Errors of UAV Autonomous Landing System for Different Radio Beacon Configurations

Abstract

At the turn of the 20th and 21st centuries, development of microelectronics and microwave techniques allowed for minimization of electronic devices and systems, and the use of microwave frequency bands for modern radio communication systems. On the other hand, the global navigation satellite system (GNSS) have contributed to the popularization of radio navigation in civilian applications. These factors had a direct impact on the development and dissemination of unmanned aerial vehicles (UAVs). In the initial period, the UAVs were used mainly for the army needs. This results also from the legal aspects of the UAV use in the airspace. Currently, commercial UAVs for civilian applications, such as image recognition, monitoring, transport, etc., are presented increasingly. Generally, the GNSS system accuracy for the UAV positioning during a flight is enough. However, the GNSS use for automatic takeoff and landing may be insufficient. The extensive, ground-based navigation support systems used at airports by manned aircraft testify to these. In the UAV case, such systems are not used due to their complexity and price. For this reason, the novel dedicated take-off and landing systems are developed. The proposal of the autonomous landing system, which is based on the Doppler effect, was presented in 2017. In this case, the square-based beacon configuration was analyzed. This paper shows the influence of various beacon configurations in the Doppler-based landing system on the positioning error during the UAV landing approach.

Full text

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1 INTRODUCTION
Attheendofthe20thcentury,thedynamic
developmentofmicroelectronicscausedthe
popularizationofunmannedaerialvehicles(UAVs),
alsocalleddrones.TheUAVisanaircraftthatdoes
notrequirecrewon‐boardandisunabletotake
passengers.TheUAVcanberemotecontrolbya
humanoperatororautonomouslybyon‐board
computers.Hence,theideaoftheUAVhasitsrootsin
thesecondhalfofthelastcentury,whentheremote‐
controlledmodelsofaircraft,cars,orshipswere
mainlyofahobbynature.TheV‐1flyingbombandV‐
2rocketfromthe2ndWorldWarcanberegardedas
theUAVprototypes.Inthepost‐warperiod,research
intothedevelopmentofthistechnologyforthearmy
needswasconductedmainlyintheUSAandUSSR.
Thisdevelopmentwasdirectlyrelatedtotheconquest
ofthecosmosinwhichunmannedspacecrafts,i.a.,
artificialsatellites,wereused.
Initially,duetolegalrestrictions,theUAVswere
usedmainlyinthearmedforces[1–3].Originally,
theirmainareaofapplicationswasimage,optical,
andradarrecognition.Theseareso‐called
surveillanceUAVs,e.g.,theNorthropGrummanRQ‐
4GlobalHawk.Then,theUAVswerealsousedto
transposingweapons.Thiskindiscalledasa
unmannedcombataerialvehicle(UCAV),e.g.,the
GeneralAtomicsMQ‐9Reaperalsocalledthe
Errors of UAV Autonomous Landing System for
Different Radio Beacon Configurations
J
.M.Kelner&C.Ziółkowski
M
ilitaryUniversityofTechnology,Warsaw,Poland
ABSTRACT:Attheturnofthe20thand21stcenturies,developmentofmicroelectronicsandmicrowave
techniquesallowedforminimizationofelectronicdevicesandsystems,andtheuseofmicrowavefrequency
bandsformodernradiocommunicationsystems.Ontheotherhand,theglobalnavigationsatellitesystem
(GNSS)havecontributedtothepopularizationofradionavigationincivilianapplications.Thesefactorshada
directimpactonthedevelopmentanddisseminationofunmannedaerialvehicles(UAVs).Intheinitialperiod,
theUAVswereusedmainlyforthearmyneeds.ThisresultsalsofromthelegalaspectsoftheUAVuseinthe
airspace.Currently,commercialUAVsforcivilianapplications,suchasimagerecognition,monitoring,
transport,etc.,arepresentedincreasingly.Generally,theGNSSsystemaccuracyfortheUAVpositioning
duringaflightisenough.However,theGNSSuseforautomatictakeoffandlandingmaybeinsufficient.The
extensive,ground‐basednavigationsupportsystemsusedatairportsbymannedaircrafttestifytothese.Inthe
UAVcase,suchsystemsarenotusedduetotheircomplexityandprice.Forthisreason,thenoveldedicated
take‐offandlandingsystemsaredeveloped.Theproposaloftheautonomouslandingsystem,whichisbased
ontheDopplereffect,waspresentedin2017.Inthiscase,thesquare‐basedbeaconconfigurationwasanalyzed.
ThispapershowstheinfluenceofvariousbeaconconfigurationsintheDoppler‐basedlandingsystemonthe
positioningerrorduringtheUAVlandingapproach.

