accelerations become complex. They essentially
consist of two major accelerations--the actual
acceleration of the vehicle and the gravitational
acceleration of the earth. For navigation purposes
only aircraft accelerations are required and
wanted; therefore, the gravitational accelerations
must be canceled. Yet, this cancellation is difficult
to obtain because the earth's gravitational
acceleration is not uniform. Therefore, the
computer must store an enormous amount of data
to effect this cancellation.
The significant disadvantage of analytic
inertial navigation is the result of maintaining the
accelerometer referenced to a fixed point in
inertial space. As the stable element navigates
about the earth, the accelerometers must sense
aircraft acceleration and the earth's gravitational
TYPES OF INERTIAL
field component. The accelerometers for this
NAVIGATION SYSTEMS
system must have a wide dynamic range as well
Learning Objective: Recognize the two
as a high overall accuracy. The most serious
problem, however, is the cancellation of the
types of inertial navigation systems and
gravitational accelerations. Irregularities in the
discriminate between systems within those
earth's shape and mass cause variations in the
two types.
gravitational field. Therefore, the cancellation of
these variations requires a complex computer with
You can classify inertial navigation systems
a very large storage capacity.
under two broad types--pure and hybrid. The
types of pure inertial navigation systems are
analytic, semianalytic, geometric, and strap-
Semianalytic Inertial Navigation System
down. The types of hybrid inertial navigation
systems are radio inertial, Doppler inertial, and
stellar inertial.
The semianalytic system is the inertial
navigation system most commonly in use today.
All naval aircraft that use inertial navigation
systems have this type of system. It may either
PURE INERTIAL
be a pure system or work with another navigation
NAVIGATION SYSTEMS
system as a hybrid system. This system's chief
advantages are the simple platform gimbal
As the name implies, a pure inertial naviga-
structure and computer functions are easily
tion system is not combined with other equipment
attained by either analog or digital means.
to improve its operating performance.
The semianalytic system always maintains the
Analytic Inertial Navigation System
stable element normal to the earth's gravitational
vector just as in other systems already discussed
in this chapter. In this system, the computer
The analytic INS uses a platform with a fixed
converts the output accelerations of the stable
angular reference to some point in inertial space.
The system makes no attempt to force the
element to angular velocities. These angular
accelerometer input axes into a preferred
alignment with respect to the earth. This method
to maintain the platform normal to the earth's
gravity vector.
does not require gyro torquing. As a result,
this platform is subjected to errors of gyro drift
The computer also develops signals to prevent
only.
the platform from processing off level due to the
earth's rotation about its polar axis. These signals
Because the platform remains rigid in
are equal to the angular velocity of the earth
space and rotates about the earth, the output
