A. CELESTIAL NAVIGATION 



The hand -held sextant and tables, historically the basic tools of the 

 navigator, are inexpensive and easy to use . With this equipment a navigational 

 fix can be established to an accuracy of about ±3 nautical miles in most geo- 

 graphical areas throughout the world. The navigator's position can only be 

 established to this precision, however, under the best of conditions at sunrise 

 and at sunset, when the sun and horizon are clearly visible . The average us- 

 ability of this method on a world-wide basis is less than 50%, and in some geo- 

 graphical areas many days may pass before a celestial sight can be taken and a 

 fix established. 



The Geon (Gyro Erected Optical Navigation System) is a develop- 

 mental high -accuracy celestial system which uses a Mark 19 gyrocompass to 

 maintain the astronomical meridian and indicate the vertical. A small equatorial 

 telescope is mounted on the system and sighted on a star. By setting the polar 

 axis the navigator determines the altitude of the pole and the latitude is found 

 directly. The longitude is calculated by noting the local hour angle and com- 

 puting the Greenwich hour angle . The method makes use of celestial coordinates 

 and avoids use of the visual horizon. The system requires manual setting and 

 computation. About three minutes are needed to establish a fix. At the present 

 state of development navigational accuracy is between 0.2 and 0.3 mile. 



The recently developed Automatic Star Tracking equipment uses a 

 very accurate inertial platform as a vertical reference system . It has the 

 ability to lock on and continuously track a selected celestial body, thus produc- 

 ing position information continuously. Equipment of this type has been used 

 successfully on a range ship for special applications . While the Geon and 

 Automatic Star Tracking systems are passive and provide greater accuracy than 

 the hand -held sextant, they have the same limitations with respect to cloud 

 cover . 



The Radiometric Sun and Moon Tracker is a celestial navigation sys- 

 tem which uses only the sun and moon as celestial reference bodies . When only 

 one body is available only lines of position maybe plotted. Although cloud cover 

 is not a limitation, the coverage is limited by the geometry of the moon-sun 

 orbits, so that world-wide availability is again below 50% „ Because of their 

 complexity and high cost, this system and the Automatic Star Tracker system 

 can usually be justified only for very special applications requiring a high 

 degree of navigational accuracy .(2) 



REFERENCES 



1 . W .S . Von Arx, "GEON: A New Navigation System, " Journal of the Institute 

 of Navigation, Vol. 9, No. 3, 1962. 



2. B.J. Baecher, Naval Navigational Trends , paper presented at Institute of 

 Navigation Meeting, June 1963 . 



33 



artbur l.Hittlcilnr. 



