Time Varied Gain (TVG) range correction: Its impact on single target detection algorithms and the calculation of Sv.
Single target detection algorithms
There is a range dependent error in the Echoview Single Target Detection Method 1 algorithm when applied to data that has not had a TVG range correction applied.
There is no error with Single Target Detection Method 2 when the data is non-TVG range corrected nor when the single target variable’s TVG range correction is correctly set.
Most Echoview users are using the correct algorithm for their data, however some may have used (or be using) the Method 1 algorithm when the Method 2 algorithm is more accurate. If you have been using an algorithm that is not recommended for your data the following pages will enable you to determine whether or not any error is likely to be significant compared to other errors in your survey1.
Are my Echoview Single Target Detection results affected?
What is the error if my Single Target Detection results are affected?
I am interested in the theory and would like to understand the background to this issue.
Sv calculations
Simmonds and MacLennan2 discuss the calibration of echosounders that use analogue or digital signal processing. Both the receiver delay and the TVG start time delay are identified as important range corrections for the calculation of TVG for accurate Sv at short ranges.
Some echosounder manufacturers have used equations that are good "long range" approximations that do not take into account the range correction required for accurate Sv at short ranges. Echoview version 4.30 and earlier uses the long range approximations (i.e. ignores the TVG range correction) for calculating Sv for data from these echosounders.
TVG range correction for Sv variables is possible for some types of data. The long range approximation may bias Sv calibrations if the calibration sphere is close to the transducer (even if outside 2 times the near field range) and may affect shallow water work especially when using long pulse lengths.
What is the error if my Sv data is affected?
I am interested in the theory and would like to understand the background to this issue.
1.Simmonds and MacLennan, Fisheries Acoustics Theory and Practice (2005) Table 9.2 Sources of error in acoustic surveys.
2.Simmonds and MacLennan, Fisheries Acoustics Theory and Practice (2005) Acoustic Instruments: Section 3.8.2, Section 3.8.3