79
1 INTRODUCTION
The goal of performed research was to familiarize
with functionality of RTKLIB (Takasu 2007) and its
possibilitytouseinprecisepositioning.Thistoolisa
free open‐source package containing applications
designed for real‐time navigation and post‐process
positioning. RTKLIB uses several modes such as
Single,PPPandcarrier‐pha
sebased.Thosemodesare
describedindetailinnextchapter.
In process of gathering data LEA‐6T module (Fig.1)
andantenna (Fig.2) fromu‐blox wereused.LEA‐6T
isalow‐costprecision‐timingGNSSmodule.Itallows
to gather lots of type of data including raw
measurements,whichwereused.Datagatheredusing
themodulearelist
edinchapter3.
Figure2.u‐bloxGPSAntennawithmagneticbase.
Evaluation of RTKLIB's Positioning Accuracy Using
low-cost GNSS Receiver and ASG-EUPOS
B.Wiśniewski,K.Bruniecki&M.Moszyński
GdańskUniversityofTechnology,Poland
ABSTRACT:Thepaperfocusesona comparisonofdifferentpositioningmethodsprovidedbyfree andopen
source software (FOSS) package called RTKLIB. The RTKLIB supports real‐time and post‐processed
positioning. The most important modes of operation tested by the authors are Kinematic, Static, Fixed and
Precise Point Positioning (PPP). The data for evaluation were obtained from low‐cost Gl
obal Navigation
SatelliteSystem (GNSS) receiver. The tested receiver was based on the u‐bloxʹs LEA‐6T GNSS module. This
receiver provides different types of information including raw carrier phase measurements. It gives the
possibilityforcentimeter‐levelprecisionofpositioning.AsthesupportingsourceofdataASG‐EUPOSsystem
wasused.ASG‐EUPOSisaPolishnetworkofGNSSreferencestationsprovidingthereal‐ti
mecorrectionsand
postprocessingservicesfortheentireterritoryofPoland.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 7
Number 1
March 2013
DOI:10.12716/1001.07.01.10
80
Figure1. u‐blox LEA‐6T mounted on the printed circuit
board.
ASG‐EUPOS (Bosy et al. 2007) was used as
supporting source of data. This Polish network of
reference stations provides different type of
positioning services. Available services deliver
supportingdata forreal‐time navigation(NAWGEO,
KODGIS, NAWGIS) and for post‐processing
(POZGEO,POZGEOD).IntheresearchPOZGEOand
POZGEO D were used. First one allows to post‐
processgiven data filein orderto calculateposition.
The second one provides data ab
out base‐stations
(position, raw observation data), which are used in
post‐processtoolthatispartofRTKLIBpackage.
2 POSITIONINGMETHODSANDACCURACY
Nowadays, satellite navigation is a non‐negligible
technology when developing an inexpensive,
locat
ion‐basedsystemaccessibleformanypeople.For
example, surveysconducted by theauthors with the
visually impaired in the “Voice Maps” project
(Kamiński & Bruniecki 2012) showed the desired
accuracyshouldbeataleveloffewmetersorbetter–
dependingontheact
ualapplication.
Inordertoimprovetheaccuracyofpositioningby
a smartphone‐based GPS receiver in the “Voice
Maps” project, the authors developed and
implemented the Differential GPS (DGPS)‐based
module. A DGPS approach reduces some sources of
errors occurring during the GPS‐based positioning
(Prasad&Ruggieri2005).Amongtheseerrorsonecan
dist
inguishtheimpactofionosphereandtroposphere
effects, which could result in inaccuracies reaching
tens of meters (Prasad & Ruggieri 2005). The
implemented system uses additional information
fromPolishground‐basedaugmentationnetworksi.e.
ASG‐EUPOS.
The Figure 3 shows the hardware for a DGPS
solution. It consists of a GAR
MIN eTREX H GPS
receiver (capable of interpreting DGPS corrections
sentviatheRTCMprotocol)andaBluetoothadapter
responsible for communication between the mobile
deviceandtheGPSreceiver.
Figure3.DGPSmeasurementunit.
