UAV Shipboard Landing with RTK.docx
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UAV Shipboard Landing with RTK.docx
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UAVShipboardLandingwithRTK
UAVShipboardLandingwithRTK
May2,2014 - By GPSWorldstaff2Comments
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CarrierPhaseCompensatesforWindandWaveMotion
Limitedlandingareaaswellasinterferenceduetowinddisturbanceandwavemotionmakeshipboardlandingsofunmannedaerialvehicles(UAVs)extremelydifficult.UseofUAVsatseacanenhancetheefficiencyofintelligencegatheringandsurveillance,andcouldalsoincreaselong-rangeair-strikecapability.Tosuccessfullylandaircraftinsuchachallengingenvironmentrequiresahigh-precisionnavigationsystem;thisprototypeappliesRTKmeasurements.
ByChiu-JungHuangandShau-ShiunJan
UAVscanperformfunctionssuchassurveying,imaging,detection,sensorwork,rescue,andgeographicinformationsystems(GIS)datacollection.TheexploitationofUAVswithportablelaunchingandrecoverysystemsusinganautomaticguidanceequipmentcanenhancetheirflexibilityinmanypracticalapplications.Inparticular,UAVscanachievegreateffectivenessfromlaunchandrecoveryaboardshipsatsea.However,thelandingareaisnarrowonaship,andinterferencerelatedtothemaritimeenvironmentduetowinddisturbanceandwavemotionsvariesgreatly,makingmaritimeUAVlandingsquitedifficult.Recoveringtheseaircraftinsucharapid-dynamicenvironmentrequiresahigh-precisionUAVnavigationsystem.
Generally,UAVsuseadifferentialGPS(DGPS)aidingstationtocontinuouslytransmitpositioningcorrectioninformationduringlandingapproach;thismethodcanprovideabout0.7to1-meteraccuracy.However,shipboardlandingsrequiremorestringentaccuracy.AccordingtheJointPrecisionApproachandLandingSystem(JPALS),therequirementsofshipboardlandingincludeverticalaccuracyontheorderof0.3meters,andtherequirementfortheverticalprotectionlevelis1.1meters.Tofulfilltheseaccuracyrequirements,wehavechosenthereal-timekinematic(RTK)technique.Recently,researchershavestudiedtheuseofRTKsatellitenavigation.TheBoeingUnmannedLittleBirdprogramhasbeenexaminingshipboardlaunchandrecoveryusingrelatednavigationtechniques.
TheaccuracyofusingRTKnavigationis1centimeter+1partpermillion.
Figure1.Flowchartforsoftware-in-the-loop.
Sincedevelopmentofshipboardlandingiscostlyintermsoftimeandmanyresources,includinghumanresources,thisresearchisanattempttoevolveasoftware-in-the-loop(SIL)simulationsystemtoanalyzetheaccuracyofusingRTKforlandingnavigation.TheSILsystemusestheMATLABSimulinkinterfacebecasueofitshelpfulgraphicuserinterfaceandblockdiagrams.AflowchartoftheSILsystemisshowninFigure1.
ThesimulatedRTKmessageprovidesthenavigationaldatausedastheanalysisresultsfromtheexperiments.Toensurethestabilityofthelandingprocess,theaircraftmodelswerecontrolbyalinearquadraticGaussianregulator(LQG),whichisabletorejecttheenvironmentaldisturbancesencounteredinthelandingprocess.TheshipmotionsweresimulatedusingthefactorsandthemodelformulatedbytheInternationalTowingTankConference.AcombinedpositionerrorconsistingoftheaircraftcontrolsandshipmotionswascalculatedandthenfedbacktotheRTKnavigationmessage.
RTKPerformance
RTKnavigationprovideshighpositioningperformanceintherangeofafewcentimeters;thetechniquecaneliminatemainerrors,includingionosphericandtroposphericerrorsandsatelliteclockerrors,amongothers.Abasestationandaroverstationcancoveraserviceareaofabout10to20squarekilometers.ThedatatransitionshouldbeinrealtimeusingawirelessVHForWi-Fimodem.
Becausedataforshipboardlandingsaredifficulttoacquire,thenavigationmessageintheSILwassimulatedusingexperimentsinvolvingavarietyofconditions.Inthisarticle,fourkindsofexperimentswereincludedtohelpverifytheavailabilityandreliabilityofusingRTKinformationasanavigationalmessage.
Westartedwithabasickinematicexperiment,whichwassimplyusedtoassesstheRTKperformance.Next,arelativepositioningexperimentwasconductedtoensuretheRTKrelativepositioningaccuracywasadequate.Afterthat,anantennareversalexperimentwasdesignedinordertounderstandtheship’sswingeffectinwhichaircraftaltitudemightcausealackofcommonviewsatellites.Finally,anantennaforwardflipexperimentwasconductedintendedtoshowthedifferentRTKpositioningresultsforavarietyofseastateeffects.
Alloftheexperimentaldatawerecollectedbyaworkshopcomputerthroughaprogramdatafile.Theanalysesoftheresultsincludedthemean,standarddeviationsofpositioningerror,unavailableRTKpercentagesandthepositioningaccuracywhenRTKwasunavailable.AlloftheanalysisresultswereimportedtotheSILsimulationusingtheGaussianrandomvariablemodel.
