13 Inertial Navigation Systems

8/7/2019 13 Inertial Navigation Systems

http://slidepdf.com/reader/full/13-inertial-navigation-systems 1/34

INERTIAL NAVIGATIONINERTIAL NAVIGATION

SYSTEMSSYSTEMS

INSINS



WORD TO THE WISEWORD TO THE WISE

The inertial system uses deviations to generateThe inertial system uses deviations to generatecorrective commands to drive the system fromcorrective commands to drive the system froma position where it is, to a position where ita position where it is, to a position where it

isn¶t, arriving at the position where it wasn¶t, itisn¶t, arriving at the position where it wasn¶t, itnow is. Consequently the position where it is,now is. Consequently the position where it is,is now the position where it wasn¶t and itis now the position where it wasn¶t and itfollows the position where it was is thefollows the position where it was is the

position where it isn¶t.position where it isn¶t.

Copyright: ARINC 561 Manual, August 1976Copyright: ARINC 561 Manual, August 1976



INERTIAL NAVIGATION SYSTEMSINERTIAL NAVIGATION SYSTEMS

Completely self Completely self--contained navigation systemcontained navigation systemcapable of providing great circle tracks over capable of providing great circle tracks over random routes without reference to externalrandom routes without reference to externalinformation sources.information sources.

-- The most complex and expensive flightdeckThe most complex and expensive flightdecknavigation system currently in use.navigation system currently in use.

-- Still the nav system of choice for many operations.Still the nav system of choice for many operations.

-- Developed for the militaryDeveloped for the military ± ± accurate, reliable, notaccurate, reliable, not

susceptible to signal jamming or erroneous signalsusceptible to signal jamming or erroneous signaltransmission.transmission.

-- Extremely simple in concept, extremelyExtremely simple in concept, extremelycomplicated in execution.complicated in execution.

-- Sometimes described as a very accurate deadSometimes described as a very accurate dead--

reckoning system.reckoning system.



INERTIAL NAVIGATION SYSTEMSINERTIAL NAVIGATION SYSTEMS

Starts from a known point, advances estimated position basedStarts from a known point, advances estimated position basedon speed, direction and time.on speed, direction and time.

-- Uses acceleration (changes in speed & direction) in place of Uses acceleration (changes in speed & direction) in place of speed itself.speed itself.

-- Movement detected byMovement detected by accelerometersaccelerometers mounted on amounted on a stablestableplatformplatform (Stabilized gyroscopically).(Stabilized gyroscopically).

-- Accelerometers are like pendulums but more sophisticated,Accelerometers are like pendulums but more sophisticated,using sliding shutters with frictionless bearings and nowusing sliding shutters with frictionless bearings and nowsolid state technology. Can detect velocity changes tosolid state technology. Can detect velocity changes to1,000ths of a G force.1,000ths of a G force.

-- This is the basic principal of INS.This is the basic principal of INS.-- Most critical element is platform stability.Most critical element is platform stability.

-- Gyros are of primary importance.Gyros are of primary importance.

-- Accelerometer technology fairly static; advances now areAccelerometer technology fairly static; advances now aremostly in gyro technology, especiallymostly in gyro technology, especially Ring Laser Gyros.Ring Laser Gyros.



LASER-light amplification by stimulated emission of radiation

The basic operating principle of a laser is to use light or electrical

impulses to excite atoms of a crystal, gas, liquid, or other substance. The

atoms release light energy (photons) to return to their original state.Atoms of the same type will release light energy of the same frequency.

Mirrors are used to contain the photons which further excite the atoms

into releasing even more photons; some of these photons escape through

a partially silvered mirror as coherent light. This light energy is emitted

as a directional beam.



RING LASER GYROSRING LASER GYROS

They utilize the principle of the Doppler effect.They utilize the principle of the Doppler effect.