Theadapterprovidesawirelessconnectiontothe
smartphone and the RS232‐based connection to the
GPS receiver. A mechanism implemented in the
smartphone application is responsible for
communication with the ASG‐EUPOS network, and
forsendingcorrectionstothereceiverviatheadapter.
Inresponse,thereceiverusestheadaptertoprovidea
more accurate position for the smartphone
applicat
ion.
HowevertheresultsobtainedwithDGPSwerenot
sufficiently good. Measurements of the DGPS
accuracyshowedthattheKODGIS‐basedpositioning
gives better results than the NAWGIS service. A
comprehensive comparison of the KODGIS and the
NAWGIS approaches is given in Figures 4 and 5.
ThoseFiguresshowspercentagesofmeasurementsof
three GNSS a
pproaches within selected ranges of
positioning error and the cumulative distribution of
positioningerrorforallthemeasurements(Kamiński
&Bruniecki2012)respectively:
| { : error( ) } |
( ) 100%
||
dD d x
Fx
D
(1)
where
D isthesetofallmeasurements.
Figure 4. Comparison of DGPS based KODGIS, NAWGIS
andembeddedGPSreceiverpositioningaccuracy.
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Figure 5.Comparison of DGPS based KODGIS, NAWGIS
andembeddedGPSreceiverpositioningaccuracy.
That was the reason for this research on
inexpensive carrier phase measurement based
positioningalgorithms.
2.1 SinglePointPositioning
In RTKLIB this mode is used to calculate position
using Single Point Positioning (SPP) or Space Based
AugmentationSystemDifferentialGPS(SBASDGPS)
whenusageofSBASdataisenabled.Configurationof
thistoolallowtousemanysourcesofdata,although
onlyroverdataisrequired inprocess ofpositioning.
Additionalsourcesofdataareephemeriscorrections,
ionospherecorrections,andprecisesatelliteclock(all
threecanalsobeusedinothermodes).
InSPPthepositioniscalculated usingonlygiven
satellites positions. Using at least four satellites is
necessary to find solution of navigation equations
with four unknowns representing receiver position
anderrors(NRC1995).
SBAS is a solution that use one or more
geostationary satellites to transmit corrections
obtained using observations from several to tens of
measurement stations. Few SBAS
exists at the time.
Territory of Europe is covered by European
GeostationaryNavigationOverlayService(EGNOS).
AccuracyofresultsobtainedusingSinglemodein
RTKLIB are at level of meter to dozen of meters for
SPPandlevelofmeterforSBASDGPS.Single mode
testresultsareshownin
chapter4.
2.2 Referencedependantcarrier‐phasemethods
RTKLIB provides variety of modes using carrier‐
phase based positioning. Modeswhich were usedin
this evaluation are Kinematic, Static and Fixed. All
three are described below. Required sources of data
arerawobservationsfromreceiverandbasestation.
Kinematiccarrier‐basedmethod
isusuallyapplied
when positioning of moving object is required. This
technique allow to achieve decimeter level of
accuracy (NRC 1995). For proper work base station
position is required to be set in configuration. This
position can be obtained from observations file,
obtained from RTCM message or by other manners.
In performed research base station position was
availableinRINEXobservationfileprovidedbyASG‐
EUPOS.
Instaticcarrier‐phasemeasurementsolution,there
isneedforlongobservationsanddatacollection.This
requirementisrelatedtogeometrychangeinsatellite
track, which assist in ambiguity resolution (NRC
1995). Like in all
carrier‐phase based methods two
sourcesofdataarenecessarywhichofonehavewell
knowncoordinatestoserveasbasestation.
Fixed is special mode provided by RTKLIB. It’s
similartopreviouslymentionedcarrier‐phasemodes,
with the difference that rover’s initial position is
known.Theroverpositioncan
beconfiguredinexact
mannerasbasestationposition.
2.3 PrecisePointPositioning(PPP)
PPP is a positioning technique in which only single
receiveris required,withoutneed forreferencefrom
basestations.Tocalculatepositioninputcode, phase
observations, precise satellite orbits and precise
satelliteclockisused.Bestresults
areachievedforat
leastdualfrequency(L1+L2)receiver.Itisknownthat
good quality results can be acquired for single
frequency (L1) receiver, but these measurements are
based mostlyonpseudo‐range. Highdependence on
pseudo‐range causes also bigger influence of
ionosphere behavior and increased need for its
corrections(Witchayangkoon2000).
