Vaisala RVP900

Chapter 6 ______________________________________________________ Processing Algorithms
VAISALA______________________________________________________________________ 213
filter bias, rather the filter simply "digs a hole" into overlapped
weather.
- Step 5: Check for Appropriate Window and Recalculate the
Moments, if necessary
The clutter power is known from the spectrum components that were
removed in Step 3. Since the weather spectrum moments and the noise
are also known from Step 4, the CSR can be calculated. The value of
the CSR, is used to decide whether the Hamming window is the most
appropriate. The scenarios are described in Figure 43 on page 207.
The end result is that very weak clutter is processed using a
rectangular window, moderate clutter a Hamming window, while
severe clutter requires a Blackman window. Note that if no clutter
were removed in Step 3, then the spectrum is processed with a
rectangular window.
The benefit of adaptive windowing is that the least aggressive window
is used for the calculation of the spectrum moments, resulting in the
minimum variance of the moment estimates
GMAP Configuration
The mf command in the dspx TTY setups is used to configure GMAP
filters. In the section for the spectrum filters select filter "Type 2" and
specify the width of the ground clutter in m/s. This width is determined
largely by your antenna rotation rate so you will want to con-figure several
widths to deal with the different rotation rates in your operational scenario.
An example might be filters indexed 1-5 corresponding to widths from 0.1
to 0.5.
A good practice is to make a scan on a clear day while using ascope or
other utility and ob-serve what the actual width of the clutter is for your
various scan rates. You will need to turnoff the clutter filtering to do this
(pick "filter 0" for the all pass filter).
Example of Implementation
GMAP has undergone extensive evaluation for use in the US WSR88D
ORDA network up-grade (Ice et al, 2004). They conclude that GMAP
meets the ORDA requirements. Their study was based on a built-in
simulator that is provided as part of the RVP900 and the ascope utility. The
simulator allows users to construct Doppler spectra, process them and
evaluate the results (Sirmans and Bumgarner, 1975). This is an essential
tool for evaluating the system performance.
Figure 44 on page 213 shows an example of the simulations for the very
difficult case when the weather has zero velocity, that is, it is perfectly
overlapped with clutter. The upper left graph shows the weather signal