a c c e l e r o m e t e r s don't automatically make
surface have a constant tangential velocity. This
allowance for this variation, the system must take
velocity is maximum along the equator and lessens
it into account.
at higher latitudes.
The need for such an allowance is illustrated
Tangential velocity is a linear quantity. It
in the following discussion. Put an aligned INS,
refers to the speed and direction an object would
devoid of any Coriolis corrective mechanism,
travel in a straight line if freed from the earth's
aboard a train in the Northern Hemisphere, and
gravity. An object near the equator travels about
transport it north. The earth's tangential velocity
1,000 miles per hour in a circular path. Assume
at the latitude where the INS aligned is 800 knots
the object is free from earth's gravity and
east. It is obvious the train's eastward velocity
atmosphere. Now it will travel at that speed in
must be constantly reduced as it progresses north.
a straight line away from the earth (on a tangent
This progressive reduction in velocity represents
to the earth). Its tangential velocity is 1,000 miles
an acceleration.
per hour. If the object moves toward the North
Since the tracks constrain the train, a force
Pole, its speed in circular travel will decrease as
from the east will be exerted on the wheel flanges.
it approaches the pole. The speed will be zero
This force is sensed by the east-west accelerometer
when placed exactly over the pole.
as acceleration to the west. The system begins
Although the earth has a trajectory in
computing west velocity that will grow. Refer to
space, this motion is not important to the
figure 7-47. An aircraft flying north directly along
inertial navigation system because every point
a moving longitude meridian subjects the east-west
on the sphere shares this trajectory. The only
accelerometer to this same force. This happens
variable involved is the variation in the earth's
as its course in space alters to the left,
tangential velocity at different latitudes. While the
Figure 7-47.-Coriolis effects.
7-39