ThemaindrawbackofPPPalgorithmsislongtime
ofconvergence.Toreachgood qualityof positioning
measurements have to be taken for over dozen
minutes. When using single frequency this time can
extend to tens of minutes or few hours. It happens
that convergence won’t be achieved
at all
(Witchayangkoon2000).
TousePPPmodeinRTKLIBsinglesourceofdata
is required‐raw observations from rover. The tool
willautomaticallydetecthowmanyfrequencieswere
used in measurement. RTKLIB implements three
methodsofPrecisePointPositioning.These methods
areKinematic,StaticandFixed.
3 MEASUREMENTSSETTING
ANDGATHERED
DATA
In order to evaluate RTKLIB’s positioning accuracy
two measurements were taken. First at “Pachołek
Gdański”(54º24’41”N;18º33’02”E,Fig.6)secondone
at parking lot near “Real” Shopping Center
(54º24’11”N;18º35’24”E,Fig.7).
First measurement was failure due to poor
weather condition and suboptimal location of
antenna.
Results showed that algorithms were not
able to hold stable fix for a longer period.
Furthermore when tool has been reaching another
fixed solutions, the positions of point clusters were
scattered.
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Figure6.Firstmeasurementsite.
While performing second measurement optimal
location of antenna was assured, which provided
better sky view. Antenna was attached directly to
center of car roof so it prevented from gathering
signals reflected from ground. Results of second
measurement analysis are described in detail in the
nextchapter.
In order to gather necessary data proper
configurat
ionofLEA‐6Tmodulehasbeenperformed.
Figure7.Secondmeasurementsite.
Configurationtookthefollowingsteps:
1 Connection between computer and receiver has
been established with baud rate set at 115,000,
whichwasenoughvalueformaximumfrequency
ofincomingsamples,
2 InView‐>ConfigurationView‐>RATE(Rates)‐>
Measurement Period value was set to 200 ms,
whichgave5HzMeasurementFrequency.
3 InView‐>Configurat
ionView‐>MSG(Messages)
‐>Messagesof type02‐10RXM‐RAWand02‐11
RXM‐SFRB were enabled at UART1 and USB
ports.
4 Recording wasturned ON. This wasachieved by
selectingRecordButtononPlayerBarandcreating
newlogfile.
This setup allowed to collectfive samples of raw
data andSFRB measurementsper second forfurt
her
analysis.FilewassavedinUBXformat.
4 RESULTSOFPOSITIONINGWITHTHERTKLIB
ANDASG‐EUPOS
As the experiment at the first site was unsuccessful
only the results from the second experiment will be
describedhere.
Therawdatafromthereceiverwasacquiredinthe
UBX format
. The essential part were carrier phase
measurements inside the UBX\RXM\RAW frames.
The Figure 8 presents the sample UBX\RXM\RAW
frame.SuchaframecontainsCarrierPhase,Pseudo
Range, Doppler Signal, Noise Ratio, Satellite
designation and the int
erior receiver Lock Signal
status.
Beside the UBX\RXM\RAW frames the essential
for further processing with RTKLIB are
UBX\RXM\SFRB data. These frames gives the
information on actual navigational massages from
eachsatellite.
In order to test the navigational algorithms the
acquired UBX stream was decomposed into the
RINEX2.10formatpa
rts.Thedecompositiongavethe
observation file (OBS), GPS satellites navigation
message file (NAV), Geostationary satellites
navigation message file (HNAV) and the satellite
basedaugmentationfile(SBS).Forfurtherprocessing
onlytheOBSfileisneededineverymode.Restofthe
files are optional depending on the algorithm
selected.
Figure8.SampleUBX\RXM\RAWframefromtheu‐center.
The simplest configuration of RTKLIB is for the
Singlesolution.ItneedsonlytheOBSfile.Theresults
of Single solution algorithm is presented on the
Figures9and10.
83
Figure9.Theresultofsinglesolutionpositioning.
Much better results were achieved when
computing the Static solution. Its computation
demands availability of reference data. Performed
evaluation included the use of reference data from
singleASG‐EUPOSstation(GDAN1)aswellasfrom
virtual reference station (VRS) prepared for the
nearby location (54º24’11”N; 18º35’24”E; h_el=40m).
