312 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



PRACTICAL ASPECTS OF INERTIAL REFERENCES 



In practice, inertial references involve many factors that cannot 

 be mentioned in this paper because of space limitations. Some of 

 these factors are associated with design details, wliile others are com- 

 mon to all systems. One of these common factors is the necessity for 

 establishing the desired inertial-reference-member orientation prior 

 to any period of use for the system containing the member. This 

 orientation must be based on inputs generated by means outside the 

 gyro miits of the reference member and supplied to these units as 

 angular-velocity conmiand signals that become zero when the proper 

 orientation is reached. For example, an optical system using a light- 

 sensitive cell tracker pointed toward Polaris and a servomechanism 

 arrangement can be used to drive an axis of the inertial reference 

 member toward alinement with the earth's axis of rotation. Simi- 

 larly, pendulum signals can be used for properly setting the reference 

 member in a selected angular position about the polar axis. 



Any actual gyro unit is imperfect in the sense that its rotor can 

 never be completely torque- free, with the result that the spin axis does 

 not hold its direction perfectly with respect to inertial space. How- 

 ever, by proper design and construction, it is possible to make the re- 

 sultant of undesired torque components very small and to keep the 

 uncertainty parts of these components at still lower levels. Drift 

 rates having an order of magnitude of one one-thousandth of earth's 

 rate (earth's rate is 15° per hour) can now be expected from commer- 

 cially available gyro units. For high performance, the rotors of these 

 units cannot be used directly to generate motion-resisting torques 

 sufficient to control heavy supporting gimbals. However, teclmiques 

 are available that make it feasible for the member carrying the spin- 

 axis bearings of the rotor to operate under conditions of substantially 

 complete freedom from interfering torque components. In units that 

 take advantage of this possibility, deflections of the case of the unit 

 with respect to the rotor do not provide heavy torque but are used 

 only for the generation of control signals, a service that imposes 

 virtually zero torque loads. By using these signals as command 

 inputs to amplifiers controlling the servomotors that drive the sup- 

 porting gimbals, the effects of friction and other disturbing torques 

 may be overcome so completely that the structure carrying the gyro 

 units remains accurately in an orientation determined by the gyro spin 

 axes. 



SPECIFIC-FORCERECEIVER SYSTEMS FOR INDICATIONS OF THE 

 VERTICAL AND THE DISTANCE TRAVELED 



Inertial navigation systems designed to operate on or near the 

 earth's surface must have some provision for indicating the direction 



