GMAP Step Description
Step 3: Remove the Clutter
Points
The inputs for this step are the Doppler power spectrum, the assumed
clutter width in m/s and the noise level, either known from noise
measurement or optionally calculated from the previous step. First the
power in the three central spectrum components is summed (DC ±1
component) and compared to the power that would be in the three central
components of a normalized Gaussian spectrum having the
specified clutter
width and discretized in the identical manner. This serves as a basis for
normalizing the power in the Gaussian to the observed power. The Gaussian
is extended down to the noise level and all spectral components that fall
within the Gaussian curve are removed. The power in the components that
are removed is the "clutter power".
A subtle point is the use of the three central points to do the power
normalization of the actual vs the idealized spectrum of clutter. This is more
robust than using a single point since for some realizations of clutter targets
viewed with a scanning antenna, the DC component is not necessarily the
maximum. Averaging over the three central components is a more robust
way to characterize the clutter power.
The very substantial algorithmic work that has been done so far is to
eliminate the proper number of central points. The operator only has to
specify a nominal clutter width in m/s. This means that the operator does
not need to consider the PRF, wavelength or number of spectrum points-
GMAP accounts for these automatically.
A key point is that in the event that the sum of the three central components
is less than the corresponding noise power, then it is assumed that there is
no clutter and all of the moments are then
calculated using a rectangular window. If the power in the three central
components is only slightly larger than the noise level, then the computed
width for clutter removal is so narrow that only the central (DC) point shall
be removed. This is very important since, if there is no clutter then we want
to do nothing or at worst only remove the central component.
Because of this behavior, there is no need to do a clutter bypass map, that
is,
turn-o the clutter filter at specific ranges, azimuths and elevation for
which the map declares that there is no clutter. Because of the day-to-day
variations in the clutter and the presence of AP, the clutter map is often
incorrect. Since GMAP determines the
no-filter case automatically and then
processes accordingly, a clutter map is not required.
RVP900 User Guide M211322EN-J
196
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