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cidence angles are different because one group is in the
near range and the other in the far range. It is found
that for buildings in the near range where the inci-
dence angle is about 309, the backscattering is dom-
inated by single bounce scattering. According to our
observation, the roofs of most of buildings, consisting
of plain tiles, are tilted about 30 — 35° from vertical.
Therefore, strong specular reflections from the roofs
are expected. Figure 1 shows the measured P-band
polarisation signatures by these buildings at the inci-
dence angle of 30°. Shown in the figure are also the-
oretical polarisation signatures by single bounce scat-
tering for comparison. In contrast, when the incidence
angle becomes 60° for the buildings in the far range,
the roofs are not perpendicular to the radar any more.
What the radar measures in this case is a strong dou-
ble bounce scattering from the wall-ground structures.
Figure 2 shows the observed P-band polarisation sig-
nature by these buildings at the incidence angle of
60°. Shown in the figure are also theoretical polari-
sation signatures by double bounce scattering from a
wall-ground structure for comparison.
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Figure 2: Top: Measured P-band polarisation signa-
tures from buildings facing radar at incidence angle
of 60°. Bottom: theoretical polarisation signatures by
double bounce scattering.
Tables 1 and 2 list the percentages of single, double
and Bragg scattering components of the HH polari-
sation for these two groups of residential buildings at
P-, L- and C-bands. The error between the predicted
and the measured values is also given in the tables.
It can been seen that while a dominant single bounce
scattering component is decomposed for the buildings
at the incidence angle of 30°, a strong double bounce
scattering component is attributed to the buildings at
the incidence angle of 60°.
One can see from Table 1 or Table 2 that the decom-
Table 1: Scattering components as a percentage of
the total HH backscattering response for residential
buildings at incidence angle of 30°.
Band | Single% Double % Bragg % Error %
P 65 35 0 3.4
L 63 37 0 5.1
C 59 41 0 2.8
Table 2: Scattering components as a percentage of
the total HH backscattering response for residential
buildings at incidence angle of 60°.
Band | Single % Double % Bragg % Error %
p 25 75 0 3.2
L 22 78 0 -5.4
C 3 70 0 6.8
positions are similar at all three bands, which implies
that the polarisation signatures at all three bands are
similar for each group of buildings. One of the possible
reasons to explain this is that the large rigid structures
of built targets may appear not significantly different
to P-, L- and C-band radars. Therefore, statistically,
all three band radars capture similar polarisation sig-
natures. As will be seen in the following subsections,
the situation for forested areas is totally different.
4.2 Forests
The decomposition for native eucalypts is listed in
Table 3. The components of double bounce scatter-
ing decrease from P- to C-band, because foliage at-
tenuation increases with an increase of the frequency.
It is the foliage attenuation that masks the double
bounce scattering from the trunk-ground structures.
On the other hand, the single bounce components in-
crease from P- to C-band, as the backscattering from
the top layer increases with increase in frequency. Fig-
ure 3 shows the co- and cross-polarisation signatures
for the native eucalypts at P-, L- and C-bands. It can
be observed that while the double bounce scattering
dominates the P- and L-band signatures, the single
bounce scattering features in the C-band signature.
In addition, pedestals (co-polarisation at x — 2:45?)
are high for all bands. Physically, this means that
the responses of RR (right-handed transmission, right-
handed receiving) and LL (left-handed transmission
and left-handed receiving) circular polarisations are
significant. A field of right-handed circular rotation
becomes a field of left-handed circular rotation after a
single bounce of the specular reflection and vice versa
for pure conducting material. The VH (HV) compo-
199
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996
