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This technique should go a long way toward providing a satisfactory means for 

 comparing observations taken at widely different places and times. Other ex- 

 amples of the use of these instruments are cited under other topics below. 



ACOUSTIC RANGING TECHNIQUES 



Distances in open water can be measured by acoustic ranging with con- 

 siderably greater precision than is possible with any other technique. The in- 

 strumentation required is exceedingly simple. A sharp transient sound such as 

 an explosion is initiated at one of two points whose separation is to be measured 

 and a listening hydrophone is placed at the other point. The instant of detona- 

 tion of the explosion, if that is the sound source employed, is either transmitted 

 by radio to the receiving point, or compared very accurately with a break cir- 

 cuit navigation chronometer or with a radio time signal such as from WWV. In 

 this latter case the arrival of the sound at the hydrophone is compared with a 

 similar break circuit chronometer or radio signal at the listening point. If the 

 initial sound is received by radio at the receiving station the acoustic travel 

 time is easily read from a single record at the receiver. If radio time com- 

 parison is employed the start of the sound and its arrival can be directly com- 

 pared simply by comparing the two records. If break circuit chronometers are 

 used than the necessary procedure is to compare both of the chronometers with 

 such a common time base as WWV or carefully compare the chronometers them- 

 selves as often as is indicated during the survey. The technique of transmitting 

 the shot instant by radio works well with radio equipment ordinarily available 

 for scientific work at sea only to rather short ranges, say to the order of 50 to 

 75 miles. At greater ranges it is usually necessary to employ the comparison 

 method. In any case the data provided is the travel time between the two points 

 for some acoustic transmission path. The computation of range from this trav- 

 el time is exceedingly simiple in some instances, i.e. , when it is known that the 

 first sound to arrive follows a relatively simple, direct path from the source to 

 the receiver. The distance is obtained simply by multiplying the speed of sound 

 in water by the elapsed time between the start of the sound and its arrival at the 

 hydrophone. In many other cases the situation is considerably complicated by 

 there being many paths over which sound energy will travel and be readily dis- 

 tinguishable by the receiver. Hence it is very important to know the nature of 

 the paths and the sound intensity associated with them as well as their relation- 

 ship to the direct line distance between the two points. Another complication of 

 the problem may be that low frequency energy will arrive at the hydrophone be- 

 fore higher frequency energy. This means of course that it travelled by a dif- 

 ferent, more rapid path between the two points. In order to measure range 

 adequately in such a situation it is first of all necessary to understand the trans- 

 mission properties of the location and, second, to make proper use of filters in 

 the electronic receiving equipment at the receiver in order properly to interpret 

 the acoustic records in terms of distance. It seems quite unnecessary to go 

 into details of the instrumentation required for this sort of use of underwater 

 sound. The receiving hydrophones and annplifiers and indeed the analysis 

 equipment, when any complicated analysis is required, are exactly similar to 

 those described under passive listening techniques. It might be worthwhile to 

 point out that crude ranging gear can be utilized for certain problems where the 

 underwater sound transmission properties are not unduly complicated and the 

 precision of the distance measurenment does not have to be very great. The 

 simplest sort of receiver with earphones or a loud speaker and a comparing 

 watch synchronized with a similar timepiece at the sending point may be quite 

 sufficient for obtaining distances precise to within a mile over distances up to 

 easily 20 or 30 miles, and considerably greater under the right circumstances. 



