Owner's manual




NAV440UserManual
7430‐0131‐01Rev.F
Page31



Asshownin
Figure2onpage21,theIntegrationtoOrientationandtheIntegrationtoVelocitysignalprocessing
blocksreceivedriftcorrectionsfromtheExtendedKalmanFilter(EKF)driftcorrectionmodule.Thedriftcorrection
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gyroscope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toVG440functions:theEKFformulationwillcontinuewithoutGPSvelocity.The
UTCpacketsynchronizationwilldriftduetointernalclockdrift.
ThestatusofGPSsignalacquisitioncanbemonitoredfromthehardwareStatusBIT(referto
in
)
Chapter11.BuiltIn
Test(BIT)
.Fromacoldstart,ittypicallytakes40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
propagationat100HzandsynchronizesthesensorsamplingwiththeGPSUTC(UniversalCoordinatedTime)second
boundarywhenavailable.
AswiththeAHRS440andVG440,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softwareStatus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IMU440data.Referto
Chapter9.Communicatingwiththe440andChapter10.ProgrammingGuidelines
forfullpacketdescriptions.
NAV440Advanced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 below:
F
r

Table8NAV440AdvancedSettings
Table8NAV440AdvancedSettings
Setting DefaultValue Comments
BaudRate 38400 9600,19200,57600alsoavailable
PacketType N1 S0,S1,S2,A0,A1,A2,N0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.