Chapter 6 ______________________________________________________ Processing Algorithms
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ratio of 10 dB, then the 2nd trip echo will have a signal-to noise-ratio no
better than -10 dB. This is the fundamental problem with using phase alone
to separate the 1st and 2nd trip echoes.
Filtered 1st and 2nd Trip Echoes
Since the strong echo generates noise that obscures the weaker echo, the
approach used in the RVP900 is to filter the echo from the other trip — the
whitening filter. This is shown in the figure. The adaptive whitening filter
removes both the clutter and the weather. All of the phase information for
the other trip is then contained in the white noise portion of the spectrum.
The phase information under the coherent echo that is removed will be
dominated by the coherent echo, that is, the other trip phase information
will be contaminated. For this reason, the filtering should effect as small a
region of the spectrum as possible.
6.8.3 Tuning for Optimal Performance
The Random Phase algorithms are controlled by the same collection of
setup and operational parameters that apply to all of the other processing
modes, for example, choice of sample size, clutter filter, angle sync,
calibration, etc. However, a few parameters are special to Random Phase
mode, and these are described below.
Secondary SQI Threshold
In standard Doppler processing, an SQI threshold is normally not applied
to reflectivity data, because it would cause those data to be rejected in
regions of high spectral width. However, in Random Phase mode, if SQI is
applied to reflectivity or dual pol data, we need to relax this convention,
specially in random phase processing, because reflected power can only be
assigned to a particular trip when it is coherent within that trip. Incoherent
echoes, regardless of their strength, can not be placed into either trip.
Thus, an SQI threshold is required to qualify reflectivity and dual pol data
in all the processing modes. The RVP900 defines a secondary SQI
threshold SQI
2
which is computed from the standard threshold value
simply as:
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
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