almost equal error in the computed position of a vessel. For navigation pur- 

 poses, the position of the satellite is predicted up to 24 hours in advance. An 

 analysis of test data from Transit 4A(1) indicates an error in satellite position 

 not greater than 0.5 nautical mile for satellite coordinate predictions made 12 

 hours in advance. Errors increase rapidly for periods greater than 12 hours. 



A second form of error arises from poor information about the navi- 

 gator's own velocity. With a two -knot velocity error during a 15 -minute pass 

 of the satellite, the estimated positional error is 0.3 - 0.5 nautical mile' ' 

 This does not appear to be a severe limitation for surface ships and submarines, 

 where accurate pit -log or inertial system information is normally available. 

 This form of error could be of much greater importance in the case of high- 

 speed aircraft, where a greater error in velocity might be expected. 



Navigational errors of about 1 nautical mile could be produced if the 

 satellite frequency or the local oscillator were to drift about one part in 10° cps 

 However, recent developments indicate stabilities of one part in 10-*^^ per hour 

 or better may be expected. Consequently the frequency stability of either oscil- 

 lator should not make any significant contribution to the navigational error, if 

 a constant frequency shift is monitored and corrected for in the computation. 



Simplified methods of computing the navigator's position integrate 

 the total number of cycles of Doppler shift over a minimum of three two -minute 

 timing intervals . Some additional error over that experienced in the original 

 method of using up to 50 data points can be expected. The error produced with 

 data of this type, using refraction -corrected Doppler, should not exceed 1-2 

 nautical miles .'^^ 



It has been shown that position errors due to atmospheric refraction 

 are reduced by the transmission of two frequencies from the satellite. With 

 this technique navigational errors of about 0.5 nautical mile might be expected. 

 By taking advantage of the higher precision (more digits) information being 

 transmitted, one can reduce this error to less than 0.1 nautical mile. The 

 higher accuracy requires more elaborate receiving and computing equipment. 

 Currently such data is not available to the nonmilitary navigator, because it is 

 coded and specialized receiving and computing equipment is required. 



c. The Present Transit System 



The Transit plan requires a minimum of four satellites in polar orbits 

 of about 600 miles with the orbital planes separated by about 45° . An orbiting 

 period of 108 minutes has been calculated for each satellite. To date a number 

 of experimental Transit satellites have been built by Johns Hopkins University 



90 



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