The most common form of composite inertial system is one in which 

 velocity, measured by some external source, such as an electromagnetic log, is 

 used to damp platform oscillations and limit the amplitude of periodically varying 

 errors. However, this measure is ineffective against the cumulative distance 

 error caused by constant gyro drift or by random errors in velocity and distance 

 caused by other phenomena. To eliminate these non- periodic errors--essential- 

 ly to reset the inertial system --an additional system is required to obtain accu- 

 rate navigational fixes. Such auxiliary systems can be designed to determine 

 the ship's position either continuously or at discrete intervals. Use may be 

 made of such navigational aids as Loran or Decca, of course, but of particular 

 interest are systems which take advantage of the availability of the stable plat- 

 form and which use celestial bodies for determining the ship's position. Com- 

 posite systems such as these include a separate computer and an optical and/ or 

 radiometric sextant slaved to the stable platform. In effect the computer com- 

 pares the actually observed altitude and azimuth of celestial bodies with those 

 it has computed on the basis of the ship's position and heading as determined by 

 the inertial navigator. Differences between the observed and computed values 

 are used to provide corrective signals for latitude, longitude, and heading. 



3. ACCURACY 



Almost all inertial navigation systems for shipboard use have been 

 installed on nuclear submarines and on missile range tracking ships, and data 

 on their performance is classified. However, some notion of the accuracy ob- 

 tainable can be derived from published information on airborne systems . For 

 example, records of a large number of recent flights indicate that one system 

 installed on jet airliners produces an error probability of 2.35 nautical miles 

 per hour. It is likely that the accuracy demanded of inertial navigation systems 

 (SINS) in Polaris submarines is considerably greater. 



98 



artbttr Zn.l-ittlc.Ilnr. 



