INS: Inertial Navigation System IRS: Inertial Reference System

Question 1

INS: Inertial Navigation System

Answer 1

● An Inertial Navigation System is just what the name implies:
○ A navigation system based on the principles of inertia.
● It is independent of ground based or space based navigation aids.
● Even without the outside reference to these aids, it is still able to give us an accurate position; along with our, speed, heading, and attitude

Question 2

INS: Basic Principles

Answer 2

● To simplify things, we will first consider motion along one axis.
(Be sure to watch the presentation video, and the whiteboard work for this part!)
○ Accelerometer and the Pendulum
○ Null Position
○ Torque Motor
○ 1st Integrator for Velocity
○ 2nd Integrator for Distance
● These values then go to the INU
● This, however, has been a simplified view.
○ The accelerometers, of course, will be mounted along each and every axis

● In short, the INS system senses the acceleration of the aircraft, and then integrates them into a new speed and then a new distance.
○ We then get a new position
● There is one big problem with only using accelerometers.
○ They can be fooled by an aircraft that is pitching.
○ They assume that we have a stable platform

Question 3

Gimbaled Inertial Platform

Answer 3

● These are then mounted in a gimbal with three degrees of freedom.
● Conventional gyroscopes will maintain the accelerometer alignment while the aircraft is then free to rotate around the platform

Question 4

Transport Wander of an Uncorrected Gyro

Answer 4

● Due to rigidity in space, this gimballed system will remain horizontally aligned with the Earth’s surface at the departure point.
○ It won’t compensate for travel over the Earth as a sphere.
● Left uncorrected, the gyro will tilt relative to aircraft and horizon as the aircraft moves along Earth’s arc
● To correct for this, the gimbal will also have an earth rate compensation motor built in

Question 5

Strapdown Inertial Platform

Answer 5

● A Strapdown Inertial Platform is attached directly to the structure of the aircraft.
○ This means that it rotates directly with the aircraft.
● Our computer will then be able to use both linear and angular data to calculate position.

Question 6

Ring Laser Gyro in an IRS

Answer 6

● Such devices are meant to measure rotation.
● However, it does not maintain rigidity in space.
● Instead, it will mathematically determine the movement of the aircraft through differences in beam speed

● Advantages:
○ No moving parts.
○ No precession/drift.
○ Light and reliable

Question 7

INS Initialization

Answer 7

● During the startup or initialization, the gyros will spin up (gimbaled system).
● The platform will also be aligned with the aircraft’s attitude (gimbaled system).
● Initial position will be entered by the pilot.
○ It will be entered right down to the gate that you are at.
● The system will then find true North via Gyro Compassing

Question 8

Finding True North (Azimuth)

Answer 8

● INS finds true north by using a process called Gyro Compassing.
○ On initialization, with the aircraft stationary, the system senses the Earth’s rotation.
○ Since the aircraft is stationary during the alignment process, any angular rate of change must be due to the earth’s rotation.
○ The magnitude of this rotation is used to estimate the aircraft’s latitude.
● The estimated latitude is then compared to the latitude entered into by the pilot during initialization
● Gyrocompassing may not be possible at higher latitudes.
○ This is because the earth’s rotation may be too slow for the gyros to sense it

Question 9

INS Components

Answer 9

The complete system is comprised of the following components:
○ The Inertial Navigation Unit (INU)
○ The Battery Unit (BU)
○ The Inertial System Display Unit (ISDU)
○ The Mode Selector Unit (MSU)