Flashcards in INS Deck (36):
What does INS stand for?
Inertia Nav system
INS operates on ....
A platform with 3 integrated gyros and 3 accelerometers on a platform using a pendulous swing method to measure acceleration in one dimension
INS is aligned to....
Meaning the gyros are also aligned ˚T North
How are the gyros and accelerometers aligned?
3 Accelerometers mounted at right angles to each other:
- one to sense E/W acceleration
- one to sense N/S acceleration
- one to sense vertical acceleration
3 Rate indicating gyros aligned at right angles to each other:
- one to sense E/W rotation
- one to sense N/W rotation
- one to sense vertical rotation
What is a rate gyro and what instruments use this?
This is a gyro with only one gimbal.
TI/TC and INS use the rate gyro.
Why are the INS gyros on a platform?
Because gyros are very sensitive. Where a gyro misalignment in a AI is insignificant, a slight misalignment for an INS cause huge errors. E.g 0.5˚ misalignment can cause a 300nm error after one hour. Thus a platform is used to detect and eliminate alignment errors.
Explain briefly the gimbal system within an INS.
The gyros, although has only 1 gimbal, is encased inside 2 cases which act as the gimbal. Between the 2 cases is a liquid. The liquid acts as a secondary precession. Unlike the TI/TC though, the secondary precession doesn't stop the rotation, only slows it.
Because the INS is being feed data electronically, INS can also adjust for apparent drift via the torque motors.
What are the alignment phases for an INS?
1. Initial power up (caging)
And aircraft MUST be stationary before process can take place.
In INS, what is initial power up (caging)?
This is when the power is switched on and the liquid between the cases (gimbals) within each gyro is heated. An amber light comes on and the gimbals are locked (caged). Once the gyros have had time to spin up to its operating RPM, the amber light switches off and levelling; proper alignment can begin.
In INS, what is levelling?
This is phase 2 of initial alignment and is the process of aligning the platform to local horizontal.
Since the A/C is stationary, then the accelerators (E/W and N/S) should have an output of zero. If there is a higher indication then this 'supposed acceleration' is due to platform misalignment. The appropriate torque motors are then moved to drive the platform to horizontal. Once E/W and N/S accelerators both read zero, this is when the platform has been levelled.
In INS, what is gyro-compassing?
This is the 3rd and final phase of alignment and is where the system detects for any E/W topple.
Due to earth's rotation, N/S topple is normal, but E/W topple is not. If there is E/W topple then the platform is misaligned. Once corrected by the azimuth motor i.e. E/W rotation falls to zero, then the alignment is complete.
When the alignment process is complete, a green complete ready light comes on and the pilot can now go into 'nav' mode.
For an INS, how is lat calc?
1˚ Lat = 60nm
Thus 1 min = 1NM
For INS, how is longitude calc?
Dept (nm) = chlong' (mins) x cos lat
In an INS system, the correct longitude MUST be input into the system otherwise a position error will occur.
What speed does INS not calc?
INS system calcs ground speed but not TAS so this must be manually input into the system.
How does INS calc drift?
By comparing HDG with TRK, as the angle between the 2 is drift angle (DA)
How does INS calc wind velocity?
The system must have a TAS input from the ADC to calc this.
What are the errors that can be experienced in an inertial system?
• Topple and drift due to earth rotation
• Topple and drift due to transport wander
• A/C manoeuvres (changes of attitude or HDG)
What inertial system errors occur in an INS?
• Coriolis effect
• Centripetal acceleration
• Schiller oscillation
For an INS, how does it correct for Coriolis effect?
For inertial systems, they account for coriolis effect by applying an appropriate lateral acceleration related to groundspeed and latitude.
For an INS, how does it correct for centripetal accelerations?
Adjustment of position to follow the curvature of the earth.
Need to account for the centrifugal force pulling us out of position as we fly around the curvature of the earth, thus a centripetal correction needs to be applied.
What is Schiller oscillation?
The platform of an INS tries to maintain local horizontal, however as A/C travels this is increasingly hard to do. As no platform/system is perfect, therefore giving false acceleration errors and thus further fixed position errors.
Sculler discovered that a platform trying to maintain local horizon is no different from a pendulum swinging and returning to equilibrium. He found that 1 cycle of error will take 84.4 mins. So apparent wander corrections are made on the Schiller model.
How long is 1 Schiller oscillation?
What is the ave position error of an INS?
21 mins after switching to Nav mode there is a 1 miles unaccounted for error. What will be the error after 63mins?
1 mile and is called bounded error.
What is bounded error?
This is where an error occurs, and the magnitude of this error remains the same over time.
So with Sculler oscillation, peak to min error will occur at 1.5NM/hr and 1 cycle will last 84.4mins
Think neutral dynamic stability.
What is unbounded errors?
This is where an error occurs, and over time this error grows in magnitude. This is also called cumulative error.
This negative dynamic stability.
What would happen to INS if power is lost or the system is switched off during flight?
The system will lose all its data and will have limited attitude capabilities. This reversion called altitude mode. This is ATT/REF facility.
As need to be stationary to align, can't do this in the sky. When system is switched to ATT/REF mode, it is NO longer in NAV mode.
What does DIS/TIME setting give you on the INS CDU?
distance and time until next way point
What does WIND setting give you on the INS CDU?
Wind directions, however to allow INS to compute this, you must manually or is fed by the ADU, the TAS as the INS can only calc ground speed.
What does DSTRK/STS setting give you on the INS CDU?
Desired track and the status for the current leg i.e. Between the 2 way current way points.
What does XTK/TKE setting give you on the INS CDU?
Cross track position and tracking error
XTK: distance in NM left or right of track
TKE: is the angular difference between current and desired trk.
How long does an INS alignment take?
12 to 15 mins
In total; for all 3 phases to be complete.
What is double integration?
Integration is a complex maths calc that will give you velocity. Integrate it again, double integration will give you distance travelled along E/W (longitude).
What is the correct order of modes on an INS MCU?
Off, standby, align, Nav
MCU: Mode control unit
What is the largest source of an error in INS?