To establish a navigational fix two intersecting lines of position are 

 required. Where a single variable is being measured, such as the rate of change 

 of range in the Transit system, for example, a number of successive measure- 

 ments are required. Where two or more satellites are available simultaneously, 

 a measurement of the vessel's position with respect to each satellite can gener- 

 ate the necessary lines of position to establish a fix. Orbital correction informa- 

 tion is required of both satellites . In other systems which are designed to meas - 

 ure two variables simultaneously, only a single measurement is required to 

 determine a fix . 



A large number of possible combinations of range, angle, and Doppler 

 measurements are possible for the determination of a navigational fix. How- 

 ever, evaluation studies have been made of only the more promising systems 

 directed toward both military and civilian requirements . 



3 . MILITARY SATELLITE NAVIGATION SYSTEM 



a. Description 



The Navy's navigation requirements are quite different from those of 

 the nonmilitary user. Since the navigator's position must not be betrayed, he 

 must remain passive. For this reason the Navy's original Transit satellites 

 were designed to give position information to vessels equipped with data- 

 processing and accurate inertial guidance navigation equipment . An inertial 

 guidance or similar system is required to supply the velocity information needed 

 to compute navigational fixes of high accuracy by range -rate methods . 



In the Transit system the satellite orbit is determined by ground 

 stations and a parametric description is transferred to the satellite. (See 

 Figure IV -13.) This information is stored in a memory and is updated at least 

 once each 24 hours . It is continuously transmitted by the satellite and is re- 

 peated at precise two -minute intervals . 



The relative motion between the satellite and the navigating vessel on 

 a rotating earth produces the well-known Doppler frequency shift. These fre- 

 quency variations are an accurate measure of the rate of change of the slant 

 range between the satellite and the navigator. By measuring the Doppler shift 

 one may obtain either of two results: (1) if the ground station position is known, 

 the satellite orbit may be determined, or (2) if the orbit of the satellite is known, 

 the position of the ground station may be calculated. 



87 



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