Vaisala RVP900

USER’S MANUAL__________________________________________________________________
250 _________________________________________________________________ M211322EN-D
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-off 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 will usually match up with
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 dB
to 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 difficulties 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. The RVP900 already 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 likewise be used
to place similar limits on the depth of visibility of the adaptive filters.
As an example, suppose that the 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 to 117.8 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.