1. Inertial Navigation Advantages
instantaneous output of position and velocity
completely self contained
all weather global operation
very accurate azimuth and vertical vector measurement
error characteristics are known and can be modeled quite well
works well in hybrid systems

2. Inertial Navigation Disadvantages
Position/velocity information degrade with time (1-2NM/hour).
Equipment is expensive ($250,000/system) - older systems had relatively high failure rates and were expensive to maintain
newer systems are much more reliable but still expensive to repair
Initial alignment is necessary - not much of a disadvantage for commercial airline operations (12-20 minutes)

3. Inertial Navigation – Basic Principle
If we can measure the acceleration of a vehicle we can
integrate the acceleration to get velocity
integrate the velocity to get position
Then, assuming that we know the initial position and velocity we can determine the position of the vehicle at ant time t.

4. Inertial Navigation – The Fly in the Ointment
The main problem is that the accelerometer can not tell the difference between vehicle acceleration and gravity
We therefore have to find a way of separating the effect of gravity and the effect of acceleration

5. Inertial Navigation – The Fly in the Ointment
This problem is solved in one of two ways
Keep the accelerometers horizontal so that they do not sense the gravity vector This is the STABLE PLATFORM MECHANIZATION
Somehow keep track of the angle between the accelrometer axis and the gravity vector and subtract out the gravity component This is the STRAPDOWN MECHANIZATION

6. Inertial Navigation – STABLE PLATFORM
The original inertial navigation systems (INS) were implemented using the STABLE PLATFORM mechanization but all new systems use the STRAPDOWN system
We shall consider the stable platform first because it is the easier to understand

7. Inertial Navigation – STABLE PLATFORM
There are three main problems to be solved:
The accelerator platform has to be mechanically isolated from the rotation of the aircraft
The aircraft travels over a spherical surface and thus the direction of the gravity vector changes with position
The earth rotates on its axis and thus the direction of the gravity vector changes with time

8. Inertial Navigation – Aircraft Axes Definition
The three axes of the aircraft are:
The roll axis which is roughly parallel to the line joining the nose and the tail Positive angle: right wing down
The pitch axis which is roughly parallel to the line joining the wingtips Positive angle: nose up
The yaw axis is vertical Positive angle: nose to the right

9. Inertial Navigation – Aircraft Axes Definition
ROLL
PITCH
YAW

10. Inertial Navigation – Platform Isolation
The platform is isolated from the aircraft rotation by means of a gimbal system
The platform is connected to the first (inner) gimbal by two pivots along the vertical (yaw) axis. This isolates it in the yaw axis
The inner gimbal is the connected to the second gimbal by means of two pivots along the roll axis. This isolates the platform in the roll axis.
The second gimbal is connected to the INU (Inertial Navigation Unit) chassis by means of two pivots along the pitch axis. This isolates it in the pitch axis.

11. Inertial Navigation – Platform Isolation
Now the platform can be completely isolated from the aircraft rotations

12. Inertial Navigation – Gyroscopes
To keep the platform level we must be able to:
Sense platform rotation and
Correct for it
To do this we mount gyroscopes on the stable platform and install small motors at each of the gimbal pivots.
The gyroscopes sense platform rotation in any of the three axes and then send a correction signal to the pivot motors which then rotates the relevant gimbal to maintain the platform at the correct attitude

13. Inertial Navigation – Alignment
Before the INS can navigate it must do two things:
Orient the platform perpendicular to the gravity vector
Determine the direction of True North
Also it must be given:
Initial Position: Input by the Pilot (or navigation computer)
Velocity: This is always zero for commercial systems

14. Inertial Navigation – Orientation
In the alignment mode the INU uses the accelerometers to send commands to the pivot motors to orient the platform so that the output of the accelerometers is zero.
Note that the earth (and therefore the INU) is rotating so that it will be necessary to rotate the platform in order to keep it level.

15. Inertial Navigation – Gyrocompassing
The rotation of the platform to keep it level is used to determine the direction of True North relative to the platform heading.