The Figures 11 and
12 presents the configuration of
RTKLIBforstaticpositioningwiththereferencedata
fromthe GDAN1reference station.The computation
is based only on the L1 frequency and only the
forwardanalysisisselected.
Figure10.Singlesolutionpositioningfluctuations.
Figure11.ConfigurationoftheRTKLIBinputfiles.
Figure12.RTKLIBalgorithmconfiguration‐themajorpart.
The time of computation lasts about 50 seconds.
The time of whole observation is about 1 hour 20
minutesandtheobservationrateis5Hz,whichgives
about 24,000 epochs. The general solution of static
computationispresentedintheFigures13and14.
Figure13.TheresultsofstaticpositioningontheCartesian
plane.
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Figure14.Staticpositioningfluctuations.
Figure15.Staticpositioninginbackwardpositioningmode.
The statisticsgave approximately 62% successful
fixrate.Thefirstfixisobtainedafterabout8minutes.
The second half of observation gave much worse
results. Such effect made much less promising the
resultsfromthebackwardcomputationresultsonthe
samedata.Theresultsobtainedinthebackwardand
combined computation mode are presented in the
Figures 15 and 16. The successfully fix rate in the
backwardmodeisonthelevelof35%.
Figure16.Staticpositioningincombinedpositioningmode.
Figure17.TheresultsofPPP‐staticpositioningincombined
mode.
Figure18. The results of PPP‐kinematic positioning in
combinedmode.
Thefigureshowsalsothefalsefixesobtainednear
the end of observation. Processing in the combined
modegavemuchbetterresults.Unfortunatelyevenin
the combined mode near the end of observation
period is unsuccessful. The fixes obtained near the
end are probably false fixes. Fortunately in the
combinedmode
observationsfromthebeginningare
being fixed. This give the biggest success rate for
combinedmodealthoughneartheendtherearesome
falsefixes.
The PPP‐static processing without the reference
datawasalsotested.TheFigures17and18showsthe
results of combined mode of computation for
PPP‐
staticandPPP‐kinematicalgorithms.
TheRTKLIB inPPP‐staticmode fixesthe location
withrelativelyhighstability.Analysisoftheaccuracy
ofthisfixresultsinthe2mhorizontalerrorwhenthe
vertical inaccuracy is at a level of 3 meters. In the
PPP‐kinematic mode dynamic fluctuations
can be
seen.HowevercomparingPPP‐kinematicpositioning
accuracytotheSinglesolutionwithSBASasshowin
the Figure 19 may be seen that PPP‐kinematic
positioningismorecohesive.
85
Figure19. The comparison of Single and PPP‐Kinematic
positioningresults.
5 CONCLUSIONS
RTKLIB found to be a tool that allow to perform
broad variety of post‐analysis of gathered data.
Furthermore it’s a tool that allows real‐time
navigation(subjectforfurtherresearch).
Most important modes were checked in order to
evaluate tool’s positioning accuracy. Though all of
tests gave worse
outcomes near the end of
measurement, obtained results were very promising
and satisfactory – for Static mode tool achieved
centimeter level of precision (Fig. 13) when fix was
reached.Thereasonofcalculatedresultsdeterioration
wasprobablysignalreflectionsfromnearbybuildings
(the measurement site wasn’t perfectly open
environment).The
effectofprobablereflectionsisbest
seeninFigures9and19.
6 SUMMARYANDFUTUREWORK
Thegeneralresultsarepromisingspeciallywhenlook
attheinexpensivenessofthereceiverandtheresults
ofstatic positioning.Asthe usageof ASG‐EUPOSin
the real‐time is also possible so
taking into
considerationthetimeofcomputation(50secondsin
the combined mode) porting the RTKLIB to the
mobiledeviceanddoingthecomputationinthefield
isevenmoreattractiveforthefutureexperiments.
The next step is to perform research of the tool
accuracy in real‐time kinematic
navigation. Future
workcouldincludedevelopmentofprecisekinematic
navigation for vehicle/pedestrian, most preferably
integrationwiththe“VoiceMaps”project.
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