SEISMIC METHODS 909 



from aircraft. Here the full range of the equipment can be utilized because 

 of the great height of the aircraft above the ground. Shoran has also been 

 successfully used for offshore surface control. 



For geophysical operations a simple and more portable apparatus for 

 Shoran type of work is now under field test. This equipment in its present 

 form is especially designed for mobile use on land. Because of the horizon 

 limitation, it is seldom practical to operate over distances greater than 15 

 or 20 miles. For this type of service the bulky high-power transmitters 

 of regular Shoran equipment are not necessary. A compact, low-power, 

 experimental system of this sort has given approximately the same accuracy 

 as the regular Shoran equipment up to distances of 10 or 20 miles. The 

 equipment of the master and the beacon stations each weighs about 25 

 pounds and each requires about 100 watts power. This portable system 

 will probably find considerable application in future geophysical land and 

 near-shore operations. When good base maps are not available, it is believed 

 that the services of this equipment would prove extremely valuable. 



Experimental work has demonstrated that the attenuation between sta- 

 tions may not be too great at the high frequencies required and that satis- 

 factory results can be obtained at short ranges through considerable dense 

 foliage growths and other small obstacles to the direct line of sight. 



The Ratran is a radar triangulation system, and determines position by 

 using ranges from two fixed beacon points. Aboard the moving station are 

 carried two radar transmitters ; each operates at a different frequency and 

 triggers one of the beacon stations. Location of the moving station is 

 computed with reference to the base-line connecting the two fixed stations. 



C.W. Phase Systems. — Several c.w. systems offer considerable 

 promise for off-shore location surveying. Although there may be consider- 

 able differences in the techniques involved in these systems, they all will 

 probably depend basically on setting up a standing radio wave pattern 

 between two or more fixed transmitters. The distances may then be meas- 

 ured by counting wave lengths and comparing phase relationships. The 

 Decca system developed in England during World War II for aircraft 

 navigation was probably the first system of this type used. 



The Lorac (Long Range Accuracy System) is one of the better-known 

 methods for determining the location of a mobile unit by comparing the 

 phase relationship between two pairs of received signals. In this system, 

 two c.w. transmitters are located at known points, and operate on the same 

 fixed frequency. For any given distance apart of the two fixed stations, 

 a chart may be plotted showing lines of constant carrier frequency phase 

 difference. This is of a hyperbolic pattern. The indicating instrument in 

 the mobile station measures the phase difference between the two fixed 

 stations, and each 360 degree change in phase is called a lane. To establish 

 position on any hyperbola of the chart it is necessary only to count the 

 number of lanes when traveling from a known point. If a fix or position 

 is desired, a second pair of transmitters is necessary, to allow a second set 



