Instruction Manual

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Page32
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GNAV540UserManual
7430‐0808‐01Rev.B
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moduleusesdatafromtheaidingsensors,whentheyareavailable,tocorrecttheerrorsinthevelocity,attitude,and
headingoutputs.Additionally,whenaidingsensorsareavailablecorrectionstotherategyroscopesand
accelerometersareperformed.
TheNAVFunctionblendsGPSderivedheadingandaccelerometermeasurementsintotheEKFupdatedependingon
thehealthandstatusoftheassociatedsensors.IftheGPSlinkislostorpoor,theKalmanFiltersolutionstops
trackingaccelerometerbias,butthealgorithmcontinuestoapplygyroscopebiascorrectionandprovidesstabilized
angleoutputs.TheEKFtrackingstatesarereducedtoanglesandgyroscopebiasonly.Theaccelerometerswill
continuetointegratevelocity.However,accelerometernoise,bias,andattitudeerrorwillcausethevelocity
estimatestostartdriftingwithinafewseconds.Theattitudetrackingperformancewilldegrade,theheadingwill
freelydrift,andthefilterwillreverttotheVGFunction;ifthemagnetometerisrunning,thenthefilterwillrevertto
.TheUTCpacket
theAHRSfunction.Ineitherfunction,theEKFformulationwillcontinuewithoutGPSvelocity
synchronizationwilldriftduetointernalclockdrift.
ThestatusofGPSsignalacquisitioncanbemonitoredfromthehardwareStatusBIT(referto
Chapter11.BuiltInTest
(BIT)
).Fromacoldstart,ittypicallytakes40–90secondsforGPStolock.Theactuallocktimedependsonthe
antenna’sviewoftheskyandthenumberofsatellitesinview.TheDSPperformstime‐triggeredtrajectory
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propagationat100HzandsynchronizesthesensorsamplingwiththeGPSUTC(UniversalCoordinatedTime)second
boundarywhenavailable.
AswiththeAHRSandVGFunctions,thealgorithmhastwomajorphasesofoperation.Immediatelyafterpower‐up,
theunitusestheaccelerometersandmagnetometerstocomputetheinitialroll,pitchandyawangles.Therolland
pitchattitudewillbeinitializedusingtheaccelerometer’sreferenceofgravity,andyawwillbeinitializedusingthe
leveledmagnetometersXandYaxisreferenceoftheearth’smagneticfield.Duringthefirst60secondsofstartup,
theunitshouldremainapproximatelymotionlessinordertoproperlyinitializetheratesensorbias.The
initializationphaselastsapproximately60seconds,andtheinitializationphasecanbemonitoredinthe
softwareStatusBITtransmittedbydefaultineachmeasurementpacket.Aftertheinitializationphase,theunit
operateswithlowerlevelsoffeedback(alsoreferredtoasEKFgain)fromtheGPS,accelerometers,and
magnetometers.
TheNAVFunctionprovidesadditionaloutputmeasurementpacketsincludingthedefault
N1NavigationPacket,
which
outputstheLatitude,Longitude,Altitude,X,Y,Zvelocities,accelerations,androllangle,pitchangle,yawangle,
anddigitalIMUdata.Formoreinformationaboutpackets,referto
Chapter9.CommunicatingwiththeGNAV540
Unit
andChapter10.ProgrammingGuidelines.
NAVAdvancedSettings
Inadditiontotheconfigurablebaudrate,packetrate,axisorientation,andsensorlow‐passfiltersettings,theNAV
unctionprovidesadditionaladvancedsettingswhichareselectablefortailoringtheunittoaspecificapplication
equirements.Theadvancedsettingsarelistedin
F
r
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Table10GNAV540AdvancedSettingsbelow:
Table10GNAV540AdvancedSettings
Setting DefaultValue Comments
BaudRate 38400 9600,19200,57600alsoavailable
PacketType N3 S0,S1,S2,A0,A1,A2,N0,N3,N4alsoavailable
PacketRate 25Hz
ThissettingsetstherateatwhichselectedPacketType,packetsareoutput.If
polledmodeisdesired,thenselectQuiet.