simulations, at the single frequency of 150 MHz, in-
terns relative to those in the previous figure. In this nose-
cluding soil properties as specified above but adding a
down case, increasing the frequency causes the scatter-
ground surface. That is, the material below the X = 0
ing lobes to coalesce into smaller events on one larger
has the aforementioned electrical properties, while that
background lobe, shifted towards Za < 0. This is even
above it has the electromagnetic characteristics of free
more evident relative to the nose-up case when the Ey
space.
component is examined as well. This background lobe
Figure 20 shows bistatic scattering patterns for inci-
dence at subsurface angles of 45, zero, and 45 rela-
impinges most directly on the flattened target end. At
tive to the vertical axis, where negative incidence angles
the same time, we must note that this end orientation
are taken to be in the first quadrant (positive x and z).
effect is not solely due to localized reactions induced
Analysis based on Fresnel coefficients and superposi-
on the flattened end itself. The dramatic rise of the back-
tion of multiple reflections explains some apparently
scatter for Za < 0 at frequencies corresponding to inte-
gral numbers of L/(λ/2) indicates that this signal mag-
baffling features in the total scattered field shown: as
the wave initially reflected from the target strikes the
nitude enhancement involves the current pattern down
underside of the ground surface, it reflects back down-
the entire length of the target.
wards. This downwards reflection interferes construc-
Overall, in this set of examples we note that there
tively with the upgoing component; that is, it does not
are dramatic shifts in the magnitude of the response,
change sign on reflection. This means that we see val-
depending on which frequencies come to the fore. We
ues of |Ez| increase near the ground surface. We also
also note that the strongest response is sometimes
see an overall pattern of two minima and two maxima
shifted spatially off to the side of the target location,
along certain lines between the target and the surface.
depending on its orientation.
This is basically a standing wave interference pattern
In conclusion, we show three bistatic scattered field
values (|Ez|) over the (x,z) cutting plane, for an inclined
between upgoing and downgoing waves, and possible
projectile. They were obtained from 3-D finite element
examples of this were seen in Figure 7 for transects G
frequencies, when the projectile is inclined at 45 with nose down (θd = 45,
27
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