reference-heterodyne frequency transmitted from T3 is measured. The measured 

 phase angle determines the location of the mobile station on one hyperbolic line 

 of position. 



To obtain a second line of position, the complete system of Figure V-6 

 is duplicated. A second base line which results in the second LOP is formed. 

 The two patterns thus generated are conventionally known as the red and green 

 patterns . 



The phase meter at the mobile receiver is geared to a revolution 

 counter which indicates the number of lanes traversed in both patterns. The 

 readings can be transferred to charts of the area on which the equiphase contours 

 are drawn, and a position fix can be established. Lane identification is not pro- 

 vided in the basic LORAC system. 



The basic system requires six transmitters, six receivers, and four 

 frequency channels to locate one mobile station. This is a multi-user system, 

 and there are no limitations on the number of mobile receivers using the system 

 simultaneously. However, the other LORAC systems provide the same capability 

 and use less equipment. 



a. LORAC Type "A" System 



In the LORAC type "A" (Alpha) system the equipment required is re- 

 duced by the use of a time-sharing procedure. The system is similar to the basic 

 system except that the T^ and To ground stations alternate as reference stations, 

 as illustrated in Figure V-7. This system requires three transmitters and two 

 receivers to establish a hyperbolic grid system. A receiver is required at both 

 the red and the green ground stations. Each mobile station using the system re- 

 quires two receivers and two phase meters along with the required frequency 

 selective equipment. 



The central shore station, T2, alternately transmits two frequencies. 

 The transmitter and receiver at both the red and the green ground stations are 

 continuously energized. In this manner, a heterodyned reference signal is al- 

 ternately transmitted as modulation on a carrier from each ground station and 

 received at the mobile station. Simultaneously, aheterodyne position signal is 

 received on a second receiver at the mobile station. These two signals are 

 compared and displayed on an integrating-type phase meter. In this manner 

 red and green lines-of-position are alternately established, with each phase 

 meter operating for one-half of the complete cycle. The switching rate is suf- 

 ficiently rapid to produce the effect of continuous transmission at the position 

 indicators. 



118 



artbvtr ai.littlejnr. 