Two laser beams travel around a closed circuit (made with threeTwo laser beams travel around a closed circuit (made with threeor four mirrors) in opposite directions and are sensed by aor four mirrors) in opposite directions and are sensed by a

detector.detector.

When the gyroscope is not turning, the two beams are both atWhen the gyroscope is not turning, the two beams are both atthe same frequency and the detector senses a level attitude.the same frequency and the detector senses a level attitude.

As the gyroscope turns, the two beams have to travel different As the gyroscope turns, the two beams have to travel differentdistances around the circuit.distances around the circuit.

As viewed from a reference point inside the gyro (the detector), As viewed from a reference point inside the gyro (the detector),there is a shift in the frequencies of the two laser beamsthere is a shift in the frequencies of the two laser beams



RING LASER GYROSRING LASER GYROS

An electronic processor calculates the difference between An electronic processor calculates the difference betweenthe frequencies of the two laser beams.the frequencies of the two laser beams.

The rate of rotation of the gyro determines the phaseThe rate of rotation of the gyro determines the phasedifference of the frequencies. Each particular phasedifference of the frequencies. Each particular phasedifference coincides with a unique rate of turn which thedifference coincides with a unique rate of turn which theprocessor can thus calculate.processor can thus calculate.

Each ring laser gyroscope only rotates on one axis,Each ring laser gyroscope only rotates on one axis,

therefore three of them are required to register changes intherefore three of them are required to register changes inpitch, roll, and yaw.pitch, roll, and yaw.



ADVANTAGES ADVANTAGES

Few moving partsFew moving parts

Small size and light weightSmall size and light weight

Rigid constructionRigid construction High tolerance to shock, acceleration,High tolerance to shock, acceleration,

and vibrationand vibration

High level of accuracyHigh level of accuracy Low cost over the lifetime of the gyroLow cost over the lifetime of the gyro



ADVANTAGES ADVANTAGES

Because there are no rotating gimbals as inBecause there are no rotating gimbals as in

a mechanical gyro, there is no friction, anda mechanical gyro, there is no friction, and

therefore no errors caused by realtherefore no errors caused by realprecessionprecession

Less power consumed than mechanicalLess power consumed than mechanicalgyros because there are fewer movinggyros because there are fewer moving

parts.parts.



DISADVANTAGESDISADVANTAGES

Base cost of Laser ring gyros is moreBase cost of Laser ring gyros is more

expensive than mechanical gyros.expensive than mechanical gyros.

Laser ring gyros are susceptible to an error Laser ring gyros are susceptible to an error

known as ³LOCKknown as ³LOCK--IN´IN´



LOCKLOCK--ININ

When the rate of turn is very small, theWhen the rate of turn is very small, the

frequency difference between the twofrequency difference between the two

beams is small.beams is small. There is a tendency for the two frequenciesThere is a tendency for the two frequencies

to ³couple´ together and ³lockto ³couple´ together and ³lock--in´ with eachin´ with each

other.other.

As a result of lock As a result of lock--in, a zero turning rate isin, a zero turning rate is

indicated.indicated.



LOCKLOCK--ININ

While lockWhile lock--in errors are not substantial, theyin errors are not substantial, they

can be accounted for by using morecan be accounted for by using more

complex ring laser gyro systems.complex ring laser gyro systems. By mechanically moving or twisting theBy mechanically moving or twisting the

system, the coupling of frequencies doessystem, the coupling of frequencies does

not occur.not occur.

This mechanical adjustment is calledThis mechanical adjustment is called

DITHERING.DITHERING.









STABLE PLATFORMSTABLE PLATFORM

Stable platform has 2 functions:Stable platform has 2 functions:

To keep accelerometers aligned with the surfaceTo keep accelerometers aligned with the surface

of earth despite changes in aircraft attitude. This isof earth despite changes in aircraft attitude. This isaccomplished by mounting the platform onaccomplished by mounting the platform ongimbals.gimbals.

