2. Inertial Navigation Flashcards
What is Dead Reckoning?
Estimating one’s own position based on a previous position. You keep track of speed and time to determine your new position relative to the previous position.
Define Inertia
Resistance to change in velocity.
Newton’s 3 Laws
- Objects in motion stay in motion and objects at rest stay at rest unless acted upon by an external force.
- F = ma
- For every action, there is an equal and opposite force
How does a linear accelerometer work?
How does a gyroscope work?
A flywheel is spun, giving it angular momentum. The flywheel maintains its orientation IOT conserve its angular momentum.
When the flywheel is mounted in a frame which prevents one from imparting torque on the flywheel, it will remain in the same position while the frame moves around it. The movement of this frame is measured.
If torque is applied to the flywheel, the flywheel will rotate in a direction 90 degress to the applied force and the angular momentum.
What is Gimballed Stabilization?
Uses a system of gyroscopes in a gimbal frame and a platform servo system.
More expensive and may cause Gimbal Lock but more accurate.
Describe Strap Down Stabilization
Accelerometers and Gyro’s are strapped down to the fixed frame. It is cheaper but difficult to calibrate.
What are Carouselling and Indexing and why are they used?
Carouselling is rotating the gyro around an axis.
Indexing is changing the orientation by 90 degrees periodically.
Removes error and biasing.
What is a Gyrocompass?
A compass using gyro stabilization to point to true north.
Describe the Sagnac Effect in LRGs
The speed of light is independant of the velocity of the medium through which it travels. If you send a beam of light around a ring but the ring is rotated, the light will take slightly longer (or shorter depending on the direction of rotation) to come around the ring. We can measure that difference to determine the angular velocity.
The better way to look at it is that the two waves are send in opposite directions around the ring. They combine to form a standing wave with nodes and anti-nodes. When the ring moves, the fluid moves, but the standing wave does not move with it. There is a sensor in the ring which moves and to the sensor, it appears that the standing wave is moving past. The sensor counts the standing waves.
Describe how the LRG in the Mk-49 works
Polygon shape using mirrors instead of circular rings.
When the ring is stationary, it produces a standing wave.
When the path lengths are different due to rotation of the medium, there is a phase difference between the component waves, which is supplied to a photomultiplier tube which converts this into electrical signals.
Describe Electron Drift. How is it compensated?
The gas used inside the ring is ionized IOT ____
It is ionized using a voltage between the cathode and anode.
This causes electrons to drift towards the positive anode, causing the gas to flow, leading to error.
This is compensated by using one cathode and two anodes evenly spaced, and adjusting the voltages such that they are equal in each direction, eliminating flow of electrons and therefore the gas.
What is Frequency Locking and how is it compensated for?
At low rotation rates, the standing wave tends to lock ot the ring and move with it due to back scatter of photons at the mirrors.
This is compensated by mechanically rotating (dithering) the optical path back and forth at a high rate.
How do we ensure high quality reflection off the mirrors?
Dyanimically positioning the mirrors such that the laser hits the mirror surface ideally.
What are Systematic Errors?
Errors which can be calibrated and compensated
Gravitational Error
Gravity is not constant through the world
What types of filters do we use?
Kalman
Alpha-beta
Kalman Filter vs Alpha-Beta Filter
A Kalman Filter is a Recursive Filter which uses the previous inputs and outputs and the current input to estimate the current position. It constantly adjusts the Gain (weighting) of the estimates depending on how accurate the previous estimates were.
An Alpha-Beta filter has static gain.
Mk 49 LRG Modes of Operation
Standby: From Power On to entering Align mode
Align: Takes 4h to align itself. Takes 16 hours to calibrate.
Navigate: Gyro is settled.
Interfaces for Nav System (7)
DSL
GPS
Alarm Panels
VHDP
Navigation Switchboards
Other INS
CCS
Types of Datums (4)
Master Datum Plane: Horizontal plane for systems throughout the ship
Master Training Datum: Perpendicular to the master datum
Gunnery Datum Plane: For the weapon/sensor/nav suite, defined by the roller path of the 57mm
Benchmark: temporary individual reference to check alignment.
Floating Stabilization
Achieves stability using a neutral buoyant sensor suspended in a liquid.
When would be the maximum intensity of the laser and therefore the maximum efficiency?
If the length of the lazing path was held constant at exactly some multiple of the wavelength.
What are the advantages of a RLG?
No high spin precision elements, no gyro bearing, almost instant reaction time.
Zero-mean errors
Error which whose distribution is centered around zero. ie. white noise.
Hyperbolic Navigation
3 signaling ground stations allow you to triangulate your position based on the time stamp of the signal.
Hyperbolic Navigation: Pros and Cons
Pro: No transmission required by user
Cons: Vulnerable to sabotage, not global, must operate and maintain stations
List some GNSS systems:
GLONASS (RU)
Galileo (EU)
Compass (China)
GPS (US)
What system was used for Hyperbolic Navigation?
LORAN-C
3 segments of a GNSS
Ground Segment
User Segment
Space Segment
Ground Segment
Tracks the satelites and sends corrections
Within the Navigation Data Message: Ephemeris Data:
Price satelite position and clock error for each satelite. Valid for 4h
Within the Navigation Data Message: Almanac Data
Course orbital parameters from satelites. valid for 24h.
Space Segment
24 Satellites
6 orbital paths
20 to 200km altitude
6 satellites are in view at all times
Broadcasts two spread-spectrum RF signals
User Segment
Users receive signals and calculate position, speed, and time themselves.
How to receivers know the precise location of the satellites they receive from?
Control Station updates the Navigation Data Mesasge for each satellite as it passes overhead.
How is the time tracked on satellites?
UTC (Coordinate Universal Time) is the universally recognized time based on atomic clocks on earth.
Each satellite has its own atomic clock kept to 1 microsecond of UTC.
The 4th satellite is used to reduce clock error.
Trilateration vs Triangulation
Triangulation uses angles.
Trilateration uses the distance. This is what GPS uses.
Why do we need a fourth satellite with time correction?
The receiver does not have an atomic clock, it has a quartz clock which is not very accurate. The fourth satellite sends a timing signal based on its atomic clock IOT increase the accuracy of the trilateration.
