18-08-2012, 04:57 PM
Scattering of C-band RADARSAT signal on sloping Earth’s surface affected by forest fire areas
Scattering of C-band RADARSAT signal.pdf (Size: 156.02 KB / Downloads: 24)
INTRODUCTION
The Kangwon province is located in the mountainous
eastern part of Korean peninsular, and provides us with a
typical terrain for geological mapping and geological exploration.
Estimation of the C-band microwave backscattering
coefficient is a difficult process and it makes even more difficult
to identify subtle changes across large scale geological
boundaries. In this study, we will investigate the forest
fire burnt areas against the unburnt areas, in mixed forest
mountains with several lithological types of ground surfaces.
It is also important to distinguish the actual geological
boundaries and the forest fire affected changes in the
backscattering coefficients from a set of space-borne SAR
data.
STUDY AREA AND DATA
Among several fire-damaged areas in Kangwon province
in April 2000, the area near Samchuk city (Fig. 1a), is
heavily damaged. The area of steeply dipping mountains, as
shown in Figure 1b, was selected for this investigation. Figure
2 shows the burnt study area. The small branches and
leaves (pine needles) of the tree canopy are mostly consumed
during the fire, but the large tree trunks remain
standing. The trees in the study area are mostly spruce and
pines but there were also broad leaf trees in the valleys.
The SAR on-board RADARSAT has variable data acquisition
modes, presenting a range of choices in incidence
(imaging) angles (from 10o to 59o), footprints (from 50 km
to 500 km), and resolutions (from 8 m to 100 m).
CHANGE DETECTION
The detection of burnt area was basically connected to the
multi-temporal evaluation of the changes that occurred
between two RADARSAT data, before and after the fire.
Multi-temporal Principle Component Analysis (PCA) was a
useful technique in observing changed area. The changed
area due to fire was successfully observed using this method
(Siegert and Ruecker, 2000). After the PCA transformation,
the first component is related to geomorphology and some
texture information in both images, while the second principle
component depicts the differences between the two
images and thus indicates changes.
CHANGES IN BACKSCATTERING COEFFICIENT
Several areas, which show high relief, were sampled with
layover/shadow masks for investigating the effects of surface
slopes on the radar backscatter over mountainous burnt
scattering surfaces. The incidence angle of RADARSAT
data can be estimated for the spherical earth model (Srivastava
and Shepherd, 1998). The local incidence angle can be
calculated using the local slopes from available DEM using
Equation 3. The forest fire induced changes in the estimated
0 over the burnt area are shown in Figures 6 and 7. The
local incidence angle rather decreases in the front-slope and
increases in the back-slope with respect to the incidence
angle of the RADARSAT F3 data imaging geometry. The
angular trends of changes in 0 show a different aspect in
the front- and back-slope, which increases to the maximum
of 3.3 dB in the front-slope, where the local incidence
angles can range 0-30o, but decreases to the maximum of
4.5 dB in back-slope, where the local incidence angles are
about 60-90o, as shown in Figure 6. Based on the histogram
analysis for the two kinds of slope in RADARSAT data
(Fig. 7), 72% of the estimated backscattering coefficients
increase in the front-slope as shown in Figure 7a, while
93% of the backscattering coefficients decrease in the backslope
as shown in Figure 7b.
CONCLUSION
The effect of steep slopes on the backscattering of C-band HHpolarized
microwave signal, RADARSAT in this case, with
respect to the local incidence angle variations in mountainous
burnt forest terrain are investigated. Because of the
steering capability and the selectable incidence angles in the
RADARSAT SAR system, the optimum mode selection for
a specific application requires careful circumspection. For
geological applications and for the monitoring of land cover
changes in mountainous areas, the RADARSAT imagery collected
at a higher incidence angle should minimize the local
topographic effects in the front-slope.