AN ACOUSTIC OCEAN-CURRENT METER 
by F, J. SUELLENTROP, A. E, BROWN, and ERIC RULE 
Lockheed Missiles and Space Company 
Palo Alto, California 
ASSTRACT 
An ocean-current meter has been developed 
which is essentially two sing-around velocity- 
of-sound meters in which the directions of 
pulse transmission are opposite in sense. 
The instrument is oriented so that the direc- 
tion of acoustic pulse transmission is parallel 
to that of the flow to be measured, so that 
the time of pulse translation is ereater in 
one velocimeter than in the other. The dif- 
ference in sing-around frequencies is then 
provortional to ocean-current velocity. 
Electronic circuitry is described by means of 
which a signal with frequency proportional to 
flow velocity is extracted. It is shown that 
the current flow measurement is independent 
of variations in the velocity of sound. 
INTRODUCTION 
The mechanical-impeller tyoe of instrument 
most commonly used to measure ocean-current 
speeds is generally unsatisfactory because of 
bearing-friction problems and because inherent 
high-inertia leads to slow response to chang- 
ing rates of flowe Acoustic flowmeters which 
effectively measure the results of the veloc- 
ity of propagation of sound in a fluid and 
the velocity of the fluid with respect to a 
transmitting and receiving transducer have 
been described in the literature.! These in- 
struments are subject to error when used in 
a fluid in which the velocity of provagation 
can vary. In the case of a single-path in- 
strument the error in flow-velocity measure- 
ment is equal to the deviation in the veloc- 
ity of sound from the value pertaining when 
the instrument was calibrated. Refinement of 
the instrument to a two-path type reduces 
this error to the extent that a given percent- 
age variation in the velocity of sound from 
calibration conditions will result in the 
same percentage error in flow measurement. 
The possible variation in the velocity of 
sound over the complete range of oceanographic 
conditions is about 12 per cent so that an un- 
acceptable error in flowmeter readings can be 
introduced in this way. A further objection 
to the type of acoustic flowmeter usually 
described is that the technique involves a 
measurement of phase difference; therefore, 
the requirement of providing sufficient sensi- 
190 
tivity, on the one hand, and the need to avoid 
ambiguity, on the other, become contradictory 
in a wide-range instrument. 
The instrument described in the present 
paper, being developed by Lockheed Missiles 
and Space Company, avoids the difficulties of 
the acoustic-type instruments described above. 
The instrument is basically simple and con- 
sists of two sing-around velocimeters arranged 
so that the transmission paths in the liquid 
are side-by-side and of equal length. The 
directions of pulse travel are opposite in the 
two velocimeters. One velocimeter measures 
the sum of the speed of sound and the speed of 
current flow while the other velocimeter meas- 
ures the difference between the two speeds. 
Therefore, by taking the difference of the 
sing-around frequencies of the velocimeters, 
we achieve a signal having a frequency voropor- 
tional to current flow. 
THHORY OF OPERATION 
In the case of the ideal velocimeter, the 
output frequency (f) is given by 
-c 
oor 
where C is the velocity of propagation and L 
is the separation between transmitter and re- 
ceiver. If one uses two velocimeters sending 
pulses in opposite directions and introduces 
a current flow (v), the two sing-around fre- 
quencies are 
fee Cat 
cy ve 
and 
_CeVv 
i ap 
Then, by taking the difference of the sing- 
around frequencies, we obtain the frequency 
= Suen 
f, = 
