Vaisala RVP900 - Tuning for Optimal Performance

Raw 1st and 2nd Trip Echoes
This figure shows how the echoes from the first trip and second trip interfere with each
other. For the case of a standard magnetron system, the first trip echo is coherent, while the
second trip echo is incoherent (white noise) since the phase of the second trip echo is
random. This is because the receiver is phase locked only to the first trip.
Another way to implement a magnetron system is to let the COHO free- run (rather than
phase locking to the transmit pulse), measure the phase of each transmit pulse and digitally
correcting for the transmit phase. Using this digital phase locking technique, the RVP900
can phase lock or "cohere" to either the
first or the second trip.
Using this technique alone, it is possible to distinguish between 1st and 2nd trip echoes for
the case when the echoes are not overlapped. The echoes appear as the idealized first and
second trip echoes. This range de-aliasing
eectively doubles the range of the radar. The
problem is that when echoes are overlapped, the noise contamination from the stronger
echo make it impossible to measure the weaker echo. This is illustrated in the figure. Thus if
the
first trip echo has a good signal-to-noise ratio of 10 dB, then the 2nd trip echo has 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
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 is dominated by
the coherent echo, that is, the other trip phase information is contaminated. For this reason,
the
filtering should eect as small a region of the spectrum as possible.
7.9.2
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, and so on.
However, a few parameters are special to random phase mode, 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, especially 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, cannot be placed in either trip.
An SQI threshold is required to qualify
reflectivity and dual pol data in all the processing
modes. RVP900 defines a secondary SQI threshold SQI
2
that is computed from the
standard threshold value as:

2
=  +    × 
Chapter 7 – Processing Algorithms
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