multiple path transmission and since the errors 
which were measured can be accounted for by the 
multiple path effect, the errors due to very 
turbulent water are assumed to be small or at 
least rare. 
Water tank tests have been conducted which 
indicate that selective fading of the carrier or 
side bands does not occur, or at least does not 
detract significantly from the accuracy of the 
system. Also, there does not seem to be a 
sufficient density of scatterers in the ocean at 
these frequencies to seriously affect these 
systems. 
RESULTS 
The meters were tested at David Taylor Model 
Basin. Fig. 4 shows a graph of the output of the 
two frequency 10-foot model of the flow meter vs 
carriage speed for two different alignments to the 
flow. The graph shows that the meter is accurate 
to better than 1% and is linear to its maximum 
velocity. Similar results were obtained for the 
other two meters. 
The two-frequency meter has also been tested 
at Scripps Institute and at sea aboard the USS 
Redfin. Under operating conditions at sea the 
error in the readings can be measured by putting 
the flow meter in the "zero calibrate" mode. 
Any indicated velocity is then a noise signal or 
error of the system. Here it was found that the 
accuracy varied with thermal conditions from 
0.5 ft/sec or 3% of full scale for severe thermal 
conditions, to better than .08 ft/sec in iso- 
thermal water. The average meter reading under 
all sea conditions approaches the accuracy of the 
electronic equipment or about 0.3%. Indications 
are that the 2-foot meter will perform better 
under nonisothermal conditions, but it has not 
yet been tested. 
The accuracy obtained by these meters is 
sufficient for many applications, especially at 
depths where isothermal water is found. The 
"zero calibration" and the nonfouling feature 
makes them ideally suited to long-term installa- 
tions. Their directivity indicates the flow 
direction. Work is proceeding to improve oper- 
ation under inhomogeneous conditions. 
ACKNOWLEDGMENT 
The contributions of 0. J. Allen to the 
development of the two-foot flow meter are grate- 
fully acknowledged. 
(A part of this work was done under 
Contract NOrd 18783) 
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