INS / IRS Flashcards
What is an inertial navigation system
A fully self contained navigation system which needs to external reference
Developed in the 70’s / 80’s
How does a INS work
Track distance change calculated by measuring a/c accelerations
How many time do you integrate acceleration to get velocity
Once
How many times do you integrate acceleration to get distance
Twice
What direction is the INS platform orientated
Relative to true north
How many rate gyros does a INS have
3 rate gyros
How many accelerometers does a INS have
3
How many gimbals does the INS platform have
3
What for a east gyro in the INS detect
Movement around east west axis
What does the north gyro detect in the INS
Movement around the N/S axis
What does the azimuth gyro detect in the INS
Movement up and down
3 key phases of INS
1) initial power up (caging)
Locks gyros
Warms oil
Gets up to speed
2) levelling
3) align with true north
If a/c flies north what happens to the accelerometer weight bob
Moves southward
Steps of the mode selector unit for the INS
1) off
2) SBY - allow to set up / warm oil
Green light = ready nav
3) align or nav
If align = more accurate but can go straight to nav
What is the red batter annunciator light for the more selector unit INS
Power supply for INS has failed
How long can a INS take to align
Up to 15 mins
How does the gyro know it’s aligned
No n/s accelerations due to earth rate
Where does INS align quickest
Equator
Where can the INS not be aligned
Above 70°N
What happens in the INS when switched from align to nav mode
Keeps platform level and aligned
Control display unit functions: TK
True track
Control display unit functions: GS
Ground speed
Control display unit functions: HDG
True heading
Control display unit functions: DA
Drift angle
Control display unit functions: XTK / TKE
Distance off track / angle between track / desired track
Control display unit functions: POS
Position as latitude / longitude
Control display unit functions: dist / time
Distance and time to next waypoint
Control display unit functions: WAYPT
Lat / long of selected waypoint
Control display unit functions: WIND
Wind direction / speed
Control display unit functions: DARTK / STS
Desired track and system Status
How is the component of gravity sensed if there is a error during alignment of the INS
Horizontal acceleration
= unbound error
How to calculate TAS from INS
INS need the addition of information from ADC to create TAS as INS on its own is just groundspeed
How does INS create magnetic heading output
Converts true heading using a database of variation magnetic and track values
What are unbound errors in the INS
Ones that get worse
Topple / drift
Apparent wander
A/c manoeuvre errors
What is Schuler oscillations
The inertial platform behaves like a pendulum with length equal to earth radius
What is the period of schuler oscillations
84.4 minutes
What type of error is a Schulers error
Bound error
What is Kalman filtering
Blending of inertial positions with gps + radio nav
In modern a/c
What power supply is used by a INS
Ac for normal operations
Dc for backup
If INS fails what can is be used as
Attitude reference
How many IRS does a short / medium haul aircraft have
2
How many IRS does a long range aircraft have
3
How is a IRS similar to a INS
Has accelerometers and gyros
Except they are laser gyros
3 solid state accelerometer
3 ring laser gyros
How is IRS different to INS
Platform it fixed directly to airframe = strap down system
Actual platform = generated mathematically to the a/c axis by computer
= knows where true north is by calculation
What at the components of a accelerometer in the IRS
Small proof weight attached to free end
Piezo- electric layer is located at the point of max strain on the beam
Acceleration perpendicular to the beam will cause it to bend
What are the components of a ring laser gyro IRS
3 mirrors
1 one way murror
1 laser
How does a ring laser gyro work
If not rotating then the laser beams both reach the detector at the same time / frequency
If rotation = one length longer than the other / frequency change
What is laser lock
If rotating rate measured is very small /slow (in the IRS) sometimes not registered
How to fix laser lock
A dither motor is used to vibrate the laser ring gyro
Doesn’t cause rotation just vibration + active hum
What is ADIRS
Air data computer + inertial reference unit
Do strap down systems suffer from schuler oscillations
Yeh
How long does the IRS take to align
8-10 mins
What happens if the IRS loses alignment in flight
Navigation ability is lost
IRS May be re aligned in flight for attitude data only
