USER’S MANUAL__________________________________________________________________
210 _________________________________________________________________ M211322EN-D
Note that if the requested number of samples is exactly a power of 2,
then an FFT is used.
As mentioned in Section 6.2.2 Frequency Domain Processing-
Doppler Power Spectrum on page 198, when there is no or very little
clutter, use of a rectangular weighting function leads to the lowest-
variance estimates of intensity, mean velocity and spectrum width.
When there is a very large amount of clutter, then the aggressive
Blackman window is required to reduce the "spill-over" of power
from the clutter target into the sidelobes of the impulse response
function. The Hamming window is used as the first guess. After the
first pass GMAP analysis is complete, a decision is made to either
accept the Hamming results, or recalculate for either rectangular or
Blackman depending on the clutter-to-signal ratio (CSR) computed
from the Hamming analysis. The recalculated results are then checked
to determine whether to use these or the original Hamming result (see
Figure 43 on page 207 for details).
- Step 2: Determine the Noise Power
In general, the spectrum noise power is known from periodic noise
power measurements. Since the receiver is linear and requires no STC
or AGC, the noise power is well-behaved at all ranges. The only time
that the spectrum noise power will differ from the measured noise
power is for very strong clutter targets. In this case, the clutter
contributes power to all frequencies, essentially increasing the
spectrum noise level. This occurs for two reasons: in the presence of
very strong clutter, even a small amount of phase noise causes the
spectrum noise level to increase, and there is significant power that
occurs in the window side-lobes. For a Hamming window, the
window side lobes are down by 40 dB from the peak at zero velocity.
Thus 50 dB clutter targets will have spectrum noise that is dominated
by the window sidelobes in the Hamming case. The more aggressive
Blackman window has approximately 55 dB window sidelobes at the
expense of having a wider impulse response and larger negative effect
on the variance of the estimates.
When the noise power is not known, it is optionally computed using a
dynamic approach similar to that of Hildebrand and Sekhon (1974).
The Doppler spectrum components are first sorted in order of their
power. As shown in Figure 43 on page 207, the sorting places the
weakest component on the left and the strongest component on the
right. The vertical axis is the power of the component. The horizontal
axis is the percentage of components that have power less than the y-
axis power value. Plotted on a dB scale, Poisson distributed noise has
a distinct shape, as shown by the curved line in Figure 43 on page 207.
This shape shows a strong singularity at the left associated with taking
the log of numbers near zero, and a strong maximum at the right where
To Next Page
Loading...