A cw transmittex- located at the red station operates at a frequency 

 400 cps higher or lower than the mobile station frequency. A special -purpose, 

 dual-frequency receiver is also used at both the red and green shore stations. 

 The receiver at each shore station accepts the transmission from both the mobile 

 station and the other shore station, doubles the lower frequency and heterodynes 

 it with the higher frequency to generate the 400 cps audio beat note. Separate 

 audio notes are then returned to the mobile station by the red and green stations 

 as a modulation of two additional, completely independent frequencies. The 

 Navigator phase-compares these signals with the beat note it produces locally. 

 A comparison of the red signal with the local mobile frequency yields a reading 

 on the phase meter of the distance to the red station. Comparing the green signal 

 with the local signal yields a measurement in elliptic coordinates which the phase 

 meter then converts into distance to the green station. 



The system employed permits more than one user. A second Navigator- 

 equipped vessel, for example, would transmit on a slightly different frequency, 

 and produce therefore a slightly different beat note, A composite of both notes 

 appears at the two shore stations and is returned to both Navigators. However, 

 the local signal generated as the heterodyne of the vessel's own transmitter with 

 the red transmitter will be a pure tone, since each shipboard transmitter is so 

 close to its own receiver as to preclude reception of other transmissions on 

 nearby frequencies. The mobile station phase meters respond only to the tone in 

 the composite frequency corresponding to the pure tone and thus indicate the dis- 

 tance to the red and green stations. 



b. Type PR Raydist 



In this system, two independent DM systems are employed. Conse- 

 quently, it is unnecessary to make transmissions from one shore station to an- 

 other, an advantage when such transmissions are difficult because of a large 

 interstation distance or rough terrain. 



c. Type R Raydist 



This system corresponds to DM Raydist but employs only the red shore 

 station. As a consequence, two-dimensional fixes cannot be obtained; only direct 

 distance to the red station can be obtained. 



d. Type E Raydist 



A group of stations are utilized to set up an intersecting net of hyper- 

 bolic coordinates. The shore station-Navigator combination of Type R is 



128 



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