Time-Frequency Analysis of Radar Range Profiles - Time-Frequency Transforms for Radar Imaging and Signal Analysis - page 88

and only the scattering center part of the original signal remains in the
image, as expected. Figure 4.13(c) shows the frequency-aspect display of the
high-variance Gaussians. Several very distinct equispaced vertical lines are
observed. They correspond to the resonant frequencies of the cavity, which
should occur at 5.30, 8.39, 11.86, and 13.52 GHz based on the dimensions
of the cavity. Indeed, we see that they occur close to these frequencies and
are almost aspect independent.
The algorithm is next demonstrated using the chamber measurement
data of a 1:30 scale model Lockheed VFY-218 airplane provided by the
Electromagnetic Code Consortium [30]. The airplane, shown in Figure
4.14(a), has two long engine inlet ducts that are rectangular at the open

Figure 4.14 (a) The VFY-218 model; (b) its standard ISAR image obtained for f = 8 to 16

GHz and a 40-degree angular window centered at 30 degrees from nose-
on; (c) enhanced ISAR image obtained by applying the adaptive Gaussian
representation to the ISAR image (the inlet cloud has been removed from
the original ISAR image); and (d) the extracted resonant features of the inlet
are shown in the frequency-aspect plane. ( Source: [31] Ó 1997 IEEE.)