(9)  
Present position (POS) is the actual latitude and longitude position of the airplane.

 (10)
 Cross track distance (XTK) is the shortest distance between the airplane's present position and the desired track.

 (11)
 Track angle error (TKE) is the angle between the airplane's actual ground track and the desired ground track.

 (12)
 Distance (DIS) is the great circle distance between the present position of the aircraft and the next waypoint or destination.

 I.  An attitude slew test procedure provides the capability to slew the INS platform about the pitch and roll axes. The purpose of the slewing is to simulate airplane attitude changes while monitoring the avionics instruments to observe the resulting changes in attitude displays. Since the INS platform attitude angles are simultaneously displayed on the Control/Display Unit and also routed to airplane interfaces, attitude slew can be used to verify proper aircraft interface wiring to calibrate the weather radar antenna and the HSI and ADI displays, or to verify line maintenance activities such as replacement of ADI's, HSI's, weather radar antennas and autopilot flight director pitch and roll computers.
 2.  Basic INS Principles
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 EFFECTIVITY AIRPLANES WITH LTN-72 INS  
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STAB RELAY

DME-2 NAVIGATION UNIT ACCELERATION
INS Simplified Schematic
Figure 2

EFFECTIVITY CONFIG 2AIRPLANES WITH LTN-72 INS

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TRUE NORTH

OFF TRACK
TK - TRACK ANGLE POS - PRESENT POSITION GS - GROUND SPEED WPT - WAYPOINT TH - TRUE HEADING DIS - DISTANCE-TO-GO
DA - DRIFT ANGLE
TIME - TIME-TO-GO XTK - CROSS TRACK DISTANCE WIND - WIND SPEED/DIRECTION TKE - TRACK ANGLE ERROR
DSR TK - DESIRED TRACK ANGLE
Inertial Navigation Relationship
Figure 3

EFFECTIVITY
AIRPLANES WITH LTN-72 INS
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TORQUER

Basic Accelerometer and Integrators
Figure 4

FEET PER
SECOND PER
SECOND

FEET
PER
SECOND

FEET

Accelerometer Output Intergration
Figure 5

ACCELERATION
VELOCITY
DISTANCE

EFFECTIVITY
AIRPLANES WITH LTN-72 INS
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INPUT AXIS (IA) FORCE

OUTPUT AXIS (OA) PIVOT POINT

OUTPUT
AXIS

Gyro Principles
Figure 6

EFFECTIVITY
AIRPLANES WITH LTN-72 INS
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 A.  The basic components of any inertial navigation system include accelerometers, gyros, inertial platforms, and computers. An understanding of INS operation is predicated upon an understanding of these basic devices.
 B.  Accelerometers (Fig. 4)
 (1)  
An essential component of any inertial navigation system is the accelerometer which senses changes in airplane velocity. The accelerometer is basically a pendulous device and when the aircraft accelerates the small pendulum tends to swing off the null position. At one end of the pivot axis of the pendulum is a signal pickoff device that tells how far the pendulum is off the null position. The signal from this pickoff device is sent to an amplifier and current from the amplifier is sent back into a torquing device located at the other end of the accelerometer pivot axis. A torque is generated that tends to restore the pendulum to the null position. The amount of current going into the torquer is a function of the acceleration the device is experiencing.

 (2)  
As long as the accelerometer remains with its sense axis perpendicular to the local vertical axis of the earth's gravitational field, only accelerations due to airplane horizontal velocity changes are sensed. Therefore, a simple accelerometer must be kept level at all times so that it will not misinterpret the force of gravity as an acceleration. An accelerometer at rest can be used, however, to sense the direction of gravity as a vertical acceleration. For example, an accelerometer senses components of gravitational acceleration as its sense axis is tilted toward the local vertical axis. In this instance it functions as a level detector.

 (3)  
Mathematically, it is possible to derive functions of velocity and distance from an original acceleration function through a process of successive integration (Fig. 5). In this manner, pulse signals representing airplane acceleration are electronically integrated to obtain airplane velocity. The airplane velocity is then integrated to obtain distance traveled.
　
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