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

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