proportional to the velocity of the aircraft along
alignment in yaw and roll. This is necessary so
the circumference of the earth. The system
the sensitive axis of the north-south accelerometer
aligns true north-south. It is also necessary for
applies aircraft rate torquing to both gyros so they
the east-west accelerometer to align true east-
will precess about all three axes (x, y, and z). Thus,
west. The stable element is then accurately aligned
the system continuously maintains the frame of
to the three coordinates--north (y-axis), east
reference.
(x-axis), and up or true vertical (z-axis).
Deriving Velocity and Distance
This arrangement allows the accelerometers to
accurately detect aircraft motion.
The inertial navigation system can accurately
MAINTAINING THE VERTICAL REF-
detect aircraft acceleration, and using precision
ERENCE. --The stable element must remain
integrators, determine aircraft velocity and
measure distance traveled. Accelerations are
perfectly level or the accelerometers will sense a
measured in units of ft/sec2 by the accelerometers.
false acceleration due to the earth's gravity. The
However, for analog computation, the accelera-
gyros try to maintain their inertial position in
tions are developed in volts per g of accelerating
space and not with respect to the local vertical.
force. The velocity integrator integrates the ac-
This causes the stable element to drift off level
celerating force to obtain velocity. The distance
as the aircraft moves over the curvature of the
integrator integrates velocity to obtain the distance
earth (fig. 7-40). If this situation were allowed to
traveled.
build up, very large errors in velocity and distance
would occur. This condition develops whether or
There are two velocity integrators in the system
not the aircraft is moving over the earth's surface.
to obtain VX and VY along the two horizontal axes.
There are two distance integrators to obtain
The earth's rotation alone will develop the same
distance traveled along the two horizontal axes.
type of errors.
The stable element must always be perpen-
In addition, some inertial navigation systems
dicular to the local vertical. Therefore, the system
employ one other velocity integrator to obtain VZ
must make the gyros precess in such a manner as
along the vertical axis.
to maintain the stable element level. With the
Accelerometer Output Corrections
stable element level, the sensitive axes of the
accelerometers are maintained horizontal to the
The arrangement of the accelerometers is
earth at all times. Now they respond only to the
perfectly suited to navigation over a stationary
horizontal component of acceleration.
plane or over flat terrain moving at uniform speed
in a straight line. The earth, however, is a rotating
MAINTAINING THE FRAME OF REF-
ERENCE. --Accuracy in maintaining the stable
sphere. As far as the inertial platform is
concerned, only points along the equator can be
element to the frame of reference determines the
considered to possess uniform linear motion.
overall performance capabilities of the system.
Here, and only here, the accelerometer signals can
Gyro torquing rate signals are continuously
translate directly into position information. For
computed to maintain the frame of reference.
this reason, it is necessary to provide an automatic
After alignment, the inertial navigation system is
device to alter the accelerometer signals, The
manually sequenced to its operating condition
automatic device allows the system to report
(navigate). If the aircraft were to remain
meaningful information. The corrective device is
stationary, the gyro torquing rate signals would
purely electrical. All or part of the circuitry
consist of earth rate only. However, as the
is active whenever a velocity signal voltage is
aircraft moves over the curvature of the earth, the
present anywhere north or south of the equator.
stable element earth reference would be lost. This
These circuits use the velocity signals, modified
happens because vehicle movement causes the
according to the latitude of the aircraft position,
gyros to precess. (Refer to figures 7-39 and 7-40.)
They insert artificial acceleration signals to those
Therefore, the system continuously computes
already in the accelerometer output circuits.
additional gyro torquing signals to compensate
The circuits are divided logically--some are
for the vehicle's movement. These signals are
for centripetal effect, some for Coriolis. Note,
aircraft rate torquing signals. They depend on the
however, that the corrections are complementary.
velocity of the aircraft wth respect to the frame
Although it is convenient to assign a separate
of reference; that is, east-west velocity and north-
purpose to the circuits, as in the following
south velocity. The angular rate is directly
7-36
