Page 653 radio acoustic ranging 6853 



top of the chronograph desk or table where it will be convenient for the chronograph 

 attendant to compare any other tape with it when numbering the time offsets. 



An R.A.R. chronograph scale (4825) is used to measure fractions of a second on a 

 Gaertner chronograph tape. With the horizontal lines of the scale parallel to the center- 

 line on the tape, the scale is adjusted until the extreme converging lines on each side 

 of the scale coincide with the time offsets where they leave the centerline. Where the 

 signal offset leaves the centerline of the chronograph tape, the fractional part of the 

 second may be read directly in tenths and estimated to hundredths. For a Dorsey 

 chronograph (6733) no scale is necessary because the time offsets are registered every 

 tenth second on the tape. 



In connection with the determination of time intervals from a chronograph tape 

 the following terminology is used: 



(a) The initial is the first time offset after the signal offset caused by the bomb 

 explosion. It is marked zero on the chronograph tape and all time intervals are 

 reckoned from it. 



(6) The initial interval is the interval between the bomb signal offset and the 

 initial or zero. It is always measured to the left from the initial, so that this interval 

 may be applied as a plus correction to the measured times of the radio returns (see 

 (c) below) . Errors are occasionally made by measuring the initial interval to the right 

 from the time offset preceding the bomb explosion. 



(c) The scaled time is the interval between the initial and the signal offset indicating 

 the receipt of the sound at the R.A.R. station. It is always measured from the mitial, 

 from left to right. It is equal to the fractional part of a second, measured from the time 

 offset preceding the return, added to the total number of seconds from the initial. 

 Scaled times are illustrated in figure 141. 



(d) Ship's run correction. — The position that is determined is the position where 

 the bomb strikes the water (6811), but by the time of the explosion, the ship, if under- 

 way, will be some distance from this position and a measurable time will be required for 

 the sound of the explosion to reach the ship's hydrophone. The actual time of the 

 explosion is always earlier than is indicated on the chronograph tape. All tape times 

 must, therefore, be corrected by an interval based on the speed of the ship, the fuse 

 interval (see 6843a) , the distance between the bombing station and the position of the 

 ship's hydrophone, the depth of the ship's hydrophone, the depth of detonation, and 

 the velocity of sound in sea water. 



A table for various fuse intervals and ship speeds is convenient for obtaining the 

 ship's run correction. This table should be prepared in advance and posted at the 

 chronograph station. It can be computed from the formula: 



V 



in which 7=ship's run correction in seconds; 6=depth of bomb explosion in meters; 

 /i,= depth of ship's hydrophone in meters; (/= distance in meters the ship (hydrophone 

 location) has traveled during the fuse interval; and «= velocity of sound in meters per 

 second. It is sufficiently accurate to base the table on an average rate of sinking, 

 regardless of bomb size, and on an average surface velocity of sound expected during 

 the period, so that the same table of corrections may be used throughout a season. 



The ship's run corrections given in table 22 were computed by using a uniform rate 

 of bomb sinking of }2 fathom per second, a depth of the ship's hydrophone of 2 fathoms, 