To keep the stable platform aligned with surface of To keep the stable platform aligned with surface of

earth to compensate for transport and earth rateearth to compensate for transport and earth rateprecession. These apparent errors are correctedprecession. These apparent errors are correctedbyby torquingtorquing ± ± a feedback process that keeps thea feedback process that keeps thestable platf orm level with the local vertical throughstable platform level with the local vertical throughthe application of real precession.the application of real precession.



STRAPDOWN PLATFORMSTRAPDOWN PLATFORM

-- In a strapdown platformIn a strapdown platforminstallation the platform is notinstallation the platform is not

gimbaled; instead ring laser gimbaled; instead ring laser gyro feedback allows agyro feedback allows acomputer to electronicallycomputer to electronicallymonitor orientation of themonitor orientation of theplatform.platform.





INERTIAL REFERENCE SYSTEMINERTIAL REFERENCE SYSTEM

--The ring laser gyros are much more accurate thanThe ring laser gyros are much more accurate thanmechanical gyros. It makes sense to use them for mechanical gyros. It makes sense to use them for other onboard instruments as well.other onboard instruments as well.

--An Inertial Reference System (IRS); mechanicalAn Inertial Reference System (IRS); mechanicalgyros become backup units.gyros become backup units.

--The IRS is used for AI, HI, radar antennaThe IRS is used for AI, HI, radar antennastabilization, and autopilot control.stabilization, and autopilot control.

--If the navigation portion of the INS goes down it isIf the navigation portion of the INS goes down it isstill possible to utilize the IRS by selecting ATTstill possible to utilize the IRS by selecting ATTinstead of NAV.instead of NAV.

-- Inertial Navigation UnitInertial Navigation Unit ± ± box containingbox containingaccelerometers & gyroaccelerometers & gyro--stabilized platformstabilized platform



CONTROL DISPLAY UNIT (CDU)CONTROL DISPLAY UNIT (CDU)

2 Flavours:2 Flavours:-- Older 2Older 2--window display & more modernwindow display & more modernCRT displayCRT display

Older displays easier to read but CRT units provideOlder displays easier to read but CRT units provide

much more informationmuch more information ± ± Multiple flight plans, >9 waypointsMultiple flight plans, >9 waypoints

Used in conjunction with mode select panelUsed in conjunction with mode select panel

Standby BatteryStandby Battery ± ± Absolutely critical to avoid completeAbsolutely critical to avoid completeloss of unit during power variationsloss of unit during power variations

These form the system packageThese form the system package







OPERATIONOPERATION

Initialization:Initialization:

Major process for computer Major process for computer

Leveling; internal block Temp adjustment (toLeveling; internal block Temp adjustment (to ± ±15oC min); gyro15oC min); gyrocompassing (to True or Mag); latitude testing;compassing (to True or Mag); latitude testing;

Total 10 to 45 minutes, depend on temp, gyro type & latitude (takesTotal 10 to 45 minutes, depend on temp, gyro type & latitude (takeslonger @ higher latitudes to sense rotation & find True North which islonger @ higher latitudes to sense rotation & find True North which isessential to the alignment processessential to the alignment process

Easy for pilotEasy for pilot

Can use NAV or ALIGN modeCan use NAV or ALIGN mode

ALIGN does a more thorough job but process can be negated if don¶tALIGN does a more thorough job but process can be negated if don¶tswitch to NAV before moving acftswitch to NAV before moving acft

Also want to be careful NOT to leave NAV mode in flight because can¶tAlso want to be careful NOT to leave NAV mode in flight because can¶tinitialize in flightinitialize in flight

Military can air initialize but civilian units must be stationary, on ground,Military can air initialize but civilian units must be stationary, on ground,in a known positionin a known position



OPERATIONOPERATION

Enter initial coEnter initial co--ordinates (From A/D Diagram)ordinates (From A/D Diagram)

-- Only as accurate as the information input.Only as accurate as the information input.

