A DOPPLER CURRENT METER 



F. F. KOCZY, M. 



KRONENGOLD and J. 



University of Miami 

 Miami, Florida 



M. LOEWENSTEIN 



ABSTRACT 



A Doppler current meter designed and assembled 

 at the Institute of Marine Science, University 

 of Miami, was tested in model tanks and in the 

 ocean. The output signal under representative 

 oceanographic conditions was recorded and 

 analyzed. 



INTRODUCTION 



An acoustic current meter based on the Doppler 

 shift principle was designed and assembled early 

 in I96I and tested in order to study character- 

 istics of the output signal and evaluate the limi- 

 tations of this meter. Doppler shift meters are 

 of interest because they have high sensitivity, 

 are inherently self-calibrating, have good tran- 

 sient response and have no moving parts. During 

 a period from June 1961 to June 1962 tests were 

 conducted in a towing tank and in representative 

 oceanographic environments . 



The instrument consists of a transmitting and 

 receiving transducer operating on a 5 Mcps 

 acoustical signal. The receiver is a single con- 

 version superheterodyne type which provides a 

 good signal to noise ratio. The intermediate 

 frequency passband was shifted upwards to 

 uniquely select the upper sideband signals 

 derived from flow towards the receiver. Addi- 

 tion of double conversion and double sideband 

 detection could provide instantaneous data on 

 flow direction and magnitude. 



THEORY 



The Doppler shift or frequency change that 

 results when a transmitted wave is reflected 

 from a moving object may be expressed as: 



where c is velocity of sound, f is transmitted 

 frequency, f ^ is transmitted frequency shifted by 

 the Doppler effect, c is positive when movement 

 is towards the receiver and K is unity if the 

 transmitter and receiver heads are identically 

 located and the reflecting object moves in direc- 

 tions parallel to the transmitted wave. For other 

 angles, the angular relationship of the trans- 

 mitting and receiving heads to the direction of 

 the reflecting object must be recognized in the 

 formula. In the interest of simplicity, K com- 

 bines these angular functions since our objective 

 was not to prove the Doppler theory but study the 

 meter characteristics. 



The instrument is self-calibrating if f is 

 held constant, K is accurately determined and c 

 is known. Frequency is held to a 0.002$ tolerance 

 using a precision quartz crystal and instability 

 is not a significant source of error. If a vane 

 arrangement is used to align the instrument into 

 the direction of current flow and the relative 

 head angles are accurately measured, K becomes a 

 known constant . 



At a current speed of 2 fps an error in sound 

 velocity determination of 5 fps results in an 

 error of less than 0.002 fps in speed indication. 

 When sound velocity is determined indirectljr- 

 the resulting errors introduced are shown in 

 Table I. 



Table I. 



Measurement Error C Error 



Temperature: 0.036°F 0.18 fps 



Salinity: 0.02 parts 



per thousand 0.086 fps 



Depth: 5 feet 0.091 fps 



fK(c+v) 



(l) The error magnitudes are in excess of those 

 generally incurred in oceanographic tests. 



Contribution No. ^21 from the Marine Laboratory, University of Miami . 



Superior numbers refer to similarly numbered references at the end of this paper. 



127 



