Correlation is a statistical measure of similar behavior between two observables, which in our case is how similar the transmitted signal is to itself at a delayed time (when the signal is received after reflection). Correlation is output in %, where 100 % means perfect correlation and 0 % means no similarity. The magnitude of the correlation is thus a quality measure of the velocity data, and as the correlation decreases so does the data accuracy. Correlation decreases with distance from the instrument and establishes the maximum usable profiling range. A commonly accepted threshold for range when considering correlation data is 50 %. Figure 1 shows how a threshold of 50 % can be justified, by how the standard deviation increases as the correlation drops.
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