Page 651 



RADIO ACOUSTIC RANGING 



6852 



The next offset recorded by the signal stylus under normal conditions will be caused 

 by a radio dash transmitted by an R.A.R. station at the instant the sound wave reaches 

 its hydrophone. This radio signal may be heard through a loud speaker or indicated 

 by the needle of a milliammeter (see 6724). As each radio return is recorded on the 

 tape, it should be identified by a pencil mark. After the radio return from the most 

 distant R.A.R. station has been recorded, the chronograph is stopped and that part of 

 the tape with the record is torn off. Figure 141 illustrates an R.A.R. record on a tape 

 from a Gaertner chronograph, with the tape broken into four parts for convenience in 

 illustration, and the correct method of marking the returns. 



Scaled time to GOBY • 27.57 sec. 

 Scaled time to DAGO ■ 18.95 sec. 



Scaled time to ESAU • 8.16 sec. 

 "1 



^ 





^ 



1 /; 



Bomb offset 



Offset caused by Signal 

 from ESAU 



Offset caused by Signal Offset caused by Signal 

 from DAGO from GOBY 



Figure 141.— Elapsed times obtained from a chronograph tape. 



During heavy atmospheric static it is often difficult, and occasionally impossible, 

 to record radio returns on the chronograph tape. A heavy crash of static will cause a 

 prolonged offset of the signal stylus and a radio signal transmitted during this time will 

 be lost under the static. The duration of an offset caused by static may sometimes be 

 reduced by pushing the stylus gently toward the center of the tape, provided the static 

 is not too heavy. 



The ship's whistle must not be blown while bomb returns are being received. 

 The whistle not only prevents the radio returns from being heard but also causes a 

 prolonged offset of the signal stylus, a phenomenon which is called "whistle static." 



6852. Identification of Bomb Returns 



When R.A.R. was first used^ the shore station equipment included an automatic 

 key that transmitted a series of radio dashes, when activated by the bomb signal. A 

 different setting of the key at each R.A.R. station varied the length of, and the interval 

 between, the radio dashes, thereby furnishing a positive means of identification on the 

 chronograph tape. However, the series of radio dashes interfered with the reception of 

 returns in close succession and the use of the automatic key was abandoned. Other 

 means must be used by the chronograph attendant to identify the returns from the 

 various R.A.R. stations. 



The best clue to the identification of radio returns is the tone of the transmitted 

 signal as heard through the loud speaker. When the radio receiver is properly tuned, 

 the return from each station usually has a characteristic tone which is caused by a slight 

 difference in the crystal frequency of the R.A.R. station, resulting in a small difference 

 in the audible beat frequency on reception (see 6433). The radio technician and the 

 chronograph attendant soon learn to recognize the different tones and are thus able 

 to identify the stations. 



The chronograph attendant should always know the sequence in which the radio 

 returns should come. For the first position in a new locality the oflScer-in-charge 