16. Inertial Navigation – Gyrocompassing

17. Inertial Navigation – Gyrocompassing
The platform is being rotated around the X and Y axes at measured rates:
RX=ΩcosΦcosα
RY=ΩcosΦsinα
Since Ω is known ( º/hour) we have two equations in two unknowns and can calculate
Φ (Latitude) and α (platform heading)

18. Inertial Navigation – Gyrocompassing
The platform is being rotated around the X and Y axes at measured rates:
RX=ΩcosΦcosα
RY=ΩcosΦsinα
Since Ω is known ( º/hour) we have two equations in two unknowns and can calculate
Φ (Latitude) and α (platform heading)

19. Inertial Navigation – Navigation
Once the INU has been aligned it can be put into NAVIGATE mode .
In navigate mode, the outputs of the accelerometers are used to determine the vehicle’s position and the gyroscopes are used to keep the platform level.
This involves
compensating for the earth’s rotation
compensating for travel over the earth’s (somewhat) spherical surface

20. Inertial Navigation – Schuler Oscillation
To compensate for the travel over the surface of the earth the platform must be rotated by an amount d/R where d is the distance travelled and R is the radius of curvature of the earth R s θ

21. Inertial Navigation – Schuler Oscillation
This leads to a phenomenon know as Schuler oscillation
At the end of the alignment procedure the accelerometers are almost never perfectly level.

22. Inertial Navigation – Schuler Oscillation
Assume for now that the aircraft remains at rest
The measured acceleration causes the INU to compute a velocity and hence a change in position.
This in turn causes the gyros to rotate the platform

23. Inertial Navigation – Schuler Oscillation
Assume for now that the aircraft remains at rest
The measured acceleration causes the INU to think that it is moving an it computes a velocity and hence a change in position.
This in turn causes the gyros to rotate the platform

24. Inertial Navigation – Schuler Oscillation
The direction of the rotation tends to level the accelerometer but when it is level, the computer has built up a considerable speed and thus overshoots. (this is like pulling a pendulum off centre and letting it go)

25. Inertial Navigation – Schuler Oscillation
Characteristics of the oscillation:
a=-gsinθ or –gθ for small angles
θ = s/R where R is the radius of curvature
differentiating twice

26. Inertial Navigation – Schuler Oscillation
This is a second order differential equation whose solution is:
θ = θ0cos(ωt)
where θ0 is the initial tilt angle and
The period of this oscillation is 84 minutes

27. Inertial Navigation – Accelerometers
Requirements:
high dynamic range (10-4 g to 10g)
low cross coupling
good linearity
little or no asymmetry
Exacting requirements dictate the use of Force-Rebalance type of devices

28. Inertial Navigation – Accelerometers
Types:
Pendulum
floating
flexure pivot
Vibrating String or Beam
MEMS (micro electromechanical systems)

29. Inertial Navigation – Accelerometers
Floated Pendulum

30. Inertial Navigation – Accelerometers
Flexure Pivot Pendulum

31. Inertial Navigation – Accelerometers
Vibrating Beam

32. Inertial Navigation – Accelerometers
MEMS

33. Inertial Navigation – Gyroscopes
Three main types:
Spinning Mass
Ring Laser
MEMS

34. Inertial Navigation – Gyroscopes
Spinning Mass:
Rigidity in Space:
A spinning mass has a tendency to maintain its orientation in INERTIAL space
Its rigidity (or resistance to change) depends on its moment of inertia and its angular velocity about the spin axis (INU gyros spin at around 25,000 RPM)
Precession;
If a torque τ is applied perpendicular to the spinning mass it will respond by rotating around an axis 90 degrees to the applied torque. I.e. ω× τ

35. Inertial Navigation – Gyroscopes
Construction:

36. Inertial Navigation – Gyroscopes
Spinning Mass Gyros:
Disadvantages:
sensitive to shock during installation and handling (Pivots can be damaged)
requires several minutes to get up to speed and temperature
expensive

37. Inertial Navigation – Gyroscopes
Ring Laser Gyro: (RLG) in service since 1986
Advantages over spinning mass gyros:
more rugged
inherently digital output
large dynamic range
good linearity
short warm up time

