Chapter 6 ______________________________________________________ Processing Algorithms
VAISALA______________________________________________________________________ 205
6.2.5 Clutter Filtering Approaches
Each major mode implements clutter filtering as follows:
- FFT Major Mode uses frequency domain clutter filters, including
GMAP. The power spectra are restored by interpolating across the gap
of removed spectral points.
- PPP Mode is used only for dual polarization. For efficiency, PPP
computations are performed using DFT techniques that are
algebraically equivalent to the traditional time-domain algorithms.
The fixed width and variable width spectral clutter filters can be used
in PPP mode; however, the power spectra and the spectra of cross-
correlations are not interpolated.
- Random Phase Mode uses frequency domain clutter filters, including
GMAP. The power spectra are restored by interpolating across the gap
of removed spectral points.
- Batch Mode uses a simple DC removal for the small batch clutter
filter. The high PRF large batch is then processed using frequency
domain clutter filters, including GMAP. The power spectra are
restored by interpolating across the gap of removed spectral points.
Earlier vintages of radar signal processors sometimes 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 sufficiently 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 will be significantly attenuated by the
filter.
With the advent of the high-speed processors, such as the RVP900, there
is sufficient 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
the 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).
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