Theory of Operation

TM 5-6675-250-34

the pendulous gyro attempts to maintain its

e. Test Support Equipment. T e s t s u p p o r t

orientation in space because of its inertia as it is

equipment includes an interconnect breakout box

carried from (a) to (b) because of the earth's rotation.

and a printed circuit card cable extender. The in-

This requires that the spin axis tilt, resulting in the

terconnect breakout box contains test jacks and

departure of the pendulum from the local vertical, as

switches for checking subcircuits of the instrument

shown in (b). The acceleration of gravity, however,

and is connected between the GRU and ECU during

exerts a downward force on the center of mass of the

testing and troubleshooting operations. The cable

pendulum and forces it to remain in the local ver-

extender is provided to connect the printed circuit

tical. A torque, therefore, is present about

card to the ECU when the printed circuit card is

a horizontal axis normal to the gyro spin axis. This

removed. The printed circuit card is removed to aid

torque produces precession about the vertical axis

c h e c k s of the system components during

turning the eastern end of the spin axis coun-

troubleshooting.

terclockwise until position (c) is reached and the spin

1-9. Theory of Operation

axis is aligned. While thus aligned the earth's

a. The operation of SIAGL is based on the

rotation will cause further precession because the

principle that a pendulous gyro suspended with its

rotation is about the spin axis of the gyro. However,

spin axis horizontal tends to align this axis with the

the gyro in processing towards north builds up

horizontal component of the earth's spin velocity

momentum which carries it through the meridian to

(fig. 1-4). The gyro is suspended on a flexible, low

position (d). At position (e) the same sequence oc-

torque metal band to permit minute pendulous and

curs as at (a) but because the spin axis is now

rotational torques to affect the motion of the pen-

pointing west, precession drives the spin axis back

dulous gyro. With the pendulous gyro in position (a)

toward north.

with the angular momentum vector (H) pointed east,

Figure 1-4. Principle of operation.

consists of a theodolite, upper housing and follow-up

b. The meridian-seeking action described in the

mechanism, a cylindrical pendulum containing the

preceding paragraph is accomplished with the

gyro wheel, and a supporting tripod.

SIAGL through the GRU (figure 1-5). The unit

1-6