C. LORAC 



1. GENERAL 



The LORAC (Long Range Accuracy) ship -positioning systems were 

 developed by the Seismograph Service Corp. for survey work in the shelf area 

 of the Gulf of Mexico. The LORAC system was designed to extend survey work 

 beyond the line of sight distances of Shoran without sacrificing the accuracy of 

 position determination. 



LORAC is a hyperbolic cw phase comparison system using frequencies 

 in the 1700-2500 kc band. At these medium frequencies rf energy tends to fol- 

 low the curvature of the earth, permitting a high degree of accuracy. 



However, both lower and higher frequencies have been used. At 2 mc 

 the operating range is of the order of 135 miles during the day, but is limited to 

 about 50 miles at night due to sky-wave interference. Over water, accuracy 

 is of the order of ±2.5 feet along the base line and is gradually degraded as the 

 mobile receiver moves away from the base line. 



Both LORAC A and LORAC B are position-fixing systems, and both 

 supply a double hyperbolic grid. In both systems transmitting stations are set 

 up at fixed shore sites. The basic receiving set displays grid coordinates in 

 the form of dial and digital counter readings. Special indicating equipment 

 including a course plotter, digital printer, tape punch, and distance-heading 

 indicator is available. 



2. DESCRIPTION 



In the basic LORAC system, illustrated in Figure V-5, cw trans- 

 mission is used and a phase difference measurement is made at the mobile 

 receiver to obtain the coordinates of its position. Transmitters Tj^ and T^ are 

 located at the ends of a base line and operate on frequencies i^ and fn which 

 differ by an audio frequency. A reference signal is established at T3 by detect- 

 ing the heterodyne beat between T, and T^. This detection takes place at a fixed 

 point and, therefore, has a constant phase angle. It is transmitted to the mobile 

 receiver as amplitude modulation on a third rf frequency fq. 



The receiving station contains two separate receivers. The direct- 

 heterodyne difference frequency when received at the mobile station has a phase 

 angle dependent upon the station position relative to the base line. The differ- 

 ence in phase between the direct-heterodyne difference frequency and the 



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artbur ai.littlfjnr. 



