Both amplitude and correlation are used as quality indicators for velocity data, and either can be applied to determine the maximum profiling range. Since they are based on different physical principles, they are sensitive to different types of abnormalities.
Typically, the correlation decreases to around 50% at the same range where the amplitude approaches the noise floor, corresponding to a signal-to-noise ratio (SNR) of approximately 3 dB.
In post-processing, applying a correlation threshold of 50% is a robust method for filtering out data points dominated by noise and for identifying the effective maximum range.
Note that not all instruments measure correlation, for these the focus has to be on the amplitude.
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