The basic radio receiver is also adjustable over the frequency range 

 of 1.7-2.5 mc. The position indicators are part of the receiver and display the 

 grid coordinates as dial and counter readings. The receiver also provides out- 

 puts to drive special indicating and course-plotting equipment. 



e. Recording and Readout 



The two phase meter indicators of both the Alpha and the Beta systems 

 provide outputs for a digital printer and tape punch unit so that a ship's course 

 can be recorded. Additionally, an Actrac course plotter may be used with the 

 indicators to provide instantaneous position and track on preplotted charts; a 

 distance -heading indicator also may be used to help navigate between preplotted 

 points. 



3. ACCURACY AND RANGE 



In cw phase-measuring systems such as LORAC, the one-half wave 

 lengths are counted and then a phase measurement is made of the remaining 

 fractional one-half wave length. The accuracy is a function of the frequency 

 employed. For a frequency of 2.0 mc, the wave length is approximately 

 492 feet. On the base line a lane (one -half wave length) is 246 feet, which con- 

 stitutes a phase-difference reading of 360°. The phase meter is calibrated to 

 0.01 lane or ±3.6°, which is about ±2.5 feet on the base line. At distances 

 farther from the base line, the lanes widen and the accuracy of the position fix 

 is reduced. Field reports have indicated accuracies of 1 : 50,000 are achieved 

 without difficulty. ^^' 



Over water, operating ranges up to 135 nautical miles during the day 

 have been reported. At night, operation is restricted to base line lengths of 

 less than 55 miles, because of sky-wave interference. 



Propagation effects may produce other errors where part of the 

 propagation path is across land masses. The magnitude of these errors has not 

 been completely investigated, but an error of 0. 1 lane would not be unusual. 

 Larger errors can be expected where the propagation path is over rough terrain 

 or where hills produce a shadow effect. A constant velocity of propagation is as- 

 sumed for most LORAC navigational problems. 



Because of the method of transferring a reference heterodyned signal 

 to the mobile station, transmitter frequency synchronization is not needed in 

 LORAC ship -positioning systems. However, in a cw positioning system of this 

 type an ambiguity does exist. The two phase measurements which are required 

 identify the position of the receiving station relative to the two intersecting pairs of 



123 



Arthur ZD.littlcJnr. 



