User manual

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 DocumentMT0100P.N
©XsensTechnologiesB.V. MTiandMTxUserManualandTechnicalDocumentation
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3.3 NorotationassumptionforXKF‐3 
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This section describes the background of the so‐called 'NoRotation' message and filter initialization setting.
TheMTcanbeconfiguredtoestimatethebiasesoftherategyroscopesandotherstatesassumingthattheMT
isnotrotating(i.e.quasi‐static)
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foracertainperiodoftime.This‘NoRotation’procedurecanbeconfiguredto
beinvokedautomaticallyatpower‐onand/orReset,orcanbeinvokedmanuallybesendingamessagetothe
MT(SetNoRotationmessage).
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PleasenotethatundernormalcircumstancesandnormalusetheMTwillsuccessfullyautomaticallyestimate
the rate gyroscope biases and there is no need to use the NoRotation feature. Please also note that if
 the
NoRotationprocedureisused,theMTmustabsolutelyNOT
berotatingduringthegivenperiodoftimethat
theprocedureisactive.Otherwiselargeerrorscanbeintroducedintheestimatedorientationoutput.Some
error checking is performed to estimate the validity of the NoRotation assumption. Please refer to the MT
Data Statusbyte,fordetailssee[LLCP]orforMTManagersee[MTM].Ultimately,theusermustbeable
to assure the validity of the assumption if NoRotation is used. If the validity can not be assured, it is not
advisabletousethisfeature.
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There are some very particular situations where the NoRotation feature can be considered. Specifically, in
applicationsthatcannotusethelocalmagneticfieldtoestimateheading,see[1.5.4],andatthesametime
doesnot
havesignificant(>10deg),andregular,variationsinrollandpitch,thegyrobiasofthe“vertical”rate
gyroscopeisnotobservablebyXKF‐3.Inpracticethiswillmeanthattheheadingwilldriftbytherateofthe
verticalgyrobiasatthatgiventime.Theheadingdriftratewillnotbe reducedovertimebecauseXKF‐3can
notestimatethe“vertical”gyrobias.
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UsingtheNoRotationfeatureappropriatelywillmakethe“vertical”gyrobiasobservableforashortperiodof
time,givingXKF‐3theopportunitytoquicklyestimatethe“vertical”gyrobias.Inpracticethiswillsignificantly
reduceheadingdrift.However,notethatthe“vertical”gyrobiasisonlyobservableduringtheperiodof time
thattheNoRotationupdateisapplied.So,headingdriftovertimeinsuchasituationcanfundamentallynotbe
prevented,butitcanbereducedgreatlyusingtheNoRotationfeatureatleastonce(atpower‐on)or,ideally,
regularlyif
youknowtheMTisnotrotating.
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IftheMTexperiencessignificantandregularvariationsinrollandpitchusingtheNoRotationfeatureshould
not be necessary. XKF‐3 continuously estimates the gyroscope biases and accounts for them. In case no
magnetometer can be used, the gyro bias of only two of the three axes can be estimated in a given
orientation. By using the MTi/MTx in different roll and pitch orientations, the gyro bias will slowly be
observableinall three axes, sinceallrategyro axes will at some pointbethe vertical one, at least to some
degree. Again, this will reduce the rate of heading drift, but some degree of heading drift will always be 
presentunlessthemagneticfieldcanbeusedasaheadingreference.
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Asdiscussed,theNoRotationfeature,canbeappliedbydefaultonpower‐onand/orReset,orcanbeactivated
duringMeasurementMode.Inthefirstcase,thedurationoftheNoRotationisfixedto2seconds.Inthelatter
caseadurationinsecondsmaybespecified,dependingonknowledgeaboutthedurationthattheMTisstill
(toamaximumof255seconds).Althoughthebiasestimatewillimproveforlongerintervals,intervalslonger
than3secondswillnotsignificantlyimprovebiasestimateandarethereforenotrecommendedasthechance
oferror(i.e.notconformingtotheabsoluteassumptionofnorotation)willincrease.
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Fordetails  onhow to use the NoRotation feature using the SDK,see[SDK] and [LLCP], for MT manager see
[MTM].
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
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TheEarthrotationcanbeneglectedforpracticalpurposes.