along the meridian and directed toward the equator. If not compensated, this 

 component and the one resulting from the earth's ablateness would combine to 

 produce a spurious acceleration signal from the latitude accelerometer. There- 

 fore, the computer supplies a correction signal based on geographical position. 



The remaining operation which the computer must perform is that of 

 correcting for Coriolis acceleration, the phenomenon resulting when a vehicle 

 moves in the direction of either pole and thus encounters variation in the linear 

 velocity of the earth's surface. Corrections for this effect are computed and 

 applied to the longitudinal accelerometer. 



b. Alternate Arrangements 



The stable platform of the system described above was maintained in 

 a plane tangent to the earth's surface. Thus each of the gyros sensed a different 

 component of the earth's rotational velocity. It was necessary for the computer 

 to determine these components (as functions of latitude) and precisely to torque 

 each gyro accordingly. To avoid the necessity for this demanding computation, 

 some systems (typically not those used for ship navigation) incorporate a fourth 

 gimbal, known as the latitude gim^bal, and maintain the platform parallel to the 

 equatorial plane. This orientation places the input axis of only one of the gyros 

 parallel to the earth's spin axis; thus the other two are insensitive to earth ro- 

 tation. The third gyro has its input axis parallel to the earth's spin axis. It 

 therefore responds to total earth rate and can be compensated through the appli- 

 cation of a fixed torque. 



Systems have been built which employ five gimbals in such a way that 

 the platform remains completely fixed with respect to space . 



c. Composite Systems 



The principal sources of errors in inertial systems are unbalance or 

 bias in accelerometers and drifting of gyros . In the gravity-erected shipboard 

 systems described above, a constant value of accelerometer bias will result in 

 velocity and distance errors which vary sinusoidally with an 84 -minute period. 

 A constant value of gyro drift produces an oscillation of the platform about the 

 vertical, and thus produces velocity and position errors varying sinusoidally 

 with an 84 -minute period. However, because of consequent errors in azimuth, 

 a constant value of gyro drift will also bring about a distance error which in- 

 creases monotonically. 



97 



arthiir ai.^itttlcilnr. 



