910 EXPLORATION GEOPHYSICS 



of intersecting hyperbola to be plotted. In practice, two phase meter indi- 

 cators are used aboard the mobile station to give the intersection of the two 

 sets of hyperbolas. 



The basic Lorac system requires six frequencies. A recent modification, 

 the type-A Lorac, uses only four frequencies, so chosen that they occupy 

 but two channels. The system uses four receivers and four transmitters. 



The accuracy of the Lorac system depends upon the frequency. When 

 operating in the 2000 kc band, a distance determination of ± 25.00 feet is 

 possible. Two important characteristics of the system may be mentioned : 

 (1) a slight shift in phase can be tolerated, which obviates the necessity of 

 precise phase synchronization, and (2) the system can accommodate any 

 number of users. 



The Raydistf is another system based on phase measurement. The 

 indicators may be located either in the mobile unit or at a fixed station. 

 The equipment may be operated either as a simple range system or as an 

 intersecting hyperbolic system. 



For determining location along a given hyperbola, four transmitters 

 and four receivers are required. Receivers Rx and R2 are preferably 

 mounted aboard the mobile unit, with transmitter Tx. T2 and R3 are located 

 at one fixed beacon station ; while T3, T4 and R4 are at another fixed beacon 

 station. The frequency of Ti differs from T4, by an audio note, and the 

 waves from both are heterodyned at a shore station Rz. This beat note 

 modulates transmitter T2, which signal is received by R2. The phase differ- 

 ence of this signal depends upon the relative distance between T4 to Rz 

 and Ti to R3. Since the distance of T4 and Rz remains fixed, the phase 

 therefore depends upon the distance from the moving transmitter Ti to R3. 

 A similar heterodyned note is received at Ri from transmitters Ti and T4, 

 and it is transmitted by T3 to the receiver i?i. Likewise, the phase at i?4 

 depends upon the distance separating R4 from Ti. An indication of the 

 phase relationship between the two sets of receivers is obtained by feeding 

 the output of Ri into the rotor, and the output of R2 into the stator winding 

 of a synchro. A revolution counter geared to the synchro records the num- 

 ber of lanes traversed from any given point. In order to obtain a fix, a 

 duplicate set of equipment is employed. 



A more simple range Raydist system has been developed, using a c.w. 

 transmitter and receiver at the fixed station and a similar transmitter and 

 receiver aboard the mobile station. At each receiver a heterodyned note is 

 produced between the distant transmitter and the adjacent transmitter. 

 This beat note is then transmitted to the other receiver by means of a third 

 transmitter, and is compared in phase with the locally produced signal. 

 Distance of travel of the mobile unit is determined by recording the number 

 of phase changes encountered from the known point of starting. 



When listening to these beat frequencies, the rate of movement of the 



t C. J. Deegan, "A Method of Surveying by Use of Radio Waves," The Oil and Gas Journal, 

 July 7, 1949, pp. 69-91. 



