthisarticle,theGPSsystemis

composed of 31 operational satellites, including 12

IIR,7 IIR‐Mand 12IIF satellites.GPS‐IIR‐Mand IIF

satellites are transmitting the new enhanced L2C

signal,whileGPS‐IIFarealsotransmittinga

newcivil

signalonthethirdfrequencyL5.

The Glonass system is composed of 24 satellites,

mainly Glonass‐M satellites, but also including one

newgenerationGlonass‐K1satellite.

In addition, the Chinese system BeiDou is

currently composed of 5 Geostationary (GEO), 5

Inclined Geosynchronous (IGSO) and 4 Medium

Earth

(MEO)orbit satellites.Allthesesatelliteswere

putinorbitby2012.Inaddition,Chinahaslaunched

7additionalsatellitesin2015and2016,buttheseare

not considered part of the operational constellation

yet.

Lastbutnotleast,theGalileosystemwasdeclared

readyforoperationaluseonDecember15th

2016.In

January 2017, the system consists of 11 usable

satellites,including3IOV(InOrbitValidation)and8

FOC (Full Operational Capability) satellites.

Additionally,thereisoneIOVsatellite(E20)whichis

only transmitting on E1 frequency, and cannot be

used for dual‐frequency PPP. Two additional

satelliteswere

placedin ellipticorbitsaftera launch

failure in 2014 (PRNs E14 and E18). These satellites

startedtobroadcasttestephemerisin2016,although

they are still reported as unhealthy. Finally, four

additional satellites (E03, E04, E05 and E07) were

launched in November 2016 and are under

commissioningatthetime

ofwritingthisarticle.

Therefore, the G4 service currently generates

correctionsforover80satellitesanditisexpectedthat

it will support about 115 satellites in 2020 once

BeiDouandGalileoconstellationsarefullydeployed.

G4offersseamlessintegrationofallsatellitesystems

todeliverarobustsolutionfor

theend‐user.

3 G4SYSTEMARCHITECTURE

The G4 systemhigh‐levelarchitecture is depicted in

Figure1.TheG4networkconsistsof45multi‐GNSS

reference tracking stations, worldwide distributed

(Figure 2). For redundancy reasons, two different

receiver brands are used in the network. This

guarantees system continuity should an

 anomaly

appearinoneofthereceiverbrands.



Figure1.G4systemarchitecture

Real‐timeobservationandnavigationdataissent

to the processing centres, located in Norway, which

areincharge ofestimating real‐time orbitandclock

estimates for all GNSS satellites. These are then

forwardedsimultaneouslytotwoNetwork

Control Centres (NCCs), located in Houston (USA)

and Perth (Australia). The NCCs

 generate the final

correction streams (including UPDs) and broadcast

them to the users via eight geostationary satellites,

whosecoverageisdepictedinFigure2.



Figure2. G4 tracking network and coverage area for

geostationarysatellites

Atanygivenlocationbelowabout75°latitude,the

usercantypicallyobservetwogeostationarysatellites

simultaneously. Corrections are broadcast in L‐band

andcanbereceivedviastandardGNSSantennas.At

theuserend,Fugro‐enabledreceiverscandecodethe

L‐band correction signal and, using Fugro’s

proprietary PPP processing

 engine, obtain high‐

accuracyreal‐timepositioning.

The system has been designed with significant

levels of redundancy, in order to avoid any end‐to‐

endsinglepointoffailureandmeethighavailability

requirementsforoffshorenavigationandoperations.

4 GALILEOACTIVATION

Asdescribedinsection2,theGalileosystem

hasbeen

developed significantly in the last years, and Initial

Services were declared on December 15th, 2016.

Fugro has been actively preparing for supporting

GalileoinitsPPPservice[9],andstartedtobroadcast

corrections for the 11 Galileo satellites immediately

afterInitialServicesweredeclared.Figure3showsa

display

ofaG4solutioninsideaFugro9205receiver

using Galileo corrections broadcast over ESAT

satelliteonDecember15th,2016.



Figure3.DisplayofaG4solutioninaFugroreceiverafter

GalileoactivationonDecember15th,2016

427

5 ACCURACYPERFORMANCE

Figure 4 shows real‐time G4 kinematic performance

forastationaryreceiverlocatedinOsloonDecember

15th,2016.ItshouldbenotedthatGalileocorrections

startcontributingtothepositionsolution from10:30

UTC when the first operational Galileo corrections

werebroadcastviageostationarysatellites.

