Signal Quality Index (SQI threshold)

Using the signal quality index (SQI), RVP10 can eliminate signals which are either too weak to be useful, or which have widths too large to justify further analysis.

SQI is defined as:

S Q I = \frac{| R_{1} |}{R_{0}}

The SQI is the normalized magnitude of the autocorrelation at lag 1 and varies between 0 for an uncorrelated signal (white noise) and 1 for a noise-free zero-width signal (pure tone).

Mean velocity estimates are degraded when the spectrum, width is large or when the signal-to-noise ratio is weak.

The SQI is a good measure of the uncertainty in the velocity estimates and is a convenient screening parameter to compute.

For very large signal-to-noise ratios (SNR) the SQI is a function of the spectrum width only. For a zero-width pure tone (W=0), the SQI is a function of the SNR only (for example, for W=0, an SNR of 1 corresponds to SQI=0.5). The SQI threshold is typically set to a value of 0.4 ... 0.5.

In terms of the Gaussian model, the SQI is :

S Q I = \frac{S N R}{S N R + 1} e^{\frac{- π^{2} W^{2}}{2}}

where the SNR is the signal-to-noise ratio.