13 Inertial Navigation Systems_2

8/2/2019 13 Inertial Navigation Systems_2

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INERTIAL NAVIGATION

SYSTEMS

INS


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WORD TO THE WISE

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

isnt, arriving at the position where it wasnt, itnow is. Consequently the position where it is,is now the position where it wasnt and itfollows the position where it was is theposition where it isnt.

Copyright: ARINC 561 Manual, August 1976


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INERTIAL NAVIGATION SYSTEMS

Completely self-contained navigation systemcapable of providing great circle tracks overrandom routes without reference to externalinformation sources.

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

- Still the nav system of choice for many operations.- Developed for the military accurate, reliable, not

susceptible to signal jamming or erroneous signaltransmission.

- Extremely simple in concept, extremelycomplicated in execution.

- Sometimes described as a very accurate dead-

reckoning system.


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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.


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RING LASER GYROS

They utilize the principle of the Doppler effect.

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

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

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

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


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RING LASER GYROS

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

The rate of rotation of the gyro determines the phase

difference of the frequencies. Each particular phasedifference coincides with a unique rate of turn which theprocessor can thus calculate.

Each ring laser gyroscope only rotates on one axis,

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


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ADVANTAGES

Few moving parts

Small size and light weight

Rigid construction

High tolerance to shock, acceleration,and vibration

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


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ADVANTAGES

Because there are no rotating gimbals as ina mechanical gyro, there is no friction, andtherefore no errors caused by realprecession

Less power consumed than mechanicalgyros because there are fewer movingparts.


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DISADVANTAGES

Base cost of Laser ring gyros is moreexpensive than mechanical gyros.

Laser ring gyros are susceptible to an errorknown as LOCK-IN


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LOCK-IN

When the rate of turn is very small, thefrequency difference between the twobeams is small.

There is a tendency for the two frequenciesto couple together and lock-in with eachother.

As a result of lock-in, a zero turning rate isindicated.


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LOCK-IN

While lock-in errors are not substantial, theycan be accounted for by using morecomplex ring laser gyro systems.

By mechanically moving or twisting thesystem, the coupling of frequencies doesnot occur.

This mechanical adjustment is calledDITHERING.


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STABLE PLATFORM

Stable platform has 2 functions:

To keep accelerometers aligned with the surfaceof earth despite changes in aircraft attitude. This is

accomplished by mounting the platform ongimbals.

To keep the stable platform aligned with surface of

earth to compensate for transport and earth rateprecession. These apparent errors are correctedby torquing a feedback process that keeps thestable platform level with the local vertical throughthe application of real precession.


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STRAPDOWN PLATFORM

- In a strapdown platforminstallation the platform is not

gimbaled; instead ring lasergyro feedback allows acomputer to electronicallymonitor orientation of theplatform.


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INERTIAL REFERENCE SYSTEM

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

-An Inertial Reference System (IRS); mechanical

gyros become backup units.-The IRS is used for AI, HI, radar antenna

stabilization, and autopilot control.-If the navigation portion of the INS goes down it is

still possible to utilize the IRS by selecting ATTinstead of NAV.

- Inertial Navigation Unit box containingaccelerometers & gyro-stabilized platform


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CONTROL DISPLAY UNIT (CDU)

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

Older displays easier to read but CRT units provide

much more information Multiple flight plans, >9 waypoints

Used in conjunction with mode select panel

Standby Battery Absolutely critical to avoid completeloss of unit during power variations

These form the system package


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OPERATION

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

- Only as accurate as the information input.

- Acft can then be moved & INS will begin sensing &correlating all movements

Limitations

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

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

latitude but work OK there once aligned farthersouth. Problem is earths rotation too slow to bedetected at those latitudes.


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DATA PRODUCED

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

Track and Position as a series of fixes

Ground speed Drift angle

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

Distance to go/ Time to go

Errors & Recommended Actions

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


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DUAL SYSTEMS

Provide redundancy in case of failure

Can be used to check each other

Esp when inputting information. Best to use 2 pilotsfor more cross-checking

1 pilot put info into each system, then check results; or

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

1 enters for both systems (into 1st then electronically

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


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Disagreement between systems must be resolved by: Other (Land-based?) navaids GPS, VOR, DME, NDB, RADAR mapping

- Dead Reckoning adequate if error is very large Better to predict system most likely to err by keeping complete

performance records on each system; more accurate system will becomeapparent over time.

On military acft, the navigator usually keeps this gyro log Usually also have supporting evidence (other navaids or warning

indications) so can avoid this decision Averaging the errors is NOT usually a good choice unless there is

absolutely no way to identify the more accurate system Errors usually appear quickly ie rapid increase in drift rate

Can switch out of NAV Mode if unit is acting up and still use attitudefunctions- Cant switch back, though need to initalize


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MIXED SYSTEMS

-Another way to provide redundancy is

by adding a complimentary system suchas GPS. This system can be used toupdate the INS.


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FUTURE

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

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

Can tell Mag North Provides gyro info + slaving info to flight instruments Becomes an Inertial Reference System for all

instruments position, hdg, attitude, autopilotreferences, etc

Can select INS or Attitude/Heading Reference System(AHRS) Can measure acceleration changes on approach to

detect wind shear


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Advantages:

-completely self contained system whichoperates independently of an outsidenavigation signal.

-can be used as an inertial reference system forother systems ADI and HSI.

Disadvantages:-INS systems are technically involved and

expensive.-only as accurate as the information input; a data

entry error eliminates all accuracy.

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13 Inertial Navigation Systems_2