by a range measurement from the shore station to the navigating vessel . This 

 method would involve measuring the time difference in arrival of two signals , 

 one being the vessel's automatic response via the satellite to the ground station's 

 coded address signal and the second being the range signal from the vessel via 

 the satellite to the ground station. From these measurements the distance from 

 thesatellite to thevessel is found anda line of position is established. A second 

 line of position would be derived from a measurement of the elevation angle or 

 bearing of the satellite with respect to the vessel. Other methods for finding 

 the second line of position are also being considered. 



Such a system using a single satellite per fix would require approxi- 

 mately eight station -keeping satellites at about a 6000 -mile altitude. Each 

 would require two or more microwave channels for relay purposes . A fix 

 accuracy of about ± 1 nautical mile is anticipated for general use, and a fix 

 accuracy of ±0.1 nautical mile is predicted for special applications . 



Another approach to the "cooperative" system which is under consid- 

 eration would use two or more satellites to generate each individual fix. Posi- 

 tional information offering about the same accuracy achieved with the single 

 satellite system would be somewhat easier to obtain. This system would require 

 16 -24 nonstation -keeping satellites in 6400 -mile circular orbits . Digital mes - 

 sages and ranging pulses would be transmitted to the two satellites within the 

 user's view and relayed to the navigating vessel. The signals would be auto- 

 matically retransmitted in reverse order via the same path to the computer 

 station, where the range of the navigating vessel from the shore station would 

 be determined and one line of position would be established. An identical meas - 

 urement from a second satellite would generate an intersecting line of position, 

 and thus a navigation fix would be established. This is called the active mode of 

 operation . 



Where the user did not wish to reveal his position, a passive mode 

 might be selected and a general address code utilized. At periodic intervals 

 the position and altitude of the satellites and then the required pulse information 

 would be transmitted through the satellite to all passive users in the area. Each 

 navigating vessel would then compute its own location. Each user would need 

 pulse receiving, timing, and display equipment for passive mode operation. 



Another suggested approach to using satellite systems for navigation 

 and ship -positioning is a modification a geodetic survey system in use by the 

 U.S. Army Map Service, Corps of Engineers. This system known as Secor 

 (Sequential Collation of Range) ^^^ uses a single transponder -type satellite and 

 three precisely positioned ground stations on base lines of 500-1000 miles in 

 length. (See Figure IV -14.) Accurate positioning at ranges of several thousand 

 miles is possible, depending on satellite altitude. 



92 



arthur Jl.littlcKnr. 