-- Acft can then be moved & INS will begin sensing &Acft can then be moved & INS will begin sensing &

correlating all movementscorrelating all movementsLimitationsLimitations

Some older systems unable to accommodate extremeSome older systems unable to accommodate extremelongitudinal convergence at >80o N or S latitudelongitudinal convergence at >80o N or S latitude

Most systems do not align properly >70o N or SMost systems do not align properly >70o N or S

latitudelatitude ± ± but work OK there once aligned farther but work OK there once aligned farther south. Problem is earth¶s rotation too slow to besouth. Problem is earth¶s rotation too slow to bedetected at those latitudes.detected at those latitudes.





DATA PRODUCEDDATA PRODUCED

INS always knows where True North is and senses allINS always knows where True North is and senses allmovement, so it always knows what acft hdg is and it is easymovement, so it always knows what acft hdg is and it is easyto derive other information:to derive other information:

Track and PositionTrack and Position ± ± as a series of fixesas a series of fixes

Ground speedGround speed

Drift angleDrift angle

Cross track error/Cross track distance (Degrees or Miles)Cross track error/Cross track distance (Degrees or Miles)

Distance to go/ Time to goDistance to go/ Time to go

Errors & Recommended ActionsErrors & Recommended Actions

Normally has a separateNormally has a separate air data systemair data system to provide fullyto provide fullycorrected TAS information used to determine W/V. Air datacorrected TAS information used to determine W/V. Air datasystem can also provide INS with altitude information for system can also provide INS with altitude information for systems with only 2 accelerometers.systems with only 2 accelerometers.



INS DRIFTINS DRIFT

Since each new position is a function of the last position, INS is aSince each new position is a function of the last position, INS is arelativerelative system. Any initial entry error remains as a constant butsystem. Any initial entry error remains as a constant butsubsequent errors are cumulative & increase over timesubsequent errors are cumulative & increase over time

Total of errors is called ³Drift´ (acft ³drifts´ from true position)Total of errors is called ³Drift´ (acft ³drifts´ from true position)

measured in NM/hour measured in NM/hour 2 NM/Hour is historical industry norm2 NM/Hour is historical industry norm ± ± Ring laser gyros reduce toRing laser gyros reduce to

about 0.2 NM/hour about 0.2 NM/hour

Sounds like a lot for a super Sounds like a lot for a super--accurate (&super accurate (&super--expensive) system butexpensive) system butsystem can be ³updated´ with landsystem can be ³updated´ with land--based navaidsbased navaids ± ± max is about 4.5max is about 4.5hours out of rangehours out of range

SFOSFO ± ± HNL, where req acft separation is 50NM/1,000 ftHNL, where req acft separation is 50NM/1,000 ft

Ring Laser Gyro equipped INS probably more accurate than VOR fixRing Laser Gyro equipped INS probably more accurate than VOR fix ± ± suitable for RMNPS (Required Minimum Navigation Performancesuitable for RMNPS (Required Minimum Navigation PerformanceAirspace)Airspace)

Drift can be corrected byDrift can be corrected by updatingupdating: : reentering aircraft position over areentering aircraft position over aknown fix (navaid or visual checkpoint)known fix (navaid or visual checkpoint)



DUAL SYSTEMSDUAL SYSTEMS

Provide redundancy in case of failureProvide redundancy in case of failure

Can be used to check each other Can be used to check each other

Esp when inputting information. Best to use 2 pilotsEsp when inputting information. Best to use 2 pilots

for more crossfor more cross--checkingchecking 1 pilot put info into each system, then check results; or 1 pilot put info into each system, then check results; or

1 reads info, 1 enters, switch for 2nd system, then1 reads info, 1 enters, switch for 2nd system, thencompare; or compare; or

1 enters for both systems (into 1st then electronically1 enters for both systems (into 1st then electronically

xfer), then other checks against flt planxfer), then other checks against flt plan in any case, must ALWAYS crosscheck to verify.in any case, must ALWAYS crosscheck to verify.