http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 13
Number 2
June 2019
DOI:10.12716/1001.13.02.22

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PredatorB.Currently,theUAVsarewidelyusedon
thecivilianmarket.Manyprivatecompaniesprovide
variousservicesusingtheUAVs,e.g.,inthefieldof
energetics[4–7],agriculture[8,9],forestryandfire
detection[10,11],watermanagement[12]andflood
detection[13],environmentalprotection[14],search
andrescue[15],radio‐communication[16],transport
[17],etc.
ThegrowthoftheUAVmarkethasalso
contributedtothedevelopmentofotherunmanned
platforms,suchasunmannedground(UGVs),surface
(USVs),andunderwatervehicles(UUVs).Themain
recipientsofthismarketsectorarestillthearmed
forcesofvariouscountries.Inareport[18],the
EuropeanCommissionindicatestheimportanceof
thistechnologyintheeconomicandtechnological
developmentofcountries,especiallyinthecivilian
sector[19].Accordingtothepresentedforecasts,the
estimatedvalueoftheUAVmarketin2019willbe
expectedtoreacharound11‐12billiondollars.In2015
and2016,thedomesticmarketwasestimatedat165
and200millionPolishzlotys,respectively[20].
Inliterature,wecanfindsynonymousfortheUAV
suchasunmannedaerialsystem(UAS)orremotely
pilotedaircraftsystems(RPAS).Sometimes,the
differencesbetweentheUAVandUASareindicated.
Then,theUAVisreferredtoitselfaircraftplatform,
whiletheUASincludesalsoothersystem
components,suchastheground‐basedflightcontrol
system.Inthiscase,theRPASisasynonymofthe
UAS.Currently,theRPASismorewidelyusedin
militaryterminology,especiallyintheNorthAtlantic
TreatyOrganization(NATO)andEuropeanDefense
Agency(EDA).Intheliterature,various
classificationsoftheUAVsarepresented.They
considerdifferenttechnicalaspectsorapplications.
Fromtheviewpointofthispaper,theUAVtermis
referencedtoaverticaltake‐offandlanding(VTOL)
aircraft,unlikeaconventionaltake‐offandlanding
(CTOL)aircraftrequiringarunway.Intheremainder
ofthepaper,theVTOLisconsideredasasynonymof
theVTOLUAV.
Globalnavigationsatellitesystems(GNSSs)[21,22]
arecommonlyusedinUAVnavigation.Inaddition,
remotecontrolandvideotransmissionfromthe
aircrafttotheremoteUAVoperatorallowssafe
displacementofthedrone.However,thissolution
maynotbesufficienttotake‐offandlanding
approach.Thisisasignificantproblem,notablyfor
theautonomousUAVs.Thetake‐offandlandingare
theaircraftflightstagesthatrequirespecialprecision
insteeringandnavigation.Formannedaircraft,the
piloton‐boardhasmorecontrolovertheplaneor
helicopter.Ontheotherhand,dedicatedtake‐offand
approachsystemsareusedatlargecivilandmilitary
airports.Theinstrumentlandingsystem(ILS),tactical
airnavigationsystem(TACAN)[23],orEuropean
GeostationaryNavigationOverlayService(EGNOS)
[21,22]areexamplesofradiolocal‐areaaugmentation
systems(LAASs).Theyareveryimportantinbad
weatherconditionswithlimitedvisibility,e.g.,fog,
snowfall,orrain.Generally,theLAASsarenot
availabletoUAVmajority.Therefore,itisimportant
todevelopsuchsolutions,especiallyforautonomous
drones.
In2016,weproposedalandingsystemonavessel
forthemannedandunmannedVTOL[24].This
systeminvolvestheuseofterrestrialradio‐beacons
(RBs)andadedicatednavigationreceiver(NR)placed
onboardaircraft.Inthiscase,thesignalDoppler
frequency(SDF)locationmethod[25–27]isusedto
estimatetheaircraftpositionrelativetoalandingpad
ontheship.Thissolution[24]isbasedontheSDF
applicationsdedicatedtoin‐flightnavigationand
CTOLlandingapproach,whichareshownin[28]and
[29],respectively.Here,theanalyzedconceptofthe
landingapproachsystemfortheunmannedVTOLis
asystemmodificationshownin[30].Theproposed
solutionhasbeendevelopedforthelandingpadin
hard‐to‐reachplacessuchasoilplatforms,vessels,
islands,orskyscraperroofs.In[30],weassumedthat
RBsarelocatedbasedonasquare.Thepurposeofthis
paperistoassesstheimpactofselected
configurationsoftheRBsontheVTOLpositioning
accuracy.
Theremainderofthepaperisorganizedas
follows.Section2describestheSDF‐based
autonomouslandingapproachsystem.Assumptions
andsimulationscenariosarepresentedinSection3.In
Section4,theobtainedsimulationresultsareshown.
Inthiscase,thepositioningaccuracyoftheVTOL
UAVfordifferentRBconfigurationsisanalyzed.The
summaryisinthefinalpartofthepaper.
2 AUTONOMOUSLANDINGAPPROACH
SYSTEMFORVTOLUAV
ThespatialstructureoftheSDF‐basedautonomous
landingapproachsystemfortheVTOLisshownin
Figure1.

