1
Introduction
Navigation systems are used for land, sea, airborne, and space vehicles. These
systems provide an operator
and/or control system with the necessary information
to effect some action in response to data provided by these systems. For example,
this action can be a course correction indication for an aircraft pilot or a feedback
control signal to guide an autonomous vehicle. These systems incorporate
onboard
sensors coupled with a computer, permitting self-contained operations with little
or no assistance required from sources external to the vehicle.
The core of the navigation system is a set of sensors combined with a computer
that can provide a relatively stable and accurate source of navigation. These sys-
tems output navigation state data, which usually include position, velocity, and
attitude. As a result of imperfections in navigation sensors and computational
errors, errors develop in the navigation state data and grow in time. Tht: host vehi-
cle's operating environment also influences the error growth rate. Long-term error
growth is minimized by including other sensors that provide independent redun-
dant navigation data,
i.e., position, in an integrated system that optimally combines
this independent data source with the core navigation system. These independent
sensors, referred to as navigation aids, are characterized by long-term
error stabil-
ity, which can compliment the short-term error stability of the navigation system's
sensors. When combined within a computer algorithm, such as a Kalman filter,
errors from both the core sensors and navigation aids can be estimated to reduce the
integrated navigation system's errors. The resulting navigation system will exhibit
improved performance, even if independent data are used intermittently or are not
available for a short timespan.
The majority of this book addresses navigation systems that are
mechanized
with accelerometer and gyro inertial sensors. These inertial sensors provide sensed
accelerations (velocity changes over a time interval) and rates (attitude changes
over a time interval). Accelerometers and gyros are mounted in orthogonal triad
clusters and enclosed within an inertial measurement unit (IMU) to provide three
components of acceleration and rate outputs. These outputs are provided to a
computer-implemented numerical integration process that computes a navigation
solution yielding a complete set of navigation state data,
i.e., position, velocity,
and attitude. These mechanizations are generally referred to as an inertial naviga-
tion unit when enclosed within a case that can be easily removed and replaced.
Implementations that include the inertial sensors, computer, and navigation aids
are referred to as an inertial navigation system (INS).
Other navigation systems that use fewer than the full three-axis accelerometer
and gyro sensors are presented. These systems include a Dead-Reckoning system
and attitude reference systems. The Dead-Reckoning system uses speed (distance
traveled) and heading sensors to compute a dead-reckoning navigation solution.
This solution is less complete and generally provides only position and heading