Figure2.Kinematicexperimentalsetup.
KinematicExperiment. Thebasestationsetupincludedanantenna,tripod,andreceiver.Theroverstationsetupincludedaportablevehiclewithabattery,antenna,andreceiverplacedasshowninFigure2.Thedataweretransmittedandreceivedusingawirelessmodemforwhichthetransmittedratewas115200bps.Thereceiverwasconnectedtoalaptopusedasaworkshoptomonitorsatellitequalityandcollectthedata.TheregioninwhichtheexperimenttookplaceisshowninFigure3:
ontheroofoftheAeronauticsandAstronauticsdepartmentbuildingatNationalChengKungUniversityinTaiwan.Theredstaristheknownpositionofthebasestation.Thebrokenrectangularredlineis25metersby10metersalongwhichthemovingroverstationmovedclockwise.
Figure3.Kinematicexperimentalregion.
However,itisdifficulttoshowthetruepositionsoftheexperiment.Inthisarticle,wetriedtogetthetruepositionbyusingalinearregressionmethodwhichusedthetime, t,astheexplanatoryvariableandtheposition, y(t),asthedependentvariable.Thelinearregressionusedthepastfiveepochpositionsasthedependentvariablesbywhichtoobtainthelinearpolynomial,andthefifthpositionwasputintothepolynomialtogetthepositionerror.Forexample,inordertocalculateanerrorat t=4,thepositionresultsfrom t=0to t=4mustbetakenintoEquation
(1)toformthesecondorderpolynomialswithparameters P,Q,and R
(1)
TheexperimentalresultsareshowninFigure4,whichistheENUpositioningerror,andTable1showstheanalysiserrormeanandstandarddeviations.Theexperimentalresultsshowthatthehorizontalpositioningaccuracyis0.037meters(95percent).
Figure4.ENUerrorresultsforthekinematicexperiment.
Table1.Positioningresultsforthekinematicexperiment.
RelativeExperiment. ThisexperimenthadonebasestationasbeforeandincludedtworoverstationswhichwereplacedonaT-bar,therelativedistancebeingknown,onaportablecartasshowninFigure5.TheregionoftheexperimentisshowninFigure6,wherethestarmarksthelocationofthebasestation,withtheroverstationmovingalongtheblackarrow.
Figure5.Experimentalsetup.
Figure6.Relativeexperimentalregion.
Therelativeerrorwascalculatedusingaknowndistance,0.72meters,tocomparethetworoverstationpositions.Figure7showstherelativeresultsoftheexperimentforwhichthemeanvalueandstandarddeviationswererecordedinTable2.Inthisexperiment,onlyabout4.5percentofthepositioningresultsfailedtomeettherequirementof0.3meters.
Figure7.Relativeerrorresults.
Table2.Positioningresultsfortherelativeexperiment.
Common-ViewSatelliteExperiment. Aircraftlandingaltitudeandtheship’sswingmotioncausedbythestateoftheseamightaffectGNSSinformationreceivedbytheantenna.Thisexperimenthadonebasestationandoneroverstationatfixedpositionsasbefore,butweattemptedtofliptheantennaofthebasestationtowardthenorthby80degrees,asshowninFigure8,andtheroverstationchangeddirectionaccordingtoTable3.Theantennadirectionalchangeof80degreeswerechosenfortheextremecasethatthebasestationandroverstationcouldexperiencecompletelydifferentsatellitesinview.
Table3.Commonviewsatelliteexperimentalsetupforantenna.
Figure8.Commonviewsatelliteexperimentalsetup.
ResultsoftheexperimentareshowninFigure9,inwhichtheverticallinesindicateantennadirectionalchanges.Forthisexperiment,everychangeis30seconds.Thisexperimentdemonstratesthatthepositionperformancedefinitelyvaries.ThepositionanalysisisshowninTable4,whichshowsahorizontalerrorof0.116meters(95percent).
Figure9.ENUresultsofthecommonviewsatelliteexperiment.
Table4.Positioningresultsforthecommonviewsatelliteexperiment.
Sea-StateExperiment. Inthisexperiment,onebasestationandoneroverstationwererequiredinafixedposition,buttheroverstationchangedthedirectionoftheantenna,asshowninFigure10,wheretheangleof x isdecidedaccordingtotheseastateinTable5.Ontheotherhand,theantennachangingtowardadifferentdirectionsimulatedtheswingmotionoftheboat.
Figure10.Swingexperimentalsetup.
Table5.Antennaangleintheswingexperiment.
TheexperimentalresultsshowninTable6arethemeanvalues,andTable7showsthestandarddeviations.Thesimulationprovidestheanalysisresultsinordertoauthenticatetheintegrationsimulations.TheresultsshowthattheseastateslightlyinfluencesRTKpositioning.
UAVandShipMotionSimulations
Duringshipboardlandingprocessing,manycomplicatedconditionsmustbetakenintoaccount,includingcrosswinds,anair-wakemodel,windgusts,anddeckmotion.Theshipdeckmotionandcrosswindeffectsaretwo
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