Figure1.Spatialstructureofautonomouslandingapproach
systemforVTOL[30]
ThegroundpartofthesystemconsistsoffourRBs
andameasuringreceiver(MR).Inthesolution
discussedin[30],weassumedthatfourRBsare
locatedbasedonthesquarearoundthelandingpad.
EachRBisequippedwithasignalgenerator,power
amplifier,andtransmittingantennaplacedatthe
stand.Additionally,theRBcanbeequippedwitha
rubidiumorcesiumfrequencystandardthatwill
increasethefrequencystabilityoftransmittedsignals.
ThisisimportantfromtheviewpointoftheSDFused
[31].ThreeRBs,i.e.,RB‐1,RB‐2,andRB‐3,transmit
harmonicsignalsatdefinedfrequenciesf1,f2,andf3,
respectively.Atfrequencyf4,theRB‐4transmitsa

431
modulatedsignalusingdifferentialphaseshiftkeying
(DPSK).Ineachtransmittedframe,informationabout
thelocationcoordinatesoftheindividualRBsand
theirfrequencycorrectionsaresent.Thesecorrections
aredeterminedbasedonlocalmeasurementscarried
outbytheMRlocatednearRB‐4.
TheNRsplacedontheVTOLsarethereceiving
partofthesystem.EachVTOLisequippedwith
typicalelementsofthenavigationsystem,namelya
GNSSreceiverandinertialnavigationsystem(INS).
Thisallowstocarryoutacontrolledorautonomous
UAVflightphase.Whereas,theNRprovides
positioningtheVTOLnearthelandingpadandits
landingapproach.TheNRistunedtothefrequency
bandonwhichoperatetheRBs.Thismeansthatthe
NRoperationbandincludesthecarrierfrequenciesf1,
f2,f3,andthemodulated‐signalbandatthefrequency
f4.TheNRismadeinsoftware‐definedradio(SDR)
technology[32,33].ThismeansthattheNRprovides
signalprocessinganddeterminingtheestimated
positionsoftheUAVrelativetothelandingpad.For
eachRB,theDopplerfrequencyshift(DFS)is
determinedeveryspecifiedtime‐periodΔTbasedon
thereceivedsignalwiththedurationofTS.Then,the
UAVcoordinatesaredeterminedbasedondiscrete
instantaneousDFSsaggregatedinatimewindowTA.
ThemethodoftheDFSdeterminationforthe
harmonicsignalispresentedin[25–27].Inthecaseof
themodulatedsignalfromRB‐4,aftersub‐band
filtering,informationframesaredemodulatedandthe
instantaneousDFSsareestimatedbasedona
methodologyshownin[34].Adetaileddescriptionof
theautonomouslandingapproachsystemand
estimatingtheVTOLcoordinatesbasedontheSDFis
containedin[30].
Thepresentedsystemcanbeclassifiedasprecise
short‐rangeradionavigationsystems.Itcanbeused
duringtheUAVlandingapproach,aswellasitstake‐
offfromthelandingfieldandin‐flightinanarea,
whereisaradio‐rangebetweentheNRandRBs.A
keyadvantageoftheproposedsolutionisitsnarrow
bandwithrelativelyhighpositioningprecision.Inthe
caseofsystemsbasedontimemeasurement,e.g.,
[35,36],obtainingcomparableaccuracywouldrequire
