Vaisala RVP900 - Page 192

Some earlier vintages of radar signal processors used an IIR (infinite impulse response) filter
for clutter rejection. The IIR filter, while requiring minimal storage and computation, has
three major drawbacks:
• The infinite impulse response requires a settling time when a transient occurs such as a
PRF change, or a spike clutter target. During the settling time, the transient response
degrades the performance of the
filter.
• The filter is fixed width in the Nyquist interval. This means that it may be suciently
wide to remove moderate or weak clutter, but may not be wide enough to remove all of
the clutter when the clutter power is very strong and consequently wider in the Nyquist
interval. This causes operators to select wider
filters than necessary so that strongest
clutter is adequately removed.
• The
filter does significant damage to overlapped (zero velocity) weather signals, that is,
these are significantly attenuated by the filter.
High-speed processors such as RVP900 oer sucient storage and computational power to
implement frequency domain
filters that, in some cases, are adaptive. Because of the
superiority of these filters, the legacy time domain IIR approach is no longer used in
RVP900. The only mode that uses time domain
filtering is the Batch mode for the low PRF
pulses (subtraction of the average I and Q to remove the DC component).
The frequency domain filters available in the RVP900 are configured using the mf setup
command (See 5.2.3 Mf — Clutter Filters (page 104)):
• Type 0: Fixed width filters with interpolation
• Type 1: Variable width single slope adaptive processing
• Type 2: Reserved (not used at this time)
• Type 3: GMAP
7.3.5.1 Fixed Width Clutter Filters
This filter removes a specified number of spectrum components (5 in the following example)
and then interpolates across the gap using the minimum of a specified number of "edge
points" (2 in the example) to anchor the interpolation at each end of the gap.
This is a fairly simple legacy approach that uses interpolation to repair the damage caused
by the removal of components.
RVP900 User Guide M211322EN-J
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