NAVIGATION — CANOES TO SPACESHIPS — DRAPER 315 



mercial organizations and tested far enough to prove tlie feasibility of 

 inertial navigation, but it is to be expected that several years of effort 

 will have to pass before linal decisions as to the best type of equipment 

 can be made. 



INERTIAL GUIDANCE SYSTEMS FOR BALLISTIC MISSILES AND SATELLITES 



Ballistic missiles are subjected to high accelerations for short pe- 

 riods of high thrust during the phase of boosting through the atmos- 

 phere and then coast in free fall until they reenter the air before strik- 

 ing target areas. In this situation, it is not feasible to base guidance on 

 indications of the local vertical. During the boost phase, the inputs 

 for specific- force receivers are greatly different from gravity, both in 

 direction and in magnitude. During the coasting phase, free fall 

 reduces the net specific force to zero, so there are effectively no inputs 

 for specific-force receivers. Reentry and the terminal phase are 

 again subject to accelerations due to air resistance that cause the 

 specific force available as the input for guidance equipment to be 

 greatly different from gravity. These facts combine to prevent systems 

 that operate by indicating the local direction of gravity from being 

 useful in equipment for guiding ballistic missiles. Some other mode 

 of operation must therefore be used to meet the operating requirements 

 of such equipment. 



Inertial guidance for ballistic missiles is achieved by eliminating 

 terrestrial-space reference equipment and solving the guidance prob- 

 lem by means of specific-force receivers fixed in an artificially oriented 

 space that is associated with a gyroscopic reference number (pi. 3, fig. 

 1). To mechanize this arrangement, a rigid body member supported 

 by a servodriven gimbal system carries the necessary gyro units and, 

 in addition, serves as the momiting for a set of three single-degree-of- 

 freedom specific-force receivers with their input axes set at right 

 angles to each other (pi. 3, fig. 2) . With this configuration, the total 

 specific force acting on the gyro-oriented reference member is sensed 

 in three components in a special set of coordinates having known geo- 

 metrical relationships to terrestrial space and to celestial space. Sig- 

 nals representing these components are generated by the specific-force 

 sensors and transmitted to a computer. This computer, acting on 

 these inputs and on information stored in its memory banks, works 

 out the instantaneous location of the missile under guidance, compares 

 this location with the desired location on the proper path, and gen- 

 erates command signals for correcting the direction of the missile. 

 These command signals are received by the missile control system, 

 which changes the orientation of the thrust vector with respect to the 

 missile so that the necessary changes in the missile path are made. 



Ballistic-missile guidance is carried out in artificially established 

 inertial-space coordinates, with the target considered as a moving 



