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