A navigator who wishes to establish a position fix using Transit during 

 one of its passes overhead receives from the satellite: 



(1) Correct time 



(2) Updated orbital information 



(3) Doppler frequency information. 



Transit radiates two phase -modulated rf frequencies in the UHF region 

 which are received by the navigating vessel with an apparent variation in each 

 frequency. At the receiver a beat frequency is formed for each of the trans - 

 mitted frequencies , which are then suitably combined in a manner which reduces 

 errors due to atmospheric refraction. 



The satellite transmits its position continuously during precisely timed 

 two -minute intervals . A minimum of three intervals is required for an accurate 

 position determination. From this information and the time that the Doppler fre- 

 quency crosses through zero, the latitude can be determined. By measuring the 

 rate of change of the Doppler frequency the longitude can be calculated. The 

 satellite also transmits orbital and time reference information as phase modu - 

 lation on the rf carrier during each interval . 



A number of methods exist for computing the navigator's position 

 once the Doppler frequency vs time has been measured and the orbital informa- 

 tion received. To carry out the required calculations present plans require a 

 specialized receiver and computing equipment with a pre-established program. 

 By a precision analysis of all Doppler information the vessel can determine its 

 position with respect to the satellite. Evaluation of the time and orbital infor- 

 mation yields the vessel's position with respect to fixed references on the earth's 

 surface . 



b. Accuracy 



Reliable figures of Transit system accuracy are classified and not 

 readily available. However, the more important errors may be generally 

 classified as follows: 



(1) Inaccurate orbital information 



(2) Lack of knowledge of the navigator's own velocity 



(3) Errors in the received frequency from the satellite. 



The principal orbital errors arise from unpredictable short -comings 

 in the satellite period. Because of the earth's atmosphere and nonuniform gravi- 

 tational field, the satellite may advance beyond or be retarded from its predicted 

 position. An error in the assumed coordinates of the satellite will result in an 



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