systematic errors can be removed by applying suitable corrections. However, 

 certain defects can occur under field conditions which cause errors which 

 would be significant even in a tactical situation. The most serious of 

 these defects are spurious triggering due to precursors (i.e., the second 

 and later echoes of previous pulses ) and triggering on cycles other than 

 the first in the transmitted pulse. 



Thus with the usual N.B.S. type of velocimeter, a triggering 

 error of one period of the carrier frequency leads to a velocity error of 

 about 3 meters per second. The causes of spurious triggering are listed 

 below: 



a. Depending on the geometric arrangement of the transducers 

 and reflectors that determine the sound path, multiple re- 

 flections of past pulses could arrive almost simultaneously 

 with the desired directly transmitted pulse. 



b. Random noise could add or subtract from the signal amplitude, 

 thus shifting the moment of triggering. 



c. The attenuation in the sound path can change in operation, 

 either by misalignment of the transducers or reflectors, by 

 changes in the gain of the electronics or of the transducers, 

 or by changes in the attenuation of the sea water. The re- 

 sulting change in signal level at the receiver input will 

 then change the moment of triggering. 



d. A transmitting transducer that is relatively undamped will 

 put out a train of waves rather than one pulse and these 

 extra oscillations can confuse the receiver input. 



44 



Arthur ZD.IUttlcJnr. 



