## 4 Performance
### Performance in a Field Environment  
Bonneville Dam spans the Columbia River between Oregon and Washington, approximately 234.3 km from the mouth of Columbia River and approximately 64 km east of Portland, OR. The spillway, constructed between 1933 and 1937, is usually acoustically the noisiest environment among the lower Columbia River dams.  
In the field, the received waveform may be distorted by its passage through water, which in almost all cases is not homogeneous, and by multipath. Multipath distortion of a microtransmitter message is caused by the near simultaneous arrival at the hydrophone of two copies of a microtransmitter message that travel by different paths and differ in time of arrival by less than the transmit duration of the message. Most commonly, multipath results from an overlap in time at the receiver of a direct path and an indirect path message in which the indirect path message has been reflected off the water surface, bottom, or structures in the water. In all cases, the indirect path message lags the direct path signal. The message length in water is approximately 1.1 m; thus, an echo whose path length is less than 1.1 m  
Figure 8: Decoding efficiency of a JSATS cabled hydrophone in an ideal environment.  
longer than the direct path will overlap the direct path signal. If multipath signals do not overlap, they can result in more than one valid detection of a single transmission. For actual field deployment, individual hydrophones are usually baffled by plastic cones lined with an anechoic material to exclude loud noises emanating from structures downstream of the hydrophones. Baffling greatly increased the ratio of tag signals relative to background noise levels and significantly increased the percentage of successful tag decodes. For the performance results reported in this article, the hydrophones were not baffled.  
The detection efficiency is defined as the number of detections divided by the number of transmissions. The transmitter was suspended in the water column at various distances from the receivers. The number of transmissions was estimated directly using the first and last detection time and the transmit interval of 1.04 s. If the transmitter was not decoded at the time expected, the original waveform would be reviewed manually to determine if it was a valid message. It would be counted as a detection if it was a valid message from the manual review. The detection efficiency was above 99.5% up to 107 m and 71.3% at 122 m.  
Decoding efficiency was the number of valid decodes divided by the number of transmissions. If the time gap between decodes for the same tag code was less than 0.3 s (usually 0.001 to 0.005 s), then there was a multipath signal. Multipath decodes were subtracted from the raw total to compute the number of total valid decodes. One DSP + FPGA card was used to control two channels. The decoding efficiency was above 53% for channel 1 and 76% for channel 2 up to 107 m [Figure 9(a)].  
Figure 9: Field performance results at Bonneville Dam spillway: **(a)** efficiency _vs._ range; **(b)** SNR _vs._ range.  
Background noise was computed as the mean value of the cross-correlation values of rejected packets that did not meet the criteria of the JSATS detector. SNR was defined as the ratio of the peak cross-correlation value of a candidate message over the background noise. For a given environment, SNR is a function of detection range and decreases as range increases. At Bonneville Dam spillway, SNR started decreasing quickly after 50 m and was only approximately 4 at 122 m [Figure 9(b)].