Vaisala RVP900 - Adjusting Threshold Qualifiers; Table 56 Default Threshold Combinations; Table 57 Adjusting Threshold Qualifiers

Table 56 Default Threshold Combinations
Parameter Description Threshold
dBZ
Reflectivity with clutter correction LOG and CCOR
dBT
Reflectivity without clutter correction
LOG
V
Mean velocity SQI and CCOR
W
Spectrum width SQI and CCOR and SIG
Dual Pol
Dierential reflectivity
LOG
7.5.2 Adjusting Threshold Qualifiers
When optimizing thresholds for your application, it is recommended that you change only
one parameter (level or criterion) at a time so that you can verify the
eect.
The following table shows some tips for optimizing the levels for the default criteria.
Table 57 Adjusting Threshold Qualifiers
Qualifier Adjustment
LOG
To optimize the LOG level, display dBT or dBZ and select the lowest value of the threshold
that eliminates the display noise. If the LOG level is set too high you lose sensitivity. Note that
if you average more pulses or ranges, then the threshold level can usually be reduced.
SQI
To optimize the SQI level, display velocity and select the lowest value of the threshold that
eliminates the display noise. If the SQI level is set too high you lose sensitivity. In general, you
should see a greater area covered by velocity than reflectivity since the velocity is more
sensitive. If you do not, you should reduce your SQI threshold. Note that if you average more
pulses or ranges, then the threshold level can usually be reduced.
CCOR
This is used to eliminate clutter targets that are very strong. It should not be set to eliminate
all clutter targets on a clear day since this means that you are losing sensitivity. To optimize
the CCOR threshold it is best to know your system coherency in terms of dB of clutter
cancelation. Start at a value of 10 dB greater (closer to 0) than this. Now display a PPI of dBZ
at an antenna elevation of approximately1 degree. The display should be relatively clean of
any clutter targets since most are rejected. Now reduce the CCOR (more negative) to increase
the number of clutter targets on the display until the number of clutter targets does not
increase. The optimum value of the CCOR is approximately 5 dB more (closer to zero) than
this point. For example, if the number of clutter targets is a maximum at -35 dB, then set the
CCOR to approximately - 30 dB. Note that your clutter
filter selection eects the result.
SIG
This should be done last. To optimize the SIG level, display the width W and select the lowest
value of the threshold that eliminates the display noise. If the SIG level is set too high you lose
sensitivity. If you average more pulses or ranges, then the threshold level can usually be
reduced.
Chapter 7 – Processing Algorithms
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