Figure 7-30.-Two-axis inertial platform in a plane coordinate system.
motion must be along a straight line passing
axis lies along the x-axis. The other accelerometer
mounts on the same platform so its sensitive axis
through the reference or initial point. Obviously;
lies along the y-axis. This will maintain their axes
using this simple INS, a person must navigate
mutually perpendicular. The accelerometers will
along a straight line.
then sense any rate of change of velocity along
Two-Axis Inertial Navigation System
the coordinate axes. Figure 7-29 is a block
Suppose, for example, that the earth is flat.
diagram of a simplified two-axis inertial naviga-
If so, you can determine position by using a
tion system.
system of coordinate axes. This system of
Figure 7-30 is an illustration of the inertial
coordinates uses two sets of parallel lines
platform mounted on a vehicle moving over a
(x and y). One set of lines is perpendicular to the
plane coordinate system. Note that the platform
other set of lines. These lines form a grid network
and accelerometers remain oriented with the
over the earth's surface.
coordinate axes regardless of vehicle heading. The
vehicle's ground track represents the vehicle's
If you use two single-axis inertial navigation
displacement over the grid system. You can locate
systems, you can determine position on the plane
(flat surface). You simply maintain proper
the vehicle at any given time by the x and y
coordinates. You plot the x-displacement left to
orientation of each accelerometer's sensitive axis
right, and the y-displacement is top to bottom on
relative to the coordinate system. One acceler-
the page. You reference time to the x-axis.
ometer mounts on a platform so that its sensitive