(4)  
Only one accelerometer is shown in Fig. 4. Actually there are two in the INS, one in the north/south direction, the other in the east/west direction. The computer associated with the inertial system knows the latitude and longitude of the takeoff point and how far the airplane has traveled in a north direction and how far in an east direction. It is a fairly simple matter for the computer, then to compute the new present position of the aircraft.

 C.  Gyros (Fig. 6)
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 (1)  
Another essential component of an inertial navigation system is the gyro. In its simple form the gyro is an accurately balanced spinning flywheel or rotor (Fig. 6). The rotor spins about a central or spin axis (SA) which passes through its center of gravity. The rotor shaft and its bearings are suspended within a gimbal. The gimbal is free to rotate about an axis which is perpendicular to the rotor spin axis called the gyro output axis (OA). The gyro operates on the principle of gyproscopic inertia which is the characteristic of a rotating mass to resist any forces which tend to change the direction of its spin axis. When a torque is applied around an axis perpendicular to both the SA and OA axis, namely the input axis (IA), the spin axis will tip but not in the direction of the torquing force as would be expected. Instead, the spin axis rotates around the output axis at right angles to the applied torque. This is known as induced gyroscopic precession.

 (2)  
Gyroscopic inertia fixes the spin axis of the gyro in space. However, because the earth rotates in space, the space-oriented gyro appears to rotate with respect to an earth-bound observer. This apparent rotation of the gyro spin axis is called apparent gyroscopic precession. Except for two special cases, this makes the gyro unsuitable for use as an earth-fixed reference unless the gyro is deliberately torqued to rotate at a rate proportional to the earth's rotation rate (earth rate). When torqued in this manner, the spin axis appears stationary and the gyro is effectively slaved to the earth's coordinate system.

 (3)  
A second form of apparent precession is due to vehicle motion over the curved surface of the earth. Consider the gyro to be in an airplane flying north along a meridian from the equator to the pole. If the gyro spin axis were oriented horizontal and parallel with the meridian when at the equator, the gyro spin axis (being space stabilized) would appear to rotate to a vertical orientation relative to earth as the airplane flies north. Consequently, in order to use the gyro as an earth-reference device in a moving airplane, it must be torqued to compensate for north-south airplane motion over the earth's surface in addition to earth rate.

 (4)  
Similarly, when flying east or west and starting with the gyro spin axis oriented in the horizontal and aligned east-west, the spin axis will appear to tilt due to travel east or west. This tilt is in addition to that caused by earth rate. Therefore, a gyro precessing torque is also necessary to maintain the earth reference orientation during east-west travel.

 (5)  
The total gyro torquing signals necessary to slave a moving gyro to an earth-related coordinate system consist of an earth rate component, a vehicle velocity component, and a gyro drift rate compensation component.

 D. Inertial Platform
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Mode Selector Unit
Figure 7

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EFFECTIVITY
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ALERT BATT
FROM/TO ANNUNCIATOR
LEFT ANNUNCIATORWAYPOINT (AMBER)NUMERICAL DIM CONTROL (AMBER)
RIGHT NUMERICAL DISPLAY

WARNWPT SELECTOR ANNUNCIATORSWITCH (AMBER)
TK CHG
(GREEN) DATA KEYBOARD
DISPLAY  INSERT  PUSHBUTTON 
SELECTOR  PUSHBUTTON  (GREEN) 
SWITCH 

Control Display Unit
Figure 8

EFFECTIVITY
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 (1)  
An airplane moving in three dimensions over the earth's surface has 6 degrees of freedom, three translational (north-south, east-west, and up-down) and three rotational (roll, pitch, and azimuth). Actual airplane motion is a changing combination of all 6 degrees of motion. Sensing of this motion is accomplished by a gyro stabilized frame of reference known as an inertial platform. The platform consists of a tilt table suspended in a servo-driven gimbal system. The gimbal system isolates the acceleration sensing devices from rotational motion of the airplane and maintains the platform parallel to the earth's surface regardless of airplane motion or earth rotation. The gimbal system effectively decouples the inertial platform from the airframe and provides all-attitude freedom of motion.
　
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