AS 11 - Inertial Navigation Flashcards
Review Questions
If unable to tune the HF radio, what are the troubleshooting procedures?
Change the volume setting, put it back to mid range and try again
Recycle power
Have another station take control of HF, then take control back
Have pilots take control of the HF, then take it back after 1-2 mins
Define Inertial Navigation System
A navigation aid that uses computer and motion sensors (accelerometers) to continuously calculate via dead reckoning the position, orientation and velocity of a moving object without the need for external references.
What are the advantages of INS?
Self contained Does not radiate Unjammable All weather operation World wide operation Relatively accurate position, very accurate attitude information
What are the generic INS components?
Accelerometers
Computers
Stable platform
Control and display unit
Explain the Law of Inertia
A body at rest tends to stay at rest, a body in motion remains in uniform motion with constant speed in a straight line unless acted on by an unbalanced external force.
Describe Newton’s 2nd law
Law of Motion
Acceleration produced by a particular force acting on a body is directly proportional to the magnitude of the force, and inversely proportional to the mass of the body.
Using measured acceleration: Integrate acceleration as a function of _____ to get _______.
Time & velocity
Using measured acceleration: Integrate velocity as a function of _____ to get _______.
Time & Displacement
True of false
Displacement equates to distance travelled.
True
How can we determine where we are now?
If we know where we started from and if we can measure acceleration in three dimensions.
Where we are = __________ + __________
Where we started + how far and in which direction we went
Y Axis or N/S = ____
X Axis or E/W = _____
Z Axis or U/D = _____
Roll
Pitch
Yaw
How does a Ring Laser Gyro work?
Lasers are fired simultaneously and travels in opposite directions around a cavity
The phase relationship between the two beams when they arrive at the detector determines motion of the AC.
If the RLG is stationary, how do the beams behave?
They complete the trip around the block at the same time since the optical path is the same.
The phase’s of both beam are the same when they arrive at the detector.
If an angular or azimuthal displacement (rotation) exists, the path length within a RLG will be?
Unequal
If a phase difference is measured within a RLG, what is implied?
A time difference between both beams arriving at the same point.
Explain the Differential Path Length Error. How is it eliminated?
If there exists a temperature difference between CERVIT cavities by even a fraction of a degree, there will be a time/ phase difference.
Eliminated by using one CERVIT cavity for both lasers
Explain the RLG Lock In error.
Non perfect optics within the cavity and mirrors will cause a certain amount of backscattering.
Backscattering tends to reinforce the beam in the traveling in the opposite direction.
A RLG Lock In error causes what at low rates of rotation?
Frequencies are combined, producing a dead zone.
A RLG lock In error, if left undetected for an hour, will yield what degree of drift?
40 - 50 Deg
What component is responsible for eliminating the Lock In error for the RLG? How does it do so?
The dither motor, it shakes the CERVIT block at its resonant frequency about the input axis.
How are errors created by the dither motor dealt with?
Averaged out and eliminated.
Dithering will cause two what every cycle?
Two extremes, but the dithering motion is averaged and the average is the end result while preventing lock in.
What is INS’s main disadvantage? How is it fixed?
Degrading accuracy over long periods.
To fix this, hybridization or integration with other navigation sensors is used to improve accuracy over time.
What are the two main advantages to INS integration?
Much improved accuracy
Higher mission success rate as the sensors may be used independently.
What are the two main disadvantages to INS integration?
Loss of complete self-containment
Loss of covertness, dependence on external radiation.
What are the four ways to integrate a pure INS?
Ground Referenced hybrid system
GPS - INS hybrid
Doppler - INS hybrid
Celestial - INS hybrid
What is an accelerometer and what does it measure regarding inertia?
A basic device for measuring acceleration
Measures the “inertial pushing back” of a known mass in response to an externally applied force.
What are an Accelerometer’s desired characteristcs?
Low threshold of sensitivity
Wide range of sensitivity
Linear output
High resolution
What are the two most common accelerometers?
Pendulum and Vibrating String
For leveling and alignment, what 10 initial conditions are required?
2 initial coordinates - Lat & Long
2 initial velocities - N & E
3 initial orientations X, Y and Z gyros
3 orientation rates
What is the alignment sequence?
Warm up - No moving parts for strapdown system, alignment performed by computer based on inputs from accelerometers and RLG
Coarse Leveling - Accelerometer readings determine initial aircraft attitude
Coarse (azimuth) Alignment
Fine Leveling
Fine Alignment AKA Gyro Compassing
What are two high altitude alignment problems?
Inability to detect true north - Becomes a problem at latitudes 70 Deg and above
Undetectable tilt - Prevents initiation of gyro compassing
What are the two types of errors? Briefly explain them
Unbounded - Errors that increase with time
Bounded - Errors that oscillate about a mean value with time
Which errors dominate first? Which introduce the greatest errors?
Bounded errors dominate first, then unbounded errors introduce the greatest errors.
What are three sources of bounded errors?
Initial leveling (platform tilt computer error)
Accelerometers (acceleration errors)
First integrator errors (velocity errors)
Explain the Bounded Error - Schuler Loop
think gyros, accelerometers and integrators
A stable platform has an initial indication of acceleration
The platform believes it is travelling across the earth, gyros are then torqued to keep the platform level to earth where the platform “thinks” it is
Due to this “correction” torque, the accelerometers feel a “real” component of acceleration due to gravity
Integrators come up with a velocity in the opposite direction
Then, the platform re-tilts to the original direction and goes back to where it was.
Regarding the Bounded Error - Schuler’s Loop, the tilt error makes the platform do what?
Rock back and forth so that its vertical axis is swinging like a pendulum.
Schuler’s Bounded Error Loop cycle has a period of what?
84.4 minutes.
What are the three unbounded errors?
Leveling gyro lift
Initial azimuth misalignment
Azimuth gyro drift
Describe the Leveling Gyro drift.
The platform will be torqued out of level and the accelerometer will sense a gravity component, which is integrated into velocity.
This torques the levelling gyro in the opposite direction, but the erroneous torqueing overshoots the gyro drift
As a result, position error grows continuously
Which unbounded errors are the largest sources of INS errors?
Worst = Leveling gyro drift
2nd Worst = Azimuth gyro drift
Review Questions
What are the 10 initial conditions required to initialize an INS?
2 initial position co-ordiantes - Lat and Long
2 initial velocities - N and E
3 initial orientations - X Y Z gyros
3 orientation rates
Review Questions
What are the advantages of INS integration?
Much improved accuracy
Higher mission completion rate since sensors may be used independently
Review Questions
What are the desired characteristics of accelerometers?
Low threshold to sensitivity
Wide range of sensitivity
Linear output
High resolution
