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need to be accurately aligned with the aircraft. Eventually software will perform all the functions of the
space stable platform. Despite all these modern advances, we can better learn about and understand
inertial systems by studying the original systems.
16.12. Principles. The basic principle behind inertial navigation is straightforward. Starting from a
known point, you calculate your present position (a continuously running DR) from the direction and
speed traveled since starting navigation. The difference between other navigation systems and INS is
how it determines direction, distances, and velocities. Accelerations are detected by the three linear
accelerometers. These accelerations are integrated over time to determine changes in velocity. Velocity
is integrated a second time to determine distance traveled. Changes in vector direction are detected with
angular accelerometers. As sensors detect changes in gyroscope orientation, correction signals are
generated to reorient the stable platform to the original position and determine new vector direction. INS
requires no other inputs. It avoids all environmental inputs such as indicated or true airspeed, magnetic
heading, drift, and winds that are necessary for dead reckoning.
16.13. Components. The five basic components of an INS are:
16.13.1. Three linear accelerometers arranged orthogonally to supply the x, y, and z axis components of
acceleration.
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16.13.2. Gyroscopes to measure and use changes in aircraft vector to maintain and orient the stable
platform.
16.13.3. A stable platform oriented to keep the x and y axis linear accelerometers oriented north-south
and east-west to provide azimuth orientation and to keep the z axis aligned with the local gravity vector.
The stable platform is necessary to prevent either the x or y axis accelerometer from picking up the force
of gravity and interpreting it as an acceleration on the aircraft.
16.13.4. Integrators to convert raw acceleration data into velocity and distance data.
16.13.5. A computer to continuously calculate position information.
16.14. Linear Accelerometers. Acceleration-measuring devices are the heart of all inertial systems. It is
important that they function reliably for all maneuvers within the capability of the aircraft, and that all
possible sources of error are minimized. Very slight accelerations and changes in heading in all
directions must be detected. Changes in temperature and pressure must not affect INS operation. To do
this, INS requires two types of accelerometers, linear and angular. The simplest type of linear
accelerometer consists of a pendulous mass that is free to rotate about a pivot axis in the instrument.
There is an electrical pickoff that converts the rotation of the pendulous mass about its pivot axis into an
output signal. This output signal is used to torque the pendulum to hold it in the original position and,
since the signal is proportional to the measured acceleration, it is sent to the navigation computer as an
acceleration output signal (Figure 16.1). To obtain acceleration in all directions, three accelerometers are
mounted mutually perpendicular in a fixed orientation. To convert acceleration into useful information,
the acceleration signals must be integrated to produce velocity and then the velocity information is
integrated to get the distance traveled. One of the forces measured by the linear accelerometers is
gravity. This acceleration may be incorrectly interpreted as an acceleration of the aircraft if the stabilized
platform is tilted relative to the local gravity vector. The accelerometers cannot distinguish between
actual acceleration and the force of gravity. This means that the linear accelerometers on the stable
platform must be kept level relative to the earth's surface (perpendicular to the local gravity vector). The
gyroscopes keep the stabilized platform and the accelerometers level and oriented in a north-south and
east-west direction.
Figure 16.1. A Basic Inertial System.
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16.15. Gyroscopes. Gyroscopes are used in inertial systems to measure angular acceleration and
changes in orientation and heading. While the types of gyros are briefly discussed here, the function of
the gyro is discussed in great detail in the next section on the stable platform. The original gimbaled
gyroscope has been replaced by newer designs.
16.15.1. Electronically Suspended Gyros. These gimbal-less gyros consist of a ball that is suspended
in a magnetic field and spun electronically. Evacuating the air in the gyro cavity further reduces friction.
The result is a near frictionless gyro with precession rates measured in years. Optical sensors measure
the ball's orientation from symbols etched on the surface of the ball.
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