Figure4. Real‐time G4 performance in Oslo on December

15th,2016



Figure5.G4positioningstatisticsforOsloandPerth

Figure 5 presents position statistics for Oslo

(Norway)andPerth(Australia)forthefirst weekof

2017.Itcanbeobservedthatpositionaccuracyisvery

stable and consistent, and horizontal accuracy is

about1.2cmhorizontaland4.0cmvertical(1‐sigma).

6 KINEMATICMARITIMEPOSITIONING

In order to assess

 G4 performance in a realistic

maritime environment, a dual antenna GNSS

positioning system has been installed on‐board the

Baronen vessel,ahighspeed passengerferry inOslo

fjord (depicted in Figure 6). The vessel is equipped

withtwoGA810antennas,eachofthemisconnected

toa Fugro9205

multi‐GNSSreceiver.Both on‐board

receivers have been configured to deliver G4+

solutions, including GPS, GLONASS, Galileo and

BeiDou.



Figure6. Baronen vessel including location of GNSS

antennas



Figure7.TrajectoryoftheBaronenvesselonJanuary 14th,

2017

The real‐time absolute positions are stored on‐

board in NMEA format and retrieved routinely for

performance assessment. Figure 7 depicts the vessel

trajectory in the Oslo fjord on January 14th, 2017.

Figure8displaysthenavigationsolutionfortheday

underanalysis,includingantennaheights,measured

antenna distance, vessel speed

 and number of

satellitesusedinthesolution.Fromthespeedplot,it

can be observed that the vessel is doing short trips

428

between 4‐8 UTC and 14‐23 UTC. The average

cruisingspeedisabout14m/s(27knots).Therestof

thetimethevesselisdocked.

In order to assess the quality of the positioning

solution, the baseline (distance) between the two

antennas has been computed, by differencing the



absolute antenna positions ateveryepoch. It can be

observedthatthecomputedantennadistanceisvery

stableoverthewholeday,eveninthehigh‐dynamic

conditions. The standard deviation of the antenna

distanceis1.9cm,forthe24hoursunderanalysis.

7 MULTI‐GNSSBENEFITS

Multi‐GNSS

PPP solutions deliver higher‐levels of

accuracycomparedtoGPS‐onlysolutionsduetothe

increased number of satellites. However, the main

benefitofmulti‐GNSSisincreasedrobustnessincase

of poor satellite tracking, in cases of partial sky‐

visibility, ionospheric scintillation or interference.

Thanks to additional satellites in view

 and different

frequencies used by different GNSS, multiGNSS

solutions perform significantly better in situations

withpoorsignalquality.

As an example, Figure 9 displays signal to noise

ratio(SNR)measuredbyaFugromonitoringreceiver

located in Port of Spain (Trinidad and Tobago) on

December29th,2016.Thelocationwas

affectedbya

20minute long RF interference that impacted

significantlythequalityofreceivedGNSSsignals.In

particular,bothGPSandGLONASSL1andL2bands

wereaffected byabout10dB. GalileoE1 signalwas

alsoaffected,andGalileoE5toalesserextent.Fugro

hasobservedsimilareventsin

otherlocationsandthis

kindofinterferencecanaffectmaritimeusers,causing

eithertotallackoftrackingforGNSSsatellitesor,as

in this case, poor signal quality and increased

frequencyofcycleslips.

Inthepositiondomain,figure10 showsG2 (GPS

and GLONASS) and G4 (including Galileo) height

error in Port of Spain for the same time span,

includingalsothenumberofsatellitesthatwasused

inthePPPsolution.Itcanbeobservedthatatagiven

moment in time, only three GPS and no GLONAS

satellitesareusable,andthisresultsinapositionreset

for

theG2solution.

Thanks to the addition of Galileo, 2‐3 additional

satellitescanbeusedwhentheinterferenceappears,

and continued good accuracy can be computed

during the whole interval, without impact for the

end‐user.Galileoisparticularlyusefulinthesecases

asitprovidesadditionalfrequencyredundancy

inthe

E5band.

8 CONCLUSIONS

Fugro’s G4 system represents state‐of‐the‐art Precise

Point Positioning technology for off‐shore

positioning, and it has been proven to deliver

outstanding results in both static and dynamic

conditions.WiththeadditionofGalileoandBeiDou,

navigation users can use today as many

 GNSS

satellites as never before, and this allows

unprecedented levels of accuracy, availability and

continuityformaritimeapplications.



Figure8.Navigationsolutionon‐boardtheBaronenvessel



Figure9.SignaltoNoiseRatio(SNR)recordedfordifferent

GNSSsignalsinPortofSpainonDecember29th,2016

429



Figure10. G2 (GPS+GLONASS) and G4

(GPS+GLONASS+Galileo)heightaccuracyforPortofSpain

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