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INTRODUCTION
This manual is intended to help in understanding the installation and operation requirements of
the Watson Attitude and Heading Reference System.
PRODUCT DESCRIPTION
Aircraft attitude and heading gyros need frequent correction to compensate for the 15º per hour
of earth rotation. Watson Industries has produced a solid state gyro system, which models these
functions for an attitude gyro and a slaved heading gyro. This system uses a microprocessor to
integrate angular rate sensor data and provides a closed loop system of error correction to adjust
biases from earth rotation and from instrument offsets. Interface to the microprocessor is done
through a 16-bit A/D and a 12-bit D/A converter. The solid state vibrating structure angular rate
gyros used in this system provide extremely high reliability, low power consumption, shock
resistance and low cost.
OPERATION
The angular rate sensor signals are coordinate transformed and then integrated to produce
attitude and heading outputs that reflect normal aircraft attitude coordinates. These attitude and
heading signals are compared against three accelerometers and a triaxial fluxgate magnetometer
to derive short-term absolute errors. These errors are filtered over a long time constant and are
used to adjust biases in the system so that the long-term convergence of the system is to the
vertical references and the magnetic heading. Compensations for centrifugal forces and velocity
changes on the accelerometers are used to improve overall stability and accuracy.
INSTALLATION
Orientation:
This device utilizes an earth's magnetic field orientation sensor, so orientation is important. The
base plate of the AHRS is intended to be mounted on top of a horizontal surface with the
connector toward the forward direction of the vehicle.
Mounting:
A mounting plate is provided for a flat surface mount. Use non-magnetic hardware. Ideally, the
unit should be installed at least 4 feet away from all significant magnetic materials. Some highly
magnetic materials require even greater separation from the AHRS unit. The unit may be
adhesively mounted at any of its surfaces. If high shock loads are expected (greater than 20G or
repeated shocks greater than 10G), the appropriate shock mounting should be used to prevent
damage. Vibration isolation should be used for use in 4G or greater vibration environments.
AHRS-E304 Watson Industries, Inc
09/06/05
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