57 INS Alignment

Question 1

INS alignment definition

Answer 1

Levelling (local level): no component of gravity sensed by X and Y
(Azimuth) alignment: alignment of the azimuth sensitive system axis. Referenced to True north

Question 2

INS initialization 10 initial conditions

Answer 2

2 initial position coordinates – LAT/Long
2 initial velocities – N & E
3 initial orientations – X,Y and Z gyros
3 orientation rates

Question 3

Types of INS alignments

Answer 3

Self alignment – support equipment not available, most popular method
Reference Alignment – all methods involving external data source (runway heading)
Moving alignment – carrier-based aircraft
In-flight alignment – ability to align airborne

Question 4

INS Self alignment sequence

Answer 4

initialization
warm-up – no moving parts for strapdown system, alignment performed by the computer based on inputs from accelerometers and RLGs
Coarse levelling – accelerometer readings determine initial aircraft attitude
Coarse (azimuth) alignment
Fine levelling
Fine alignment AKA gyro compassing

Question 5

INS High Latitude Alignment Problems

Answer 5

1. undetectable tilt (simulated by computer) preventing initiation of gyro compassing
2. inability to accurately resolve True North

Question 6

INS errors 2 types

Answer 6

Unbounded: increase with time
Bounded: oscillate about a mean value with time
Bounded errors dominate first, then unbounded errors introduce the greatest errors

Question 7

Bounded errors

Answer 7

initial levelling (platform tilt-computer error)
Accelerometers (acceleration errors)
First integrator errors (velocity errors)

Question 8

Unbounded errors

Answer 8

Levelling gyro drift: produces the largest single error source within an INS
Initial azimuth misalignment: operator input
Azimuth gyro drift: second largest source of total error