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
