Figure 8-4. The RSSI output as a function of source level. The coefficients 

 derived in Equation 8-1 are based on linear regression. 



the following coefficients for a linear regression: 

 db = counts* 0.46 + constant 



(8-1) 



The response of the log amplifier is close to linear over the full range. The 

 maximum deviation observed in Figure 8-4 is 2 db in the range between 170 

 and 180 counts. 



Instrument 2. The 600-kHz system uses a single transducer with a diam- 

 eter of 10.16 cm. It processes all information linearly, with about 42 db of 

 instantaneous dynamic range in any one configuration, and a total range of 

 approximately 100 db. To use the full dynamic range, the system must be 

 stopped and restarted. 



Signal processing in the 600-kHz system provides considerable flexibility. 

 The return signal from the transducer is fed into a linear pre-amplifier, 

 providing between and 50 db of gain. The pre-amplifier gain is software 

 selectable before each data collection period. After amplification, the signal is 

 fed into an 8-bit analog-to-digital (A/D) converter, sampling at 5 MHz. The 

 output from the A/D is stored directly onto the computer hard drive. The 

 high sampling rate allows for complete resolution of the return signal and 

 considerable flexibility in post-processing. It is also computationally 

 intensive, producing as much as 12 KB/sec. Backscatter from the water 

 column and from the bottom cannot be resolved within a single ping since the 

 difference in scattering strength exceeds the instantaneous dynamic range of 

 42 db. 



142 



Chapter 8 ADCP Measurements at SUPERTANK 



