Strapdown INS

Introduction
•
Modern systems have removed most of the
mechanical complexity of platform systems by
having the sensors attached rigidly, or 'strapped
down', to the body of the host vehicle.

The Northrop Grumman Navigation Systems Division (NSD) LN-260 is a
Form, Fit, and Function replacement INS/GPS for the F-16.
Aerial Surveying
Applications

Basic Principles
Strapdown Technology:
Body fixed
3 Accelerometers
3 Gyroscopes

Basic Principles
Strapdown Technology:
The measurement principle
SENSORS FASTENED DIRECTLY ON THE VEHICLE
BODY FIXED COORDINATE SYSTEM
ANALYTIC SYSTEM

Mechanization of Strapdown INS

Inertial
Navigation
System
(INS)
Attitude&heading reference system
(AHRS)
Inertial measurement unit
(IMU)
strapdown inertial navigation system
building blocks
Inertial
instrument
block
Instrument
support
electronics
Attitude
computation
Navigation
computation

Properties of Platforms
Advantages
Simpler gyros (platform rotates at small rates, lower
dynamic range).
High accuracy (North and East accelerometers do not see a
component of gravity).
Self alignment by gyro compassing.
Sensor calibration by platform rotations.
Disadvantages
Complexity and cost.
Gimbal magnetics (torquers must not leak magnetic flux).
Reliability (bearings and slip rings tend to wear).

3/12/02 - 9
Properties of Strapdown Systems
Advantages
Simple structure, low cost.
More rugged and lighter.
Reliability (no gimbal magnetics, no slip rings,
no
bearings, electronics more reliable then
machinery).
Disadvantages
More difficult to align.
More difficult to calibrate.
Motion induced errors which can only be partly
compensated for.
Accelerometer errors (each accelerometer may
feel
1 g from gravity).
Requires a computer that can perform
coordinate
rotations in <.01 sec).

A simple two-dimensional strapdown
navigation system
•
Much simpler than a 3-D navigation
problem.
•
The system contains two
accelerometers and a single axis rate
gyroscope, all of which are attached
rigidly to the body of the vehicle.

Fig. 6 Reference frames for two-dimensional navigation

A simple two-dimensional strapdown
navigation system
•
Navigation is required to take place
with respect to a space-fixed
reference frame denoted by the axes
and
.
•
The reference and body axis sets are
shown
where represents the angular
displacement between the body and
reference frames.

Two-dimensional strapdown navigation system equations

Fig.5 Two-dimensional strapdown inertial navigation system

A simple two-dimensional strapdown
navigation system
•
For a vehicle moving at a velocity,
,
in a
single plane around a perfectly spherical
Earth of radius
this rate is given by
/(
+
z)
where
z is the height of the vehicle
above the surface of the Earth
.
This is
often referred to as the transport rate.

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- Fall '20
- Centrifugal Force, Rotation, Frame of reference, Inertial frame of reference, Gimbal lock, Northrop Grumman Navigation Systems Division