Dual-PRF unfolding

Dual-PRF velocity unfolding is computed within the 3x3 filter when both are enabled. There are two steps to the process:

1. The most recent and the previous ray are used. For every valid point in the most recent ray, the algorithm performs a search among the three nearest neighbors in the previous ray to find a valid velocity. The search pattern is shown at the bottom of 2D 3x3 Filtering Concept Examples. This larger selection of alternate-PRF bins makes it more likely that the algorithm finds the pairs of Low/High PRF data that are required for unfolding.
2. The unfolded velocities are subjected to standard 3x3 filtering.

When performing Dual-PRF scans, it is possible to collect data to the maximum unambiguous range of the low-PRF radials. Normally, this would result in the high-PRF radials showing no data as their maximum unambiguous range are shorter. However, the 3x3 filter is used to interpolate across this gap and fill in the missing data of the higher-PRF radials. The process is the same as described in 2D 3x3 Speckle Filter Rules, but because there are two input bins less for the evaluation, the number of valid bins required to interpolate is also lowered by two in this case.