6853 HYDROGKAPHIC MANUAL PaGE 652 



should give this information. The length of the signal offset may occasionally be used 

 as a means of identification. The duration of a signal depends principally on the 

 strength of the bomb signal at the R.A.R. station, which is variable with distance, and 

 on the sensitivity of the R.A.R. station, and to a lesser degree on the strength of the 

 batteries. If one of the stations is more sensitive than the others, it will transmit a 

 longer signal which will be recorded on the chronograph tape as a longer offset and 

 furnish a means of positive identification. 



In routine operations, after a few positions, the identification of the various 

 stations is simple when the positions are obtained at equal time intervals and the ship 

 is maintained on approximately the same course. The chronograph attendant can 

 take the differences between successive time intervals from an R.A.R. station and 

 from these determine whether the time intervals are increasing or decreasing and the 

 magnitude of the change. By extrapolation he can closely estimate the time inter »^al 

 for a station at the next position, and he may often be able to identify the first of two 

 returns where the position is near a bisectrix between two stations (see 6823). The 

 ofRcer-in-charge should also notify the chronograph attendant when a major change in 

 course is made and about how this will affect the differences between successive time 

 intervals. 



In rough weather a sono-radio buoy that is too sensitive will transmit radio signals 

 as the buoy structm-e rises and falls with the swell or waves; or the noise from the 

 propeller of a nearby vessel may cause an R.A.R. station to transmit signals at various 

 intervals. Such signals are called strays and the sounds received are short and sharp, 

 resembling somewhat the chirping of small birds or crickets. Because of their short- 

 ness, such signals are usually easy to distinguish from the bomb returns but, when such 

 a stray is recorded just before the bomb return, too close to be distinguished by ear or 

 recorded as a separate offset on the chronograph tape, an error may result in the meas- 

 ured time interval. During periods when strays are being transmitted by any station, 

 the chronograph attendant must pay particular attention to the returns from that sta- 

 tion, and any which he suspects of stray interference should be questioned in the Record, 

 and the plotting station should be warned when the time intervals are communicated. 



After the returns have been identified, the identity of each should be indicated on 

 the chronograph tape. The name of the station may be written in full or it may be 

 merely indicated by using the first letter of the station name, as in figure 141. 



6853. Scaling Chronograph Tapes 



A complete record on a chronograph tape contains a series of signal offsets on one 

 side of the centerline, which have been identified by pencil marks when received, and 

 time offsets at regular intervals for its entire length on the opposite side of the center- 

 line. 



To measure the time intervals it is only necessary to determine, in fractions of a 

 second, where each signal offset occurs between two adjacent time offsets, first counting 

 the time offsets. The time offsets on the tape should be numbered, unless this is done 

 automatically as on the Dorsey chronograph (6733), using as an initial the first time 

 offset after the bomb explosion. This initial is numbered zero and, counting from this 

 offset, each fifth one is numbered, as well as the offset preceding each radio return 

 (see fig. 141). To aid in numbering the time offsets on a Gaertner chronograph tape, 

 a length of tape with seconds marked on it may be run off, and the offsets numbered 

 consecutively for use as a standard scale. This standard tape may be secured to the 



