Super tank Project - Offshore Group 



Univ. of Florida (ACP / OBS) 



^ 



Wqvo Oaugo 



Offahox* 



Ohio State Dnlv. N 



KHe Single Beam 



Figure 8-3. The relative position of offshore instruments 



OSU were mounted on the mobile carriage, approximately 1 m shoreward of 

 the center pole. Appendix H (contained in Volume II of this report) includes 

 a figure showing the six locations where data were collected and the relative 

 instrument alignments resulting from each location. The fde listings in the ap- 

 pendix include the position of the instruments for each file. 



Instrument Characteristics 



Instrument 1. The 2.4-MHz system processes the data for both velocity 

 and amplitude information. In the configuration used in SUPERTANK, the 

 system had three operational transducers, each with a ceramic diameter of 

 3.18 cm. The total dynamic range is approximately 70 db. 



The acoustic return signals from the 2.4-MHz transducers are fed into a 

 pre-amplifier and then processed using a logarithmic amplifier (Signetics 

 SA 604). The amplifier has two outputs: a hard-limited signal that preserves 

 the phase information of the input and a Receive Signal Strength Indicator 

 (RSSI). The first output is used to calculate the Doppler shift. The RSSI is a 

 0- to 5-V signal that is proportional to the logarithm (base 10) of the filtered 

 input signal. The RSSI is low-pass filtered twice. The time constant of the 

 filters is 50 /isec, equivalent to a 4-cm propagation distance. After filtering, 

 the output signal is fed into an 8-bit A/D converter (0-255 "counts") and 

 recorded. The sampling rate is set such that the vertical resolution in the 

 RSSI data is roughly 2 cm. 



The response of the logarithmic amplifier was calibrated by feeding a 

 signal directly into the receiver and monitoring the output, as the input level 

 changed. The calibration curve is shown in Figure 8-4. The input has units 

 of decibels and is referred to a 1-V reference signal. A least-squares fit gives 



Chapter 8 ADCP Measurements at SUPERTANK 



141 



