906 EXPLORATION GEOPHYSICS 



erally give unique solutions to the problems without reference to a record 

 of the portion of the traverse followed to the unknown point. In other 

 words, this solution does not depend on any previous recording of informa- 

 tion. The c.w. systems require that two or more stations be in continuous 

 communication and that the results be integrated or accumulated at one 

 station. This is somewhat analogous to the difference between navigation 

 by astronomical fixes and navigation by dead reckoning. In the first case 

 the unknown position is determined independently of any other observa- 

 tions. In the second case, to determine position it is necessary to keep a 

 continuous log of the course, including speed and direction. The courses 

 and the distances traversed must be computed vectorally in order to deter- 

 mine the location of a new position. 



The pulse system consists of sending out a sudden burst of radio fre- 

 quency energy and measuring the time for a reflection to return from a 

 marker target or from a beacon or transpounder station. If the distance is 

 measured from two points of known location in this manner, or the dis- 

 tance and azimuth are measured to any known point, then the position of 

 the observer may be established by simple trigonometry or arithmetic. 



In the c.w. system an interference pattern of radio waves is set up by 

 two or more transmitter stations at known locations. This is accompHshed 

 by transmitting on the same frequency. The mobile station observer then 

 counts and records the number of standing wave patterns which he crosses 

 by means of an accumulating phase meter. In this way the observer may 

 keep track of his course and position relative to the starting point in terms 

 of standing wave patterns. If the observer moves as much as one-half of 

 a wave length during the time when there is a loss of communication 

 between units, an error will be introduced into the counting system. This 

 error cannot be removed from the system without resetting the phase meter 

 dials at a known location. 



A disadvantage of the pulse system is that it requires a wide spectral 

 band width in order to give accurate results. This means that pulsed systems, 

 to have an accuracy of the order of 50 feet or better, must be operated in 

 the v.h.f. (very high frequencies) or microwave region of the radio 

 spectrum in order to conserve valuable space for communications in the 

 lower portion of the radio spectrum. When these high frequencies are 

 used, operation is confined to the line of sight conditions. This usually 

 limits the range of the equipment to th^ horizon distance which is dependent 

 upon the particular antenna heights. In the c.w. system with its narrow 

 band width, it is practical to operate at lower frequencies, and as a conse- 

 quence the range of the apparatus will not be limited to the horizon dis- 

 tance. Also the longer wave lengths follow the curvature of the earth. 



JS avigational Radar. — Radar equipment operating in the 3 cm. and 10 



cm. bands has been used for offshore surveying. The general operating 

 principle of radar has long been known to geophysicists. Like the reflection 

 seismograph, radar is an echo ranging device, but unlike the seismograph. 