Disagreement between systems must be resolved by:Disagreement between systems must be resolved by:

Other (LandOther (Land--based?) navaidsbased?) navaids ± ± GPS, VOR, DME, NDB, RADAR mappingGPS, VOR, DME, NDB, RADAR mapping

-- Dead Reckoning adequate if error is very largeDead Reckoning adequate if error is very large

Better to predict system most likely to err by keeping completeBetter to predict system most likely to err by keeping completeperformance records on each system; more accurate system will becomeperformance records on each system; more accurate system will become

apparent over time.apparent over time. On military acft, the navigator usually keeps this gyro logOn military acft, the navigator usually keeps this gyro log

Usually also have supporting evidence (other navaids or warningUsually also have supporting evidence (other navaids or warningindications) so can avoid this decisionindications) so can avoid this decision

Averaging the errors is NOT usually a good choice unless there isAveraging the errors is NOT usually a good choice unless there isabsolutely no way to identify the more accurate systemabsolutely no way to identify the more accurate system

Errors usually appear quicklyErrors usually appear quickly ± ± ie rapid increase in drift rateie rapid increase in drift rate

Can switch out of NAV Mode if unit is acting up and still use attitudeCan switch out of NAV Mode if unit is acting up and still use attitudefunctionsfunctions

-- Can¶t switch back, thoughCan¶t switch back, though ± ± need to initalizeneed to initalize



TRIPLE SYSTEMSTRIPLE SYSTEMS

Ultimate & most expensive formUltimate & most expensive form

Provides double redundancy + second check on route entry &Provides double redundancy + second check on route entry &initialization + supporting information when systems divergeinitialization + supporting information when systems diverge

-- Voting identifies weakest system by matching 2 closestVoting identifies weakest system by matching 2 closesttogether & rejecting the 3rdtogether & rejecting the 3rd

-- 3rd could be most accurate one but odds are in3rd could be most accurate one but odds are infavour of the other 2favour of the other 2

-- Eliminates ³Rogue system´Eliminates ³Rogue system´

Triple Mixing is flip side of votingTriple Mixing is flip side of voting ± ± uses middle Lat & Long couses middle Lat & Long co--ordinates to produce a single, optimum position estimateordinates to produce a single, optimum position estimate

Typical drift of 1.7 NM/Hour is about cut in half to 1.0 NM/Hour Typical drift of 1.7 NM/Hour is about cut in half to 1.0 NM/Hour

with modern gyroswith modern gyros







FUTUREFUTURE

Can Expect INS to stick around for a while because itCan Expect INS to stick around for a while because itknows so much & can derive so much moreknows so much & can derive so much more

Knows Lat + Long + True North & can be loaded withKnows Lat + Long + True North & can be loaded withmagnetic variation infomagnetic variation info

Can tell Mag NorthCan tell Mag North Provides gyro info + slaving info to flight instrumentsProvides gyro info + slaving info to flight instruments

Becomes an InertialBecomes an Inertial ReferenceReference System for allSystem for allinstrumentsinstruments ± ± position, hdg, attitude, autopilotposition, hdg, attitude, autopilotreferences, etcreferences, etc

Can select INS or Attitude/Heading Reference SystemCan select INS or Attitude/Heading Reference System(AHRS)(AHRS)

Can measure acceleration changes on approach toCan measure acceleration changes on approach todetect wind shear detect wind shear



Advantages:Advantages:

--completely self contained system whichcompletely self contained system whichoperates independently of an outsideoperates independently of an outside

navigation signal.navigation signal.--can be used as an inertial reference system for can be used as an inertial reference system for other systems ADI and HSI.other systems ADI and HSI.

Disadvantages:Disadvantages:

--INS systems are technically involved andINS systems are technically involved andexpensive.expensive.

--only as accurate as the information input; a dataonly as accurate as the information input; a dataentry error eliminates all accuracy.entry error eliminates all accuracy.

Documents

13 Inertial Navigation Systems