Inertial navigation systems

Question 1

an inertial navigation system is?

Answer 1

a fully self contained system that requires no external inputs regardless of location, time flow

Question 2

two types of inertial navigation systems

Answer 2

inertial nav sytem

inertial reference system

Question 3

INS

Answer 3

uses 3 rate gyros
integrates acceleration and time to get speed and distance
has 2 or 3 accelerometers measuring accelerations in different axis
primarily used for navigation, some attitude as well
stabilised platform system
1.5Nm of drift per hour
15min alignment time

Question 4

integrations questions: speed

Answer 4

speed: integrates once the acceleration in time and know the initial speed

Question 5

integrations questions:distance

Answer 5

distance: integrating acceleration in time twice and needs to know the initial speed, or/ integrate speed in time once

Question 6

integrations questions:position

Answer 6

position: integrate acceleration in time twice and know initial sped and initial position, or/ integrate speed in time once and know initial position

Question 7

INS accelerometer design

Answer 7

pendulum based electromechanical accelerometer
E&I bar linked to electromagnets which return pendulum back after a disturbance
amount of electrical signal needed to restore from e bar is sensed - higher electrical value needed= higher acceleration
errors in acceleration will cause position errors- inertial drift- 1.5Nm per hour
acceleration based altitude is the least accurate kind

Question 8

rate integrated gyro

Answer 8

used for alignment
gyro in a cylinder inside another cylinder
lubricating oil between the cylinders reduces frictional error
only disturbed in one axis
move around the sensitive axis- inner cylinder rotates(due to precessed force) and this is measured
3 rate gyros always needed
very small and sensitive

Question 9

sensitive axis

Answer 9

input axis

axis about which rotation will cause precession

Question 10

output axis

Answer 10

axis about which the precessed force acts

Question 11

keeping accelerometers pointed N/S E/W

Answer 11

accelerometers & gyros on stabilised platform
3 torque motors
when the a/c pitch rolls or yaws the gyros rotate and the synchros measure this and send a signal to the relevant torque motors which pitch roll or yaw the platform by the exact opposite to keep it level and pointing north

Question 12

aligning the platform

Answer 12

3 phases:
initial power up caging
levelling
gyro compassing

takes ‘about’ 15 mins

Question 13

aligning the platform: initial power up

Answer 13

gyros spun to operating RPM

lubricating oil of gyro warmed to operating temp

Question 14

aligning the platform: levelling

Answer 14

when level the N/S & E/W accelerometers should sense nothing
torque motors rotate the platform until this is achieved
requires the aircraft to be stationary

Question 15

aligning the platform: gyro compassing

Answer 15

system will wait to sense some topple
topple depends on latitude- system can self determine latitude
topple occurs around the N/S axis- self determines true north

Question 16

final platform alignement step: pilot input

Answer 16

needed to establish starting location
enters lat as a cross check
enters longitude as undeterminable by the system
system saves the last longitude before shutdown to compare with the one the pilot enters as a check

Question 17

if the wrong longitude was entered and accepted:

Answer 17

the same error would persist for the whole flight. wouldn’t get better or worse
bounded error

Question 18

if the wrong latitude was entered and accepted:

Answer 18

wrong topple & drift corrections would be applied and position would drift more & more
unbounded error

Question 19

INS errors

Answer 19

most corrected for 
2 biggest contributors & which are unbounding:
inertial error from accelerometers 
random error from the gyros 
other errors:
earth rotation (topple & drift)
transport wander (topple & drift)
Coriolis effect
earths curvature 
schuler oscillation

Question 20

INS error: ER & TW

Answer 20

predictable

calculated by computer which sends rate corrections to torque motors to tilt platform to oppose these effects

Question 21

INS error: Coriolis effect

Answer 21

aircraft has lateral velocity with earths rotation, changes with lat and groundspeed
computer applies a lateral acceleration based on groundspeed & latitude

Question 22

INS error: earths curvature

Answer 22

system thinks its heading a straight line gradually getting further from surface of earth
compute applies acceleration towards centre of the earth dependant on your groundspeed

Question 23

INS error: schuler oscillation

Answer 23

system that returns to a datum after being displaced will oscillate
different systems will have different oscillations
INS ends up in a scholar oscillation- behaves like a pendulum been suspended to the centre of the earth

Schuler period is 84.4 minutes
error is bounded and cancelled out by the computer

Question 24

INS power source

Answer 24

AC- primary
DC- secondary
battery- backup- allows time to get power back

Question 25

pilot INS controls

Answer 25

CDU - control display unit

MSU- mode select unit

Question 26

INS CDU functions:
TK/GS
HDG/DA
XTK/TKE
POS
WAY PT
DIS/TIME
WIND
DSRTK/STS
DIM

Answer 26

TK/GS
Track and Groundspeed.
HDG/DA
Heading and Drift Angle.
XTK/TKE
Distance of Track angle between track and desired track.
POS
Position as Lat & Long.
WAY PT
Lat & Long of selected Waypoint.
DIS/TIME
Distance and Time to current waypoint. or destination
WIND
Calculated Wind Velocity. only with ADC input
DSRTK/STS
Desired Track and Status.
DIM
Brightness control.

Question 27

INS CDU lights & buttons

Answer 27

alert light- 2 mins to next waypoint
warning light- INS failed
amber bat light - using internal battery
hold button- freezes display
rotary knob- chooses what's displayed

Question 28

INS MSU

Answer 28

off = off
STBY= stby, spins up gyro and warms oil
ALIGN = aligns
NAV= normal operating mode- nav and attitude (if ou have attitude info)
ATT= attitude mode- used to regain attitude info after loss of alignment
green ready light illuminates when ready- goes out when you switch to nav
red batteries light- tells us system is on battery power AND battery voltage low- system has or about to fail

Question 29

IRS inertial reference systems

Answer 29

need 3 accelerometers
3 ring laser gyros
strap down system 
alignment time- about 10 mins
drift 0.1 Nm per hour
must have FMS connection
have 2 or 3 for redundancy 
main contribution is attitude information

Question 30

INS vs IRS

Answer 30

standalone nav: 
INS yes
IRS nope needs FMS
velocity and position data:
INS yes
IRS yes
other data to other systems
INS maybe 
IRS yes

Question 31

IRS gyros

Answer 31

uses ring laser gyros RLG
triangular cavity, 2 laser anode, 1 laser cathode, mirrors & prism guide lasers to light detector
one mirror controlled by motor to align lasers
lasers travel opposite directions around cavity
helium and Neon gas create laser

any rotation round sensitive axis- one laser contracts and one stretched, causes disturbance in interference pattern
gyro measure frequency difference and translates to rotation rate

Question 32

how many RLG does a IRS have?

Answer 32

3
at 90° to each other
Any pitch roll and yaw detected - attitude measurement

Question 33

IRS errors

Answer 33

same as INS
random wander- imperfections, reduced with longer path for laser to travel
laser lock- wavelengths paired at small rotations and no change detected- Dither motor reduces by oscillating RLG so almost never slow enough for laser lock- computer cancels this out when determining attitude

Question 34

strap down system

Answer 34

not mounted on a platform but directly bolted to the aircraft

Question 35

IRS alignment

Answer 35

takes about 10 min (between about 5-11 in EASA)
system aligns to aircraft reference trihedron

levelling: uses accelerometer to determine local vertical
gyro compassing: uses RLG to detect earth rotation which is used to find lat and true north

Question 36

IRS panel

Answer 36

off, align, nav and att

align light- aligning; flashing- waiting for longitude input
on DC light- AC power failed
DC fail light- DC power failed, AC only
fault- fault detected