theuseofamuchwiderband.Wepointoutthatthe
frequencyallocationforthistypeofdedicatedsystem
isaseriousproblem.Forthedevelopedsystem,
unlicensedfrequencybands,e.g.,dedicatedtotheWi‐
Fiincludedinindustrial,scientificandmedical(ISM)
bands,canbeusedforthispurpose.Inthiscase,the
emissionoftheharmonicsignalswithmorepower
thantheemissionaverageinthebanddoesnot
constituteasignificantinterferencetoothersystems.
3 SCENARIOANDASSUMPTIONSFOR
SIMULATIONSTUDIES
Inascenarioshownin[30],weassumedthattheRBs
areplacedonthebasisofthesquarewithaside
lengthequalr(seeFigure1).Inthispaper,weanalyze
threeconfigurationsoftheRBpositionsbasedon
otherregularpolygons,i.e.atriangle,pentagon,and
hexagon.Acommonfeatureofallconfigurationsis
theradiusRofacircumscribedcircle.Figure2
presentstheanalyzedRBconfigurationstogetherwith
areferenceconfigurationbasedonthesquare(see
Figure2(b)).

Figure2.AnalyzedspatialconfigurationsofRBsbasedon
regularpolygons:a)triangle,b)rectangle,c)pentagon,d)
hexagon
Inaddition,insimulationstudies,weassume
similarassumptionsasin[30],i.e.,
 landingpointatOistheoriginofthelocal
coordinatesystem;
 thesystemconfigurationbasedontheregular
polygonconsistsofKRBs,whereK=3,4,5,6,for
theregulartriangle,rectangle(i.e.,square),
pentagon,andhexagon,respectively(seeFigure
2);
 assumingthedistancer=40m[30]between
neighboringRBsinthesquareconfiguration,the
radiusR≈28.3misabasefordeterminingtheRB
coordinateswithrespecttothepointOineach
configuration;thelocationcoordinatesofthe
individualRBsfortheanalyzedconfigurationsare
containedinTable1;theheightoftheRB
transmittingantennasishT=zk=2mfork=1,..,K;
 thesystemoperatesintheISMbandusedbythe
Wi‐Fi,i.e.,2.4GHz;ineachconfiguration,thek‐th
RBtransmitstheharmonicsignalatfk,where
k=1,..,K–1;while,theK‐thRBemitstheDPSK
signalatfK;thesefrequenciesaredeterminedas
follows:fK(kHz)=2399800+50∙K+100and
fk(kHz)=2399800+50∙(k–1)fork=1,..,K–1;the
bandwidthoftheDPSKsignalisequalto
BT=80kHz;
 theNRoperatesatthefrequencyfR=2.4GHzwith
thereceptionbandBR=500kHz;
foraDopplercurve(DFSsversustime)analysis,
thetimewindowTA=5.0sisused;
 inanelectromagneticenvironment,anadditive
whiteGaussiannoise(AWGN)isoccurred,anda
leveloftheemittedsignalsatthefarthestpoint(L)
ofananalyzedtrajectoryisensuredbyasignal‐to‐
noiseratioequaltoSNR=8dB;
 theVTOLflightbetweentheLandPpointsis
carriedoutataconstantaltitudehL=50mwitha
velocityv=72km/h=20m/s(seeFigure3);then,
theflightceilingislowered;
 thelengthoftheanalyzedVTOLflightroute,i.e.,
thedistancebetweentheLandPpoints,isequalto
d=400m.


