376 



DIRECT MEASUREMENT TECHNIQUES 



2.0 



40 



60 80 



ECHO NUMBER 



100 



rzo 



140 



Figure 10. Variations and fluctuations in sphere echoes. 



istics of the ocean (see Chapter 7). The remainder 

 may be ascribed to such external causes as changes in 

 the performance of equipment and changes in target 

 aspect. 



21.6.1 



Variation 



Variations occurring over a sufficiently long time 

 are very difficult to detect. Sometimes they result 

 from gradual changes in the characteristics of the 

 echo-ranging gear employed and may be detected 

 each time the system is calibrated. 



More often, however, variation may be most promi- 

 nent during a long run in the course of a single day 

 or on successive days. At long ranges, changes in the 

 transmission conditions in the water may be responsi- 

 ble for some of the variation observed; horizontal 

 temperature gradients may occur and cause changes 

 in the value of the transmission anomaly. This effect 

 may be most conspicuous at long ranges for two in- 

 terrelated reasons. First, if the ranges are long the 

 operating area is much larger, and horizontal differ- 

 ences in temperature may be more likely. Second, 

 since the transmission anomaly increases with 

 range, variations attributable to slow changes in 

 the transmission anomaly will be greatest at long 



ranges. At short ranges, much less is known about 

 variation. 



Marked variation in the echo level was observed 

 during the course of a number of runs during early 

 echo-ranging tests on a sphere in San Diego. ^ The re- 

 sults of one reel of film exposed to the sphere echoes, 

 as shown on a cathode-ray oscilloscope, are repro- 

 duced in Figure 10; pulses were sent out at intervals 

 of 1.2 sec and the range of the sphere was about 

 109 yd. Here, the ratio of the observed echo ampli- 

 tudes to the echo amplitudes predicted from theory 

 (in which transmission loss is taken into account) is 

 plotted for each individual echo received. The short- 

 term changes are most noticeable, but the slow up- 

 ward slope of the average of the points is evidence of 

 variations as defined here. The cause of this variation, 

 however, is not known. 



Changes in the calibration of the equipment over 

 a period of time, known as "drift," are also responsi- 

 ble for some of the variation observed. As pointed out 

 in Section 21.4.2, slow drifts of 3 to 4 db have been 

 observed between calibration checks at San Diego, 

 at approximately four-month intervals, in a crystal 

 projector. Just how much of the variation normally 

 encountered can be attributed to drift, however, can- 

 not be estimated very accurately. 



