908 EXPLORATION GEOPHYSICS 



Radar Beacons Pulse Systems. — A marked improvement in the 

 application of conventional reflection radar for survey purposes is found 

 in the radar beacons or transpounder systems, one type uses a pulse method 

 and the other a continuous wave phase or standing wave pattern. 



In one pulse method, termed the Shoran, a pulse coming from the single 

 moving station triggers two separated fixed beacon stations and they in 

 turn retransmit to a receiver aboard the moving station. The time intervals 

 representing the pulses from the two fixed beacons are indicated. The 

 moving transmitter and the two beacons each operate on a different fre- 

 quency, in the range of 220 to 310 mc. (megacycles). At these frequencies, 

 the range is limited to line-of -sight. Aboard ship this range is about 15 to 

 20 miles, while from an aircraft flying at 1,000 feet elevation it may be 

 40 miles. At these distances, the accuracy is ± 25 to 75 feet. In both the 

 Shoran and the Ratran systems auxiliary equipment can be provided 

 whereby the pilot can be guided to fly a given course by means of a PDI 

 (pilot direction indicator) for the Shoran, and a PLI (pilot location indica- 

 tor) for the Ratran. 



Since the two beacons are at known point locations, the position of the 

 mobile station can be determined by direct intersection, without measuring 

 angles. The resolution of these data comes from a solution of a triangula- 

 tion problem ; given, the fixed distances between two stationary beacons 

 and the measured distances from the mobile or master station (location 

 unknown). This is an advantageous feature over the radar, because radio 

 distances can be measured much more accurately than azimuth angles. A 

 further advantage of the beacon system is that lower frequencies can be 

 used, because the narrow beams for angle measurements are not required 

 and simple compact antennae may be used. Moreover, the required trans- 

 mitter power for two-way beacon systems is less than that necessary for 

 the operation of reflection radar. This comes from the fact that in beacon 

 transmitter work the necessary power is proportional to the square of the 

 distance, whereas in the case of reflected radar signals the required power 

 is proportional to the fourth power of the distance. 



The Shoran systemf is a typical example of a method which has been 

 successfully used in geophysical location surveying. This system originally 

 was developed for precision aerial bombing during World War II. The 

 navigator in an aircraft determines his position to an accuracy of 25 feet 

 by measuring the range to two fixed beacon locations on the ground. The 

 equipment of the airborne station weighs approximately 250 pounds 

 installed and requires about 1.2 kilowatts of power. The fixed ground 

 station equipment weighs nearly 1200 pounds installed and requires 1.6 

 kilowatts of power. 



The Shoran method has found a ready application in the survey control 

 for flying magnetometer and for base control in photographic mapping 



t Edgar B. Stern, Jr., "Shoran Radar," Oil and Gas Journal, July, 1948, p. 70-74. 

 Carl I. Aslakson, "Can the Velocity of Propagation of Radio Waves be Measured by 

 Shoran?", Transactions, Amer. Geoph. Union, Aug., 1949. 



