NAVIGATION — CANOES TO SPACESHIPS — DRAPER 311 



to zero. Therefore, the orientation chosen for any given situation 

 may be the one that allows the simplest or most convenient configura- 

 tion of the required mechanism. For example, causing one of the 

 support axes of the gyroscopic package to operate in parallelism with 

 the earth's axis of rotation makes it possible for a simple sidereal clock 

 drive (such as represented in fig. 6) to compensate for the rotation of 

 the earth. The orientation of the reference member remains fixed with 

 respect to inertial space about this polar axis, so that the first support 

 gimbal outside of the sidereal time drive remains parallel to some 

 meridian on the earth. By mechanical adjustments, this artificial 

 meridian may be alined -with any selected earth meridian. The com- 

 bination of this artificial meridian inside the navigation equipment and 

 the artificially established direction of the earth's polar axis provides 

 an adjustable earth reference space that is derived from an adjustable 

 inertial reference space and a time drive. Positions on the earth are 

 fixed by determining the direction of the local gravitational vector 

 with respect to this mechanically established earth reference space 

 (pi. 2).^ 



The inertial-system configuration just described was chosen be- 

 cause of its simplicity for the purposes of explanation. Various other 

 configurations are possible. For example, the gyro units of the iner- 

 tial reference member may bo designed to receive contiimous torque 

 inputs that cause the member to change its orientation with respect 

 to inertial space so that it indicates the direction of the local vertical 

 as the system moves over the earth's surface. In a system of this 

 kind, changes in position with respect to the earth are indicated by 

 integrations of torque-controlling signals to calibrated gyro units. 

 Each signal corresponds to an angular velocity component of the 

 reference member with respect to inertial space, and one integration 

 gives the associated angular displacement. Correction of this dis- 

 placement for rotation of the earth by a time signal and multiplica- 

 tion by the radius of the earth gives a component of distance traveled 

 over the earth's surface. The same procedure applied to the other 

 components of inertial-reference-member rotation gives the corre- 

 sponding components of travel. Combining all the travel components 

 gives the indicated resultant movement of an inertial navigation sys- 

 tem over the earth. Relating this indicated movement to the point 

 of departure gives the indicated position of the vehicle carrying the 

 navigation equipment at any instant. 



Another configuration of inertial navigation equipment places the 

 inertial reference member in an arbitrary orientation and employs 

 computers to produce information on indicated position. This ar- 

 rangement is often used for the guidance of ballistic missiles. 



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