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Figure 19.4 X-band VV-polarised SAR backscatter image acquired by the airborne
E-SAR system over Monks Wood National Nature Reserve, East Anglia, UK in 2000.
Spatial resolution
3 m. Data were provided by the German Aerospace Centre (DLR).
=
mapping to some extent (Imhoff, 1995). Additional errors may arise from effects
of vegetation structure, which can infl uence radar backscatter to a similar magnitude
as biomass dependence.
While radar backscatter images like the one shown in fi gure 19.4 can be acquired
from a single data acquisition, coherence-based approaches using SAR interferom-
etry (InSAR) require at least two SAR acquisitions of the same area at a suitable
spatial baseline (the distance separating the viewing positions of the sensors at the
time of the acquisitions). These techniques exploit the fact that the coherence
between the two SAR images tends to be lower for areas with high forest biomass
due to temporal and volume decorrelation processes. In boreal forest, this method
works best in winter, when the entire biome is frozen and the scattering elements
(branches, needles, stems and ground) do not change in water content or geometric
properties between the two image acquisitions. Because SAR is an active sensor, it
can acquire data during the boreal winter night-time when the use of passive sensors
is limited. Superior performance of wintertime coherence for boreal forest stem
volume retrieval was reported by Pulliainen et al. (2003). Forest stem volume was
also retrieved from ERS-1 and ERS-2 C-band tandem coherence (one day between
acquisitions) under winter conditions in Finland and Sweden by Askne and Santoro
(2005), and from L-band JERS-1 wintertime coherence (44 days between acquisi-
tions) in Siberian taiga forest (Eriksson et al., 2003).
Phase-based approaches measure the location of the scattering phase centre
within the canopy layer, from where most of the backscatter is originating. The
scattering phase centre in a forest is the 'integral of the returns from a large collec-
tion of scatterers, which include ground, stems, branches and leaves or needles'
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