I. INERTIAL NAVIGATION SYSTEMS 



1. GENERAL 



Inertial navigation systems are ship-mounted and entirely self-con- 

 tained. The vast majority of marine inertial navigation systems has been in- 

 stalled on Polaris submarines. (Until very recently each of these ships carried 

 three identical systems, but the number is being reduced to two.) Similar iner- 

 tial systems are carried on certain aircraft carriers and nuclear attack subma- 

 rines and on range tracking ships. The systems are complex and expensive, 

 but they offer unequalled accuracy in position-tracking and, being self-contained, 

 are entirely independent of external sources of navigational information. They 

 differ from other ship-mounted systems in their greatly superior ability to main- 

 tain a continuous track of the ship's position. They are not, of course, absolute- 

 ly accurate; errors occur and, generally speaking, increase with time. The 

 coupling of an inertial system with another system which is capable of correcting 

 the inertially determined position by reference to some external object or signal 

 produces a composite system having a degree of continuous accuracy unmatched 

 by any other method of navigation. 



2 . DESCRIPTION 



a . Typical Shipboard Inertial Navigation System 



To determine the motion of a vehicle over the earth's surface an iner- 

 tial navigation system first measures the vehicle's motion with respect to a ref- 

 erence system which has some known orientation with respect to the earth and 

 then translates this motion into terms of earth coordinates. The reference sys- 

 tem is provided by a device called a "stable platform, " by a system of gyroscopes 

 and by accelerometers mounted and interconnected in such a way that it maintains 

 one of its orthogonal axes parallel to the local vertical and another pointed north, 

 independent of vehicle movements . Accelerometers mounted upon this platform 

 can then measure vehicle acceleration with respect to the platform. Accelera- 

 tions are integrated to obtain velocity and distance traveled. These quantities 

 are translated into earth coordinates to maintain position data and to correct 

 platform orientation to correspond to the changing position. 



To illustrate in more detail the technique of inertial navigation, we 

 will consider a single typical shipboard system which uses as its reference sys- 

 tem three orthogonal axes, one vertical, one north-south, and the third east- 

 west. These axes are established by three single-degree-of-freedom gyros 



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