38. Inertial Navigation – Gyroscopes
Ring Laser Gyro: (RLG) in service since 1986
General Principle:

39. Inertial Navigation – Gyroscopes
Ring Laser Gyro: (RLG) in service since 1986
General Principle:

40. Inertial Navigation – Gyroscopes
Ring Laser Gyro
Problems:
Lock-in at low rotation rates due to weak coupling between the two resonant systems (coupling due to mirror backscatter)
Analagous to static friction (stiction) in mechanical systems
Causes a dead zone
Alleviated by “dithering” the gyro at a few hundred Hz
Random loss of pulses at the output ( causes “drift”)

41. Inertial Navigation – Gyroscopes
Fibre Optic Gyro
Similar concept to RLG except that amplification is not usesd
Two strands of optical fibre are wound in opposite directions on a coil form
Laser light is sent from a single source down both fibres
The outputs of the two fibres are combined at a photodiode
Rotation of the coil around its axis causes the two paths to have different lengths and the output of the photodiode provides a light dark pattern. Each cycle indicates an increment of angular rotation

42. Inertial Navigation – Gyroscopes
Fibre Optic Gyro
Has the advantage of being rugged and relatively cheap
Sensitivity increases with length of fibre
Unfortunately, the longer the fibre, the lower the output signal.
Used on low performance systems

43. Inertial Navigation – Gyroscopes
MEMS Gyro
All gyros to date have been quite large
in fact the sensitivity of spinning mass gyros and RLGs are a direct function of their size.
Efforts are being made to apply MEMS technology to gyros as well as to accelerometers

44. Inertial Navigation – Gyroscopes
MEMS Gyro
The MEMS gyro uses the Coriolis Effect
In a rotating system (such as the earth) moving objects appear to deflected perpendicular to their direction of travel.
The effect is a function of the velocity if the object and the rate of rotation

45. Inertial Navigation – Gyroscopes
MEMS Gyro
In a MEMS gyro the times of a tuning fork are the moving object
MEMS gyros exhibit high drift rates and thus are not suitable for commercial aviation use
They are used in conjunction with GPS in “coupled” systems which use the best characteristics of each

46. Inertial Navigation – Strapdown Systems
The main problem for an INS is to separate the vehicle acceleration from the effect of gravity on the accelerometers
In the stable platform, this is done by maintaining the accelerometers perpedicular to the gravity vector which allows us to ignore the effect of gravity
Another approach is to keep track of the gravity vector and subtract its effect from the outputs of the accelerometers
This is an analytical or computational implementation

47. Inertial Navigation – Strapdown Systems
As the name implies, the accelerometers are fixed or “strapped down” to the chassis of the INU and hence to the aircraft.
Since the gravity vector is three dimensional, three accelerometers are required to keep track of it.
In addition, three RLGs are mounted with their axes aligned with the x,y, and z axes (roll, pitch and yaw) of the aircraft respectively.

48. Inertial Navigation – Strapdown Systems
Alignment:
During the alignment procedure, the INS measures the direction of the gravity vector. Notice that the outputs of the accelerometers are proportional to the Direction Cosines of the gravity vector

49. Inertial Navigation – Strapdown Systems
Example:
If the outputs of the accelerometers are:
ax =
ay =
az =
What are the roll and pitch angles?

50. Inertial Navigation – Strapdown Systems
Example:
If the roll and pitch angles are Φ and Θ respectively
aX = gsin Θ Note:
aY = gsin Φcos Θ
aZ = gcos Φcos Θ
Therefore: Θ=sin-1(aX/g)
and Φ= sin-1(aY/gcos Θ)

51. Inertial Navigation – Strapdown Systems
Example:
Thus g = m/s2
Θ = sin-1( / ) = 1º
Φ = sin-1( /( x 1) = 1º

52. Inertial Navigation – Strapdown Systems
Note that during alignment the RLGs on the x and y axes give a direct readout of the two platform rates required for gyrocompassing

53. Inertial Navigation – Strapdown Systems
Note:
The sensitivity of Ring Laser Gyro is:
N=4A/λL
Where: N is the number of fringes per radian
A is the area enclosed by the path
L is the Length of the path
λ is the wave length of the light
Note that the larger the area, the more sensitive the gyro