Vaisala RVP900 - Page 232

where Slope and Offset are the secondary SQI threshold parameters defined in the Mf
setup section.
The factory default values are (Slope = 0.50) and (Offset = 0.05), that is, the secondary
threshold is a little less than half of the standard value.
The algorithms check whether the SQI of each recovered trip is less than the secondary SQI
threshold, and if so, the LOG portion of the data are rejected. This SQI test is necessary for a
clean LOG picture, but we need to use a more permissive (lower) threshold value than would
usually be applied to the reflectivity and dual pol data alone.
The Slope and Offset values should be adjusted so that the density of speckles in LOG
data is approximately the same as the density of speckles in FFT velocity data for a given
primary SQI value. You may then adjust the primary SQI threshold to achieve the
appropriate
trade-o of speckles versus sensitivity for your system in all modes of
operation. Even with proper adjustment, it is normal for dual pol, dBZ and dBT data to show
"holes" in regions of weather that have high turbulence or shear when SQI threshold is
applied to that data. These dropouts usually match similar gaps in the velocity and width
data, both of which are traditionally thresholded by SQI.
Maximum Power Ratio Between Trips
The adaptive
filtering that is performed on the data for each trip greatly extends the
visibility of a weak echo that is overlapped with a much stronger one. In practice, the
filtering process is often able to remove 25 ... 35 dB of dominant power in order to reveal a
much weaker echo in the other trip. The performance depends on many factors, primarily
the spectral width of the dominant echo, and the overall stability of the radar system.
The
diculties of removing a dominant "other trip" echo from a weather signal are
analogous to the challenge of removing a dominant clutter target from that same signal. In
both cases we are trying to extract a weak weather signature using a
filtering procedure that
relies on the spectral confinement of the stronger signal.
RVP900 has a parameter that can be adjusted to control sub-clutter visibility, that is, the
Clutter-to-Signal Ratio (CSR). Just as the CSR applies to the clutter filters, it can also be
used to place similar limits on the depth of visibility of the adaptive filters.
As an example, if RVP900 is operating in random phase mode at a PRF of 1500Hz, and is
observing widespread weather having uniform intensity in both the first 100Km trip and the
second 100Km trip. If the CSR were set too conservatively at only 15 dB, then the algorithm
would generally be blind to second-trip weather in the range interval from 100 Km ... 100
km.
The explanation for this can be found in the 1/r
2
geometric correction for weather echo
intensity. At ranges less than 17.8 km, the
first trip weather would generally dominate the
second trip weather by more than 15 dB. Thus, the initial 17.8 km ring of second trip data
would be rejected by the CSR criteria. However, if the CSR were increased to 30 dB, then the
size of this missing ring would be reduced to only 3.2 km.
If the CSR is set too low, there is an abrupt ring of missing data in the beginning of the
second trip. If set too high, there are speckles and other spurious
eects within this same
interval. The optimum setting should strike a balance between these two eects.
RVP900 User Guide M211322EN-J
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