432
Table1.CoordinatesofRBpositionsonOXYplanefordifferentconfigurations
__________________________________________________________________________________________________
RB‐k ConfigurationofRBs
__________________________________________________________________________________________________
RegularTriangleSquareRegularpentagon Regularhexagon
K=3K=4K=5K=6
r=49.0mr=40.0mr=33.3mr=28.3m
__________________________________________________________________________________________________
k xk(m)yk(m)xk(m)yk(m)xk(m)yk(m)xk(m)yk(m)
1 14.124.520.020.022.916.624.514.1
2 14.1–24.520.0–20.022.9–16.624.5–14.1
3 28.30.0–20.0–20.0–8.7–26.90.0–28.3
4 –––––––20.020.0–28.30–24.5–14.1
5 –––––––––––––8.726.9–24.514.1
6 –––––––––  –––––––––0.028.3
__________________________________________________________________________________________________


Figure3.SpatialscenarioofVTOLlandingapproachon
exampleofRBreferenceconfigurationprojectedinplane:
a)OXZandb)OXY[30]
SimulationstudiesarecarriedoutfortheUAV
movementtrajectorydepictedinFigure3.Inthis
case,twoscenariosareconsidered.Inthefirst
scenario,Sc.1,weevaluatetheVTOLpositionerror
alongtheLPtrajectoryfordifferentRBconfigurations
andtheapproachdirectionα=0.Inthesecond
scenario,Sc.2,theVTOLpositioningerrorisanalyzed
atthepointPforthevariousαdirections.
4 RESULTSOFSIMULATIONSTUDIES
FortheassumptionspresentedinSection3,we
conductedsimulationstudies.Inouranalysis,the
VTOLpositioningerrorisabasicmeasureofthe
accuracyassessmentofthedevelopednavigation
system.Thismeasureisdefinedasfollows

222
000
Δ
R
xx yy zz (1)
where(x0,y0,z0)and(x,y,z)=therealandestimated
coordinatesoftheUAVposition,respectively.
ThesimulationresultsobtainedforSc.1are
illustratedinFigure4.Inthiscase,graphsofthe
instantaneouspositioningerrorfortheVTOLlanding
approacharepresentedforfouranalyzedRB
configurations.Additionally,theaverageerrors
shownbydashedlines.

Figure4.InstantaneousandaverageVTOLpositioning
errorsversusdistancedtopointPforvariousRBs
configurations:a)K=3,b)K=4,c)K=5,andd)K=6
Theobtainedresultsshowthehighprecisionof
theVTOLpositioningbasedontheproposedsystem
andSDFmethod.Ford<100m,theinstantaneous
erroroftheUAVpositionforeachconfigurationis
lessthan1.0m.Inthelastsecondofapproachingthe
pointP,theerrorislessthan0.5m.


