314 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



cuits act to introduce the required dynamic performance. By varia- 

 tion of circuit parameters, it is possible to adjust both the period and 

 the damping so tliat optimimi resuks are achieved under operating 

 conditions. 



The sensing components required for Schuler-tmied equivalent pen- 

 dulum sj^stems are arrangements in which gravitational and inertia- 

 reaction forces acting on a seismic mass (a body designed to act as the 

 receiving element of a sensor) cause deflection against some restrain- 

 ing means in a way that generates an output signal representing 

 the resultant input force. This resultant is made up of the force 

 acting to move the seismic element in the direction of the gravity field 

 and the inertia-reaction effects that tend to cause the seismic element 

 to lag behind the linear acceleration of the sensing miit. It is con- 

 venient to consider gravity and inertia-reaction effects as combining 

 to form the specific force, which is the resultant force per unit mass 

 acting on a body due to gravity and acceleration. A simple pendulum 

 is a specific- force receiver in which the suspended mass tends to aline 

 itself with the direction of the specific-force input vector. Many 

 other kinds of specific- force receivers are in use in which the output 

 signal represents the specific-force component acting along an input 

 axis that has a fixed direction with raspect to the case of the sensing 

 component. Devices of this kind appear as essential elements in all 

 inertial navigation systems. 



INERTIAL NAVIGATION SYSTEMS FOR VEHICLES MOVING AT 

 SUBSTANTIALLY CONSTANT ALTITUDE 



Navigation systems used in vehicles supported by ground, air, or 

 water normally move in surfaces that are substantially spherical about 

 the center of the earth, so that the specific-force input for specific- 

 force receivers is practically identical with gravity. This means that 

 it is feasible to indicate the direction of gravity by means of an equiva- 

 lent pendulum system with Schuler tuning. Inertial equipments for 

 use in ground vehicles, aircraft, surface ships, and submarines all take 

 advantage of this fact by basing their indications of vehicle location 

 on the orientation of an indicated vertical member with respect to an 

 earth reference member positioned by a gyroscopic inertial reference 

 system and sidereal time. In some cases, the inertial reference mem- 

 ber holds its physical orientation among the stars, while in other cases 

 the member moves, and its rate of change of orientation with respect 

 to inertial space acts as the input for a computer whose output is 

 navigational information. 



Many designs are possible for inertial systems based on indications 

 of the local vertical by an equivalent pendulum with Schuler tuning. 

 Several different equipments have been constructed by various com- 



