H. NAVIGATION SYSTEMS USING SATELLITES 



I. GENERAL 



A number of high -precision navigation networks using earth satellites 

 are under study by the Navy, NASA, and FAA. To date, only the Navy's Transit 

 program, being carried out by the Applied Physics Laboratory of Johns Hopkins 

 University, under the cognizance of BuWeps , has been implemented. The more 

 promising systems which have so far been investigated promise true global 

 coverage, all-weather operation, relative immunity to interference, unlimited 

 traffic -handling ability, the availability of frequent fixes , and very high positional 

 accuracy. Mid -latitude positional accuracies exceeding 0.50 nautical miles are 

 predicted on the basis of an early analysis of Transit 4A experimental data . 



There is some commonality to all navigation systems which would 

 utilize satellites . The satellite's position relative to fixed points on the earth's 

 surface is communicated to the navigating vessel. The navigation system then 

 determines the vessel's position with respect to the satellite at a given time. . 

 This information can then be combined with the known position of the satellite, 

 and the vessel's longitude and latitude may be computed. 



All satellite navigational systems will require precise orbital infor- 

 mation so that the satellite's position at any point in time may be accurately 

 predicted. A communication link for determining the position of the navigating 

 vessel with respect to the satellite in all types of weather must be available . 

 Also, in some cases, such as high-speed aircraft, the velocity and altitude will 

 be required to obtain an accurate positional fix. 



2. METHODS OF MEASUREMENT 



A number of satellite navigational methods are possible and may be 

 classified as to type of measurement made with the instantaneous position of the 

 satellite as a reference. In all systems the satellite follows some orbital path 

 around the earth . The satellite position is accurately computed in advance and 

 corrections are made for perturbations which occur with time . The accuracy 

 of the satellite position is then dependent upon how well the orbit is known and 

 the accuracy of the time measurement. The position of a vessel relative to the 

 satellite may then be found by measuring the: 



1 . altitude (elevation angle) 



2. azimuth (bearing) 



3 . range to the satellite 



4. rate of change of altitude, azimuth, or range. 



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arthur n.HittlcJnr. 



