Inertial navigation systems Flashcards
an inertial navigation system is?
a fully self contained system that requires no external inputs regardless of location, time flow
two types of inertial navigation systems
inertial nav sytem
inertial reference system
INS
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
integrations questions: speed
speed: integrates once the acceleration in time and know the initial speed
integrations questions:distance
distance: integrating acceleration in time twice and needs to know the initial speed, or/ integrate speed in time once
integrations questions:position
position: integrate acceleration in time twice and know initial sped and initial position, or/ integrate speed in time once and know initial position
INS accelerometer design
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
rate integrated gyro
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
sensitive axis
input axis
axis about which rotation will cause precession
output axis
axis about which the precessed force acts
keeping accelerometers pointed N/S E/W
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
aligning the platform
3 phases:
initial power up caging
levelling
gyro compassing
takes ‘about’ 15 mins
aligning the platform: initial power up
gyros spun to operating RPM
lubricating oil of gyro warmed to operating temp
aligning the platform: levelling
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
aligning the platform: gyro compassing
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
final platform alignement step: pilot input
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
if the wrong longitude was entered and accepted:
the same error would persist for the whole flight. wouldn’t get better or worse
bounded error
if the wrong latitude was entered and accepted:
wrong topple & drift corrections would be applied and position would drift more & more
unbounded error
INS errors
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
INS error: ER & TW
predictable
calculated by computer which sends rate corrections to torque motors to tilt platform to oppose these effects
INS error: Coriolis effect
aircraft has lateral velocity with earths rotation, changes with lat and groundspeed
computer applies a lateral acceleration based on groundspeed & latitude
INS error: earths curvature
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
INS error: schuler oscillation
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
INS power source
AC- primary
DC- secondary
battery- backup- allows time to get power back
pilot INS controls
CDU - control display unit
MSU- mode select unit
INS CDU functions: TK/GS HDG/DA XTK/TKE POS WAY PT DIS/TIME WIND DSRTK/STS DIM
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.
INS CDU lights & buttons
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
INS MSU
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
IRS inertial reference systems
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
INS vs IRS
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
IRS gyros
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
how many RLG does a IRS have?
3
at 90° to each other
Any pitch roll and yaw detected - attitude measurement
IRS errors
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
strap down system
not mounted on a platform but directly bolted to the aircraft
IRS alignment
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
IRS panel
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