433

Figure5.VTOLpositioningerroratpointPversus
approachdirectionαfordifferentRBconfigurations:
a)K=3,b)K=4,c)K=5,andd)K=6
InSc.1,wemayusetheaverageerrorobtainedon
theentireanalyzedroutewiththelengthof400mas
acomparativemeasure.Inthiscase,theaverage
errorsareequalto2.4m,3.8m,3.4m,and3.8mfor
configurationsbasedontheregulartriangle,
rectangle,pentagon,andhexagon,respectively.
Therefore,wemayconcludethatthebestresultsare
obtainedforK=3.Thisresultmayberelatedtothe
approachdirectionα=0assumedinSc.1.Thus,for
oddvaluesofK,oneofRBisinthedirectionofthe
UAVmovement.
TheimpactoftheVTOLapproachdirection
relativetotheadoptedcoordinatesystemisanalyzed
inSc.2.Theobtainedsimulationresultsareillustrated
inFigure5.
TheobtainedgraphshapesoftheUAVposition
errorarecloselyrelatedtotheRBconfigurations
depictedinFigure2.Inthiscase,thelargesterrors
occurwhentheapproachdirectionαcoincideswith
thedirectiondeterminedbythepointOandthe
locationofatleastoneRB.Inthesignalreceivedfrom
suchRB,theestimatedDFSstakemaximumvalues
andthesedataarenotusedintheSDF.Thisis
particularlyvisibleforK=4andK=6,whenaRBpair
isalwayslocatedintheanalyzeddirections.
TheanalysisoftheresultsinFigure5showsthat
decreasingthenumberofRBsinthesystemdoesnot
necessarilyleadtohighersystemaccuracy.For
comparisonoftheindividualconfigurations,the
errorsatpointPaveragedovertheapproach
directionare0.76m,0.17m,0.55m,and0.16mfor
K=3,4,5,6,respectively.Therefore,thisisthe
oppositecasetothatpresentedinFigure4.
Generally,foreachoftheanalyzedRB
configurations,theVTOLpositioningerroratpointP
isalwayslessthan2mregardlessofthedirectionα.
Formostapproachdirections,theUAVpositionerror
islessthan0.5m.Thesevaluesareverysmallin
relationtotheassumedradiusofthelandingpad
equaltoR≈28.3m.Hence,wemayconcludethatthe
developedsystemallowsthesafeandautonomous
landingapproachevenwithsizabledimensionsof
theVTOL.
5 CONCLUSION
Inthispaper,weevaluatetheinfluenceoftheRB
configurationinthelandingapproachsystemforthe
VTOLUAVonitspositioningerror.Thedeveloped
systemisbasedontheDFSmeasurementinthe
signalsreceivedfromtheterrestrialRBsaroundthe
landingpad.TheDFSsaremeasuredinthededicated
NR,whichisplacedonboardaircraft.TheSDF
methodisusedtoestimatetheVTOLposition
relativetothelandingsite.Ouranalysisisbasedon
simulationstudies.Inthiscase,weconsidertwo
scenariosandfourRBconfigurationsbasedonthe
regularpolygons.Theobtainedresultshowsthehigh
accuracyoftheUAVpositioningforallanalyzed
configurations.Thebestresultsatthepointlocated
abovethelandingcenterareobtainedforthe
configurationconsistingofsixRBs.Atthispointand
forthisconfiguration,themeanerrorregardlessof
theapproachdirectionwaslessthan20cm.The
proposedsolutionseemsidealforuseinstand‐alone
autonomouslandingapproachsystemsfortheUAV.
However,empiricalresearchisstillrequired,which
isplannedinthefuture.
ThepresentedideaoftheSDF‐basednavigation
forUAVscanalsobeusedfortheneedsofother
typesofautonomousvehicles.Inthefuture,we
considerusingthisconcepttonavigateautonomous
USVsormannedvesselsenteringaport.Inthiscase,
theRBswillbelocatedinthecoastalzonearoundthe
port.

434
ACKNOWLEDGMENTS
Thisworkwasdevelopedwithinaframeworkofthe
ResearchGrant“Basicresearchinsensortechnologyfield
usinginnovativedataprocessingmethods”no.
GBMON/13‐996/2018/WATsponsoredbythePolish
MinistryofDefense.
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