Page 404 - Fundamentals of Radar Signal Processing

estimator also allows reduction of the noise before the estimates are calculated,

reducing the estimate bias. This process, called spectral subtraction, is
depicted in Fig. 5.32. The noise power spectrum N(ω) is estimated from a
presumed clear region of the spectrum and then simply subtracted off to form a
reduced-noise power spectrum

FIGURE 5.32 Spectral subtraction.

(5.122)

Because of the statistical variations of any given data set, it is possible that
may have some negative values; these are usually set to zero.
Figure 5.33 shows two images from the KFFC WSR-88D NEXRAD
weather radar located in Peachtree City, Georgia, just south of Atlanta and
operated by the U.S. National Weather Service. The images were collected on

March 19, 1996 and show a heavy storm in the area. While these images are
much more easily viewed in color than in grayscale, some features are visible.
The image on the left is a map of the power estimate, proportional to the volume
reflectivity η. Lighter grays represent areas of heavier rainfall. The image on the
right is a map of the radial velocity measured by the radar and thus of the wind
speed. The large area on the left and top-left of the image with the circular inner

boundary represents a range-aliased region where reliable velocity estimates
are not available. The radar itself is at the center of the circle having this
boundary. Inside that radius, the black area at the top represents a high velocity
toward the radar, while the dark gray area inside the circular contour and to the
left and bottom of the image represents high wind speeds away from the radar.
Thus, the wind is blowing from the top to the bottom in this image. The various
square and round markers in the power and velocity images are created by the

analysis software and flag various features in the storm.